Showing 1–50 of 52 results for author: Notomi, M

Nanocavity Confinement by Orthogonal Valley- and SSH- Topological Interfaces In Glide-Symmetric Photonic Crystal Structures

Authors: Takahiro Uemura, Wei Dai, Yuto Moritake, Masaaki Ono, Eiichi Kuramochi, Masaya Notomi

Abstract: Valley photonic crystals enable valley-dependent transport and chirality-selective emission, but incorporating wavelength-scale localization remains challenging. Existing valley-photonic-crystal cavities rely on finite defects or local lattice modifications that require structure-specific optimization and offer limited continuous control. Here, we theoretically and experimentally demonstrate two-d… ▽ More Valley photonic crystals enable valley-dependent transport and chirality-selective emission, but incorporating wavelength-scale localization remains challenging. Existing valley-photonic-crystal cavities rely on finite defects or local lattice modifications that require structure-specific optimization and offer limited continuous control. Here, we theoretically and experimentally demonstrate two-dimensional nanocavity confinement using two orthogonal domain walls in a glide-symmetric valley photonic crystal. A valley domain wall confines the guided interface mode transversely, while an SSH-like domain wall localizes it longitudinally. Starting from a glide-symmetry-protected Dirac point in a bearded-interface waveguide, controlled displacements of adjacent triangular holes open a topological gap in the continuous guided-mode dispersion. The displacement amplitude $ΔR$ tunes the gap, mode volume, and intrinsic radiative $Q$ factor. Implemented in a silicon photonic-crystal slab, the structure exhibits localized resonances within the topological mode gap and systematic spectral tuning with $ΔR$. The maximum measured loaded $Q$ factor is $1.2\times10^{4}$. This approach enables continuously tunable, high-$Q$ nanocavities integrated into topological waveguide networks for compact resonant devices and enhanced light--matter interactions. △ Less

Submitted 21 August, 2026; v1 submitted 17 August, 2026; originally announced August 2026.

Comments: 32 pages, 10 figures

Inverse-Designed High-Q/V Silicon Nitride Photonic Crystal Cavities for Second- and Third-Harmonic Generation

Authors: M. Takiguchi, P. Heidt, X. Z. Lim, J. Zöllner, S. Yanagimoto, K. Nakayama, T. Aihara, M. Ono, H. Sumikura, M. Notomi

Abstract: SiN photonic crystal (PhC) cavities are promising platforms for nonlinear and quantum photonics because of their wide transparency window, CMOS compatibility, and negligible two-photon absorption. However, realizing high-Q/V cavities remains challenging because of the relatively low refractive index of SiN. Here, we employ inverse design to optimize a two-dimensional SiN PhC cavity and experimenta… ▽ More SiN photonic crystal (PhC) cavities are promising platforms for nonlinear and quantum photonics because of their wide transparency window, CMOS compatibility, and negligible two-photon absorption. However, realizing high-Q/V cavities remains challenging because of the relatively low refractive index of SiN. Here, we employ inverse design to optimize a two-dimensional SiN PhC cavity and experimentally demonstrate a quality factor of approximately 80,000, the highest reported for a near-stoichiometric SiN 2D PhC cavity. Furthermore, both second- and third-harmonic generation are observed from the same cavity, providing experimental evidence of strong optical confinement and large Q/V. Our results establish inverse-designed SiN PhC cavities as a promising platform for nonlinear photonics and future heterogeneous integrated photonic devices. △ Less

Submitted 13 August, 2026; originally announced August 2026.

Mid-infrared spontaneous and stimulated emission dynamics in black phosphorus

Authors: Hisashi Sumikura, Akihiko Shinya, Masaya Notomi

Abstract: Black phosphorus (BP) has attracted attention as a light-emitting layered semiconductor for mid-infrared (MIR) photonics owing to its direct and thickness-tunable bandgap energy, highly anisotropic optical transitions, and potentially suppressed Auger recombination. However, spontaneous and stimulated emission dynamics governed by carrier recombination in BP have remained unexplored because time-r… ▽ More Black phosphorus (BP) has attracted attention as a light-emitting layered semiconductor for mid-infrared (MIR) photonics owing to its direct and thickness-tunable bandgap energy, highly anisotropic optical transitions, and potentially suppressed Auger recombination. However, spontaneous and stimulated emission dynamics governed by carrier recombination in BP have remained unexplored because time-resolved emission spectroscopy is challenging in the MIR region. Here we develop a time-resolved MIR emission microscope combining wavelength upconversion with superconducting single photon detection. This approach enables observation of emission dynamics in photoexcited BP at a wavelength around 4.6 μm with sub-100-ps temporal resolution. Temperature-dependent measurements reveal a crossover from excitonic to electron-hole plasma emission at around 70 K, supported by independent results in the characteristic transition temperature, pump-fluence dependence, and rise and decay dynamics of MIR emission. In a suspended BP structure, we further observe a nonlinear increase in the emission intensity above a well-defined pump threshold. Spectral narrowing and intense picosecond emission pulses appear above the threshold, providing solid evidence of stimulated emission assisted by optical feedback from a Fabry-Pérot cavity. These results elucidate ultrafast carrier dynamics and optical gain formation in BP and establish time-resolved upconversion spectroscopy as a powerful approach for investigating MIR photonic materials and devices. △ Less

Submitted 4 August, 2026; originally announced August 2026.

Transverse spin texture in optical non-Hermitian skin modes

Authors: Naoki Ichiji, Issei Takeda, Taiki Yoda, Yuto Moritake, Masaya Notomi, Satoshi Ashihara

Abstract: In structured electromagnetic fields, polarization textures are often closely linked to the spatial variation of the energy flow. However, this familiar picture has been established mainly for lossless and isotropic settings, and concrete examples showing how it is modified in media with gain and loss remain limited. Here, we demonstrate that optical skin modes associated with the non-Hermitian sk… ▽ More In structured electromagnetic fields, polarization textures are often closely linked to the spatial variation of the energy flow. However, this familiar picture has been established mainly for lossless and isotropic settings, and concrete examples showing how it is modified in media with gain and loss remain limited. Here, we demonstrate that optical skin modes associated with the non-Hermitian skin effect (NHSE) carry a finite transverse circular-polarization texture and further show that the accompanying in-plane electric-field spin texture deviates from the familiar lossless spin-flow picture. Using exact TE mode solutions, we separate the common exponential skin envelope from the oscillatory component. This decomposition shows that the circular-polarization texture is not generated by the skin envelope itself but by the oscillatory interference component modified by non-Hermiticity. It also reveals a handedness bias and a reshaped spatial relation between circularity and intensity. Finite-element calculations confirm that these features remain robust in loss-biased anisotropic media. These results show that gain and loss provide additional freedom for engineering electric-field spin textures beyond conventional lossless photonic settings. △ Less

Submitted 1 June, 2026; originally announced June 2026.

Comments: 13 pages, 7 figures

Chiral-Mode Control around a Hermitian Diabolic Point in Discrete Non-Hermitian Coupled Resonators

Authors: Kota Yagi, Takahiro Uemura, Yuto Moritake, Adam Mock, Masaya Notomi

Abstract: Motivated by the prospect of chiral-mode control in compact photonic systems, we analyze discrete coupled single-mode resonators. Using the minimal three-resonator model, we show that an infinitesimal complex onsite perturbation near a Hermitian diabolic point (DP) induces chiral-mode selection, governed by what we term an asymptotic exceptional point (AEP). Here, an AEP denotes a Hermitian DP equ… ▽ More Motivated by the prospect of chiral-mode control in compact photonic systems, we analyze discrete coupled single-mode resonators. Using the minimal three-resonator model, we show that an infinitesimal complex onsite perturbation near a Hermitian diabolic point (DP) induces chiral-mode selection, governed by what we term an asymptotic exceptional point (AEP). Here, an AEP denotes a Hermitian DP equipped with a non-Hermitian perturbation that induces an asymptotically defective effective Hamiltonian. The eigenvectors coalesce in the asymptotic limit toward the DP, although the Hamiltonian at the point itself remains diagonalizable. Operationally, this AEP response realizes chirality switching from an achiral state to a chiral state. The associated eigenvalue response exhibits the anomalous fractional-power scaling $Δλ \propto {\varepsilon}^{3/2}$, distinct from the square-root response of an ordinary exceptional point (EP). We further show that, in a broader two-parameter perturbation space, ordinary EPs lie on exceptional-line branches that meet at the AEP. A finitebias control sweep crosses these branches at an EP pair, enabling chirality reversal between opposite chiral states. The central message is therefore that the AEP organizes two related routes for chirality switching: direct switching from an achiral state to a chiral state via the AEP, and switching between opposite chiral states via an EP pair in the vicinity of the AEP. Within a finite-resolution averaging model, these two operating points exhibit different practical performance characteristics, and under sufficiently high control resolution, the AEP operating point can become more favorable than the EP-pair operating point, suggesting a route toward compact and low-energy chiral photonic devices. △ Less

Submitted 15 May, 2026; originally announced May 2026.

Comments: 21 pages, 11 figures

Fine Structures of Berry Curvature and Unquantized Valley Chern Numbers in Valley Photonic Crystals

Authors: Wei Dai, Taiki Yoda, Yuto Moritake, Masaya Notomi

Abstract: Valley photonics has emerged as a promising platform in topological photonic systems, yet the topological nature of valley-dependent phenomena remains unsettled. Theoretically, inter-valley scattering may occur with structural imperfections, and global Chern numbers vanish due to time-reversal symmetry. As a result, valley-dependent topology is locally defined around K(K') points in the half-Brill… ▽ More Valley photonics has emerged as a promising platform in topological photonic systems, yet the topological nature of valley-dependent phenomena remains unsettled. Theoretically, inter-valley scattering may occur with structural imperfections, and global Chern numbers vanish due to time-reversal symmetry. As a result, valley-dependent topology is locally defined around K(K') points in the half-Brillouin zone (HBZ). While half-integer valley Chern numbers have been widely assumed, their quantization and topological validity remain controversial. Here, we systematically investigate a continuous spectrum of valley photonic crystal designs by evaluating their Berry curvatures, valley Chern numbers, and angular momenta. We show that valley Chern numbers are generically unquan-tized and instead form a continuous spectrum varying with structural parameters. We further reveal previously unexplored fine structures in the Berry curvature distribution in momentum space. The unquantized valley Chern numbers are attributed to inter- and intra-valley cancellation of Berry curvature, highlighting the absence of a protecting mechanism for quantization. Our results call for a reassessment of valley-dependent topology and provide a more rigorous framework for interpreting valley-related photonic phenomena. △ Less

Submitted 28 March, 2026; originally announced March 2026.

Comments: 12 pages, 8 figures

Journal ref: Phys. Rev. B 113, 195412 (2026)

Silicon photonic optical-electrical-optical converters based on load-resistor and current-injection operation

Authors: Masaya Arahata, Shota Kita, Akihiko Shinya, Hisashi Sumikura, Masaya Notomi

Abstract: Optical-electrical-optical (OEO) converters are key primitives for low-latency, energy-efficient photonic computing because they enable nonlinear activation and optical signal regeneration on chip. We report two monolithically integrated silicon-photonic OEO converters-load-resistor (high-speed variant) and current-injection (high-gain variant) types-fabricated at a silicon photonics foundry. Each… ▽ More Optical-electrical-optical (OEO) converters are key primitives for low-latency, energy-efficient photonic computing because they enable nonlinear activation and optical signal regeneration on chip. We report two monolithically integrated silicon-photonic OEO converters-load-resistor (high-speed variant) and current-injection (high-gain variant) types-fabricated at a silicon photonics foundry. Each device combines a germanium photodetector with a micro-ring modulator (MRM). The converters exhibit reconfigurable nonlinear transfer functions and measurable on-chip RF OEO gain. The RF OEO gain scales linearly with the MRM bias power, with slopes of 0.10 mW^-1 (load-resistor of 10 kΩ) and 1.4 mW^-1 (current-injection), enabling a gain > 1 region at practical bias powers (~10 mW and ~1 mW, respectively). Eye diagrams confirm clear openings up to 4 Gb/s for a high-speed load-resistor variant with a 500-Ω load. To the best of our knowledge, this is the first experimental demonstration of a monolithically integrated, foundry-fabricated silicon-photonic load-resistor type OEO converter exhibiting reconfigurable nonlinear transfer and on-chip RF OEO gain. In the carrier-injection device, the activation slope exceeds unity, yielding 3.9 dB extinction-ratio regeneration. Short-pulse measurements yield 3-dB bandwidths of 1.49 GHz, 160 MHz (load-resistor of 500 Ω and 10 kΩ), and 76 MHz (current-injection), consistent with the RF data. Energy analysis shows an energy-bandwidth trade-off (RC-limited for load-resistor vs. lifetime-limited for injection) and outline routes to sub-pJ/bit operation via reduced capacitance and improved EO efficiency. These results establish silicon-photonic OEO converters as compact, foundry-compatible building blocks for scalable optoelectronic computing and optical neural networks. △ Less

Submitted 7 January, 2026; originally announced January 2026.

Low loss switchable topological photonic crystal enabled by submicron-scale patterning and phase-change of Sb2Se3

Authors: Takahiro Uemura, Yuto Moritake, Eiichi Kuramochi, Masaaki Ono, Hisashi Sumikura, Masaya Notomi

Abstract: Photonic topological insulators (PTIs) offer robust platforms for light manipulation, but reconfigurable control of their topological properties without degrading performance remains a major challenge. While phase-change materials (PCMs) provide large refractive index modulation, widely used materials such as Ge2Sb2Te5 (GST) have been successfully deployed in commercial applications including opti… ▽ More Photonic topological insulators (PTIs) offer robust platforms for light manipulation, but reconfigurable control of their topological properties without degrading performance remains a major challenge. While phase-change materials (PCMs) provide large refractive index modulation, widely used materials such as Ge2Sb2Te5 (GST) have been successfully deployed in commercial applications including optical data storage. However, they exhibit significant optical absorption in their crystalline state, which poses a challenge for transmissive photonic devices such as PTIs where high transparency is essential. Here, we overcome this fundamental limitation by integrating the ultra-low-loss PCM antimony triselenide (Sb2Se3) onto a silicon-based 2D PTI. We achieve submicron-scale selective patterning of Sb2Se3 on a photonic crystal for the first time, and demonstrate a topological phase transition induced by the material phase change. Owing to the transparency of Sb2Se3 in both its amorphous and crystalline states, a high Q-factor on the order of 10^3 is preserved-representing nearly an order-of-magnitude improvement over previous GST-based devices. This work resolves the absorption-loss bottleneck in reconfigurable PTIs and paves the way for practical, low-loss, tunable topological photonic devices. △ Less

Submitted 29 December, 2025; originally announced December 2025.

Observation of non-Hermitian point gap in photonic crystals

Authors: Yuto Moritake, Nozomi Ogawa, Issei Takeda, Yusuke Ohinata, Takahiro Uemura, Taiki Yoda, Kenta Takata, Eiichi Kuramochi, Hisashi Sumikura, Masaya Notomi

Abstract: Non-Hermitian point gap (NHPG) is a unique phenomenon in non-Hermitian systems and induces non-Hermitian skin effect (NHSE). In photonic crystals, NHPG and the NHSE have previously been explored mainly through material loss, where the typically low $Q$ factors make direct observation of complex frequencies challenging. Here, we demonstrate the direct experimental observation of an NHPG by using a… ▽ More Non-Hermitian point gap (NHPG) is a unique phenomenon in non-Hermitian systems and induces non-Hermitian skin effect (NHSE). In photonic crystals, NHPG and the NHSE have previously been explored mainly through material loss, where the typically low $Q$ factors make direct observation of complex frequencies challenging. Here, we demonstrate the direct experimental observation of an NHPG by using a radiation-loss-based non-Hermitian photonic crystal. Radiation loss can be engineered through structural design, enabling control of the imaginary part of the complex frequency and allowing relatively high $Q$ factors. This approach is compatible with widely used absorption-free silicon-slab photonic crystals. We developed a measurement system that can measure photonic bands along arbitrary lines in $k$-space. Our measurements demonstrated direct observation of the NHPG in photonic crystals, and the reversal of non-Hermitian topology through the flip of loop rotation in a complex plane. Our platform, which requires neither gain media nor synthetic dimensions, establishes radiation-loss engineering as a simple and versatile route for photonic functionality using an NHSE in nanophotonic systems. △ Less

Submitted 16 December, 2025; originally announced December 2025.

Journal ref: Phys. Rev. Lett. 137, 046903 (2026)

Inverse-Designed Non-Hermitian Hollow Nanowire Cavity for Generating Optical Orbital Angular Momentum

Authors: Xuen Zhen Lim, Masato Takiguchi, Ryuji Kuruma, Hisashi Sumikura, Masaya Notomi

Abstract: We designed a gallium nitride hexagonal hollow nanowire whispering gallery mode cavity that generates an |m|=6 topological light with orbital angular momentum (OAM). OAM is generated by breaking the cross-sectional mirror symmetry of the nanowire, which creates a non-Hermitian system. This is achieved by replacing the central airhole of the hollow nanowire with a cluster of 6 overlapping circular… ▽ More We designed a gallium nitride hexagonal hollow nanowire whispering gallery mode cavity that generates an |m|=6 topological light with orbital angular momentum (OAM). OAM is generated by breaking the cross-sectional mirror symmetry of the nanowire, which creates a non-Hermitian system. This is achieved by replacing the central airhole of the hollow nanowire with a cluster of 6 overlapping circular air holes with rotational offset relative to the hexagonal cross-sectional profile of the nanowire. The design parameters were then further optimized in Finite Element Method using an inverse design method to maximize the normalized OAM order |l|. We were able to realize of a cavity mode with |l| = 5.7, a mode purity of about 97%, and a Q-factor of ~250. This marks the first OAM generating active photonic device design falling within a sub-micron footprint, with additional novelties of being single component and materialistically homogeneous. △ Less

Submitted 10 May, 2026; v1 submitted 11 December, 2025; originally announced December 2025.

On-chip room-temperature CW lasing from a III-V nanowire integrated with a Si photonic crystal platform

Authors: Masato Takiguchi, Takuro Fujii, Hisashi Sumikura, Akihiko Shinya, Shinji Matsuo, Masaya Notomi

Abstract: We report the demonstration of continuous-wave (CW) lasing at room temperature from a III-V semiconductor nanowire integrated into a Si photonic crystal (PhC) cavity. Conventional hybrid nanowire lasers [M. Takiguchi. et.al., APL Photonics, 2, 046106 (2017)], which typically feature circular nanowire-cross-sections, suffer from a weak optical confinement, preventing CW lasing under ambient conditi… ▽ More We report the demonstration of continuous-wave (CW) lasing at room temperature from a III-V semiconductor nanowire integrated into a Si photonic crystal (PhC) cavity. Conventional hybrid nanowire lasers [M. Takiguchi. et.al., APL Photonics, 2, 046106 (2017)], which typically feature circular nanowire-cross-sections, suffer from a weak optical confinement, preventing CW lasing under ambient conditions. To overcome this limitation, we fabricated nanowires with rectangular cross-sections via dry etching and integrated them into the air trenches of Si PhC cavities formed using atomic force microscope tips. This configuration forms a hybrid photonic crystal cavity with an improved optical confinement. As a result, we achieved room-temperature CW oscillation from a single nanowire, representing a significant step toward on-chip nanophotonic light sources. This unique in-plane integration of the nanolaser in the same plane as the Si slab rather than on top of the substrate will contribute to the development of compact, scalable, and CMOS-compatible photonic circuits. △ Less

Submitted 6 October, 2025; originally announced October 2025.

Higher-order exceptional points unveiled by nilpotence and mathematical induction

Authors: Kenta Takata, Adam Mock, Masaya Notomi, Akihiko Shinya

Abstract: Non-Hermitian systems can have peculiar degeneracies of eigenstates called exceptional points (EPs). An EP of $n$ degenerate states is said to have order $n$, and higher-order EPs (HEPs) with $n \ge 3$ exhibit intrinsic order-scaling responses potentially applied to superior sensing and state control. However, traditional eigenvalue-based searches for HEPs are facing fundamental limitations in ter… ▽ More Non-Hermitian systems can have peculiar degeneracies of eigenstates called exceptional points (EPs). An EP of $n$ degenerate states is said to have order $n$, and higher-order EPs (HEPs) with $n \ge 3$ exhibit intrinsic order-scaling responses potentially applied to superior sensing and state control. However, traditional eigenvalue-based searches for HEPs are facing fundamental limitations in terms of complexity and implementation. Here, we propose a design paradigm for HEPs based on a simple property for matrices termed nilpotence and concise inductive procedure. The nilpotence guarantees a HEP with desired order and helps divide the problem. Our inductive scheme repeatedly extends a system and doubles its EP order, starting with a known design. Based on the nilpotence, we systematically design photonic cavity arrays operating at chiral, passive, and active HEPs with $n = 3, 6, 7$ and show their peculiar directional radiation, induced transparency, and enhanced transmittance and spontaneous emission, respectively. We inductively find lattice systems with diverging EP order originating from a well-known $2 \times 2$ parity-time-symmetric Hamiltonian. We also extend the active HEP system with $n = 7$ to another with $n = 14$ and have further magnified responses. Our work pushes the investigation and application of HEPs to previously unexplored regimes in various physical systems. △ Less

Submitted 9 November, 2025; v1 submitted 1 October, 2025; originally announced October 2025.

Comments: 23 pages, 6 figures, full version including Supplemental Information

Journal ref: Commun. Phys. 9, 274 (2026)

BIC slow light waveguides based on interband coupling

Authors: Sae R. Endo, Yuta Tanimura, Takahiro Ito, Kenta Takata, Takahiro Uemura, Masaya Notomi, Satoshi Iwamoto, Yasutomo Ota

Abstract: Harnessing bound states in the continuum (BICs) for guiding light in leaky environments has unlocked new possibilities in photonic integrated circuits. BIC confinement enables low-loss waveguiding of leaky transverse-magnetic (TM) modes in etchless waveguides based on dielectric wires loaded on plane slabs. We have recently reported BIC slow light waveguides by introducing one-dimensional photonic… ▽ More Harnessing bound states in the continuum (BICs) for guiding light in leaky environments has unlocked new possibilities in photonic integrated circuits. BIC confinement enables low-loss waveguiding of leaky transverse-magnetic (TM) modes in etchless waveguides based on dielectric wires loaded on plane slabs. We have recently reported BIC slow light waveguides by introducing one-dimensional photonic crystals into such etchless waveguides. However, they were restricted to a high-symmetry point ($X$ point), limiting their applicability. In this Letter, we propose and numerically demonstrate BIC slow light waveguides at off-high-symmetry points by exploiting Friedrich-Wintgen BICs, arising from the interband coupling of two guided modes sharing a radiation continuum. We identified a systematic approach for tuning the loss minimum position in momentum space and simultaneously achieved a high group index over $100$ and a low propagation loss of less than $5 \times 10^{-2}~\mathrm{dB/cm}$ at an off-high-symmetry point. Our findings pave the way for advanced control of light-matter interactions in non-Hermitian photonic systems. △ Less

Submitted 25 September, 2025; originally announced September 2025.

Comments: 4 pages, 4 figures

Transition from exceptional points to observable nonlinear bifurcation points in anti-PT symmetric coupled cavity systems

Authors: Takahiro Uemura, Kenta Takata, Masaya Notomi

Abstract: Exceptional points (EPs) in anti-parity-time (APT)-symmetric systems have attracted significant interest. While linear APT-symmetric systems exhibit structural similarities with nonlinear dissipative systems, such as mutually injection-locked lasers, the correspondence between exceptional points in linear non-Hermitian Hamiltonians and bifurcation phenomena in nonlinear lasing dynamics has remaine… ▽ More Exceptional points (EPs) in anti-parity-time (APT)-symmetric systems have attracted significant interest. While linear APT-symmetric systems exhibit structural similarities with nonlinear dissipative systems, such as mutually injection-locked lasers, the correspondence between exceptional points in linear non-Hermitian Hamiltonians and bifurcation phenomena in nonlinear lasing dynamics has remained unclear. We demonstrated that, in a two-cavity system with APT symmetry and gain saturation nonlinearity, an EP coincides with a bifurcation point of nonlinear equilibrium states, which appears exactly at the lasing threshold. Although the EP and the bifurcation point originate from fundamentally different physical concepts, the bifurcation point is observable and retains key EP characteristics even above the lasing threshold. Notably, the bifurcation point that originates from the linear EP also bridges linear and nonlinear dynamics of the system: it serves as an accessible transition point in the nonlinear dynamics between the limit-cycle and synchronization regimes. Furthermore, we clarified that beat oscillation that conserves the energy difference, which is a unique dynamic in the weak-coupling regime of a linear APT system, evolves into a nonlinear limit cycle with equal amplitudes in the two cavities in the presence of gain saturation. Our findings establish a direct link between EP-induced bifurcation points and nonlinear dynamics, providing fundamental insights into non-Hermitian and nonlinear optical systems. △ Less

Submitted 31 August, 2025; originally announced September 2025.

Comments: 16 pages, 12 figures

Chiral Diffraction from Aperiodic Monotile Lattice

Authors: Yuto Moritake, Masato Takiguchi, Takuma Aihara, Masaya Notomi

Abstract: Aperiodic systems such as quasiperiodic systems exhibit unique properties different from periodic structures. In 2023, Smith et al. discovered a new aperiodic structure: a single-shaped tile that can only tile space aperiodically, known as an aperiodic monotile. Although the aperiodic monotile possesses intriguing mathematical properties, its experimental investigation remains unexplored. In this… ▽ More Aperiodic systems such as quasiperiodic systems exhibit unique properties different from periodic structures. In 2023, Smith et al. discovered a new aperiodic structure: a single-shaped tile that can only tile space aperiodically, known as an aperiodic monotile. Although the aperiodic monotile possesses intriguing mathematical properties, its experimental investigation remains unexplored. In this study, we report an experimental investigation of diffraction patterns from a monotile lattice using a nanophotonic platform. We observed clear Bragg peaks, which is evidence of long-range order and a chiral structure of the diffraction patterns. Furthermore, we found exotic behavior in circular polarization dependence, which cannot be observed in conventional quasiperiodic structures. These findings establish the monotile lattice as a novel class of aperiodic systems, expanding the study of nonperiodic structures beyond conventional quasicrystals. △ Less

Submitted 9 June, 2025; originally announced June 2025.

Journal ref: Nat Commun 17, 6085 (2026)

Programmable Photonic Unitary Processor Enables Parametrized Differentiable Long-Haul Spatial Division Multiplexed Transmission

Authors: Mitsumasa Nakajima, Kohki Shibahara, Kohei Ikeda, Akira Kawai, Masaya Notomi, Yutaka Miyamoto, Toshikazu Hashimoto

Abstract: The explosive growth of global data traffic demands scalable and energy-efficient optical communication systems. Spatial division multiplexing (SDM) using multicore or multimode fibers is a promising solution to overcome the capacity limit of single-mode fibers. However, long-haul SDM transmission faces significant challenges due to modal dispersion, which imposes heavy computational loads on digi… ▽ More The explosive growth of global data traffic demands scalable and energy-efficient optical communication systems. Spatial division multiplexing (SDM) using multicore or multimode fibers is a promising solution to overcome the capacity limit of single-mode fibers. However, long-haul SDM transmission faces significant challenges due to modal dispersion, which imposes heavy computational loads on digital signal processing (DSP) for signal equalization. Here, we propose parameterized SDM transmission, where programmable photonic unitary processors are installed at intermediate nodes. Instead of relying on conventional digital equalization only on the receiver side, our approach enables direct optimization of the SDM transmission channel itself by the programmable unitary processor, which reduces digital post-processing loads. We introduce a gradient-based optimization algorithm using a differentiable SDM transmission model to determine the optimal unitary transformation. As a key enabler, we first implemented telecom-grade programmable photonic unitary processor, achieving a low-loss (2.1 dB fiber-to-fiber), wideband (full C-band), polarization-independent, and high-fidelity (R2>96% across the C-band) operation. We experimentally demonstrate 1300-km transmission using a three-mode fiber, achieving strong agreement between simulation and experiment. The optimized photonic processor significantly reduces modal dispersion and post-processing complexity. Our results establish a scalable framework for integrating photonic computation into the optical layer, enabling more efficient, high-capacity optical networks. △ Less

Submitted 22 May, 2025; originally announced May 2025.

Propagation and circulating modes of reciprocal non-Hermitian skin effect

Authors: Issei Takeda, Taiki Yoda, Yuto Moritake, Kenta Takata, Masaya Notomi

Abstract: The non-Hermitian skin effect (NHSE) is a novel localization phenomenon, in which all bulk states in a non-Hermitian system under certain conditions are localized at the edge of the system. Conventionally, most studies of NHSE have dealt with discrete lattice systems with non-reciprocal couplings. However in recent years, NHSE in a reciprocal two-dimensional continuous medium, such as photonic cry… ▽ More The non-Hermitian skin effect (NHSE) is a novel localization phenomenon, in which all bulk states in a non-Hermitian system under certain conditions are localized at the edge of the system. Conventionally, most studies of NHSE have dealt with discrete lattice systems with non-reciprocal couplings. However in recent years, NHSE in a reciprocal two-dimensional continuous medium, such as photonic crystal systems, has also been reported. In particular, we have previously shown that NHSE also occurs in two-dimensional uniform media. In such two-dimensional systems, skin modes propagate in a direction perpendicular to the localization direction, and especially, they have the property of propagating in only one direction. In this paper, we show numerically an intriguing scattering phenomenon: when a scatterer is placed in the path of a skin mode, the scattering causes the skin mode to hop between opposing edges. In addition, we propose a new method of generating circulating modes with orbital angular momentum using this scattering phenomenon. Our work paves the way for new applications of NHSE as micro-sized optical devices manipulating or generating OAM. △ Less

Submitted 2 May, 2025; originally announced May 2025.

Comments: 14 pages, 13 figures

Slow Light Waveguides based on Bound States in the Continuum

Authors: Yuta Tanimura, Yuki Ishii, Kenta Takata, Takahiro Uemura, Masaya Notomi, Satoshi Iwamoto, Yasutomo Ota

Abstract: The concept of bound states in the continuum (BIC) has been advancing light confinement technology in leaky environments. In this letter, we propose and numerically demonstrate a slow light waveguide based on a BIC mode. We considered a waveguide with a polymer core loaded on a plane slab, which supports a leaky guided mode coupled to the radiation continuum in the slab. We found that periodic mod… ▽ More The concept of bound states in the continuum (BIC) has been advancing light confinement technology in leaky environments. In this letter, we propose and numerically demonstrate a slow light waveguide based on a BIC mode. We considered a waveguide with a polymer core loaded on a plane slab, which supports a leaky guided mode coupled to the radiation continuum in the slab. We found that periodic modulation of the polymer core along the propagation direction can result in a high group index mode with a low propagation loss due to BIC confinement. The introduction of one-dimensional photonic crystals into the BIC waveguides will largely expand its functionality and applications in integrated photonics. △ Less

Submitted 12 March, 2025; originally announced March 2025.

Comments: 4 pages, 4 figures

Journal ref: Optics Letters Vol. 50, Issue 6, pp. 2013-2016 (2025)

Optoelectronic recurrent neural network using optical-electrical-optical converters with RC delay

Authors: Masaya Arahata, Shota Kita, Kazuo Aoyama, Akihiko Shinya, Hiroshi Sawada, Masaya Notomi

Abstract: Optical neural network (ONN) has been attracting intense attention owing to their low latency and low-power consumption. Among the ONNs, optical recurrent neural network (RNN) enables low-power and high-speed time-series data processing using a compact loop structure. The loop losses need to be efficiently compensated so that the time-series information is maintained in the RNN operation. For this… ▽ More Optical neural network (ONN) has been attracting intense attention owing to their low latency and low-power consumption. Among the ONNs, optical recurrent neural network (RNN) enables low-power and high-speed time-series data processing using a compact loop structure. The loop losses need to be efficiently compensated so that the time-series information is maintained in the RNN operation. For this purpose, we focus on the optoelectronic RNN (OE-RNN) with optical-electrical-optical (OEO) converters to compensate for the loop losses. However, the effect of resistive-capacitive (RC) delay of OEO converters on the RNN performance is unclear. Here, we study in simulation an OE-RNN equipped with OEO converters with RC delay. We confirm that our modeled OE-RNN achieves the high training accuracy of time-series data classification even when RC delay is comparably large to the time interval of time-series data. Our analyses reveal that the accumulation of time-series data by RC delay does not degrade the RNN performance but rather can compensate for the degraded RNN performance due to loop losses. From the theoretical analysis referring to the gradient explosion and vanishing problems, we find the region related to loss and RC delay where the high training accuracy can be achieved. In simulation, we confirm this compensation effect in the large OE-RNN circuit up to 32$\times$32 scale. Our proposed scheme opens a new way of time-series data processing by utilizing RC delay for the optical computing and optical communication. △ Less

Submitted 25 November, 2024; originally announced November 2024.

EP restoration and fast-light edge states in photonic crystal waveguide with glide and time reversal symmetry

Authors: Takahiro Uemura, Taiki Yoda, Yuto Moritake, Shutaro Otsuka, Kenta Takata, Masaya Notomi

Abstract: Exceptional points (EPs) in the propagation states give rise to the emergence of intriguing properties with the divergence of the group velocity. However, there have been no experimental reports due to the necessity of maintaining high levels of fabrication precision and the requisite high group velocity contrast. In our study, we propose a design of photonic crystal waveguide with glide and time… ▽ More Exceptional points (EPs) in the propagation states give rise to the emergence of intriguing properties with the divergence of the group velocity. However, there have been no experimental reports due to the necessity of maintaining high levels of fabrication precision and the requisite high group velocity contrast. In our study, we propose a design of photonic crystal waveguide with glide and time reversal symmetry, and derive an effective Hamiltonian for edge states to realize fast-light edge states. We adopt a systematic method to generate EPs in edge states by introducing non-Hermitian perturbations to Dirac points guaranteed by glide symmetry, which ensures that EP modes are free from out-of-plane radiation losses. Then, our study reveals the conditions for the exact EP restoration and provides an analytical solution to offset the EP smoothing due to symmetry breaking, which drastically reduces the group velocity contrast. A good symmetry property of the photonic crystal waveguide allows us to derive the effective Hamiltonian as a simple form, and the EPs can be restored by adjusting the real part of the permittivity. Furthermore, we design a feasible photonic crystal slab waveguide incorporating graphene as the absorbing material, and numerically demonstrate a group velocity reaching $v_g = 3.3c$ near the EP, which is up to 25 times that of the original structure. Thanks to the short periodicity of photonic crystals, it's possible to reach the speed of light in vacuum with group velocity contrasts on the order of one digit. Our study paves an innovative way to manipulate the group velocity of light. △ Less

Submitted 23 October, 2024; originally announced October 2024.

Comments: 17 pages, 11 figures

Photonic topological phase transition induced by material phase transition

Authors: Takahiro Uemura, Yuto Moritake, Taiki Yoda, Hisashi Chiba, Yusuke Tanaka, Masaaki Ono, Eiichi Kuramochi, Masaya Notomi

Abstract: Photonic topological insulators (PTIs) have been proposed as an analogy to topological insulators in electronic systems. In particular, two-dimensional PTIs have gained attention for the integrated circuit applications. However, controlling the topological phase after fabrication is difficult because the photonic topology requires the built-in specific structures. This study experimentally demonst… ▽ More Photonic topological insulators (PTIs) have been proposed as an analogy to topological insulators in electronic systems. In particular, two-dimensional PTIs have gained attention for the integrated circuit applications. However, controlling the topological phase after fabrication is difficult because the photonic topology requires the built-in specific structures. This study experimentally demonstrates the band inversion in two-dimensional PTI induced by the phase transition of deliberately-designed nanopatterns of a phase-change material, Ge2Sb2Te5 (GST), which indicates the first observation of the photonic topological phase transition with changes in the Chern number. This approach allows us to directly alter the topological invariants, which is achieved by symmetry-breaking perturbation through GST nanopatterns with different symmetry from original PTI. The success of our scheme is attributed to the ultrafine lithographic alignment technologies of GST nanopatterns. These results demonstrate to control photonic topological properties in a reconfigurable manner, providing an insight into new possibilities for reconfigurable photonic processing circuits. △ Less

Submitted 12 June, 2024; originally announced June 2024.

Vector beam generation from standing hollow GaN nanowire lasers on sapphire substrate

Authors: Masato Takiguchi, Sylvain Sergent, Benjamin Damilano, Stephane Vezian, Sebastien Chenot, Nicole Yazigi, Peter Heidt, Tai Tsuchizawa, Taiki Yoda, Hisashi Sumikura, Akihiko Shinya, Masaya Notomi

Abstract: We fabricated GaN based hollow nanowires standing upright on a sapphire substrate by the sublimation method and found that they exhibit laser oscillation at room temperature. These very long, hollow, nano-sized structures cannot be fabricated by other means. Furthermore, we determined the condition under which the fundamental mode is azimuthally polarized by investigating the dispersion of the hol… ▽ More We fabricated GaN based hollow nanowires standing upright on a sapphire substrate by the sublimation method and found that they exhibit laser oscillation at room temperature. These very long, hollow, nano-sized structures cannot be fabricated by other means. Furthermore, we determined the condition under which the fundamental mode is azimuthally polarized by investigating the dispersion of the hollow structure. Examination of the measured emission properties indicates that the hollow nanowire operates as a topological, vector-beam, light source. △ Less

Submitted 14 November, 2023; originally announced November 2023.

High Transmission in 120-degree Sharp Bends of Inversion-symmetric and Inversion-asymmetric Photonic Crystal Waveguides

Authors: Wei Dai, Taiki Yoda, Yuto Moritake, Masaaki Ono, Eiichi Kuramochi, Masaya Notomi

Abstract: Bending loss is one of the serious problems for constructing nanophotonic integrated circuits. Recently, many works reported that valley photonic crystals (VPhCs) enable significantly high transmission via 120-degree sharp bends. However, it is unclear whether the high bend-transmission results directly from the valley-photonic effects, which are based on the breaking of inversion symmetry. In thi… ▽ More Bending loss is one of the serious problems for constructing nanophotonic integrated circuits. Recently, many works reported that valley photonic crystals (VPhCs) enable significantly high transmission via 120-degree sharp bends. However, it is unclear whether the high bend-transmission results directly from the valley-photonic effects, which are based on the breaking of inversion symmetry. In this study, we conduct a series of comparative numerical and experimental investigations of bend-transmission in various triangular PhCs with and without inversion symmetry and reveal that the high bend-transmission is solely determined by the domain-wall configuration and independent of the existence of the inversion symmetry. Preliminary analysis of the polarization distribution indicates that high bend-transmissions are closely related to the appearance of local topological polarization singularities near the bending section. Our work demonstrates that high transmission can be achieved in a much wider family of PhC waveguides, which may provide novel designs for low-loss nanophotonic integrated circuits with enhanced flexibility and a new understanding of the nature of valley-photonics △ Less

Submitted 18 January, 2025; v1 submitted 26 October, 2023; originally announced October 2023.

Comments: 15 pages, 7 figures, and 1 table & Supplementary information (18 pages, 15 figures and 1 table). Submitted to Nature Communication

Journal ref: Nat Commun 16, 796 (2025)

Effect of shatterproof polymer film application on the fracture types and strength of glass subject to bending load

Authors: Tomohisa Kojima, Ryohei Momokawa, Takuma Matsuo, Mitsuo Notomi

Abstract: Shatterproof polymer films are widely for windows used because they can be easily installed on existing glass windows to improve safety. Applying them to glass plates has been reported to not only prevent fragments from scattering but also increase load-bearing capacity and penetration resistance. However, the clarification of their mechanism and quantitative evaluation are still insufficient beca… ▽ More Shatterproof polymer films are widely for windows used because they can be easily installed on existing glass windows to improve safety. Applying them to glass plates has been reported to not only prevent fragments from scattering but also increase load-bearing capacity and penetration resistance. However, the clarification of their mechanism and quantitative evaluation are still insufficient because the effect of film application on the strength and failure mode of glass under quasi-static loading has not been investigated. In this study, three-point bending tests and fracture surface observations were conducted on a float glass with a shatterproof polymer film. The stress field formed inside the glass was visualised during the tests using the photoelastic method. By varying the support span of the specimen, the deformation mode was varied to generate three types of failures: bending, shear caused by Hertzian contact stress, and mixed-mode failures. Under the conditions in the present study, the breaking loads of the specimens with and without film were almost the same; however, the fracture surface observation indicated that the area subjected to shear failure caused by Hertzian contact stress was larger with film application. Finally, the effect of the film thickness on the breaking load due to bending deformation was theoretically predicted. △ Less

Submitted 28 April, 2023; v1 submitted 29 March, 2023; originally announced March 2023.

Journal ref: Engineering Failure Analysis, Vol. 148, (2023), p. 107220

Optical non-Hermitian skin effect in two-dimensional uniform media

Authors: Taiki Yoda, Yuto Moritake, Kenta Takata, Kazuki Yokomizo, Shuichi Murakami, Masaya Notomi

Abstract: The non-Hermitian skin effect (NHSE) is a novel localization phenomenon in certain non-Hermitian systems with gain and/or loss. Most of previous works study the non-Hermitian skin effect in periodic systems. However, electromagnetic waves often propagate within uniform materials without periodic modulation, and it has not been clear whether the optical NHSE occurs in uniform media such as bulk mat… ▽ More The non-Hermitian skin effect (NHSE) is a novel localization phenomenon in certain non-Hermitian systems with gain and/or loss. Most of previous works study the non-Hermitian skin effect in periodic systems. However, electromagnetic waves often propagate within uniform materials without periodic modulation, and it has not been clear whether the optical NHSE occurs in uniform media such as bulk materials and electromagnetic metamaterials. Here we establish the theory of the optical NHSE in non-Hermitian anisotropic media. We show that the NHSE occurs even in uniform media with appropriate anisotropy and material loss. The localization of non-Hermitian skin modes are completely determined by an effective gauge potential caused by the anisotropy of a dielectric tensor. On the basis of the theory, we propose subwavelength multilayer metamaterials as a novel platform for the optical NHSE. We also propose a new concept of stationarily-excited skin modes whose frequencies are forced to be real in non-Hermitian systems. We find that the NHSE occurs even under the condition that the frequency is forced to be real, which implies that the NHSE we propose is observable under stationary excitation. Our work presents a general theory of the NHSE in homogeneous systems, and pave the way to realize the optical NHSE in bulk materials and metamaterials. △ Less

Submitted 9 March, 2023; originally announced March 2023.

Comments: 17 pages, 7 figures

Improved design and experimental demonstration of ultrahigh-Q C${}_\text{6}$-symmetric H1 hexapole photonic crystal nanocavities

Authors: Kenta Takata, Eiichi Kuramochi, Akihiko Shinya, Masaya Notomi

Abstract: An H1 photonic crystal nanocavity is based on a single point defect and has eigenmodes with a variety of symmetric features. Thus, it is a promising building block for photonic tight-binding lattice systems that can be used in studies on condensed matter, non-Hermitian and topological physics. However, improving its radiative quality ($Q$) factor has been considered challenging. Here, we report th… ▽ More An H1 photonic crystal nanocavity is based on a single point defect and has eigenmodes with a variety of symmetric features. Thus, it is a promising building block for photonic tight-binding lattice systems that can be used in studies on condensed matter, non-Hermitian and topological physics. However, improving its radiative quality ($Q$) factor has been considered challenging. Here, we report the design of a hexapole mode of an H1 nanocavity with a $Q$ factor exceeding $10^8$. We achieved such extremely high-$Q$ conditions by designing only four structural modulation parameters thanks to the ${\rm C_{6}}$ symmetry of the mode, despite the need of more complicated optimizations for many other nanocavities. The fabricated silicon photonic crystal nanocavities exhibited a systematic change in their resonant wavelengths depending on the spatial shift of the air holes in units of 1 nm. Out of 26 such samples, we found eight cavities with loaded $Q$ factors over one million ($1.2 \times 10^6$ maximum). We examined the difference between the theoretical and experimental performances by conducting a simulation of systems with input and output waveguides and with randomly distributed radii of air holes. Automated optimization using the same design parameters further increased the theoretical $Q$ factor by up to $4.5 \times 10^8$, which is two orders of magnitude higher than in the previous studies. Our work elevates the performance of the H1 nanocavity to the ultrahigh-$Q$ level and paves the way for its large-scale arrays with unconventional functionalities. △ Less

Submitted 5 January, 2023; originally announced January 2023.

Journal ref: Opt. Express 31, 11864-11884 (2023)

Exceptional point induced unidirectional radiation from non-Hermitian plasmonic structures

Authors: Yuto Moritake, Masaya Notomi

Abstract: Non-Hermitian (NH) photonics has attracted considerable attention from researchers owing to exotic properties that originate from the parity-time ($\mathcal{PT}$) phase transition and exceptional points (EPs). To date, the $\mathcal{PT}$ phase transition, EPs, and circling around EPs have been investigated in many photonic systems. However, few studies focused on the singular nature of the EP eige… ▽ More Non-Hermitian (NH) photonics has attracted considerable attention from researchers owing to exotic properties that originate from the parity-time ($\mathcal{PT}$) phase transition and exceptional points (EPs). To date, the $\mathcal{PT}$ phase transition, EPs, and circling around EPs have been investigated in many photonic systems. However, few studies focused on the singular nature of the EP eigenstates of the Hamiltonian matrices. Moreover, the switching of an EP eigenstate based on the sign of the coupling constant and its manifestation in physical phenomena have not yet been investigated. In this paper, we propose and numerically demonstrate a unidirectional radiation phenomena manifested by the formation of Huygens dipoles using the singular EP eigenstates in coupled plasmonic systems. Two types of EPs corresponding to positive and negative signs of the coupling constants are realized using dipole-dipole coupling. We show that the Huygens dipole is formed at the EP condition and its radiation direction can be controlled by choosing the sign of the coupling. The unique photonic functionality, which originates directly from the singular eigenstate at the EP uncovers a new aspect of NH photonic physics and devices. △ Less

Submitted 5 October, 2021; originally announced October 2021.

Imaginary couplings in non-Hermitian coupled-mode theory: Effects on exceptional points of optical resonators

Authors: Kenta Takata, Nathan Roberts, Akihiko Shinya, Masaya Notomi

Abstract: Exceptional point (EP) degeneracies in coupled cavities with gain and loss provide on-chip photonic devices with unconventional features and performance. However, such systems with realistic structures often miss the exact EPs even in simulation, and the mechanism of this EP disruption has yet to be thoroughly identified. Here, we extend the coupled-mode theory of one-dimensional non-Hermitian res… ▽ More Exceptional point (EP) degeneracies in coupled cavities with gain and loss provide on-chip photonic devices with unconventional features and performance. However, such systems with realistic structures often miss the exact EPs even in simulation, and the mechanism of this EP disruption has yet to be thoroughly identified. Here, we extend the coupled-mode theory of one-dimensional non-Hermitian resonator arrays to study the effects of the imaginary part of the inter-cavity coupling, which is a second-order term and attributed to material amplification, absorption, and radiation. By taking an appropriate gauge for the model, we clarify that the imaginary coupling components have a symmetric form in the effective Hamiltonian and hence represent non-Hermiticity. These additional factors can lift the gain- and loss-based EP degeneracies. However, they are proportional to the sum of the imaginary permittivities for involved cavity pairs. Thus, when the amplification and absorption of adjacent cavities are balanced, their contribution to the imaginary coupling is canceled, and the EP singularity can be restored. Radiation-induced imaginary couplings measure the change in net radiation loss by the interference between cavity modes. Their impact on the EP can also be counteracted by small cavity resonance detuning even in loss-biased cases. We show and analyze eligible simulation examples based on photonic crystal nanocavities, and highlight design of an ideal EP degeneracy that is protected by generalized PT symmetry and induced by radiation. △ Less

Submitted 20 January, 2022; v1 submitted 18 July, 2021; originally announced July 2021.

Comments: 19 pages, 11 figures

Journal ref: Phys. Rev. A 105, 013523 (2022)

Evaluation of dynamic fracture toughness of a bonded bi-material interface subject to high-strain-rate shearing using digital image correlation

Authors: Tomohisa Kojima, Yuta Kimura, Shuichi Arikawa, Mitsuo Notomi

Abstract: High-strain-rate shear tests were conducted on a three-layered bonded test piece comprising a central aluminum layer with PMMA resin layers bonded on both sides. Upon calculating the displacement field and the strain field using digital image correlation (DIC), the crack tip was located, and the fracture toughness was evaluated at the Aluminum/PMMA bonding interface. As a result of the DIC, it was… ▽ More High-strain-rate shear tests were conducted on a three-layered bonded test piece comprising a central aluminum layer with PMMA resin layers bonded on both sides. Upon calculating the displacement field and the strain field using digital image correlation (DIC), the crack tip was located, and the fracture toughness was evaluated at the Aluminum/PMMA bonding interface. As a result of the DIC, it was possible to determine the process by which 1) the elastic stress wave propagated to the aluminum section, 2) the wave was transmitted to the PMMA section, and 3) the crack developed at the interface. The tip of the crack was identified using displacement distributions obtained using DIC. The fracture toughness of the interface was evaluated using the stress intensity factor. The true interfacial stress was calculated by correcting the strain value at the interface obtained using DIC. The distribution of the stress suggested that mode II fracture appears in the present test method when the crack is sufficiently shorter than the length of the bonding interface, and mode I and mode II fractures appear when the crack is longer in comparison. Although the value of the stress intensity factor was disturbed by the error of the DIC analysis, it was confirmed that the obtained values were similar regardless of the difference in the crack length, upon averaging the stress intensity factor values from the crack tips to the long-range with a ratio of 1 to the subset in DIC. As the obtained stress intensity factor value was similar to the values calculated in the related literature, it can be concluded that the method proposed in this study yields a reasonable stress intensity factor. △ Less

Submitted 17 August, 2021; v1 submitted 9 November, 2020; originally announced November 2020.

Journal ref: Engineering Fracture Mechanics, Vol. 241 (2021), 107391

Emulating the local Kuramoto model with an injection-locked photonic crystal laser array

Authors: Naotomo Takemura, Kenta Takata, Masato Takiguchi, Masaya Notomi

Abstract: The Kuramoto model is a mathematical model for describing the collective synchronization phenomena of coupled oscillators. We theoretically demonstrate that an array of coupled photonic crystal lasers emulates the Kuramoto model with non-delayed nearest-neighbor coupling (the local Kuramoto model). Our novel strategy employs indirect coupling between lasers via additional cold cavities. By install… ▽ More The Kuramoto model is a mathematical model for describing the collective synchronization phenomena of coupled oscillators. We theoretically demonstrate that an array of coupled photonic crystal lasers emulates the Kuramoto model with non-delayed nearest-neighbor coupling (the local Kuramoto model). Our novel strategy employs indirect coupling between lasers via additional cold cavities. By installing cold cavities between laser cavities, we avoid the strong coupling of lasers and realize ideal mutual injection-locking with effective non-delayed dissipative coupling. First, after discussing the limit cycle interpretation of laser oscillation, we demonstrate the synchronization of two indirectly coupled lasers by numerically simulating coupled-mode equations. Second, by performing a phase reduction analysis, we show that laser dynamics in the proposed device can be mapped to the local Kuramoto model. Finally, we briefly demonstrate that a chain of indirectly coupled photonic crystal lasers actually emulates the one-dimensional local Kuramoto chain. We also argue that our proposed structure, which consists of periodically aligned cold cavities and laser cavities, will best be realized by using state-of-the-art buried multiple quantum well photonic crystals. △ Less

Submitted 5 May, 2021; v1 submitted 30 October, 2020; originally announced October 2020.

Journal ref: Sci Rep 11, 8587 (2021)

Observing exceptional point degeneracy of radiation with electrically pumped photonic crystal coupled-nanocavity lasers

Authors: Kenta Takata, Kengo Nozaki, Eiichi Kuramochi, Shinji Matsuo, Koji Takeda, Takuro Fujii, Shota Kita, Akihiko Shinya, Masaya Notomi

Abstract: Controlling gain and loss of coupled optical cavities can induce non-Hermitian degeneracies of eigenstates, called exceptional points (EPs). Various unconventional phenomena around EPs have been reported, and expected to incorporate extra functionalities into photonic devices. The eigenmode exactly under the EP degeneracy is also predicted to exhibit enhanced radiation. However, such responses hav… ▽ More Controlling gain and loss of coupled optical cavities can induce non-Hermitian degeneracies of eigenstates, called exceptional points (EPs). Various unconventional phenomena around EPs have been reported, and expected to incorporate extra functionalities into photonic devices. The eigenmode exactly under the EP degeneracy is also predicted to exhibit enhanced radiation. However, such responses have yet to be observed in on-chip lasers, because of both the limited controllability of their gain and loss and the lifting of degeneracy by pump-induced cavity detuning. Here, we report the first non-Hermitian nanophotonic platform based on two electrically pumped photonic crystal lasers and its spontaneous emission at an EP degeneracy. Systematically tuned and independent current injection to our wavelength-scale active heterostructure cavities enables us to demonstrate the clear EP phase transition of their spontaneous emission, accompanied with the spectral coalescence of coupled modes and reversed pump dependence of the intensity. Furthermore, we find experimentally and confirm theoretically the peculiar squared Lorentzian emission spectrum very near the exact EP, which indicates the four-fold enhancement of the photonic local density of states induced purely by the degeneracy. Our results open a new pathway to engineer the light-matter interaction by non-Hermiticity and explore larger reconfigurable laser arrays for further non-Hermitian features and physics. △ Less

Submitted 21 December, 2020; v1 submitted 6 July, 2020; originally announced July 2020.

Comments: 13 pages, 5 figures, and supplementary materials

Journal ref: Optica 8, 184-192 (2021)

Designs toward synchronization of optical limit cycles with coupled silicon photonic crystal microcavities

Authors: Naotomo Takemura, Masato Takiguchi, Masaya Notomi

Abstract: A driven high-Q Si microcavity is known to exhibit limit cycle oscillation originating from carrier-induced and thermo-optic nonlinearities. We propose a novel nanophotonic device to realize synchronized optical limit cycle oscillations with coupled silicon (Si) photonic crystal (PhC) microcavities. Here, coupled limit cycle oscillators are realized by using coherently coupled Si PhC microcavities… ▽ More A driven high-Q Si microcavity is known to exhibit limit cycle oscillation originating from carrier-induced and thermo-optic nonlinearities. We propose a novel nanophotonic device to realize synchronized optical limit cycle oscillations with coupled silicon (Si) photonic crystal (PhC) microcavities. Here, coupled limit cycle oscillators are realized by using coherently coupled Si PhC microcavities. By simulating coupled-mode equations, we theoretically demonstrate mutual synchronization (entrainment) of two limit cycles induced by coherent coupling. Furthermore, we interpret the numerically simulated synchronization in the framework of phase description. Since our proposed design is perfectly compatible with current silicon photonics fabrication processes, the synchronization of optical limit cycle oscillations will be implemented in future silicon photonic circuits. △ Less

Submitted 8 September, 2020; v1 submitted 22 May, 2020; originally announced May 2020.

Journal ref: Vol. 28, Issue 19, pp. 27657-27675 (2020)

Generation and Annihilation of Topologically-Protected Bound States in the Continuum and Circularly-Polarized States by Symmetry Breaking

Authors: Taiki Yoda, Masaya Notomi

Abstract: We demonstrate by breaking the $C_{6}$ symmetry for higher-order at-$Γ$ bound states in the continuum (BICs) with topological charge $-2$ in photonic crystals, (i) deterministic generation of off-$Γ$ BICs from the at-$Γ$ BIC, and (ii) a variety of pair-creation and annihilation processes of circularly-polarized states with opposite topological charge and same handedness. These processes are well e… ▽ More We demonstrate by breaking the $C_{6}$ symmetry for higher-order at-$Γ$ bound states in the continuum (BICs) with topological charge $-2$ in photonic crystals, (i) deterministic generation of off-$Γ$ BICs from the at-$Γ$ BIC, and (ii) a variety of pair-creation and annihilation processes of circularly-polarized states with opposite topological charge and same handedness. These processes are well explained by the conservation of handedness-wise topological charges, showing topologically robustness of these phenomena. The results indicate a new control of topological aspect of far-field polarization vectors. △ Less

Submitted 21 April, 2020; originally announced April 2020.

Comments: 12 pages, 9 figures

Journal ref: Phys. Rev. Lett. 125, 053902 (2020)

Photon correlation measurements of stochastic limit cycles emerging from high-$Q$ nonlinear silicon photonic crystal microcavities

Authors: N. Takemura, M. Takiguchi, H. Sumikura, E. Kuramochi, A. Shinya, M. Notomi

Abstract: We performed measurements of photon correlation [$g^{(2)}(τ)$] in driven nonlinear high-$Q$ silicon (Si) photonic crystal (PhC) microcavities. The measured $g^{(2)}(τ)$ exhibits a damped oscillatory behavior when input pump power exceeds a critical value. From comparison between experiments and simulations, we attribute the measured oscillation of $g^{(2)}(τ)$ to self-pulsing (a limit cycle) emerg… ▽ More We performed measurements of photon correlation [$g^{(2)}(τ)$] in driven nonlinear high-$Q$ silicon (Si) photonic crystal (PhC) microcavities. The measured $g^{(2)}(τ)$ exhibits a damped oscillatory behavior when input pump power exceeds a critical value. From comparison between experiments and simulations, we attribute the measured oscillation of $g^{(2)}(τ)$ to self-pulsing (a limit cycle) emerging from an interplay between photon, carrier, and thermal dynamics. Namely, the oscillation frequency of $g^{(2)}(τ)$ corresponds to the oscillation period of the limit cycle, while its finite coherence (damping) time originates from the stochastic nature of the limit cycle. From the standpoint of phase reduction theory, we interpret the measured coherence time of $g^{(2)}(τ)$ as the coherence (diffusion) time of a generalized phase of the limit cycle. Furthermore, we show that an increase in laser input power enhances the coherence time of $g^{(2)}(τ)$ up to the order of microseconds, which could be a demonstration of the stabilization of a stochastic limit cycle through pumping. △ Less

Submitted 6 August, 2020; v1 submitted 8 January, 2020; originally announced January 2020.

Journal ref: Phys. Rev. A 102, 011501 (2020)

Active topological photonics

Authors: Yasutomo Ota, Kenta Takata, Tomoki Ozawa, Alberto Amo, Zhetao Jia, Boubacar Kante, Masaya Notomi, Yasuhiko Arakawa, Satoshi Iwamoto

Abstract: Topological photonics has emerged as a novel route to engineer the flow of light. Topologically-protected photonic edge modes, which are supported at the perimeters of topologically-nontrivial insulating bulk structures, have been of particular interest as they may enable low-loss optical waveguides immune to structural disorder. Very recently, there is a sharp rise of interest in introducing gain… ▽ More Topological photonics has emerged as a novel route to engineer the flow of light. Topologically-protected photonic edge modes, which are supported at the perimeters of topologically-nontrivial insulating bulk structures, have been of particular interest as they may enable low-loss optical waveguides immune to structural disorder. Very recently, there is a sharp rise of interest in introducing gain materials into such topological photonic structures, primarily aiming at revolu-tionizing semiconductor lasers with the aid of physical mechanisms existing in topological physics. Examples of re-markable realizations are topological lasers with unidirectional light output under time-reversal symmetry breaking and topologically-protected polariton and micro/nano-cavity lasers. Moreover, the introduction of gain and loss provides a fascinating playground to explore novel topological phases, which are in close relevance to non-Hermitian and parity-time symmetric quantum physics and are in general difficult to access using fermionic condensed matter systems. Here, we review the cutting-edge research on active topological photonics, in which optical gain plays a pivotal role. We discuss recent realizations of topological lasers of various kinds, together with the underlying physics explaining the emergence of topological edge modes. In such demonstrations, the optical modes of the topological lasers are deter-mined by the dielectric structures and support lasing oscillation with the help of optical gain. We also address recent researches on topological photonic systems in which gain and loss themselves essentially influence on topological prop-erties of the bulk systems. We believe that active topological photonics provides powerful means to advance mi-cro/nanophotonics systems for diverse applications and topological physics itself as well. △ Less

Submitted 11 December, 2019; v1 submitted 11 December, 2019; originally announced December 2019.

Comments: 23 pages, 11 figures, review paper to appear in Nanophotonics

Journal ref: Nanophotonics 9, 547 (2020)

Probing the Ginzburg-Landau Potential for Lasers Using Higher-order Photon Correlations

Authors: Naotomo Takemura, Masato Takiguchi, Masaya Notomi

Abstract: Lasing transition is known to be analogous to the second-order phase transition. Furthermore, for some cases, it is possible to define the Ginzburg-Landau (GL) potential, and the GL theory predicts the photon statistical properties of lasers. However, the GL potential for lasers is surprising, because lasers are operating far from equilibrium. In this paper, we theoretically examine the validity o… ▽ More Lasing transition is known to be analogous to the second-order phase transition. Furthermore, for some cases, it is possible to define the Ginzburg-Landau (GL) potential, and the GL theory predicts the photon statistical properties of lasers. However, the GL potential for lasers is surprising, because lasers are operating far from equilibrium. In this paper, we theoretically examine the validity of the GL theory for lasers in terms of various parameters, particularly, the ratio between photon and carrier lifetimes. For this purpose, we use stochastic rate equations and higher-order photon correlation functions. With higher-order photon correlation measurements, we can check whether or not laser dynamics are described by the GL theory. We demonstrate that, for low-$β$ lasers, the GL theory is applicable even when the photon lifetime is comparable to the carrier lifetime and that photon-carrier relaxation oscillation is the fundamental origin of the breakdown of the GL theory, which can be understood in the framework of center manifold reduction. △ Less

Submitted 1 July, 2022; v1 submitted 23 August, 2019; originally announced August 2019.

Ultrafast and energy-efficient all-optical switching with graphene-loaded deep-subwavelength plasmonic waveguides

Authors: Masaaki Ono, Masanori Hata, Masato Tsunekawa, Kengo Nozaki, Hisashi Sumikura, Hisashi Chiba, Masaya Notomi

Abstract: All-optical switches have attracted attention because they can potentially overcome the speed limitation of electric switches. However, ultrafast, energy-efficient all-optical switches have been challenging to realize due to the intrinsically small optical nonlinearity in existing materials. As a solution, we propose graphene-loaded deep-subwavelength plasmonic waveguides (30 nm x 20 nm). Thanks t… ▽ More All-optical switches have attracted attention because they can potentially overcome the speed limitation of electric switches. However, ultrafast, energy-efficient all-optical switches have been challenging to realize due to the intrinsically small optical nonlinearity in existing materials. As a solution, we propose graphene-loaded deep-subwavelength plasmonic waveguides (30 nm x 20 nm). Thanks to extreme light confinement, we have significantly enhanced optical nonlinear absorption in graphene, and achieved ultrafast all-optical switching with a switching energy of 35 fJ and a switching time of 260 fs. The switching energy is four orders of magnitudes smaller than that in previous graphene-based devices and is the smallest value ever reported for any all-optical switch operating at a few picoseconds or less. This device can be efficiently connected to conventional Si waveguides and employed in Si photonic integrated circuits. We believe that this graphene-based device will pave the way towards on-chip ultrafast and energy-efficient photonic processing. △ Less

Submitted 3 July, 2019; originally announced July 2019.

Comments: 28 pages, 5 figures

Simulation technique of quantum optical emission process from multiple two-level atoms based on classical numerical method

Authors: Hideaki Taniyama, Hisashi Sumikura, Masaya Notomi

Abstract: In this paper, we report a numerical method for analyzing optical radiation from a two-level atom. The proposed method can consistently consider the optical emission and absorption process of an atom, and also the interaction between atoms through their interaction with a radiation field. The numerical model is based on a damping oscillator description of a dipole current, which is a classical mod… ▽ More In this paper, we report a numerical method for analyzing optical radiation from a two-level atom. The proposed method can consistently consider the optical emission and absorption process of an atom, and also the interaction between atoms through their interaction with a radiation field. The numerical model is based on a damping oscillator description of a dipole current, which is a classical model of atomic transition and is implemented with a finite-difference time-domain method. Using the method, we successfully simulate the spontaneous emission phenomena in a vacuum, where the interaction between an atom and a radiated field plays an important role. We also simulate the radiation from an atom embedded in a photonic crystal (PhC) cavity. As a result, an atom-cavity field interaction is sucessfuly incorporated in the simulation, and the enhancement of the optical emission rate of an excited atom is explained. The method considers the effect of the interaction between atoms through the radiated field. We simulate the optical emission process of the multiple atoms and show that an enhancement of the emission rate can occur owing to the an atom-atom interaction (superradiance)(R. H. Dicke, Phys. Rev. {\bf 93}, 99[1954]). We also show that the emission rate is suppressed by the effect of the destructive dipole-dipole interaction under an out-of-phase excitation condition (subradiance). △ Less

Submitted 24 May, 2019; originally announced May 2019.

Comments: 10 pages, 7 figures

Journal ref: Optics Express Vol. 27, Issue 9, pp. 12070-12079 (2019)

All-optical dynamic modulation of spontaneous emission rate in hybrid optomechanical cavity quantum electrodynamics systems

Authors: Feng Tian, Hisashi Sumikura, Eiichi Kuramochi, Masato Takiguchi, Masaaki Ono, Akihiko Shinya, Masaya Notomi

Abstract: Recent nanofabrication technologies have miniaturized optical and mechanical resonators, and have led to a variety of novel optomechanical systems in which optical and mechanical modes are strongly coupled. Here we hybridize an optomechanical resonator with two-level emitters and successfully demonstrate all-optical dynamic control of optical transition in the two-level system by the mechanical os… ▽ More Recent nanofabrication technologies have miniaturized optical and mechanical resonators, and have led to a variety of novel optomechanical systems in which optical and mechanical modes are strongly coupled. Here we hybridize an optomechanical resonator with two-level emitters and successfully demonstrate all-optical dynamic control of optical transition in the two-level system by the mechanical oscillation via the cavity quantum-electrodynamics (CQED) effect. Employing copper-doped silicon nanobeam optomechanical resonators, we have observed that the spontaneous emission rate of excitons bound to copper atoms is dynamically modulated by the optically-driven mechanical oscillation within the time scale much shorter than the emission lifetime. The result is explained very well with an analytical model including the dynamic modulation of the Purcell effect and the exciton population. To the best of our knowledge, this is the first demonstration of a dynamic modulation of the spontaneous emission rate by mechanical oscillations. Our achievement will open up a novel field of hybrid optomechanical CQED systems in which three body--optical transitions, optical resonance modes, and mechanical resonance modes--are strongly coupled and will pave the way for novel hybrid quantum systems. △ Less

Submitted 22 January, 2019; originally announced January 2019.

Photonic Topological Insulating Phase Induced Solely by Gain and Loss

Authors: Kenta Takata, Masaya Notomi

Abstract: We reveal a one-dimensional topological insulating phase induced solely by gain and loss control in non-Hermitian optical lattices. The system comprises units of four uniformly coupled cavities, where successive two have loss, the others experience gain and they are balanced under two magnitudes. The gain and loss parts are effectively dimerized, and a bulk bandgap, topological transition, midgap… ▽ More We reveal a one-dimensional topological insulating phase induced solely by gain and loss control in non-Hermitian optical lattices. The system comprises units of four uniformly coupled cavities, where successive two have loss, the others experience gain and they are balanced under two magnitudes. The gain and loss parts are effectively dimerized, and a bulk bandgap, topological transition, midgap topological edge and interface states in finite systems can all be achieved by controlled pumping. We also clarify non-Hermitian topological numbers and edge states in gapless conditions. △ Less

Submitted 4 July, 2018; v1 submitted 25 October, 2017; originally announced October 2017.

Comments: v3, 14 pages, 15 figures, including Appendices

Journal ref: Phys. Rev. Lett. 121, 213902 (2018)

Continuous-wave operation and 10-Gb/s direct modulation of InAsP/InP sub-wavelength nanowire laser on silicon photonic crystal

Authors: Masato Takiguchi, Atsushi Yokoo, Kengo Nozaki, Muhammad Danang Birowosuto, Kouta Tateno, Guoqiang Zhang, Eiichi Kuramochi, Akihiko Shinya, Masaya Notomi

Abstract: We demonstrated sub-wavelength (~111 nm diameter) single nanowire (NW) continuous wave (CW) lasers on silicon photonic crystal in the telecom-band with direct modulation at 10 Gb/s by optical pumping at cryogenic temperatures. To estimate the small signal response and pseudo-random bit sequence (PRBS) modulation of our CW lasers, we employed a new signal detection technique that employs a supercon… ▽ More We demonstrated sub-wavelength (~111 nm diameter) single nanowire (NW) continuous wave (CW) lasers on silicon photonic crystal in the telecom-band with direct modulation at 10 Gb/s by optical pumping at cryogenic temperatures. To estimate the small signal response and pseudo-random bit sequence (PRBS) modulation of our CW lasers, we employed a new signal detection technique that employs a superconducting single photon detector and a time-correlated single photon counting module. The results showed that our NW laser was unambiguously modulated at above 10 Gb/s and an open eye pattern was obtained. This is the first demonstration of a telecom-band CW NW laser with high-speed PRBS modulation. △ Less

Submitted 8 February, 2017; v1 submitted 12 December, 2016; originally announced December 2016.

Journal ref: APL Photonics, 2, 046106 (2017)

Dispersion and light transport characteristics of large-scale photonic-crystal coupled nanocavity arrays

Authors: Nobuyuki Matsuda, Eiichi Kuramochi, Hiroki Takesue, Masaya Notomi

Abstract: We investigate the dispersion and transmission property of slow-light coupled-resonator optical waveguides that consist of more than 100 ultrahigh-Q photonic crystal cavities. We show that experimental group-delay spectra exhibited good agreement with numerically calculated dispersions obtained with the three-dimensional plane wave expansion method. Furthermore, a statistical analysis of the trans… ▽ More We investigate the dispersion and transmission property of slow-light coupled-resonator optical waveguides that consist of more than 100 ultrahigh-Q photonic crystal cavities. We show that experimental group-delay spectra exhibited good agreement with numerically calculated dispersions obtained with the three-dimensional plane wave expansion method. Furthermore, a statistical analysis of the transmission property indicated that fabrication fluctuations in individual cavities are less relevant than in the localized regime. These behaviors are observed for a chain of up to 400 cavities. △ Less

Submitted 7 September, 2014; originally announced September 2014.

Journal ref: Opt. Lett. 39, 2290-2293 (2014)

Spontaneous emission inhibition of telecom-band quantum disks inside single nanowire on different substrates

Authors: M. D. Birowosuto, G. Zhang, A. Yokoo, M. Takiguchi, M. Notomi

Abstract: We investigate the inhibited spontaneous emission of telecom-band InAs quantum disks (Qdisks) in InP nanowires (NWs). We have evaluated how the inhibition is affected by different disk diameter and thickness. We also compared the inhibition in standing InP NWs and those NWs laying on silica (SiO2), and silicon (Si) substrates. We found that the inhibition is altered when we put the NW on the high-… ▽ More We investigate the inhibited spontaneous emission of telecom-band InAs quantum disks (Qdisks) in InP nanowires (NWs). We have evaluated how the inhibition is affected by different disk diameter and thickness. We also compared the inhibition in standing InP NWs and those NWs laying on silica (SiO2), and silicon (Si) substrates. We found that the inhibition is altered when we put the NW on the high-refractive index materials of Si. Experimentally, the inhibition factor $ζ$ of the Qdisk emission at 1,500 nm decreases from 4.6 to 2.5 for NW on SiO2 and Si substrates, respectively. Those inhibitions are even much smaller than that of 6.4 of the standing NW. The inhibition factors well agree with those calculated from the coupling of the Qdisk to the fundamental guided mode and the continuum of radiative modes. Our observation can be useful for the integration of the NW as light sources in the photonic nanodevices. △ Less

Submitted 30 May, 2014; originally announced June 2014.

Comments: 19 pages, 6 figures (Appendix is not included)

Journal ref: Optics Express, Vol. 22, Issue 10, pp. 11713-11726 (2014)

Movable high Q nanoresonators realized by semiconductor nanowires on a Si photonic crystal platform

Authors: M. D. Birowosuto, A. Yokoo, G. Zhang, K. Tateno, E. Kuramochi, H. Taniyama, M. Takiguchi, M. Notomi

Abstract: Subwavelength semiconductor nanowires have recently attracted interest for photonic applications because they possess various unique optical properties and offer great potential for miniaturizing devices. However, realizing tight light confinement or efficient coupling with photonic circuits is not straightforward and remains a challenge. Here we show that a high Q nanocavity can be created by pla… ▽ More Subwavelength semiconductor nanowires have recently attracted interest for photonic applications because they possess various unique optical properties and offer great potential for miniaturizing devices. However, realizing tight light confinement or efficient coupling with photonic circuits is not straightforward and remains a challenge. Here we show that a high Q nanocavity can be created by placing a single III/V semiconductor nanowire with a diameter of under 100 nm in a grooved waveguide in a Si photonic crystal, by means of nanoprobe manipulation. We observe very fast spontaneous emission (91 ps) from nanowires accelerated by the strong Purcell enhancement in nanocavities, which proves that very strong light confinement can be achieved. Furthermore, this system enables us to move the nanocavity anywhere along the waveguide. This configuration provides a significant degree of flexibility in integrated photonics and permits the addition and displacement of various functionalities of III/V nanocavity devices in Si photonic circuits. △ Less

Submitted 17 March, 2014; originally announced March 2014.

Comments: 19 pages, 4 figures, first version to Nature Materials 2014

Journal ref: Nature Materials 13 279-285 2014

Entangled photons from on-chip slow light

Authors: Hiroki Takesue, Nobuyuki Matsuda, Eiichi Kuramochi, Masaya Notomi

Abstract: We report the first entanglement generation experiment using an on-chip slow light device. With highly efficient spontaneous four-wave mixing enhanced by the slow light effect in a coupled resonator optical waveguide based on a silicon photonic crystal, we generated 1.5-$μ$m-band high-dimensional time-bin entangled photon pairs. We undertook two-photon interference experiments and observed the coi… ▽ More We report the first entanglement generation experiment using an on-chip slow light device. With highly efficient spontaneous four-wave mixing enhanced by the slow light effect in a coupled resonator optical waveguide based on a silicon photonic crystal, we generated 1.5-$μ$m-band high-dimensional time-bin entangled photon pairs. We undertook two-photon interference experiments and observed the coincidence fringes with visibilities $>74\%$. The present result enables us to realize an on-chip entanglement source with a very small footprint, which is an essential function for quantum information processing based on integrated quantum photonics. △ Less

Submitted 29 January, 2014; originally announced January 2014.

Comments: 5 pages, 3 figures. First draft

Journal ref: Scientific Reports 4, 3913 (2014) (full version)

An on-chip coupled resonator optical waveguide single-photon buffer

Authors: Hiroki Takesue, Nobuyuki Matsuda, Eiichi Kuramochi, William J. Munro, Masaya Notomi

Abstract: Integrated quantum optical circuits are now seen as one of the most promising approaches with which to realize single photon quantum information processing. Many of the core elements for such circuits have been realized including sources, gates and detectors. However, a significant missing function necessary for photonic information processing on-chip is a buffer, where single photons are stored f… ▽ More Integrated quantum optical circuits are now seen as one of the most promising approaches with which to realize single photon quantum information processing. Many of the core elements for such circuits have been realized including sources, gates and detectors. However, a significant missing function necessary for photonic information processing on-chip is a buffer, where single photons are stored for a short period of time to facilitate circuit synchronization. Here we report an on-chip single photon buffer based on coupled resonator optical waveguides (CROW) consisting of 400 high-Q photonic crystal line defect nanocavities. By using the CROW, a pulsed single photon was successfully buffered for 150 ps with 50-ps tunability while maintaining its non-classical properties. Furthermore, we showed that our buffer preserves entanglement by storing and retrieving one photon from a time-bin entangled state. This is a significant step towards an all-optical integrated quantum information processor. △ Less

Submitted 13 November, 2013; v1 submitted 12 November, 2013; originally announced November 2013.

Comments: 6 pages, 4 figures. First draft. Typo corrected

Journal ref: Nature Communications 4, 2725 (2013) (full version)

Slow light enhanced correlated photon pair generation in photonic-crystal coupled-resonator optical waveguides

Authors: Nobuyuki Matsuda, Hiroki Takesue, Kaoru Shimizu, Yasuhiro Tokura, Eiichi Kuramochi, Masaya Notomi

Abstract: We demonstrate the generation of quantum-correlated photon pairs from a Si photonic-crystal coupled-resonator optical waveguide. A slow-light supermode realized by the collective resonance of high-Q and small-mode-volume photonic-crystal cavities successfully enhanced the efficiency of the spontaneous four-wave mixing process. The generation rate of photon pairs was improved by two orders of magni… ▽ More We demonstrate the generation of quantum-correlated photon pairs from a Si photonic-crystal coupled-resonator optical waveguide. A slow-light supermode realized by the collective resonance of high-Q and small-mode-volume photonic-crystal cavities successfully enhanced the efficiency of the spontaneous four-wave mixing process. The generation rate of photon pairs was improved by two orders of magnitude compared with that of a photonic-crystal line defect waveguide without a slow-light effect. △ Less

Submitted 3 April, 2013; originally announced April 2013.

Comments: 14 pages, 4 figures

Journal ref: Opt. Express 21, 8596-8604 (2013)

Design for ultrahigh-Q position-controlled nanocavities of single semiconductor nanowires in 2D photonic crystals

Authors: M. D. Birowosuto, A. Yokoo, H. Taniyama, E. Kuramochi, M. Takiguchi, M. Notomi

Abstract: Using Finite-Difference Time-Domain (FDTD) simulation, we show that ultrahigh- Q nanocavities can be obtained through the manipulation of a single semiconductor nanowire (NW) inside a slot in a line defect of a two-dimensional (2D) photonic crystal. By controlling the design and its lattice parameters of the photonic crystal, we have achieved a quality factor Q larger than 106 and a mode volume Vc… ▽ More Using Finite-Difference Time-Domain (FDTD) simulation, we show that ultrahigh- Q nanocavities can be obtained through the manipulation of a single semiconductor nanowire (NW) inside a slot in a line defect of a two-dimensional (2D) photonic crystal. By controlling the design and its lattice parameters of the photonic crystal, we have achieved a quality factor Q larger than 106 and a mode volume Vc smaller than 0.11 μm3 (1.25 of a cubic wavelength in the NW) for a cavity peak in the telecommunication band. This design is useful for realizing a position-controlled cavity in a photonic crystal. Here we also discuss the small dependence of the Qfactor, the Vc, and the cavity peak in relation to the position of the NWinside the slot and the potential application to the cavity quantum electrodynamics (QED) using the embedded-emitter NW. △ Less

Submitted 13 November, 2012; originally announced November 2012.

Comments: 22 pages, 8 figures, to appear, Journal of Applied Physics 112 (2012)

Journal ref: J. Appl. Phys. 112, 113106 (2012)

Fast Purcell-enhanced single photon source in 1,550-nm telecom band from a resonant quantum dot-cavity coupling

Authors: M. D. Birowosuto, H. Sumikura, S. Matsuo, H. Taniyama, P. J. van Veldhoven, R. Nötzel, M. Notomi

Abstract: High-bit-rate nanocavity-based single photon sources in the 1,550-nm telecom band are challenges facing the development of fibre-based long-haul quantum communication networks. Here we report a very fast single photon source in the 1,550-nm telecom band, which is achieved by a large Purcell enhancement that results from the coupling of a single InAs quantum dot and an InP photonic crystal nanocavi… ▽ More High-bit-rate nanocavity-based single photon sources in the 1,550-nm telecom band are challenges facing the development of fibre-based long-haul quantum communication networks. Here we report a very fast single photon source in the 1,550-nm telecom band, which is achieved by a large Purcell enhancement that results from the coupling of a single InAs quantum dot and an InP photonic crystal nanocavity. At a resonance, the spontaneous emission rate was enhanced by a factor of 5 resulting a record fast emission lifetime of 0.2 ns at 1,550 nm. We also demonstrate that this emission exhibits an enhanced anti-bunching dip. This is the first realization of nanocavity-enhanced single photon emitters in the 1,550-nm telecom band. This coupled quantum dot cavity system in the telecom band thus provides a bright high-bit-rate non-classical single photon source that offers appealing novel opportunities for the development of a long-haul quantum telecommunication system via optical fibres. △ Less

Submitted 28 March, 2012; originally announced March 2012.

Comments: 16 pages, 4 figures

Journal ref: Sci. Rep. 2 (2012) 321

Electro-optic adiabatic wavelength shifting and Q switching demonstrated using a p-i-n integrated photonic crystal nanocavity

Authors: Takasumi Tanabe, Eiichi Kuramochi, Hideaki Taniyama, Masaya Notomi

Abstract: We demonstrate adiabatic wavelength shifting by electro-optic modulation using a p-i-n integrated high-Q photonic crystal nanocavity. The wavelength of the trapped light is adiabatically shifted by modulating the resonance of the cavity faster than the photon lifetime. The cavity resonance is changed by injecting electrons through a p-i-n junction to reduce the refractive index. In addition we emp… ▽ More We demonstrate adiabatic wavelength shifting by electro-optic modulation using a p-i-n integrated high-Q photonic crystal nanocavity. The wavelength of the trapped light is adiabatically shifted by modulating the resonance of the cavity faster than the photon lifetime. The cavity resonance is changed by injecting electrons through a p-i-n junction to reduce the refractive index. In addition we employ adiabatic wavelength shifting in a demonstration of dynamic Q tuning by electro-optic modulation. △ Less

Submitted 2 September, 2010; originally announced September 2010.

Comments: 3 pages, 3 figures, submitted to Opt. Lett

Journal ref: Opt. Lett. 35, 3895-3897 (2010)