Li Huang
Email Address
elehual@nus.edu.sg
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Publication First Demonstration of Waveguide-Integrated Black Phosphorus Electro-Optic Modulator for Mid-Infrared Beyond 4 μm(2020-02-13) Li Huang; Bowei Dong; Yiming Ma; Chengkuo Lee; Kah-Wee Ang; ELECTRICAL AND COMPUTER ENGINEERINGWe demonstrate the first black phosphorus (BP) electro-optic modulator integrated with Si waveguide for the mid-infrared (MIR) spectrum from 3.85 to 4.1 μm. Optical properties of BP were effectively modulated by a vertical electric field from a top gate through Burstein-Moss and Franz-Keldysh effects. With a gate bias of -4 V, a modulation depth of ~5 dB was achieved with a small active footprint of merely 225 μm 2 . The modulation depth was observed to increase with decreasing light power. The results are promising for operations under weak-light condition with low-power consumption and compact footprint at room temperature. Our device lays a stepping stone towards realizing on-chip MIR systems for applications such as bio-sensing and environment monitoring.Publication Black phosphorus photonics toward on-chip applications(AIP Publishing, 2020-07-08) Li Huang; Kah-Wee Ang; ELECTRICAL AND COMPUTER ENGINEERINGUnceasing efforts have been devoted to photonics based on black phosphorus ever since it came under the spotlight of two-dimensional materials research six years ago. The direct bandgap of black phosphorus is tunable by layer number, vertical electric field, and chemical doping, covering a broad spectrum for efficient light manipulation. The optical anisotropy further enables the identification and control of light polarization. Along with high carrier mobility, nonlinear optical properties, and integration capability due to its layered lattice structure, black phosphorus manifests itself as a promising multipurpose material for chip-scale optoelectronics. In this manuscript, we review the research on black phosphorus photonics, with a focus on the most fundamental active functions in photonic circuits: photodetection, electro-optic modulation, light emission, and laser pulse generation, aiming at evaluating the feasibility of integrating these black phosphorus-based components as a compact system for on-chip applications.Publication Towards low-loss waveguides in SOI and Ge-on-SOI for mid-IR sensing(Institute of Physics Publishing, 2018-04-25) Younis, Usman; Luo, Xianshu; Dong, Bowei; Huang, Li; Vanga, Sudheer K.; Lim, Andy Eu-Jin; Lo, Patrick Guo-Qiang; Lee, Chengkuo; Bettiol, Andrew A.; Ang, Kah-Wee; ELECTRICAL AND COMPUTER ENGINEERING; PHYSICSSilicon-on-insulator is an attractive choice for developing mid-infrared photonic integrated circuits. It benefits from mature fabrication technologies and integration with on-chip electronics. We report the development of SOI channel and rib waveguides for mid-infrared wavelengths centered at 3.7 ?m. Propagation loss of ~1.44 dB/cm and ~1.2 dB/cm has been measured for TE and TM polarizations in channel waveguides, respectively. Similarly, propagation loss of ~1.39 dB/cm and ~2.82 dB/cm has been measured for TE and TM polarized light in rib waveguides. The propagation loss is consistent with the measurements obtained using a different characterization setup and for the same waveguide structures on a different chip. Given the tightly confined single-mode in our 400 nm thick Si core, this propagation loss is among the lowest losses reported in literature. We also report the development of Ge-on-SOI strip waveguides for mid-infrared wavelengths centered at 3.7 ?m. Minimum propagation loss of ~8dB/cm has been measured which commensurate with that required for high power mid-infrared sensing. Ge-on-SOI waveguides provide an opportunity to realize monolithically integrated circuit with on-chip light source and photodetector. © 2018 The Author(s). Published by IOP Publishing Ltd.Publication Black Phosphorus Carbide as a Tunable Anisotropic Plasmonic Metasurface(American Chemical Society, 2018-05-18) Huang, Xin; Cai, Yongqing; Feng, Xuewei; Tan, Wee Chong; Hasan, Dihan Md. Nuruddin; Chen, Li; Chen, Nan; Wang, Lin; Huang, Li; Duffin, Thorin Jake; Nijhuis, Christian A.; Zhang, Yong-Wei; Lee, Chengkuo; Ang, Kah-Wee; BIOMEDICAL ENGINEERING; CHEMICAL & ENVIRONMENTAL ENGINEERING; ELECTRICAL AND COMPUTER ENGINEERING; PHYSICS; CHEMISTRY; MATERIALS SCIENCE AND ENGINEERINGTailoring photonics for monolithic integration beyond the diffraction limit opens a new era of nanoscale electronic-photonic systems, including graphene plasmonics which exhibits low level of losses and high degree of spatial confinement. Limited to its isotropic optical conductivity, searching for new plasmonic building blocks which offer tunability and design flexibility beyond graphene is becoming quite crucial for next-generation optoelectronic device. Here, motivated by the recent emergence of a new 2D material, we develop a mid-infrared (mid-IR) metasurface by nanostructuring a thin layer of black phosphorus carbide (b-PC) and realize efficient excitation of hybrid plasmon mode at deep subwavelength-scale. Far-field infrared spectroscopy demonstrates that the hybrid plasmon mode displays an anticrossing behavior of two splitting optical modes, which can be attributed to the Fano resonance between plasmons and IR-active optical phonons in b-PC. Significantly, it further presents a strong anisotropic behavior along different crystal orientations, which arises from its peculiar puckered lattice structure with two clearly distinguishable axes. The results illustrate that anisotropic b-PC plasmon not only represents an important advance in subwavelength optoelectronics, but also provides a viable platform for hyperbolic metamaterials, bringing widespread applications into biosensors, single-photon source, nanoantenna, and subwavelength resolution imaging. © 2018 American Chemical Society.Publication Mid-infrared modulators integrating silicon and black phosphorus photonics(Elsevier Ltd, 2021-12-01) Huang, L.; Dong, B.; Yu, Z. G.; Zhou, J.; Ma, Y.; Zhang, Y-W; Lee, C.; Ang, K-W; ELECTRICAL AND COMPUTER ENGINEERINGVast applications of mid-infrared await the realisation of integrated photonic systems for this unique spectrum. Despite its potential as a universal platform for diverse active functions in mid-infrared, black phosphorus (BP) photonics still lacks integrated modulators for completion. Here we realize a hybrid integration of mid-infrared BP modulator on silicon photonics waveguide. Through gating effect, the anisotropic absorption in armchair BP can be tuned for enabling efficient optical modulation spanning ∼3.85–4.1 μm. The integrated waveguide design further promotes light–BP interaction that achieves a modulation depth of ∼5 dB at a low bias of −4 V. Additionally, the active footprint of 225 μm2 and the switching energy of ∼2.6 pJ are remarkably smaller compared to traditional counterparts. Function diversity of such a platform is further verified by integrating BP photodetector and modulator. The combination of two-dimensional materials and silicon photonics manifests a versatile platform to realise high-performance optoelectronic devices for compact on-chip mid-infrared system. © 2021 The Author(s)Publication Pronounced Photovoltaic Effect in Electrically Tunable Lateral Black-Phosphorus Heterojunction Diode(Blackwell Publishing Ltd, 2017-12-11) WANG LIN; Huang, Li; Tan, Wee Chong; Feng, Xuewei; Chen, Li; Ang, Kah-Wee; DEPT OF PHYSICS; ELECTRICAL AND COMPUTER ENGINEERINGRecently, both lateral and vertical p–n junctions have been realized in 2D materials using various strategies, with a number of works on exploring the potential of lateral heterojunctions resulting from thickness-modulated bandgaps at the interface. Here, electrically tunable all-black-phosphorus (BP) lateral heterojunction diodes, without the need of split-gating or selective chemical doping or transfer-based vertical stacking, are experimentally demonstrated. The BP heterojunction diode, which exhibits an ultralow off-state current density of 8 pA µm−1 at a mere Vd of 100 mV and a significant gate-tunable current-rectifying behavior with the highest rectification ratio exceeding 600, is able to harvest solar energy at both visible and near-infrared wavelengths beyond the bandgap limitation of transition metal dichalcogenides. Specifically, at 660 nm, the device achieves an open-circuit voltage (Voc) of 210 mV and a short-circuit current (Isc) of 1.5 nA at 3.6 W cm−2 power density, resulting in an external quantum efficiency of 7.4% which outperforms both split-gating and chemically doped homojunctions. This work paves the way for the exploitation of BP lateral heterojunction for broadband energy harvesting towards future optoelectronic applications. © 2017 WILEY-VCH Verlag GmbH & Co. KGaA, WeinheimPublication Charge Carrier Mobility and Series Resistance Extraction in 2D Field-Effect Transistors: Toward the Universal Technique(2021-07-16) Yu-Chieh Chien; Xuewei Feng; Li Chen; Kai-Chun Chang; Wee Chong Tan; Sifan Li; Li Huang; Kah Wee Ang; ELECTRICAL & COMPUTER ENGINEERING2D semiconductor field-effect transistors (2D FETs) have emerged as a promising candidate for beyond-silicon electronics applications. However, its device performance has often been limited by the metal-2D semiconductor contact, and the non-negligible contact resistance (RSD) not only deteriorates the on-state current but also hinders the direct characterization of the intrinsic properties of 2D semiconductors (e.g., intrinsic charge carrier mobility, μint). Therefore, a proper extraction technique that can independently characterize the metal-2D semiconductor contact behavior and the intrinsic properties of a 2D semiconducting layer is highly desired. In this study, a universal yet simple method is developed to accurately extract the critical parameters in 2D FETs, including characteristic temperature (To), threshold voltage (VT), RSD, and μint. The practicability of this method is extensively explored by characterizing the temperature-dependent carrier transport behavior and the strain-induced band structure modification in 2D semiconductors. Technology computer aided design simulation is subsequently employed to verify the precision of RSD extraction. Furthermore, the universality of the proposed method is validated by successfully implementing the extraction to various 2D semiconductors, including black phosphorus, indium selenide, molybdenum disulfide, rhenium disulfide, and tungsten disulfide with top- and bottom-gated configurations.Publication Tunable black phosphorus heterojunction transistors for multifunctional optoelectronics(Royal Society of Chemistry, 2018-08-07) WANG LIN; HUANG LI; TAN WEE CHONG; FENG XUEWEI; Chen, Li; ANG KAH WEE; ELECTRICAL AND COMPUTER ENGINEERINGMany, black phosphorus (BP) based field-effect transistors, homojunctions, and vertical van der Waals structures have been developed for optoelectronic applications, with few studies being conducted on exploring the potential of their naturally formed heterojunctions. Here, we report a novel thickness-modulated, gate-tunable BP heterojunction phototransistor for multiple purposes and high performance optoelectronics. Despite its thickness of less than 5 nm, the device, whose fabrication spares the need for split-gate or chemical doping or vertical stacking requirements, achieves an excellent photoresponsivity of 383 A W-1 at 1550 nm under zero gate bias, which is among the best photoresponse performance of all-BP-based photodetectors in this spectral range. Furthermore, it exhibits a shot-noise-limited noise equivalent power (NEPshot) of less than 10-2 pW Hz-1/2, making it very promising for ultra-low power detection. Additionally, owing to the heterojunction-induced built-in electric field, the device can be readily used for infrared photovoltaic devices in the absence of source-drain bias (Vd), a feature that is distinctively superior to traditional phototransistors. The multifunctionality demonstrated in our BP heterojunction transistor paves the way towards realizing tunable improved performance optoelectronics based on 2D materials platform.Publication Fabry-Perot cavity enhanced light-matter interactions in two-dimensional van der Waals heterostructure(Elsevier Ltd, 2019-05-02) Huang, Xin; Feng, Xuewei; Chen, Li; Wang, Lin; Tan, Wee Chong; Huang, Li; Ang, Kah-Wee; ELECTRICAL AND COMPUTER ENGINEERINGDespite monolayer transition metal dichalcogenide (TMD) shows a direct band gap property, its atomic thickness causes poor light absorption that severely limits its practical applications. For improving the optical gain of TMD, however, many approaches were proposed such as complicated fabrication process that compromises the stability and reliability of two-dimensional (2D) materials, which further limits the device scalability. In this work, a simple method is reported to engineer the light-matter interactions in few-layer molybdenum disulfide (MoS2) and tungsten diselenide (WSe2) via an asymmetric Fabry-Perot cavity (FPc). The cavity is based on the hybrid integration of TMD/hexagonal boron nitride (h-BN)/Au/SiO2 heterostructure realized through layer-by-layer stacking. By modulating the underlying h-BN thickness, constructive resonant absorption can be achieved by multiple internal reflections, which significantly increases the Raman and optical absorption of MoS2 and WSe2. Leveraging on the enhanced light-matter interactions, we further integrate this asymmetric Fabry?Perot cavity into WSe2/MoS2 van der Waals heterostructure (vdWH) to realize high performance photodiode and photovoltaic devices, leading to a ~5 folds increase in photodiode responsivity and a peak external quantum efficiency (EQE) of 7.5%. This work demonstrates an effective way towards hybrid integration of Fabry-Perot cavity with 2D materials, which could offer a potential pathway for enabling novel optoelectronic devices, such as 2D light-emitting diodes (LEDs) and solar cells. © 2019 Elsevier LtdPublication Infrared Black Phosphorus Phototransistor with Tunable Responsivity and Low Noise Equivalent Power(American Chemical Society, 2017-10-18) HUANG LI; TAN WEE CHONG; WANG LIN; DONG BOWEI; LEE CHENGKUO; ANG KAH WEE; ELECTRICAL AND COMPUTER ENGINEERINGThe narrow band gap property of black phosphorus (BP) that bridges the energy gap between graphene and transition metal dichalcogenides holds great promise for enabling broadband optical detection from ultraviolet to infrared wavelengths. Despite its rich potential as an intriguing building block for optoelectronic applications, however, very little progress has been made in realizing BP-based infrared photodetectors. Here, we demonstrate a high sensitivity BP phototransistor that operates at a short-wavelength infrared (SWIR) of 2 μm under room temperature. Excellent tunability of responsivity and photoconductive gain are acquired by utilizing the electrostatic gating effect, which controls the dominant photocurrent generation mechanism via adjusting the band alignment in the phototransistor. Under a nanowatt-level illumination, a peak responsivity of 8.5 A/W and a low noise equivalent power (NEP) of less than 1 pW/Hz1/2 are achieved at a small operating source-drain bias of -1 V. Our phototransistor demonstrates a simple and effective approach to continuously tune the detection capability of BP photodetectors, paving the way to exploit BP to numerous low-light-level detection applications such as biomolecular sensing, meteorological data collection, and thermal imaging.