Showing papers by "Xinliang Zhang published in 2021"

2D materials-based homogeneous transistor-memory architecture for neuromorphic hardware.

Abstract: In neuromorphic hardware, peripheral circuits and memories based on heterogeneous devices are generally physically separated. Thus, exploration of homogeneous devices for these components is key fo...

Over 21% Efficiency Stable 2D Perovskite Solar Cells

Abstract: Owing to their insufficient light absorption and charge transport, 2D Ruddlesden-Popper (RP) perovskites show relatively low efficiency. In this work, methylammonium (MA), formamidinum (FA), and FA/MA mixed 2D perovskite solar cells (PSCs) are fabricated. Incorporating FA cations extends the absorption range and enhances the light absorption. Optical spectroscopy shows that FA cations substantially increase the portion of 3D-like phase to 2D phases, and X-ray diffraction (XRD) studies reveal that FA-based 2D perovskite possesses an oblique crystal orientation. Nevertheless, the ultrafast interphase charge transfer results in an extremely long carrier-diffusion length (≈1.98 µm). Also, chloride additives effectively suppress the yellow δ-phase formation of pure FA-based 2D PSCs. As a result, both FA/MA mixed and pure FA-based 2D PSCs exhibit a greatly enhanced power conversion efficiency (PCE) over 20%. Specifically, the pure FA-based 2D PSCs achieve a record PCE of 21.07% (certified at 20%), which is the highest efficiency for low-dimensional PSCs (n ≤ 10) reported to date. Importantly, the FA-based 2D PSCs retain 97% of their initial efficiency at 85 °C persistent heating after 1500 h. The results unambiguously demonstrate that pure-FA-based 2D PSCs are promising for achieving comparable efficiency to 3D perovskites, along with a better device stability.

Ghost hyperbolic surface polaritons in bulk anisotropic crystals.

Abstract: Polaritons in anisotropic materials result in exotic optical features, which can provide opportunities to control light at the nanoscale1-10. So far these polaritons have been limited to two classes: bulk polaritons, which propagate inside a material, and surface polaritons, which decay exponentially away from an interface. Here we report a near-field observation of ghost phonon polaritons, which propagate with in-plane hyperbolic dispersion on the surface of a polar uniaxial crystal and, at the same time, exhibit oblique wavefronts in the bulk. Ghost polaritons are an atypical non-uniform surface wave solution of Maxwell's equations, arising at the surface of uniaxial materials in which the optic axis is slanted with respect to the interface. They exhibit an unusual bi-state nature, being both propagating (phase-progressing) and evanescent (decaying) within the crystal bulk, in contrast to conventional surface waves that are purely evanescent away from the interface. Our real-space near-field imaging experiments reveal long-distance (over 20 micrometres), ray-like propagation of deeply subwavelength ghost polaritons across the surface, verifying long-range, directional and diffraction-less polariton propagation. At the same time, we show that control of the out-of-plane angle of the optic axis enables hyperbolic-to-elliptic topological transitions at fixed frequency, providing a route to tailor the band diagram topology of surface polariton waves. Our results demonstrate a polaritonic wave phenomenon with unique opportunities to tailor nanoscale light in natural anisotropic crystals.

2D Materials Enabled Next-Generation Integrated Optoelectronics: from Fabrication to Applications

Abstract: 2D materials, such as graphene, black phosphorous and transition metal dichalcogenides, have gained persistent attention in the past few years thanks to their unique properties for optoelectronics. More importantly, introducing 2D materials into silicon photonic devices will greatly promote the performance of optoelectronic devices, including improvement of response speed, reduction of energy consumption, and simplification of fabrication process. Moreover, 2D materials meet the requirements of complementary metal-oxide-semiconductor compatible silicon photonic manufacturing. A comprehensive overview and evaluation of state-of-the-art 2D photonic integrated devices for telecommunication applications is provided, including light sources, optical modulators, and photodetectors. Optimized by unique structures such as photonic crystal waveguide, slot waveguide, and microring resonator, these 2D material-based photonic devices can be further improved in light-matter interactions, providing a powerful design for silicon photonic integrated circuits.

Real-time observation of frequency Bloch oscillations with fibre loop modulation.

Abstract: Bloch oscillations (BOs) were initially predicted for electrons in a solid lattice to which a static electric field is applied. The observation of BOs in solids remains challenging due to the collision scattering and barrier tunnelling of electrons. Nevertheless, analogies of electron BOs for photons, acoustic phonons and cold atoms have been experimentally demonstrated in various lattice systems. Recently, BOs in the frequency dimension have been proposed and studied by using an optical micro-resonator, which provides a unique approach to controlling the light frequency. However, the finite resonator lifetime and intrinsic loss hinder the effect from being observed practically. Here, we experimentally demonstrate BOs in a synthetic frequency lattice by employing a fibre-loop circuit with detuned phase modulation. We show that a detuning between the modulation period and the fibre-loop roundtrip time acts as an effective vector potential and hence a constant effective force that can yield BOs in the modulation-induced frequency lattices. With a dispersive Fourier transformation, the pulse spectrum can be mapped into the time dimension, and its transient evolution can be precisely measured. This study offers a promising approach to realising BOs in synthetic dimensions and may find applications in frequency manipulations in optical fibre communication systems.

Ultrahigh-speed graphene-based optical coherent receiver.

Abstract: Graphene-based photodetectors have attracted significant attention for high-speed optical communication due to their large bandwidth, compact footprint, and compatibility with silicon-based photonics platform. Large-bandwidth silicon-based optical coherent receivers are crucial elements for large-capacity optical communication networks with advanced modulation formats. Here, we propose and experimentally demonstrate an integrated optical coherent receiver based on a 90-degree optical hybrid and graphene-on-plasmonic slot waveguide photodetectors, featuring a compact footprint and a large bandwidth far exceeding 67 GHz. Combined with the balanced detection, 90 Gbit/s binary phase-shift keying signal is received with a promoted signal-to-noise ratio. Moreover, receptions of 200 Gbit/s quadrature phase-shift keying and 240 Gbit/s 16 quadrature amplitude modulation signals on a single-polarization carrier are realized with a low additional power consumption below 14 fJ/bit. This graphene-based optical coherent receiver will promise potential applications in 400-Gigabit Ethernet and 800-Gigabit Ethernet technology, paving another route for future high-speed coherent optical communication networks. Graphene-based photodetectors have many advantages for applications. Here, the authors demonstrate a high-speed optical coherent receiver for optical communications based on graphene-on-plasmonic slot waveguide photodetectors.

On-chip terahertz isolator with ultrahigh isolation ratios.

Abstract: Terahertz isolators, one of the typical nonreciprocal devices that can break Lorentz reciprocity, are indispensable building blocks in terahertz systems for their critical functionality of manipulating the terahertz flow. Here, we report an integrated terahertz isolator based on the magneto-optical effect of a nonreciprocal resonator. By optimizing the magneto-optical property and the loss of the resonator, we experimentally observe unidirectional propagation with an ultrahigh isolation ratio reaching up to 52 dB and an insertion loss around 7.5 dB at ~0.47 THz. With a thermal tuning method and periodic resonances, the isolator can operate at different central frequencies in the range of 0.405–0.495 THz. This on-chip terahertz isolator will not only inspire more solutions for integrated terahertz nonreciprocal devices, but also have the feasibility for practical applications such as terahertz sensing and reducing unnecessary reflections in terahertz systems. Nonreciprocal devices are crucial in scientific research and practical applications at all frequencies. Here the authors demonstrate an integrated terahertz optical isolator based on the magneto-optical effect in a nonreciprocal resonator.

Simultaneous ultraviolet, visible and near-infrared continuous-wave lasing in a rare-earth-doped microcavity

Abstract: Microlaser with multiple lasing bands is critical in various applications, such as full-colour display, optical communications and computing. Here, we propose a simple and efficient method for homogeneously doping rare earth elements into a silica whispering-gallery-mode microcavity. By this method, we demonstrate simultaneous and stable lasing covering ultraviolet, visible and near-infrared bands in an ultrahigh-Q (exceeding 108) Er-Yb co-doped silica microsphere under room temperature and continuous-wave pump for the first time. The lasing thresholds of the 380, 410, 450, 560, 660, 800, 1080 and 1550 nm-bands are estimated to be 380, 150, 2.5, 12, 0.17, 1.7, 10 and 38 {\mu}W, respectively, where the lasing in the 380, 410 and 450 nm-bands by Er element have not been separately demonstrated under room temperature and continuous-wave pump until this work. This ultrahigh-Q doped microcavity is an excellent platform for high-performance multi-band microlasers, ultrahigh-precise sensors, optical memories and cavity-enhanced light-matter interaction studies.

80 GHz germanium waveguide photodiode enabled by parasitic parameter engineering

Abstract: A high-speed germanium (Ge) waveguide photodiode (PD) is one of the key components of an integrated silicon photonics platform for large-capacity data communication applications, but the parasitic parameters limit the increase of its bandwidth. Several studies have been reported to reduce parasitic parameters, at the cost of compromising other performances. Here, we propose and investigate a bandwidth-boosting technique by comprehensively engineering the parasitic parameters. Experimentally, a bandwidth up to 80 GHz is realized for vertical positive-intrinsic-negative (PIN) Ge PDs without decreasing the responsivity and dark current, indicating that parasitic parameter engineering is a promising method to promote high-speed performance of Ge PDs.

High-efficient and high-accurate integrated division-of-time polarimeter

Abstract: The characterization of the state of polarization is of great importance in broad applications, such as microscopy, communications, astronomy, and remote sensing. In this Letter, we propose and demonstrate a novel integrated division-of-time polarimeter (DOTP) based on a Mach–Zehnder interferometer and two photodetectors (PDs). The proposed DOTP achieves improved measuring efficiency and accuracy by measuring a pair of orthogonal polarization states simultaneously. The analysis matrix, which is used to recover the Stokes vector, is elaborately optimized to reduce the influence of the PD noise. Compared to the conventionally designed DOTP, the measuring efficiency is improved by 33% and the equally weighted variance, a figure of merit used to characterize the total variance of the Stokes vector, is also reduced by 33%. The performance of the proposed device is experimentally characterized by comparing with a commercial product. Furthermore, a method based on the least-squares method and singular value decomposition is adopted to quantize the deviation between the ideal optimal analysis matrix and the practical one.

Suppression and revival of single-cavity lasing induced by polarization-dependent loss.

Abstract: For most photonics devices and systems, loss is desperately averted, since it will increase the power consumption and degrade the performance. However, in some non-Hermitian systems, loss can induce a modal gain when the parity-time symmetry is broken, which offers a new way to manipulate the lasing of active cavities. Here we experimentally observe the counterintuitive phenomenon in a single laser cavity assisted by the polarization-dependent loss. A parity–time symmetric system is constituted by the two orthogonally polarized photonic loops in a single laser cavity, which can guarantee the consistency of two coupling loops. The measured output power of the cavity depends on the cross-polarization loss, which reveals virtually opposite relationships before and after the critical point. It provides a novel, to the best of our knowledge, understanding of polarization loss and shows great potential for lasing manipulation in a single cavity with polarization control.

Antenna-integrated silicon–plasmonic graphene sub-terahertz emitter

Abstract: Photo-mixing with its advantages of ultra-large bandwidth and precise tunability has emerged as an important technique for terahertz (THz) wave generation. Recently, graphene photodetectors exhibiting a large bandwidth are expected to further boost the development of integrated THz emitters. Here, we fabricate a sub-THz emitter based on a large-bandwidth silicon–plasmonic graphene (SPG) photodetector integrated with a broadband rounded bow-tie THz antenna. The SPG sub-THz emitter is experimentally demonstrated to emit sub-THz waves with a radiation spectrum from 50 to 300 GHz. A maximum sub-THz emission power of 5.4 nW is obtained at 145 GHz with only 3 mW input light power. The SPG sub-THz emitter can be fabricated by a CMOS-compatible process, which offers enormous opportunities for its use in a variety of THz applications.

Optical spatiotemporal differentiator using a bilayer plasmonic grating.

Abstract: As a key element in wave-based analog computation, optical differentiators have been implemented to directly perform information processing, such as edge detection and pulse shaping, in both spatial and temporal domains. Here, we propose an optical spatiotemporal differentiator, which simultaneously performs first-order spatial and temporal differentiation in transmission by breaking the mirror symmetry of a subwavelength bilayer metal grating. The spatial and temporal performance of the plasmonic differentiator is evaluated numerically using the output field profiles of an optical beam and pulse envelope, showing resolutions of ∼2µm and ∼50fs, respectively. Moreover, the function of spatiotemporal differentiation is demonstrated with input flat-top pulse fields. The proposed optical differentiator has potential applications in ultra-compact real-time optical multifunctional computing systems and parallel signal processing.

Precise dynamic characterization of microcombs assisted by an RF spectrum analyzer with THz bandwidth and MHz resolution.

Abstract: The radio frequency (RF) spectrum of microcombs can be used to evaluate its phase noise features and coherence between microcomb teeth. Since microcombs possess characteristics such as high repetition rate, narrow linewidth and ultrafast dynamical evolution, there exists strict requirement on the bandwidth, resolution and frame rate of RF measurement system. In this work, a scheme with 1.8-THz bandwidth, 7.5-MHz spectral resolution, and 100-Hz frame rate is presented for RF spectrum measurement of microcombs by using an all-optical RF spectrum analyzer based on cross-phase modulation and Fabry Perot (FP) spectrometer, namely FP-assisted light intensity spectrum analyzer (FP-assisted LISA). However, extra dispersion introduced by amplifying the microcombs will deteriorate the bandwidth performance of measured RF spectrum. After compensating the extra dispersion through monitoring the dispersion curves measured by FP-assisted LISA, the more precise RF spectra of microcombs are measured. Then, the system is used to measure the noise sidebands and line shape evolution of microcombs within 2s temporal window, in which dynamic RF combs variation at different harmonic frequencies up to 1.96 THz in modulation instability (MI) state and soliton state are recorded firstly. Therefore, the improved bandwidth and resolution of FP-assisted LISA enable more precise measurement of RF spectrum, paving a reliable way for researches on physical mechanism of microcombs.

Pure Temporal Dispersion for Aberration Free Ultrafast Time-Stretch Applications

Abstract: Photonic time-stretch applications overcome the speed limitations of conventional digitizers and enable the observation of nonrepetitive and statistically rare phenomena that occur on short timescales. In most time-stretch applications, large temporal dispersion with large bandwidth is highly desired to satisfy the far-field diffraction regime. However, most conventional spatial dispersers or chirped fiber Bragg gratings are constrained by their spatial volume, which can be overcome by using ultra-low-loss dispersive fibers, an ideal medium for large temporal dispersion ( β 2), but they suffer from third-order dispersion ( β 3) and aberrations. In this paper, an optical phase conjugation-based, third-order dispersion compensation scheme was introduced, with accumulated β 2 and eliminated β 3, and it achieved a ±3400-ps2 pure temporal dispersion of over 30-nm bandwidth. Leveraging this pure temporal dispersion, up to 2% of temporal aberrations were eliminated. Furthermore, Fourier domain spectroscopy achieved a record 15000 optical effective resolvable points, with a nondegraded 2-pm resolution over a 30-nm range.

Proposal and demonstration of a controllable Q factor in directly coupled microring resonators for optical buffering applications

Abstract: Optical resonators with controllable Q factors are key components in many areas of optical physics and engineering. We propose and investigate a Q-factor controllable system composed of two directly coupled microring resonators, one of which is tunable and coupled to dual waveguides. By shifting the resonance of the controllable microring, the Q factor of the system as well as the other microring changes significantly. We have demonstrated wide-range controllable Q factors based on this structure in silicon-on-insulator, for example. The influences of spectral detuning and coupling strength between two resonators on the variation of Q factors are studied in detail experimentally. Then, we explore its applications in optical buffering. Tunable fast-to-slow/slow-to-fast light has been carried out by switching the system between the high-Q state and low-Q state. Moreover, optical pulse capture and release are also achievable using this structure with dynamic tuning, and the photon storage properties are investigated. The proposed Q-factor tunable system is simple, flexible, and realizable in various integrated photonic platforms, allowing for potential applications in on-chip optical communications and quantum information processing.

High-power Si-Ge photodiode assisted by doping regulation

Abstract: High-power silicon-based photodiodes are key components in many silicon photonics systems, such as microwave photonics systems, an optical interconnection system with multi-level modulation formats, etc. Usually, the saturation power of the silicon-germanium (Si-Ge) photodiode is limited by the space-charge screening (SCS) effect and the feasibility of the fabrication process. Here, we propose a high saturation power Si-Ge photodiode assisted by doping regulation. Through alleviating the SCS effect of the photodiode, we successfully demonstrate an 85.7% improvement on the saturation power and a 57% improvement on the -1 dB compression photocurrent. The proposed high-power Si-Ge photodiode requires no specific fabrication process and will promote the low-cost integrated silicon photonics systems for more applications.

Real-time observation of the thermo-optical and heat dissipation processes in microsphere resonators.

Abstract: This work reports the real-time observation of the thermo-optical dynamics in silica microsphere resonators based on the dispersive time stretch technique. In general, the thermo-optical dynamics of silica microsphere resonators, including the thermal refraction and thermal expansion, can be characterized by the resonance wavelength shift, whose duration is at the millisecond timescale. However, this fast wavelength shift process cannot be directly captured by conventional spectroscopy, and only its transmission feature can be characterized by a fast-scanning laser and an intensity detector. With the advance of the time-stretch spectroscopy, whose temporal resolution is up to tens of nanoseconds, the thermo-optical dynamics can be observed in a more straight-forward way, by utilizing the pump-probe technology and mapping the resonance wavelength to the time domain. Here, the thermo-optical dynamics are explored as a function of the power and the scanning rate of the pump laser. Theoretical simulations reproduce the experimental results, revealing that the thermo-optical dynamics of silica microsphere resonators is dominated by the fast thermo-optical effect and the slow heat dissipation process to the surroundings, which leads to gradual regression of the resonance wavelength. This work provides an alternative solution for studying the thermo-optical dynamics in whispering gallery mode microresonators, which would be crucial for future applications of microresonator photonic systems.

High Efficiency Electro-Optic Modulation in a Graphene Silicon Hybrid Tapered Microring Resonator

Abstract: Graphene silicon devices have attracted significant attention due to their outstanding electronic and optical properties. By electrically tuning the Fermi level of the monolayer graphene sheet, electro-optic (EO) modulators have been widely analyzed. Despite significant progress has been achieved on high-speed modulation with integrated graphene silicon waveguides, it remains challenging to realize a high modulation efficiency since the light absorption in graphene is limited. Here, we propose and experimentally demonstrate a high efficiency EO modulator using a graphene-assisted tapered silicon ring resonator, where the tapered zone is covered by a graphene/graphene capacitor. This graphene-assisted tapered zone strongly enhances the graphene-light interaction that the achieved static and dynamic modulation (1KHz) depths are up to 26 dB and 12 dB, respectively. Physical mechanism of the modulation based on this nanostructure is also discussed in detail. The proposed nanostructure provides a promising alternative method for high-performance EO modulator, which is essential for th e on-chip optical communication, optical computing and optical signal processing.

Ultra-narrow passband-tunable filter based on a high-Q silicon racetrack resonator.

Abstract: An ultra-narrow narrow passband-tunable optical filter employing a high-Q silicon racetrack resonator is proposed and experimentally demonstrated on a SOI platform. The high-Q silicon racetrack resonator is realized by utilizing the multimode waveguide racetrack, and the Q factor is measured as high as 8.1×105. The structure of the device is based on a thermally tunable Mach–Zehnder interferometer coupled racetrack. The tunability of the bandwidth is realized by tuning the coupling coefficient between the racetrack resonator and the input or output ports. Finally, the bandwidth of the filter can be tuned from 1.92 to 11.00 pm (240 MHz to 1.375 GHz), and the free spectral range is about 0.28 nm (35 GHz), with the footprint of 0.21mm2.

CMOS-compatible integrated 4-f system for mode-transparent spatial manipulation.

Abstract: To exploit spatial dimension, on-chip optical modes with various spatial profiles have been utilized in optical interconnects and spatial analog computing. An integrated Fourier optical system is able to perform spatial operations. However, the reported schemes based on a subwavelength structure pose difficulty in fabrication, and the fabrication-friendly structure has been investigated only with a fundamental mode. With the complementary metal-oxide-semiconductor process, we propose an integrated 4-f system with simple geometry and a moderate minimum feature size to manipulate the mode’s spatial size and position in a mode-transparent way. A size magnification of 2.5 and center-to-center position offset of 7 µm are experimentally demonstrated. Reasonable insertion loss and low inter-mode crosstalk are measured over a 30 nm bandwidth. The work in this Letter paves the way for an on-chip Fourier optical system with convenient fabrication and broadband operation.

Broadband frequency control of light using synthetic frequency lattices formed by four-wave-mixing Bragg scatterings

Abstract: Recent exploration in synthetic frequency lattices has re-formed the methods of manipulating the light spectrum by analogizing the diffraction management in real space, which can be created by employing electro-optic modulation (EOM) in dielectric waveguides or ring resonators. In the presence of effective gauge potential, that is, the accumulated phase of mode transfer between adjacent lattice sites, the frequency spectrum of incident light can be flexibly tailored. However, the finite bandwidth and modulation depth of EOM vastly restrain the frequency shift and efficiency of mode transfer. Here we experimentally demonstrate that the synthetic frequency lattice can be created with the nonlinear process of four-wave-mixing Bragg scattering. The bandwidth of frequency manipulation is expanded up to terahertz and the frequency shift can be larger than 200 GHz. Furthermore, we can realize effects such as negative refraction and perfect imaging in frequency dimension by changing the effective gauge potentials. The study provides a powerful and promising approach to precisely control light frequency in broadband and may benefit all-optical modulation in optical telecommunication systems.

Electrical crosstalk suppression for a compact optical segmented modulator.

Abstract: Advanced coding formats can improve the spectral efficiency in optical transmission systems, while the generation can be expensive and power hungry when electrical digital-to-analog converts (DACs) are utilized. Optical segmented modulators can supersede electrical DACs with the merits of low cost and power efficiency. However, due to their compact size, the leakage current between the adjacent segments results in considerable electrical crosstalk, which impairs the linearity of the modulators and distorts the modulated signal. Here, we propose and demonstrate an electrical crosstalk suppression scheme for optical segmented modulators by introducing a complementary doped region as an insulator. Two depletion regions with high impedances are formed, resulting in the decrease in leakage current and crosstalk. Qualitative and quantitative analysis are performed, and experimentally, in a ring based segmented modulator, more than 5 dB crosstalk improvement is successfully achieved within the 30 GHz range.