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Giant optical spin-orbit interactions in ferroelectric van der Waals waveguides
Authors:
Ding Xu,
Saeed Rahmanian Koshkaki,
Vicente Galicia,
Chun-Ying Huang,
Victoria Quirós-Cordero,
Jakhangirkhodja A. Tulyagankhodjaev,
André Koch Liston,
Daniel G. Chica,
Emma Lian,
Amirhosein Amini,
Yongseok Hong,
Taketo Handa,
P. James Schuck,
Xiaoyang Zhu,
Xavier Roy,
Arkajit Mandal,
Milan Delor
Abstract:
Optical spin-orbit interactions (SOI) link photonic spin to momentum, offering a route toward on-chip polarization control and beam steering. Nevertheless, achieving sufficient optical SOI and nonlinearities on sub-micrometer scales - a prerequisite for dense photonic integration - remains an outstanding challenge. Here, we show that highly birefringent van der Waals (vdW) waveguides provide an id…
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Optical spin-orbit interactions (SOI) link photonic spin to momentum, offering a route toward on-chip polarization control and beam steering. Nevertheless, achieving sufficient optical SOI and nonlinearities on sub-micrometer scales - a prerequisite for dense photonic integration - remains an outstanding challenge. Here, we show that highly birefringent van der Waals (vdW) waveguides provide an ideal, chip-compatible platform to address this limitation. We focus on the ferroelectric semiconductor NbOI2, which exhibits record optical nonlinearities and dielectric anisotropy. Using femtosecond optical microscopy, we image light propagation and harmonic conversion beyond the total internal reflection barrier over tens of micrometers in NbOI2 slab waveguides. We report giant optical spin-splitting through the optical spin Hall effect, which facilitates spatial separation of optical spin currents on sub-micrometer scales, in quantitative agreement with a microscopic light-matter interaction model. We further leverage optical spin-momentum locking to realize polarization-controlled waveguide steering. We generalize these observations across various vdW waveguides and empirically confirm a scaling law linking dielectric anisotropy to geometric spin-splitting. Our results establish highly anisotropic vdW waveguides as an ideal platform for densely integrated opto-spintronic technologies.
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Submitted 13 May, 2026;
originally announced May 2026.
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Tunable Narrowband Terahertz Radiation from van der Waals Ferroelectrics
Authors:
Chun-Ying Huang,
Taketo Handa,
Daniel G. Chica,
Zhihao Cui,
Ding Xu,
Jeongheon Choe,
Yiliu Li,
Margalit L. Feuer,
Milan E. Delor,
Michael Fechner,
David R. Reichman,
Xavier Roy,
Xiaoyang Zhu
Abstract:
The terahertz (THz) spectral range is central to high-speed communication, precision metrology, sensing technologies, and a range of fundamental scientific investigations. Achieving these capabilities in practical systems increasingly demands chip-scale integration of THz photonic components that are typically bulky. In this context, van der Waals (vdW) materials provide a unique platform for inte…
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The terahertz (THz) spectral range is central to high-speed communication, precision metrology, sensing technologies, and a range of fundamental scientific investigations. Achieving these capabilities in practical systems increasingly demands chip-scale integration of THz photonic components that are typically bulky. In this context, van der Waals (vdW) materials provide a unique platform for integrated nonlinear photonics in the visible and near-infrared regimes, and extending this framework into the THz domain would constitute a significant advance. Here, we report tunable, intense, and narrowband THz radiation from ferroelectric niobium oxyhalides. Through halogen substitution and alloying, we achieve continuous and precise control over the emission frequency from 3.1 to 5.8 THz. We show that the narrowband THz radiation is driven by phonons associated with the ferroelectric polarization. We further demonstrate dynamic and nonvolatile control of the polarity of the coherent THz wave with external electric field. This work demonstrates efficient narrowband THz emission from vdW ferroeletrics and provides microscopic insight into its origin, paving the way for on-chip THz technology for a broad range of applications.
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Submitted 5 December, 2025;
originally announced December 2025.
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Rydberg Exciton Dynamics in the Blockade Regime of Cu2O
Authors:
Gillian E. Minarik,
Eric A. Arsenault,
Vinícius da Silveira Lan Avelar,
Taketo Handa,
X. -Y. Zhu
Abstract:
Hosting giant Rydberg excitons with principal quantum numbers up to n = 30, cuprous oxide (Cu2O) provides a rare solid-state setting for exploring Rydberg physics, as exemplified by the blockade effect. Here we access the strongly interaction regime at high excitation densities (10^14-10^16/cm^3) and resolve the corresponding blockade dynamics for n = 2-7 using time-resolved spectroscopy. We find…
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Hosting giant Rydberg excitons with principal quantum numbers up to n = 30, cuprous oxide (Cu2O) provides a rare solid-state setting for exploring Rydberg physics, as exemplified by the blockade effect. Here we access the strongly interaction regime at high excitation densities (10^14-10^16/cm^3) and resolve the corresponding blockade dynamics for n = 2-7 using time-resolved spectroscopy. We find that Rydberg blockades are primarily governed by resonant dipolar interactions and that exciton recombination is coupled to the blockade itself. These findings demonstrate the potential for manipulating Rydberg exctions in the strongly interacting blockade regime in a solid state system.
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Submitted 7 August, 2025;
originally announced August 2025.
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Observation of Coherent Ferrons
Authors:
Jeongheon Choe,
Taketo Handa,
Chun-Ying Huang,
André Koch Liston,
Jordan Cox,
Jonathan Stensberg,
Yongseok Hong,
Daniel G. Chica,
Ding Xu,
Fuyang Tay,
Samra Husremovic,
Vinicius da Silveira Lanza Avelar,
Eric A. Arsenault,
Zhuquan Zhang,
James McIver,
Dmitri N. Basov,
Milan Delor,
Xavier Roy,
X. -Y. Zhu
Abstract:
Excitation of ordered quantum phases gives rise to collective modes and quasiparticles, as exemplified by spin waves and magnons emerging from magnetic order. Extending this paradigm to ferroelectric materials suggests the existence of polarization waves and their fundamental quanta, ferrons. Here, we report the generation and transport of polarization waves, i.e., coherent ferrons, in the van der…
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Excitation of ordered quantum phases gives rise to collective modes and quasiparticles, as exemplified by spin waves and magnons emerging from magnetic order. Extending this paradigm to ferroelectric materials suggests the existence of polarization waves and their fundamental quanta, ferrons. Here, we report the generation and transport of polarization waves, i.e., coherent ferrons, in the van der Waals ferroelectric material NbOI2. Upon excitation by a short laser pulse, the polarization wave emits intense and narrow-band terahertz (THz) radiation at the ferroelectric transverse optical phonon frequency, modulates the ferroelectric order parameter, and propagates uniaxially along the polar axis at hypersonic velocities of ~105 m/s. These long-lived, uniaxial, and dipole-carrying polarization waves may find applications in narrow-band THz emission, ferronic information processing, and coherent electric control.
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Submitted 16 August, 2025; v1 submitted 28 May, 2025;
originally announced May 2025.
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A 2D van der Waals Material for Terahertz Emission with Giant Optical Rectification
Authors:
Taketo Handa,
Chun-Ying Huang,
Yiliu Li,
Nicholas Olsen,
Daniel G. Chica,
David D. Xu,
Felix Sturm,
James W. McIver,
Xavier Roy,
Xiaoyang Zhu
Abstract:
Exfoliation and stacking of two-dimensional (2D) van der Waals (vdW) crystals have created unprecedented opportunities in the discovery of quantum phases. A major obstacle to the advancement of this field is the limited spectroscopic access due to a mismatch in sample sizes (1 - 10 micrometer) and wavelengths (0.1 - 1 millimeter) of electromagnetic radiation relevant to their low-energy excitation…
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Exfoliation and stacking of two-dimensional (2D) van der Waals (vdW) crystals have created unprecedented opportunities in the discovery of quantum phases. A major obstacle to the advancement of this field is the limited spectroscopic access due to a mismatch in sample sizes (1 - 10 micrometer) and wavelengths (0.1 - 1 millimeter) of electromagnetic radiation relevant to their low-energy excitations. Here, we introduce a new member of the 2D vdW material family: a terahertz (THz) emitter. We show intense and broadband THz generation from the vdW ferroelectric semiconductor NbOI2 with optical rectification efficiency over one-order-of-magnitude higher than that of the current standard THz emitter, ZnTe. The NbOI2 THz emitter can be easily integrated into vdW heterostructures for on-chip near-field THz spectroscopy of a target vdW material/device. Our approach provides a general spectroscopic tool for the rapidly expanding field of 2D vdW materials and quantum matter.
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Submitted 14 November, 2024;
originally announced November 2024.
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Coupling of Electronic Transitions to Ferroelectric Order in a 2D Semiconductor
Authors:
Chun-Ying Huang,
Daniel G. Chica,
Zhi-Hao Cui,
Taketo Handa,
Morgan Thinel,
Nicholas Olsen,
Yufeng Liu,
Michael E. Ziebel,
Guiying He,
Yinming Shao,
Connor A. Occhialini,
Jonathan Pelliciari,
Dmitri N. Basov,
Matthew Sfeir,
Abhay Pasupathy,
Valentina Bisogni,
David R. Reichman,
Xavier Roy,
Xiaoyang Zhu
Abstract:
A ferroelectric material often exhibits a soft transvers optical (TO) phonon mode which governs it phase transition. Charge coupling to this ferroelectric soft mode may further mediate emergent physical properties, including superconductivity and defect tolerance. However, direct experimental evidence for such coupling is scarce. Here we show that a photo-launched coherent phonon couples strongly…
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A ferroelectric material often exhibits a soft transvers optical (TO) phonon mode which governs it phase transition. Charge coupling to this ferroelectric soft mode may further mediate emergent physical properties, including superconductivity and defect tolerance. However, direct experimental evidence for such coupling is scarce. Here we show that a photo-launched coherent phonon couples strongly to electronic transitions across the bandgap in the van der Waals (vdW) two-dimensional (2D) ferroelectric semiconductor NbOI2. Using terahertz time-domain spectroscopy and first-principles calculations, we identify this mode as the TO phonon responsible for ferroelectric order. This exclusive coupling occurs only with above-gap electronic transition and is absent in the valence band as revealed by resonant inelastic X-ray scattering. Our findings suggest a new role of the soft TO phonon mode in electronic and optical properties of ferroelectric semiconductors.
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Submitted 11 October, 2024;
originally announced October 2024.
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Transient Magnetoelastic Coupling in CrSBr
Authors:
Youn Jue Bae,
Taketo Handa,
Yanan Dai,
Jue Wang,
Huicong Liu,
Allen Scheie,
Daniel G. Chica,
Michael E. Ziebel,
Andrew D. Kent,
Xiaodong Xu,
Ka Shen,
Xavier Roy,
Xiaoyang Zhu
Abstract:
Recent research has revealed remarkable properties of the two-dimensional (2D) van der Waals layered crystal CrSBr, which is both a semiconductor and an A-type antiferromagnet. Here we show the role of strong magnetoelastic coupling in the generation and propagation of coherent magnons in CrSBr. Time and spatially resolved magneto-optical Kerr effect (tr-MOKE) microscopy reveals two time-varying t…
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Recent research has revealed remarkable properties of the two-dimensional (2D) van der Waals layered crystal CrSBr, which is both a semiconductor and an A-type antiferromagnet. Here we show the role of strong magnetoelastic coupling in the generation and propagation of coherent magnons in CrSBr. Time and spatially resolved magneto-optical Kerr effect (tr-MOKE) microscopy reveals two time-varying transient strain fields induced by out-of-plane transverse and in-plane longitudinal lattice displacements. These transient strain fields launch coherent wavepackets of magnons, optical and acoustic at 24.6 GHz and 33.4 GHz, respectively. These findings suggest mechanisms for controlling and manipulating coherent magnons from distinct magnetoelastic couplings in this 2D van der Waals magnetic semiconductor.
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Submitted 15 January, 2024;
originally announced January 2024.
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Visualizing moiré ferroelectricity via plasmons and nano-photocurrent in graphene/twisted-WSe2 structures
Authors:
Shuai Zhang,
Yang Liu,
Zhiyuan Sun,
Xinzhong Chen,
Baichang Li,
S. L. Moore,
Song Liu,
Zhiying Wang,
S. E. Rossi,
Ran Jing,
Jordan Fonseca,
Birui Yang,
Yinming Shao,
Chun-Ying Huang,
Taketo Handa,
Lin Xiong,
Matthew Fu,
Tsai-Chun Pan,
Dorri Halbertal,
Xinyi Xu,
Wenjun Zheng,
P. J. Schuck,
A. N. Pasupathy,
C. R. Dean,
Xiaoyang Zhu
, et al. (6 additional authors not shown)
Abstract:
Ferroelectricity, a spontaneous and reversible electric polarization, is found in certain classes of van der Waals (vdW) material heterostructures. The discovery of ferroelectricity in twisted vdW layers provides new opportunities to engineer spatially dependent electric and optical properties associated with the configuration of moiré superlattice domains and the network of domain walls. Here, we…
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Ferroelectricity, a spontaneous and reversible electric polarization, is found in certain classes of van der Waals (vdW) material heterostructures. The discovery of ferroelectricity in twisted vdW layers provides new opportunities to engineer spatially dependent electric and optical properties associated with the configuration of moiré superlattice domains and the network of domain walls. Here, we employ near-field infrared nano-imaging and nano-photocurrent measurements to study ferroelectricity in minimally twisted WSe2. The ferroelectric domains are visualized through the imaging of the plasmonic response in a graphene monolayer adjacent to the moiré WSe2 bilayers. Specifically, we find that the ferroelectric polarization in moiré domains is imprinted on the plasmonic response of the graphene. Complementary nano-photocurrent measurements demonstrate that the optoelectronic properties of graphene are also modulated by the proximal ferroelectric domains. Our approach represents an alternative strategy for studying moiré ferroelectricity at native length scales and opens promising prospects for (opto)electronic devices.
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Submitted 12 September, 2023;
originally announced September 2023.
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Smectic Pair Density Wave Order in EuRbFe4As4
Authors:
He Zhao,
Raymond Blackwell,
Morgan Thinel,
Taketo Handa,
Shigeyuki Ishida,
Xiaoyang Zhu,
Akira Iyo,
Hiroshi Eisaki,
Abhay N. Pasupathy,
Kazuhiro Fujita
Abstract:
The pair density wave (PDW) is a novel superconducting state in which Cooper pairs carry center-of-mass momentum in equilibrium, leading to the breaking of translational symmetry. Experimental evidence for such a state exists in high magnetic field and in some materials that feature density wave orders that explicitly break translational symmetry. However, evidence for a zero-field PDW state that…
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The pair density wave (PDW) is a novel superconducting state in which Cooper pairs carry center-of-mass momentum in equilibrium, leading to the breaking of translational symmetry. Experimental evidence for such a state exists in high magnetic field and in some materials that feature density wave orders that explicitly break translational symmetry. However, evidence for a zero-field PDW state that exists independent of other spatially ordered states has so far been elusive. Here, we show that such a state exists in the iron pnictide superconductor EuRbFe4As4 (Eu-1144), a material that features coexisting superconductivity (Tc ~ 37K) and magnetism (Tm ~ 15 K). We show from the Spectroscopic Imaging Scanning Tunneling Microscopy (SI-STM) measurements that the superconducting gap at low temperature has long-range, unidirectional spatial modulations with an incommensurate period of ~8 unit cells. Upon raising the temperature above Tm, the modulated superconductor disappears, but a uniform superconducting gap survives to Tc. When an external magnetic field is applied, gap modulations disappear inside the vortex halo. The SI-STM and bulk measurements show the absence of other density wave orders, showing that the PDW state is a primary, zero-field superconducting state in this compound. Both four-fold rotational symmetry and translation symmetry are recovered above Tm, indicating that the PDW is a smectic order.
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Submitted 28 June, 2023;
originally announced June 2023.
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Spontaneous Exciton Dissociation in Transition Metal Dichalcogenide Monolayers
Authors:
Taketo Handa,
Madisen A. Holbrook,
Nicholas Olsen,
Luke N. Holtzman,
Lucas Huber,
Hai I. Wang,
Mischa Bonn,
Katayun Barmak,
James C. Hone,
Abhay N. Pasupathy,
X. -Y. Zhu
Abstract:
Since the seminal work on MoS2 monolayers, photoexcitation in atomically-thin transition metal dichalcogenides (TMDCs) has been assumed to result in excitons with large binding energies (~ 200-600 meV). Because the exciton binding energies are order-of-magnitude larger than thermal energy at room temperature, it is puzzling that photocurrent and photovoltage generation have been observed in TMDC-b…
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Since the seminal work on MoS2 monolayers, photoexcitation in atomically-thin transition metal dichalcogenides (TMDCs) has been assumed to result in excitons with large binding energies (~ 200-600 meV). Because the exciton binding energies are order-of-magnitude larger than thermal energy at room temperature, it is puzzling that photocurrent and photovoltage generation have been observed in TMDC-based devices, even in monolayers with applied electric fields far below the threshold for exciton dissociation. Here, we show that the photoexcitation of TMDC monolayers results in a substantial population of free charges. Performing ultrafast terahertz (THz) spectroscopy on large-area, single crystal WS2, WSe2, and MoSe2 monolayers, we find that ~10% of excitons spontaneously dissociate into charge carriers with lifetimes exceeding 0.2 ns. Scanning tunnelling microscopy reveals that photo-carrier generation is intimately related to mid-gap defect states, likely via trap-mediated Auger scattering. Only in state-of-the-art quality monolayers14, with mid-gap trap densities as low as 10^9 cm^-2, does intrinsic exciton physics start to dominate the THz response. Our findings reveal that excitons or excitonic complexes are only the predominant quasiparticles in photo-excited TMDC monolayers at the limit of sufficiently low defect densities.
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Submitted 19 June, 2023;
originally announced June 2023.
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Charge carrier coupling to the soft phonon mode in a ferroelectric semiconductor
Authors:
Mark E. Ziffer,
Lucas Huber,
Feifan Wang,
Victoria A. Posey,
Jake C. Russell,
Taketo Handa,
Xavier Roy,
X. -Y. Zhu
Abstract:
Many crystalline solids possess strongly anharmonic soft phonon modes characterized by diminishing frequency as temperature approaches a critical point associated with a symmetry breaking phase transition. While electron-soft phonon coupling can introduce unique scattering channels for charge carriers in ferroelectrics, recent studies on the non-ferroelectric lead halide perovskites have also sugg…
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Many crystalline solids possess strongly anharmonic soft phonon modes characterized by diminishing frequency as temperature approaches a critical point associated with a symmetry breaking phase transition. While electron-soft phonon coupling can introduce unique scattering channels for charge carriers in ferroelectrics, recent studies on the non-ferroelectric lead halide perovskites have also suggested the central role of anharmonic phonons bearing resemblance to soft modes in charge carrier screening. Here we apply coherent phonon spectroscopy to directly study electron coupling to the soft transverse optical (TO) phonon mode in a ferroelectric semiconductor SbSI. Photo-generated charge carriers in SbSI are found to be exceptionally long lived and are associated with a transient electro-optical effect that can be explained by interactions between charge carriers and thermally stimulated soft-phonon excitations. These results provide strong evidence for the role of electron-soft phonon coupling in the efficient screening of charge carriers and in reducing charge recombination rates, both desirable properties for optoelectronics.
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Submitted 22 May, 2022;
originally announced May 2022.