Showing 1–17 of 17 results for author: Loh, K P

Giant nonlinear optical chirality in twisted heterobilayers

Authors: Xiang Zhang, Bo Li, Leyi Zhao, Pengzhi Wang, Luwei Zhou, Jiangbo Peng, Gan Wang, Kian Ping Loh, Tao-Yuan Du, Mingjie Li

Abstract: Twisting two dissimilar monolayer semiconductors induces structural chirality that remains largely elusive in linear optics but becomes remarkably pronounced in the nonlinear regime. Here we demonstrate that MoS2/WSe2 heterobilayers exhibit giant, twist-tunable nonlinear chirality in second-harmonic generation (SHG). The sign of SHG circular dichroism is governed by structural handedness, and its… ▽ More Twisting two dissimilar monolayer semiconductors induces structural chirality that remains largely elusive in linear optics but becomes remarkably pronounced in the nonlinear regime. Here we demonstrate that MoS2/WSe2 heterobilayers exhibit giant, twist-tunable nonlinear chirality in second-harmonic generation (SHG). The sign of SHG circular dichroism is governed by structural handedness, and its magnitude reaches 1.96 near a 30° twist angle under 1260-nm excitation, approaching the theoretical limit of 2. Furthermore, reversed chirality is observed when light is incident from opposite directions. Using a layer-resolved model, we attribute this phenomenon to helicity-dependent interference between the two monolayer SHG fields, mediated by a nonlinear Pancharatnam-Berry phase. These findings establish that the relative orientation of atomically thin layers can deterministically control nonlinear chiral responses, identifying twisted 2D heterostructures as a versatile platform for nonlinear chiral photonics, frequency conversion, and ultracompact light-matter interfaces. △ Less

Submitted 17 May, 2026; originally announced May 2026.

Local energy landscape drives long exciton diffusion in 2D halide perovskite semiconductors

Authors: Alan Baldwin, Géraud Delport, Kai Leng, Rosemonde Chahbazian, Krzysztof Galkowski, Kian Ping Loh, Samuel D. Stranks

Abstract: Halide perovskites have emerged as disruptive semiconductors for applications including photovoltaics and light emitting devices, with modular optoelectronic properties realisable through composition and dimensionality tuning. Layered Ruddlesden-Popper perovskites of the form BA2MAn-1PbnI3n+1, where n is the number of lead-halide and methylammonium (MA) sheets spaced by longer butylammonium (BA) c… ▽ More Halide perovskites have emerged as disruptive semiconductors for applications including photovoltaics and light emitting devices, with modular optoelectronic properties realisable through composition and dimensionality tuning. Layered Ruddlesden-Popper perovskites of the form BA2MAn-1PbnI3n+1, where n is the number of lead-halide and methylammonium (MA) sheets spaced by longer butylammonium (BA) cations, are particularly interesting due to their unique two-dimensional character and charge carrier dynamics dominated by strongly bound excitons. However, long-range energy transport through exciton diffusion in these materials is not understood or realised. Here, we employ local time-resolved luminescence mapping techniques to visualise exciton transport in high-quality exfoliated flakes of the BA2MAn-1PbnI3n+1 perovskite family. We uncover two distinct transport regimes, depending on the temperature range studied. At temperatures above 100 K, diffusion is mediated by thermally activated hopping processes between localised states. At lower temperatures, a non-uniform energetic landscape emerges in which exciton transport is dominated by energy funnelling processes to lower energy states, leading to long range transport over hundreds of nanometres even in the absence of exciton-phonon coupling and in the presence of local optoelectronic heterogeneity. Efficient, long-range and switchable excitonic funnelling offers exciting possibilities of controlled directional long-range transport in these 2D materials for new device applications. △ Less

Submitted 29 October, 2020; originally announced October 2020.

Learning Motifs and their Hierarchies in Atomic Resolution Microscopy

Authors: Jiadong Dan, Xiaoxu Zhao, Shoucong Ning, Jiong Lu, Kian Ping Loh, N. Duane Loh, Stephen J. Pennycook

Abstract: Progress in functional materials discovery has been accelerated by advances in high throughput materials synthesis and by the development of high-throughput computation. However, a complementary robust and high throughput structural characterization framework is still lacking. New methods and tools in the field of machine learning suggest that a highly automated high-throughput structural characte… ▽ More Progress in functional materials discovery has been accelerated by advances in high throughput materials synthesis and by the development of high-throughput computation. However, a complementary robust and high throughput structural characterization framework is still lacking. New methods and tools in the field of machine learning suggest that a highly automated high-throughput structural characterization framework based on atomic-level imaging can establish the crucial statistical link between structure and macroscopic properties. Here we develop a machine learning framework towards this goal. Our framework captures local structural features in images with Zernike polynomials, which is demonstrably noise-robust, flexible, and accurate. These features are then classified into readily interpretable structural motifs with a hierarchical active learning scheme powered by a novel unsupervised two-stage relaxed clustering scheme. We have successfully demonstrated the accuracy and efficiency of the proposed methodology by mapping a full spectrum of structural defects, including point defects, line defects, and planar defects in scanning transmission electron microscopy (STEM) images of various 2D materials, with greatly improved separability over existing methods. Our techniques can be easily and flexibly applied to other types of microscopy data with complex features, providing a solid foundation for automatic, multiscale feature analysis with high veracity. △ Less

Submitted 29 November, 2021; v1 submitted 23 May, 2020; originally announced May 2020.

Generation of 30-fs pulses from a diode-pumped graphene mode-locked Yb:CaYAlO4 laser

Authors: Jie Ma, Haitao Huang, Kaijie Ning, Xiaodong Xu, Guoqiang Xie, Liejia Qian, Kian Ping Loh, Dingyuan Tang

Abstract: Stable 30 fs pulses centered at 1068 nm (less than 10 optical cycles) are demonstrated in a diode pumped Yb:CaYAlO4 laser by using high-quality chemical vapor deposited monolayer graphene as the saturable absorber. The mode locked 8.43 optical-cycle pulses have a spectral bandwidth of ~ 50 nm and a pulse repetition frequency of ~ 113.5 MHz. To our knowledge, this is the shortest pulse ever reporte… ▽ More Stable 30 fs pulses centered at 1068 nm (less than 10 optical cycles) are demonstrated in a diode pumped Yb:CaYAlO4 laser by using high-quality chemical vapor deposited monolayer graphene as the saturable absorber. The mode locked 8.43 optical-cycle pulses have a spectral bandwidth of ~ 50 nm and a pulse repetition frequency of ~ 113.5 MHz. To our knowledge, this is the shortest pulse ever reported for graphene mode-locked lasers and mode-locked Yb-doped bulk lasers. Our experimental results demonstrate that graphene mode locking is a very promising practical technique to generate few-cycle optical pulses directly from a laser oscillator. △ Less

Submitted 19 December, 2015; originally announced December 2015.

Comments: 15 pages,6 figures

Graphene Nanobubble: A New Optical Nonlinear Material

Authors: Qiaoliang Bao, Jianqiang Chen, Yuanjiang Xiang, Kai Zhang, Shaojuan Li, Xiaofang Jiang, Qing-Hua Xu, Kian Ping Loh, T. Venkatesan

Abstract: Graphene is a rising star in nonlinear optics due to its saturable absorption and giant Kerr nonlinearity, these properties are useful in digital optics based on optical nonlinear devices. However, practical applications require large optical nonlinearities and these are inherently limited by the interaction length of atomically thin graphene. Here, we demonstrate optical bistability in a Fabry Pe… ▽ More Graphene is a rising star in nonlinear optics due to its saturable absorption and giant Kerr nonlinearity, these properties are useful in digital optics based on optical nonlinear devices. However, practical applications require large optical nonlinearities and these are inherently limited by the interaction length of atomically thin graphene. Here, we demonstrate optical bistability in a Fabry Perot cavity containing monolayer and bilayer graphene which have been restructured to form nanobubbles. We find that graphene nanobubble can act as a new type of optical nonlinear media due to its vertical side wall as well as added curvature, which enable strong non linear dispersive effects leading to a large optically induced phase change. Unlike thermally induced bistability, the all optical switching between two transmission states happens within a time scale of tens of nanoseconds. Nanobubble based optical devices with intrinsic optical nonlinearity help to overcome the optical path length limitation of atomically thin two dimensional films and allow us to explore the promise of using such elements as the building block of digital all-optical circuitry. △ Less

Submitted 23 June, 2015; originally announced June 2015.

Comments: 17 pages, 4 figures

Graphene-enhanced, internal-magnetic-field-generated Rabi oscillations in metal-coated Si-SiO2 photoconductive detectors

Authors: Venkatram Nalla, Kai Zhang, Kian Ping Loh, Wei Ji

Abstract: We report our demonstration of Rabi oscillations in Si-SiO2-Al photoconductive devices with nanosecond laser pulses of a few nJ at room temperature without external magnetic fields. Zeeman splitting of spin quantum states of dopants in silicon is achieved with internal magnetic fields produced by the Al film under excitation of laser pulses. Rabi oscillation frequency is 15 MHz and 25 MHz when pho… ▽ More We report our demonstration of Rabi oscillations in Si-SiO2-Al photoconductive devices with nanosecond laser pulses of a few nJ at room temperature without external magnetic fields. Zeeman splitting of spin quantum states of dopants in silicon is achieved with internal magnetic fields produced by the Al film under excitation of laser pulses. Rabi oscillation frequency is 15 MHz and 25 MHz when photocurrent direction is perpendicular and parallel, respectively, to the propagation direction of linearly-polarized, 532-nm, 7-ns laser pulses. Insertion of graphene buffer layer between Al and SiO2 provides a three-fold enhancement in Rabi oscillation amplitude. This simple-structured, low-cost device operated at room temperature should open a new avenue for future spin-based electronics and optoelectronics. △ Less

Submitted 27 July, 2013; originally announced July 2013.

Achieving Giant Magneto-optic Effects with Optical Tamm States in Graphene-based Photonics

Authors: Haixia Da, Cheng-Wei Qiu, Qiaoliang Bao, Jinghua Teng, Kian Ping Loh, Francisco J. Garcia-Vidal

Abstract: We manipulate optical Tamm states in graphene-based photonics to achieve and steer large magneto-optical effects. Here we report the presence of a giant Faraday rotation via a single graphene layer of atomic thickness while keeping a high transmission. The Faraday rotation is enhanced across the interface between two photonic crystals due to the presence of an interface mode, which presents a stro… ▽ More We manipulate optical Tamm states in graphene-based photonics to achieve and steer large magneto-optical effects. Here we report the presence of a giant Faraday rotation via a single graphene layer of atomic thickness while keeping a high transmission. The Faraday rotation is enhanced across the interface between two photonic crystals due to the presence of an interface mode, which presents a strong electromagnetic field confinement at the location of the graphene sheet. Our proposed scheme opens a promising avenue to realize high performance graphene magneto-optical devices that can be extended to other two-dimensional structures. △ Less

Submitted 19 March, 2013; originally announced March 2013.

Comments: 18 pages, 4 figures

Large nonlinear Kerr effect in graphene

Authors: Han Zhang, Stephane Virally, Qiaoliang Bao, Kian Ping Loh, Serge Massar, Nicolas Godbout, Pascal Kockaert

Abstract: Under strong laser illumination, few-layer graphene exhibits both a transmittance increase due to saturable absorption and a nonlinear phase shift. Here, we unambiguously distinguish these two nonlinear optical effects and identify both real and imaginary parts of the complex nonlinear refractive index of graphene. We show that graphene possesses a giant nonlinear refractive index n2=10-7cm2W-1, a… ▽ More Under strong laser illumination, few-layer graphene exhibits both a transmittance increase due to saturable absorption and a nonlinear phase shift. Here, we unambiguously distinguish these two nonlinear optical effects and identify both real and imaginary parts of the complex nonlinear refractive index of graphene. We show that graphene possesses a giant nonlinear refractive index n2=10-7cm2W-1, almost nine orders of magnitude larger than bulk dielectrics. We find that the nonlinear refractive index decreases with increasing excitation flux but slower than the absorption. This suggests that graphene may be a very promising nonlinear medium, paving the way for graphene-based nonlinear photonics. △ Less

Submitted 25 March, 2012; originally announced March 2012.

Comments: Optics Letters received 12/02/2011; accepted 03/12/2012; posted 03/21/2012,Doc. ID 159129

Journal ref: Optics Letters Vol. 37, Issue 11, pp. 1856-1858 (2012)

Monolayer Graphene as Saturable Absorber in Mode-locked Laser

Authors: Qiaoliang Bao, Han Zhang, Zhenhua Ni, Yu Wang, Lakshminarayana Polavarapu, Kian Ping Loh, Zexiang Shen, Qing-Hua Xu, Ding Yuan Tang

Abstract: We demonstrate that the intrinsic properties of monolayer graphene allow it to act as a more effective saturable absorber for mode-locking fiber lasers compared to multilayer graphene. The absorption of monolayer graphene can be saturated at lower excitation intensity compared to multilayer graphene, graphene with wrinkle-like defects, and functionalized graphene. Monolayer graphene has a remarkab… ▽ More We demonstrate that the intrinsic properties of monolayer graphene allow it to act as a more effective saturable absorber for mode-locking fiber lasers compared to multilayer graphene. The absorption of monolayer graphene can be saturated at lower excitation intensity compared to multilayer graphene, graphene with wrinkle-like defects, and functionalized graphene. Monolayer graphene has a remarkable large modulation depth of 95.3%, whereas the modulation depth of multilayer graphene is greatly reduced due to nonsaturable absorption and scattering loss. Picoseconds ultrafast laser pulse (1.23 ps) can be generated using monolayer graphene as saturable absorber. Due to the ultrafast relaxation time, larger modulation depth and lower scattering loss of monolayer graphene, it performs better than multilayer graphene in terms of pulse shaping ability, pulse stability and output energy. △ Less

Submitted 15 November, 2010; v1 submitted 13 July, 2010; originally announced July 2010.

Comments: 23 pages

Compact graphene mode-locked wavelength-tunable erbium-doped fiber lasers: from all anomalous dispersion towards all normal dispersion

Authors: Han Zhang, Dingyuan Tang, Luming Zhao, Qiaoliang Bao, Kian Ping Loh, Bo Lin, Swee Chuan Tjin

Abstract: Soliton operation and soliton wavelength tuning of erbium-doped fiber lasers mode locked with atomic layer graphene was experimentally investigated under various cavity dispersion conditions. It was shown that not only wide range soliton wavelength tuning but also soltion pulse width variation could be obtained in the fiber lasers. Our results show that the graphene mode locked erbium-doped fiber… ▽ More Soliton operation and soliton wavelength tuning of erbium-doped fiber lasers mode locked with atomic layer graphene was experimentally investigated under various cavity dispersion conditions. It was shown that not only wide range soliton wavelength tuning but also soltion pulse width variation could be obtained in the fiber lasers. Our results show that the graphene mode locked erbium-doped fiber lasers provide a compact, user friendly and low cost wavelength tunable ultrahsort pulse source. △ Less

Submitted 26 March, 2010; originally announced March 2010.

Journal ref: Laser Physics Letters, 2010

Femtosecond Pump-Probe Studies of Reduced Graphene Oxide Thin Films

Authors: Brian A. Ruzicka, Lalani K. Werake, Hui Zhao, Shuai Wang, Kian Ping Loh

Abstract: The dynamics of photocarriers in reduced graphene oxide thin films is studied by using ultrafast pump-probe spectroscopy. Time dependent differential transmissions are measured with sample temperatures ranging from 9 to 300 K. At each sample temperature and probe delay, the sign of differential transmission remains positive. A fast energy relaxation of hot carriers is observed, and is found to be… ▽ More The dynamics of photocarriers in reduced graphene oxide thin films is studied by using ultrafast pump-probe spectroscopy. Time dependent differential transmissions are measured with sample temperatures ranging from 9 to 300 K. At each sample temperature and probe delay, the sign of differential transmission remains positive. A fast energy relaxation of hot carriers is observed, and is found to be independent of sample temperature. Our experiments show that the carrier dynamics in reduced graphene oxide is similar to other types of graphene, and that the differential transmission is caused by phase-state filling of carriers. △ Less

Submitted 10 April, 2010; v1 submitted 9 March, 2010; originally announced March 2010.

Comments: 3 pages, 3 figures

Journal ref: Appl. Phys. Lett. 96, 173106 (2010)

Graphene mode locked, wavelength-tunable, dissipative soliton fiber laser

Authors: Han Zhang, Dingyuan Tang, R. J. Knize, Luming Zhao, Qiaoliang Bao, Kian Ping Loh

Abstract: Atomic layer graphene possesses wavelength-insensitive ultrafast saturable absorption, which can be exploited as a full-band mode locker. Taking advantage of the wide band saturable absorption of the graphene, we demonstrate experimentally that wide range (1570 nm - 1600nm) continuous wavelength tunable dissipative solitons could be formed in an erbium doped fiber laser mode locked with few laye… ▽ More Atomic layer graphene possesses wavelength-insensitive ultrafast saturable absorption, which can be exploited as a full-band mode locker. Taking advantage of the wide band saturable absorption of the graphene, we demonstrate experimentally that wide range (1570 nm - 1600nm) continuous wavelength tunable dissipative solitons could be formed in an erbium doped fiber laser mode locked with few layer graphene. △ Less

Submitted 28 February, 2010; originally announced March 2010.

Journal ref: Applied Physics Letters 2010

Vector Dissipative Solitons in Graphene Mode Locked Fiber Lasers

Authors: Han Zhang, Dingyuan Tang, Luming Zhao, Qiaoliang Bao, Kian Ping Loh

Abstract: Vector soliton operation of erbium-doped fiber lasers mode locked with atomic layer graphene was experimentally investigated. Either the polarization rotation or polarization locked vector dissipative solitons were experimentally obtained in a dispersion-managed cavity fiber laser with large net cavity dispersion, while in the anomalous dispersion cavity fiber laser, the phase locked NLSE solito… ▽ More Vector soliton operation of erbium-doped fiber lasers mode locked with atomic layer graphene was experimentally investigated. Either the polarization rotation or polarization locked vector dissipative solitons were experimentally obtained in a dispersion-managed cavity fiber laser with large net cavity dispersion, while in the anomalous dispersion cavity fiber laser, the phase locked NLSE solitons and induced NLSE soliton were experimentally observed. The vector soliton operation of the fiber lasers unambiguously confirms the polarization insensitive saturable absorption of the atomic layer graphene when the light is incident perpendicular to its 2D atomic layer. △ Less

Submitted 26 February, 2010; originally announced February 2010.

Journal ref: Physical Review A, 2010

Atomic layer graphene as saturable absorber for ultrafast pulsed lasers

Authors: Qiaoliang Bao, Han Zhang, Yu Wang, Zhenhua Ni, Yongli Yan, Ze Xiang Shen, Kian Ping Loh, Ding Yuan Tang

Abstract: The optical conductance of monolayer graphene is defined solely by the fine structure constant. The absorbance has been predicted to be independent of frequency. In principle, the interband optical absorption in zero-gap graphene could be saturated readily under strong excitation due to Pauli blocking. Here, we demonstrate the use of atomic layer graphene as saturable absorber in a mode-locked f… ▽ More The optical conductance of monolayer graphene is defined solely by the fine structure constant. The absorbance has been predicted to be independent of frequency. In principle, the interband optical absorption in zero-gap graphene could be saturated readily under strong excitation due to Pauli blocking. Here, we demonstrate the use of atomic layer graphene as saturable absorber in a mode-locked fiber laser for the generation of ultrashort soliton pulses (756 fs) at the telecommunication band. The modulation depth can be tuned in a wide range from 66.5% to 6.2% by varying the thickness of graphene. Our results suggest that ultrathin graphene films are potentially useful as optical elements in fiber lasers. Graphene as a laser mode locker can have many merits such as lower saturation intensity, ultrafast recovery time, tunable modulation depth and wideband tuneability. △ Less

Submitted 30 October, 2009; originally announced October 2009.

Large energy mode locking of an erbium-doped fiber laser with atomic layer graphene

Authors: H. Zhang, D. Y. Tang, L. M. Zhao, Q. L. Bao, K. P. Loh

Abstract: We report on large energy pulse generation in an erbium-doped fiber laser passively mode-locked with atomic layer graphene. Stable mode locked pulses with single pulse energy up to 7.3 nJ and pulse width of 415 fs have been directly generated from the laser. Our results show that atomic layer graphene could be a promising saturable absorber for large energy mode locking. We report on large energy pulse generation in an erbium-doped fiber laser passively mode-locked with atomic layer graphene. Stable mode locked pulses with single pulse energy up to 7.3 nJ and pulse width of 415 fs have been directly generated from the laser. Our results show that atomic layer graphene could be a promising saturable absorber for large energy mode locking. △ Less

Submitted 30 September, 2009; originally announced September 2009.

Comments: 14 pages 4 figures

Trapping of dark vector solitons in a fiber laser

Authors: H. Zhang, D. Y. Tang, L. M. Zhao, X. Wu, Q. L Bao, K. P. Loh

Abstract: We report on the experimental observation of incoherently coupled dark vector soliton trapping in a fiber laser. Dark vector solitons along the two orthogonal polarization directions though with large difference in central frequency, energy and darkness could be still synchronously trapped as a group velocity locked dark vector soliton in virtue of the incoherent interactions. Our numerical simu… ▽ More We report on the experimental observation of incoherently coupled dark vector soliton trapping in a fiber laser. Dark vector solitons along the two orthogonal polarization directions though with large difference in central frequency, energy and darkness could be still synchronously trapped as a group velocity locked dark vector soliton in virtue of the incoherent interactions. Our numerical simulation could well rebirth the experimental observations and confirm the existence of incoherently coupled dark vector soliton under certain conditions. △ Less

Submitted 7 May, 2009; v1 submitted 17 April, 2009; originally announced April 2009.

Comments: 17 pages, 4 figures. Submitted to Optics Express

Dark Soliton Fiber Laser

Authors: H. Zhang, D. Y. Tang, L. M. Zhao, X. Wu, Q. L. Bao, K. P. Loh

Abstract: We report on the experimental observation of stable dark solitons in an all normal dispersion fiber laser. We found experimentally that dark soliton formation is a generic feature of the fiber laser under strong continuous wave (CW) emission. However, only under appropriate pump strength and negative cavity feedback, stable single or multiple dark soliton could be achieved. Furthermore, we show… ▽ More We report on the experimental observation of stable dark solitons in an all normal dispersion fiber laser. We found experimentally that dark soliton formation is a generic feature of the fiber laser under strong continuous wave (CW) emission. However, only under appropriate pump strength and negative cavity feedback, stable single or multiple dark soliton could be achieved. Furthermore, we show that the features of the observed dark solitons could be well understood based on the nonlinear Schrodinger equation (NLSE). △ Less

Submitted 8 May, 2009; v1 submitted 11 March, 2009; originally announced March 2009.

Comments: 14 pages, 6 figures, submitted to optics letters