Enhancing the field-of-view of spectral-scanning FMCW LiDAR by multipass configuration with an echelle grating.

We present a spectral-scanning frequency-modulated continuous wave (FMCW) 3D imaging system capable of producing high-resolution depth maps with an extended field of view (FOV). By employing a multipass configuration with an echelle grating, the system achieves an FOV of 5.5° along the grating axis. The resulting depth maps have a resolution of 70 × 40 pixels, with a depth resolution of 5.1 mm. The system employs an echelle grating for beam steering and leverages the multipass configuration for angular FOV magnification. Quantitative depth measurements and 3D imaging results of a static 3D-printed depth variation target are demonstrated. The proposed approach offers a promising solution for enhancing the FOV of spectral-scanning FMCW LiDAR systems within a limited wavelength-swept range, thereby reducing system complexity and cost, paving the way for improved 3D imaging applications.

Subwavelength Bessel beam arrays with high uniformity based on a metasurface.

Bessel beam arrays are highly attractive due to non-diffraction properties, parallel processing, and large capacity capabilities. However, conventional approaches of generating Bessel beams, such as spatial light modulators, axicons, and diffraction optical elements, suffer from various limitations of system complexity and bulkiness, low uniformity, and limited numerical aperture (NA). The limited NA imposes constraints on achieving minimal full width at half maximum (FWHM) of the Bessel beam, ultimately compromising the resolution of the beam. In this study, we demonstrate a method for generating Bessel beam arrays with regular and random patterns via an ultra-compact metasurface. This approach integrates the phase profile of an optimized beam splitter with a meta-axicon. The Bessel beam arrays exhibit subwavelength dimensions of FWHM (590 nm, ∼0.9λ) and relatively high uniformity of 90% for N A=0.2 and 69% for N A=0.4. Furthermore, the method achieves effective suppression of background noise and zeroth-order intensity compared to methods based on Dammann grating (DG) based metasurfaces. The proposed method highlights potential applications of Bessel beam arrays in various fields, such as laser machining, optical communication, and biomedical imaging.

Resonant-cavity-enhanced p-i-n photodetector using a high-contrast-grating for 940nm.

Two novel top mirror designs of high contrast gratings (HCG) are used as the top mirrors of the resonant-cavity enhanced photodetector (RCE PD) operating at 940 nm. The bottom mirror is composed of 36-pair AlAs/GaAs, while the top mirror is a thin-layer grating providing reflectivity higher than 99%. With grating periods varying from 450 to 490 nm, different designs with FWHM of about 0.2∼3 nm are attained. A broadband HCG as top reflector can result in significantly improved manufacturing cost, as well as near unity quantum efficiency (QE). A resonator HCG can result in a new splitting responsivity spectrum with on-off ratio of 14 dB, which has the potential to serve as the basic elements of ternary system, polarization dichroism or diattenuation, and optical switch.

Resonant-antiresonant coupled cavity VCSELs.

The wavelength tuning range of a tunable vertical-cavity surface-emitting laser (VCSEL) is strongly influenced by the design of the interface between the semiconductor cavity and the air cavity. A simplified model is used to investigate the origin of the dramatic differences in free spectral range (FSR) and tuning slope observed in semiconductor cavity dominant, extended cavity, and air cavity dominant VCSELs. The differences arise from the positioning of the resonant and antiresonant wavelengths of the semiconductor cavity with respect to the center wavelength. The air cavity dominant design is realized by designing an antiresonant semiconductor cavity, resulting in a larger tuning slope near the center of the tuning range and a wider FSR toward the edges of the tuning range. The findings from the simplified model are confirmed with the simulation of a full VCSEL structure. Using an air cavity dominant design, an electrically pumped laser with a tuning range of 68.38 nm centered at 1056.7 nm at a 550 kHz sweep rate is demonstrated with continuous wave emission at room temperature. This epitaxial design rule can be used to increase the tuning range of tunable VCSELs, making them more applicable in swept-source optical coherence tomography and frequency-modulated continuous-wave LIDAR systems.

Site-Controlled Growth of Monolithic InGaAs/InP Quantum Well Nanopillar Lasers on Silicon.

In this Letter, we report the site-controlled growth of InP nanolasers on a silicon substrate with patterned SiO2 nanomasks by low-temperature metal-organic chemical vapor deposition, compatible with silicon complementary metal-oxide-semiconductor (CMOS) post-processing. A two-step growth procedure is presented to achieve smooth wurtzite faceting of vertical nanopillars. By incorporating InGaAs multiquantum wells, the nanopillar emission can be tuned over a wide spectral range. Enhanced quality factors of the intrinsic InP nanopillar cavities promote lasing at 0.87 and 1.21 µm, located within two important optical telecommunication bands. This is the first demonstration of a site-controlled III-V nanolaser monolithically integrated on silicon with a silicon-transparent emission wavelength, paving the way for energy-efficient on-chip optical links at typical telecommunication wavelengths.

Very high efficiency optical coupler for silicon nanophotonic waveguide and single mode optical fiber.

Integrated optical circuits are poised to open up an array of novel applications. A vibrant field of research has emerged around the monolithic integration of optical components onto the silicon substrates. Typically, single mode optical fibers deliver the external light to the chip, and submicron single-mode waveguides then guide the light on-chip for further processing. For such technology to be viable, it is critically important to be able to efficiently couple light into and out of the chip platform, and between the different components, with low losses. Due to the large volume mismatch between a fiber and silicon waveguide (on the order of 600), it has been extremely challenging to obtain high coupling efficient with large tolerance. To date, demonstrated coupling has been relatively lossy and effective coupling requires impractical alignment of optical components. Here, we propose the use of a high contrast metastructure (HCM) that overcomes these issues, and effectively couples the off-chip, out-of-plane light waves into on-chip, in-plane waveguides. By harnessing the resonance properties of the metastructure, we show that it is possible to spatially confine the incoming free-space light into subwavelength dimensions with a near-unity (up to 98%) efficiency. The underlying coupling mechanism is analyzed and designs for practical on-chip coupler and reflector systems are presented. Furthermore, we explore the two-dimensional HCM as an ultra-compact wavelength multiplexer with superior efficiency (90%).

Widely tunable 1060-nm VCSEL with high-contrast grating mirror.

We report tunable VCSELs emitting around 1060 nm, enabled by high-contrast grating (HCG) mirror. Single-mode continuous-wave (CW) operation up to 110 °C is demonstrated, with room-temperature single-mode output power >1.3 mW at a very low threshold of ~300 µA. The obtained thermal resistance of 0.88 °C/mW is low for VCSELs with an oxide-confined laser aperture. A wide, continuous tuning range up to 40 nm was achieved with electrostatic and thermal tuning, at a fast tuning speed up to 1.15 MHz. In addition, we developed transverse-mode control designs of HCGs to greatly improve the single-mode yield of oxidized VCSELs. The cost-effective, wafer-scale fabrication makes these VCSELs promising as tunable light sources for swept-source optical coherent tomography (SS-OCT) and LiDAR applications.

Room-temperature Fabry-Perot resonances in suspended InGaAs/InP quantum-well nanopillars on a silicon substrate.

We present a new platform based on suspended III-V semiconductor nanopillars for direct integration of optoelectronic devices on a silicon substrate. Nanopillars grown in core-shell mode with InGaAs/InP quantum wells can support long-wavelength Fabry-Pérot resonances at room temperature with this novel configuration. Experimental results are demonstrated at a silicon-transparent wavelength of 1460 nm, facilitating integration with silicon platform.

MEMS-tunable VCSELs using 2D high-contrast gratings.

We demonstrate the room-temperature operation of a two-dimensional (2D) high-contrast grating (HCG) vertical-cavity surface-emitting laser (VCSEL) at 1080 nm. To the best of our knowledge, this is the first tunable electrically pumped surface-emitting laser using a 2D HCG. Our theory successfully explains the mechanism of broadband ultrahigh reflection of 2D HCGs. Our monolithic integrated laser exhibits single-mode output power above 0.68 mW under continuous-wave operation. Wavelength tunability is demonstrated via microelectromechanical system-controlled voltage.

Surface-normal electro-optic spatial light modulator using graphene integrated on a high-contrast grating resonator.

We demonstrate efficient optical modulation of surface-normal reflection in a novel device structure integrating graphene on a high contrast grating (HCG) resonator. As high as 11 dB extinction ratio is achieved by varying the voltage applied to a single atomic layer of graphene on a HCG resonator. The device topology facilitates easy fabrication of large 2D arrays, and free-space operation. We also demonstrate a graphene-oxide-graphene structure which can potentially operate at MHz operation speed. The devices are fully fabricated by standard CMOS compatible processes indicating that the integrated structure of graphene-on-HCG shows great promise for display, imaging and interconnects applications with low-cost and large scalability.

Wurtzite-Phased InP Micropillars Grown on Silicon with Low Surface Recombination Velocity.

The direct growth of III-V nanostructures on silicon has shown great promise in the integration of optoelectronics with silicon-based technologies. Our previous work showed that scaling up nanostructures to microsize while maintaining high quality heterogeneous integration opens a pathway toward a complete photonic integrated circuit and high-efficiency cost-effective solar cells. In this paper, we present a thorough material study of novel metastable InP micropillars monolithically grown on silicon, focusing on two enabling aspects of this technology-the stress relaxation mechanism at the heterogeneous interface and the microstructure surface quality. Aberration-corrected transmission electron microscopy studies show that InP grows directly on silicon without any amorphous layer in between. A set of periodic dislocations was found at the heterointerface, relaxing the 8% lattice mismatch between InP and Si. Single crystalline InP therefore can grow on top of the fully relaxed template, yielding high-quality micropillars with diameters expanding beyond 1 µm. An interesting power-dependence trend of carrier recombination lifetimes was captured for these InP micropillars at room temperature, for the first time for micro/nanostructures. By simply combining internal quantum efficiency with carrier lifetime, we revealed the recombination dynamics of nonradiative and radiative portions separately. A very low surface recombination velocity of 1.1 × 10(3) cm/sec was obtained. In addition, we experimentally estimated the radiative recombination B coefficient of 2.0 × 10(-10) cm(3)/sec for pure wurtzite-phased InP. These values are comparable with those obtained from InP bulk. Exceeding the limits of conventional nanowires, our InP micropillars combine the strengths of both nanostructures and bulk materials and will provide an avenue in heterogeneous integration of III-V semiconductor materials onto silicon platforms.

Illumination Angle Insensitive Single Indium Phosphide Tapered Nanopillar Solar Cell.

Low cost, high efficiency photovoltaic can help accelerate the adoption of solar energy. Using tapered indium phosphide nanopillars grown on a silicon substrate, we demonstrate a single nanopillar photovoltaic exhibiting illumination angle insensitive response. The photovoltaic employs a novel regrown core-shell p-i-n junction to improve device performance by eliminating shunt current paths, resulting in a high VOC of 0.534 V and a power conversion efficiency of 19.6%. Enhanced broadband light absorption is also demonstrated over a wide spectral range of 400-800 nm.

Surface-normal coupled four-wave mixing in a high contrast gratings resonator.

We demonstrate enhanced four-wave mixing using a silicon high contrast grating (HCG) resonator on a SOI (silicon-on-insulator) wafer directly coupled with free space Gaussian beam in surface-normal direction. The measured quality factor for HCG resonator is ~7330. Peak conversion efficiency of -19.5dB is achieved at low pumping power ~900µW. Surface-normal coupling allows for easily and robust alignment system. The very small footprint and high efficiency of our device provide an effective method for wavelength conversion in chip-scale integrated optics.

Heterogeneously integrated long-wavelength VCSEL using silicon high contrast grating on an SOI substrate.

We report an electrically pumped hybrid cavity AlGaInAs-silicon long-wavelength VCSEL using a high contrast grating (HCG) reflector on a silicon-on-insulator (SOI) substrate. The VCSEL operates at silicon transparent wavelengths ~1.57 µm with >1 mW CW power outcoupled from the semiconductor DBR, and single-mode operation up to 65 °C. The thermal resistance of our device is measured to be 1.46 K/mW. We demonstrate >2.5 GHz 3-dB direct modulation bandwidth, and show error-free transmission over 2.5 km single mode fiber under 5 Gb/s direct modulation. We show a theoretical design of SOI-HCG serving both as a VCSEL reflector as well as waveguide coupler for an in-plane SOI waveguide, facilitating integration of VCSEL with in-plane silicon photonic circuits. The novel HCG-VCSEL design, which employs scalable flip-chip eutectic bonding, may enable low cost light sources for integrated optical links.

Theory and design of two-dimensional high-contrast-grating phased arrays.

Optical properties of two-dimensional (2D) high-contrast gratings are investigated. We analyze the mechanisms for high-contrast gratings to function as various high-performance optical components. Our top-down design procedure allows us to efficiently obtain initial structural parameters and engineer them for a wide range of applications, such as reflectors, filters, resonators, waveplates, and even 2D phase plates. Simulation results of our designed structures show ultra-high power efficiency, and excellent agreement with our predicted functionalities.

Metastable growth of pure wurtzite InGaAs microstructures.

III-V compound semiconductors can exist in two major crystal phases, namely, zincblende (ZB) and wurtzite (WZ). While ZB is thermodynamically favorable in conventional III-V epitaxy, the pure WZ phase can be stable in nanowires with diameters smaller than certain critical values. However, thin nanowires are more vulnerable to surface recombination, and this can ultimately limit their performances as practical devices. In this work, we study a metastable growth mechanism that can yield purely WZ-phased InGaAs microstructures on silicon. InGaAs nucleates as sharp nanoneedles and expand along both axial and radial directions simultaneously in a core-shell fashion. While the base can scale from tens of nanometers to over a micron, the tip can remain sharp over the entire growth. The sharpness maintains a high local surface-to-volume ratio, favoring hexagonal lattice to grow axially. These unique features lead to the formation of microsized pure WZ InGaAs structures on silicon. To verify that the WZ microstructures are truly metastable, we demonstrate, for the first time, the in situ transformation from WZ to the energy-favorable ZB phase inside a transmission electron microscope. This unconventional core-shell growth mechanism can potentially be applied to other III-V materials systems, enabling the effective utilization of the extraordinary properties of the metastable wurtzite crystals.

Tailoring the optical characteristics of microsized InP nanoneedles directly grown on silicon.

Nanoscale self-assembly offers a pathway to realize heterogeneous integration of III-V materials on silicon. However, for III-V nanowires directly grown on silicon, dislocation-free single-crystal quality could only be attained below certain critical dimensions. We recently reported a new approach that overcomes this size constraint, demonstrating the growth of single-crystal InGaAs/GaAs and InP nanoneedles with the base diameters exceeding 1 µm. Here, we report distinct optical characteristics of InP nanoneedles which are varied from mostly zincblende, zincblende/wurtzite-mixed, to pure wurtzite crystalline phase. We achieved, for the first time, pure single-crystal wurtzite-phase InP nanoneedles grown on silicon with bandgaps of 80 meV larger than that of zincblende-phase InP. Being able to attain excellent material quality while scaling up in size promises outstanding device performance of these nanoneedles. At room temperature, a high internal quantum efficiency of 25% and optically pumped lasing are demonstrated for single nanoneedle as-grown on silicon substrate. Recombination dynamics proves the excellent surface quality of the InP nanoneedles, which paves the way toward achieving multijunction photovoltaic cells, long-wavelength heterostructure lasers, and advanced photonic integrated circuits.

Comprehensive model of 1550 nm MEMS-tunable high-contrast-grating VCSELs.

A comprehensive theoretical model for the long-wavelength micro-electro-mechanical-tunable high-contrast-grating vertical-cavity surface-emitting lasers is presented. Our band structure model calculates the optical gain and spontaneous emission of the InGaAlAs quantum well active region. The grating reflectivity and the cavity resonance condition are investigated through optical modeling. Correlating the results with the electrostatic model for the micro-electro-mechanical system, we accurately predict the measurements on the voltage-contolled lasing wavelength. Furthermore, our calculated temperature-dependent wavelength-tunable light output vs. current (L-I) curves show excellent agreement with experiment.

A 32 × 32 optical phased array using polysilicon sub-wavelength high-contrast-grating mirrors.

We report on microelectromechanical systems (MEMS)-actuated 32 × 32 optical phased arrays (OPAs) with high fill-factors and microsecond response time. To reduce the mirror weight and temperature-dependent curvature, we use high-contrast-grating (HCG) mirrors comprising a single layer of sub-wavelength polysilicon gratings with 400 nm thickness, 1250 nm pitch, and 570 nm grating bar width. The mirror has a broad reflection band and a peak reflectivity of 99.9% at 1550 nm wavelength. With 20 × 20 µm2 pixels and 2 µm, the OPA has a total aperture of 702 × 702 µm2 and a fill factor of 85%. The OPA is electrostatically controlled by voltage and has a total field of view of ± 2°, an instantaneous field of view (beam width) of 0.14°, and a response time of 3.8 µs. The latter agrees well with the mechanical resonance frequency of the HCG mirror (0.42 MHz).

High speed optical phased array using high contrast grating all-pass filters.

We report a high speed 8x8 optical phased array using tunable 1550 nm all-pass filters with ultrathin high contrast gratings (HCGs) as the microelectromechanical-actuated top reflectors. The all-pass filter design enables a highly efficient phase tuning (1.7 π) with a small actuation voltage (10 V) and actuation displacement of the HCG (50 nm). The microelectromechanical HCG structure facilitates a high phase tuning speed >0.5 MHz. Beam steering is experimentally demonstrated with the optical phased array.