Metasurfaces for near-eye display applications

Virtual reality(VR)and augmented reality(AR)are revolutionizing our lives.Near-eye displays are crucial technologies for VR and AR.Despite the rapid advances in near-eye display technologies,there are still challenges such as large field of view,high resolution,high image quality,natural free 3D effect,and compact form factor.Great efforts have been devoted to striking a balance between visual performance and device compactness.While traditional optics are nearing their limitations in addressing these challenges,ultra-thin metasurface optics,with their high light-modulating capabilities,may present a promising solution.In this review,we first introduce VR and AR near-eye displays,and then briefly explain the working principles of light-modulating metasurfaces,review recent developments in metasurface devices geared toward near-eye display applications,delved into several advanced natural 3D near-eye display technologies based on metasurfaces,and finally discuss about the remaining challenges and future perspectives associated with metasurfaces for near-eye display applications.

Recent advances of heterogeneously integrated Ⅲ–Ⅴ laser on Si

Due to the indirect bandgap nature,the widely used silicon CMOS is very inefficient at light emitting.The integration of silicon lasers is deemed as the‘Mount Everest’for the full take-up of Si photonics.The major challenge has been the materials dissimilarity caused impaired device performance.We present a brief overview of the recent advances of integratedⅢ-Ⅴlaser on Si.We will then focus on the heterogeneous direct/adhesive bonding enabling methods and associated light coupling structures.A selected review of recent representative novel heterogeneously integrated Si lasers for emerging applications like spectroscopy,sensing,metrology and microwave photonics will be presented,including DFB laser array,ultra-dense comb lasers and nanolasers.Finally,the challenges and opportunities of heterogeneous integration approach are discussed.

Multi-channel 28-GHz millimeter-wave signal generation on a silicon photonic chip with automated polarization control

We propose and experimentally demonstrate an integrated silicon photonic scheme to generate multi-channel millimeter-wave(MMW) signals for 5 G multi-user applications. The fabricated silicon photonic chip has a footprint of 1.1 × 2.1 mm^2 and integrates 7 independent channels each having on-chip polarization control and heterodyne mixing functions. 7 channels of4-Gb/s QPSK baseband signals are delivered via a 2-km multi-core fiber(MCF) and coupled into the chip with a local oscillator(LO) light. The polarization state of each signal light is automatically adjusted and aligned with that of the LO light, and then 7 channels of 28-GHz MMW carrying 4-Gb/s QPSK signals are generated by optical heterodyne beating. Automated polarizationcontrol function of each channel is also demonstrated with ～7-ms tuning time and ～27-dB extinction ratio.

Silicon-integrated high-speed mode and polarization switch-andselector

On-chip optical communications are growingly aiming at multimode operation together with mode-division multiplex-ing to further increase the transmission capacity.Optical switches,which are capable of optical signals switching at the nodes,play a key role in optical networks.We demonstrate a 2×2 electro-optic Mach-Zehnder interferometer-based mode-and polar-ization-selective switch fabricated by standard complementary metal-oxide-semiconductor process.An electro optic tuner based on a PN-doped junction in one of the Mach-Zehnder interferometer arms enables dynamic switching in 11 ns.For all the channels,the overall insertion losses and inter-modal crosstalk values are below 9.03 and-15.86 dB at 1550 nm,respect-ively.

Preface to the Special Topic on Compound Semiconductor Materials and Devices on Si

The research in silicon photonics has been booming due to its potential for lowcost,reliable,energy-efficient and high-density chip-wise integration using widely available CMOS technology,featuring the tremendous success in modulator,detector and other passive waveguide components in industry.However,the absence of efficient and reliable electrical to optical converter on Si platform has been considered as“the last piece of the puzzle”,hindered by the in-direct bandgap property of Si bulk materials.CompoundⅢ–Ⅴsemiconductor devices offer highly efficient optical light emitting sources and optical amplifiers,hence the compound semiconductor materials and devices on Si platform are drawing more and more attention nowadays as it could make possible the long-dreamed light sources on Si substrates by combining their advantages with silicon ICs,enabling the fabrication of full functional optoelectronic circuits,chip-to-chip and even system-to-system optical chips.

Mass-produced and uniformly luminescent photochromic fibers toward future interactive wearable displays

Enabling flexible fibers with light-emitting capabilities has the potential to revolutionize the design of smart wearable interactive devices.A recent publication in Light Science&Application,an interdisciplinary team of scientists led by Prof.Yan-Qing Lu and Prof.Guangming Tao has realized a highly flexible,uniformly luminescent photochromic fiber based on a mass-produced thermal drawing method.It overcomes the shortcomings of existing commercial lightdiffusing fibers,exhibiting outstanding one-dimensional linear illumination performance.The research team integrated controllable photochromic fibers into various wearable interaction interfaces,providing a novel approach and insights to enable human-computer interaction.

Compact, submilliwatt, 2 × 2 silicon thermo-optic switch based on photonic crystal nanobeam cavities

We propose and experimentally demonstrate a 2×2 thermo-optic(TO) crossbar switch implemented by dual photonic crystal nanobeam(PCN)cavities within a silicon-on-insulator(SOI)platform.By thermally tuning the refractive index of silicon,the resonance wavelength of the PCN cavities can be red-shifted.With the help of the ultrasmall mode volumes of the PCN cavities,only～0.16 mW power is needed to change the switching state.With a spectral passband of 0.09 nm at the 1583.75 nm operation wavelength,the insertion loss(IL)and crosstalk(CT)performances were measured as IL(bar)=-0.2 dB,CT(bar)=-15 dB,IL(cross)=-1.5 dB,and CT(cross)=-15 dB.Furthermore,the thermal tuning efficiency of the fabricated device is as high as1.23 nm/mW.

Compact on-chip 1×2 wavelength selective switch based on silicon microring resonator with nested pairs of subrings

We propose and experimentally demonstrate compact on-chip 1×2 wavelength selective switches(WSSs) based on silicon microring resonators(MRRs) with nested pairs of subrings(NPSs). Owing to the resonance splitting induced by the inner NPSs, the proposed devices are capable of performing selective channel routing at certain resonance wavelengths of the outer MRRs. System demonstration of dynamic channel routing using fabricated devices with one and two NPSs is carried out for 10 Gb∕s non-return-to-zero signal. The experimental results verify the effectiveness of the fabricated devices as compact on-chip WSSs.

Ultracompact bandwidth-tunable filter based on subwavelength grating-assisted contra-directional couplers

An ultracompact,bandwidth-tunable filter has been demonstrated using a silicon-on-insulator(SOI)wafer.The device is based on cascaded grating-assisted contra-directional couplers(GACDCs).It also involves the use of a subwavelength grating(SWG)structure.By heating one of the heaters on GACDCs,a bandwidth tunability of~6 nm is achieved.Owing to the benefit of having a large coupling coefficient between SWG and strip waveguides,the length of the coupling region is only 100 pm.Moreover,the combination of the curved SWG and the tapered strip waveguides effectively suppresses the sidelobes.The filter possesses features of simultaneous wavelength tuning with no free spectral range(FSR)limitation.A maximum bandwidth of 10 nm was experimentally measured with a high out-of-band contrast of 25 dB.Similarly,the minimum bandwidth recorded is 4 nm with an out-of-band contrast of 15 dB.

High-speed electro-optic modulation in topological interface states of a one-dimensional lattice

lectro-optic modulators are key components in data communication,microwave photonics,and quantum photonics.Modulation bandwidth,energy efficiency,and device dimension are crucial metrics of modulators.Here,we provide an important direction for the miniaturization of electro-optic modulators by reporting on ultracompact topological modulators.A topological interface state in a one-dimensional lattice is implemented on a thin-film lithium-niobate integrated platform.Due to the strong optical confinement of the interface state and the peaking enhancement of the electro-optic response,a topological cavity with a size of 1.6×140μm^(2)enables a large modulation bandwidth of 104 GHz.The first topological modulator exhibits the most compact device size compared to reported LN modulators with bandwidths above 28 GHz,to the best of our knowledge.100 Gb/s non-return-to-zero and 100 Gb/s four-level pulse amplitude modulation signals are generated.The switching energy is 5.4 fJ/bit,owing to the small electro-optic mode volume and low capacitance.The topological modulator accelerates the response time of topological photonic devices from the microsecond order to the picosecond order and provides an essential foundation for the implementation of large-scale lithium-niobate photonic integrated circuits.

On-chip metamaterial-enabled high-order mode-division multiplexing

Mode-division multiplexing(MDM)technology enables high-bandwidth data transmission using orthogonal waveguide modes to construct parallel data streams.However,few demonstrations have been realized for generating and supporting high-order modes,mainly due to the intrinsic large material groupvelocity dispersion(GVD),which make it challenging to selectively couple different-order spatial modes.We show the feasibility of on-chip GVD engineering by introducing a gradient-index metamaterial structure,which enables a robust and fully scalable MDM process.We demonstrate a record-high-order MDM device that supports TE_(0)–TE_(15)modes simultaneously.40-GBaud 16-ary quadrature amplitude modulation signals encoded on 16 mode channels contribute to a 2.162 Tbit∕s net data rate,which is the highest data rate ever reported for an on-chip single-wavelength transmission.Our method can effectively expand the number of channels provided by MDM technology and promote the emerging research fields with great demand for parallelism,such as high-capacity optical interconnects,high-dimensional quantum communications,and large-scale neural networks.

Monolithic integration of embedded Ⅲ-Ⅴ lasers on SOI

Silicon photonic integration has gained great success in many application fields owing to the excellent optical device properties and complementary metal-oxide semiconductor(CMOS)compatibility.Realizing monolithic integration of Ⅲ-Ⅴ lasers and silicon photonic components on single silicon wafer is recognized as a long-standing obstacle for ultradense photonic integration,which can provide considerable economical,energy-efficient and foundry-scalable onchip light sources,that has not been reported yet.Here,we demonstrate embedded InAs/GaAs quantum dot(QD)lasers directly grown on trenched silicon-on-insulator(SOI)substrate,enabling monolithic integration with buttcoupled silicon waveguides.By utilizing the patterned grating structures inside pre-defined SOI trenches and unique epitaxial method via hybrid molecular beam epitaxy(MBE),high-performance embedded InAs QD lasers with monolithically out-coupled silicon waveguide are achieved on such template.By resolving the epitaxy and fabrication challenges in such monolithic integrated architecture,embedded Ⅲ-Ⅴ lasers on SOI with continuous-wave lasing up to 85°C are obtained.The maximum output power of 6.8mW can be measured from the end tip of the butt-coupled silicon waveguides,with estimated coupling efficiency of approximately-6.7 dB.The results presented here provide a scalable and low-cost epitaxial method for the realization of on-chip light sources directly coupling to the silicon photonic components for future high-density photonic integration.

On-chip silicon photonic 2 × 2 mode-and polarization-selective switch with low inter-modal crosstalk

Mode-and polarization-division multiplexing offer new dimensions to increase the transmission capacity of optical communications. Selective switches are key components in reconfigurable optical network nodes. An on-chip silicon 2 × 2 mode-and polarization-selective switch that can route four data channels on two modes and two polarizations simultaneously is proposed and experimentally demonstrated for the first time, to the best of our knowledge. The overall insertion losses are lower than 8.6 d B. To reduce the inter-modal crosstalk, polarization beam splitters are added to filter the undesired polarizations or modes. The measured inter-modal andintra-modal crosstalk values are below-23.2 and-22.8 d B for all the channels, respectively.

Metamaterial-enabled arbitrary on-chip spatial mode manipulation

On-chip spatial mode operation,represented as mode-division multiplexing(MDM),can support high-capacity data communications and promise superior performance in various systems and numerous applications from optical sensing to nonlinear and quantum optics.However,the scalability of state-of-the-art mode manipulation techniques is significantly hindered not only by the particular mode-order-oriented design strategy but also by the inherent limitations of possibly achievable mode orders.Recently,metamaterials capable of providing subwavelength-scale control of optical wavefronts have emerged as an attractive alternative to manipulate guided modes with compact footprints and broadband functionalities.Herein,we propose a universal yet efficient design framework based on the topological metamaterial building block(BB),enabling the excitation of arbitrary high-order spatial modes in silicon waveguides.By simply programming the layout of multiple fully etched dielectric metamaterial perturbations with predefined mathematical formulas,arbitrary high-order mode conversion and mode exchange can be simultaneously realized with uniform and competitive performance.The extraordinary scalability of the metamaterial BB frame is experimentally benchmarked by a record high-order mode operator up to the twentieth.As a proof of conceptual application,an 8-mode MDM data transmission of 28-GBaud 16-QAM optical signals is also verified with an aggregate data rate of 813 Gb/s(7%FEC).This user-friendly metamaterial BB concept marks a quintessential breakthrough for comprehensive manipulation of spatial light on-chip by breaking the long-standing shackles on the scalability,which may open up fascinating opportunities for complex photonic functionalities previously inaccessible.

All-optical silicon microring spiking neuron

With the rapid development of artificial intelligence and machine learning, brain-inspired neuromorphic photonics has emerged as an extremely attractive computing paradigm, promising orders-of-magnitude higher computing speed and energy efficiency compared to its electronic counterparts. Tremendous efforts have been devoted to photonic hardware implementations of mimicking the nonlinear neuron-like spiking response and the linear synapse-like weighting functionality. Here, we systematically characterize the spiking dynamics of a passive silicon microring neuron. The research of self-pulsation and excitability reveals that the silicon microring can function as an all-optical class Ⅱ resonate-and-fire neuron. The typical refractory period has been successfully suppressed by configuring the pump power above the perturbation power, hence allowing the microring neuron to operate with a speed up to roughly sub-gigahertz. Additionally, temporal integration and controllable inhibition regimes are experimentally demonstrated for the first time, to the best of our knowledge. Our experimental verification is obtained with a commercial CMOS platform, hence offering great potential for large-scale neuromorphic photonics integration.

Non-blocking 2×2 switching unit based on nested silicon microring resonators with high extinction ratios and low crosstalks

In this paper,we propose and demonstrate a2 9 2 optical Benes switching unit based on two nested silicon microring resonators(MRRs)monolithically integrated on a silicon-on-insulator(SOI)wafer.High extinction ratios(ERs)of about 44.7/38.0 dB and low crosstalk values of about-37.5/-45.2 dB at cross/bar states are obtained with the fabricated device.The operation principle is theoretically studied and the switching function is verified by system demonstration experiments with 10 and12.5 Gb/s non-return-to-zero(NRZ)signals.The switching speed on the order of gigahertz based on free carrier effect in silicon is also experimentally demonstrated.

Ultracompact topological photonic switch based on valley-vortex-enhanced high-efficiency phases shift

Topologically protected edge states based on valley photonic crystals(VPCs)have been widely studied,from theoretical verifcation to technical applications.However,research on integrated tuneable topological devices is still lacking.Here,we study the phase-shifting theory of topological edge modes based on a VPC structure.Benefiting from the phase vortex formed by the VPC structure,the optical path of the topological edge mode in the propagation direction is approximately two-fold that of the conventional optical mode in a strip waveguide.In experiments,we show a 1.57-fold improvement inπ-phase tuning efficiency.By leveraging the highefficiency phase-shifting properties and the sharp-turn features of the topological waveguide,we demonstrate an ultracompact 1×2 thermo-optic topological switch(TOTS)operating at telecommunication wavelengths.A switching power of 18.2 mW is needed with an ultracompact device footprint of 25.66×28.3μm in the wavelength range of 1530-1582 nm.To the best of our knowledge,this topological photonic switch is the smallest switch of any dielectric or semiconductor 1×2/2×2 broadband optical switches,including thermo-optic and electro-optic switches.In addition,a high-speed transmission experiment employing the proposed TOTS is carried out to demonstrate the robust transmission of high-speed data.Our work reveals the phase shifting mechanism of valley edge modes,which may enable diverse topological functional devices in many fields,such as optical communications,nanophotonics,and quantum information processing.

High-efficiency and broadband four-wave mixing in a silicon-graphene strip waveguide with a windowed silica top layer

We experimentally demonstrate high-efficiency and broadband four-wave mixing in a silicon-graphene strip waveguide. A four-wave mixing conversion efficiency of -38.7 d B and a 3-dB conversion bandwidth of 35 nm are achieved in the silicon-graphene strip waveguide with an optimized light-graphene interaction length of 60 μm. The interaction length is controlled by a windowed area of silica layer on the silicon waveguide.Numerical simulations and experimental studies are carried out and show a nonlinear parameter γGOSas large as 10~4 W^(-1)· m^(-1).

On-chip silicon polarization and mode handling devices

Mode-and polarization-division multiplexing are new promising options to increase the transmission capacity of optical communications.On-chip silicon polarization and mode handling devices are key components in integrated mode-and polarization-division multi-plexed photonic circuits.In this paper,we review our recent progresses on silicon-based polarization beam splitters,polarization splitters and rotators,mode（de）multiplexers,and mode and polarization selective switches.Silicon polarization beam splitters and rotators are demonstrated with high extinction ratio,compact footprint and high fabrication tolerance.For on-chip mode multiplexing,we introduce a low loss and fabrication tolerant three-mode（de）multiplexer employing sub-wavelength grating structure.In analogy to a conventional wavelength selective switch in wavelength-division multi-plexing,we demonstrate a selective switch that can route mode-and polarization-multiplexed signals.

Compact all-optical differential-equation solver based on silicon microring resonator

We propose and numerically demonstrate an ultrafast real-time ordinary differential equation （ODE） computing unit in optical field based on a silicon microring resonator, operating in the critical coupling region as an optical temporal differentiator. As basic building blocks of a signal processing system, a subtractor and a splitter are included in the proposed structure. This scheme is featured with high speed, compact size and integration on a siliconon-insulator （SOl） wafer. The size of this computing unit is only 35 μm × 45 μm. In this paper, the performance of the proposed structure is theoretically studied and analyzed by numerical simulations.