Tianbo Sun

Optical phased array using high contrast gratings for two dimensional beamforming and beamsteering

We have developed a microelectromechanical system (MEMS) optical phased array incorporating a high-index-contrast subwavelength grating (HCG) for beamforming and beamsteering in a range of ± 1.26° × 1.26°. Our approach needs only a thin single-layer HCG made of silicon, considerably improving its speed thanks to the low mass, and is suitable for high optical power applications. The measured resonant frequency of HCG is 0.32 MHz.

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.

A novel optical phased array is experimentally demonstrated with high speed (0.577 MHz) beam steering, which consists of 8×8 tunable 1550-nm all-pass filters with ultrathin high-contrast grating as the micro-electro-mechanically actuated reflector for fast tuning.

High-contrast grating resonators for label-free detection of disease biomarkers.

A label-free optical biosensor is described that employs a silicon-based high-contrast grating (HCG) resonator with a spectral linewidth of ~500 pm that is sensitive to ligand-induced changes in surface properties. The device is used to generate thermodynamic and kinetic data on surface-attached antibodies with their respective antigens. The device can detect serum cardiac troponin I, a biomarker of cardiac disease to 100 pg/ml within 4 mins, which is faster, and as sensitive as current enzyme-linked immuno-assays for cTnI.

Low-loss slow light inside high contrast grating waveguide

We propose a novel hollow-core slow light waveguide using high contrast grating (HCG). Light propagates in air along a path bounded by two HCG layers. A strong interaction between the light and the HCG leads to a large group index, and thus the slow light effect. Waveguide loss and group index can be optimized separately by tuning the HCG and waveguide parameters. High performance slow light is obtained with <0.1 dB/cm loss, >120 group index and >120 GHz bandwidth.

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.

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

We demonstrate four-wave mixing using a Si-based high contrast gratings (HCG) resonator directly coupled in surface-normal direction. Quality-factor of the resonator is ~7000 and peak conversion efficiency is -19.5dB at low pump power levels.

Integrated plasmonic refractive index sensor based on grating/metal film resonant structure

Optical biosensors with the high sensitivity is an important tool for environment monitoring, disease diagnosis and drug development. Integrating the biosensor could reduce the size and cost and is desirable for home and outdoor use. However, the integrated structure always results in the worsening of sensitivity and narrowing of sensing range, especially for small molecule sensing. In this work, we propose an integrated plasmonic biosensor based on the resonant structure composed of dielectric grating and metal film. With vertically incident light from the grating side, the surface plasmon polariton (SPP) mode could be excited at certain wavelength and the reflected light would vanish. Simulation results indicate that, when varying refractive index (ndet) of detection layer, the energy of reflected light changes dramatically. Assuming the resolution of the power meter is 0.01dB, the sensing resolution could be 4.37×10-6 RIU, which is very close to the bulk lens based SPP biosensor by monitoring the light intensity variation. Since antibody and antigen always have the size of tens of nanometers, it is necessary to check the sensing ability of the sensor in tens of nanometers. Fixing ndet and varying the thickness of detection layer, calculation result demonstrates that the reflected light energy is sensitive to the thickness change with one hundred nanometers. This attributes to the surface mode property of SPP mode.

Fast optical phased array with ultra-lightweight high-contrast-grating mirrors

We report an optical phased array (OPA) using high-contrast-grating (HCG) as an electrostatically actuated mirror for the purpose of free space fast beamsteering. In this paper, several attractive properties of HCG mirror are introduced such as single material, high reflectance along a wide bandwidth and low-mass inertia and the fabrication result is shown. ±10° × 10° with 2° beam width is observed at 1550 nm wavelength using HCG optical phased array.

Optical phased array using single crystalline silicon high-contrast-gratings for beamsteering

We present a single crystalline silicon optical phased array using high-contrast-gratings (HCG) for fast two dimensional beamforming and beamsteering at 0.5 MHz. Since there are various applications for beamforming and beamsteering such as 3D imaging, optical communications, and light detection and ranging (LIDAR), it is great interest to develop ultrafast optical phased arrays. However, the beamsteering speed of optical phased arrays using liquid crystal and electro-wetting are typically limited to tens of milliseconds. Optical phased arrays using micro-electro-mechanical systems (MEMS) technologies can operate in the submegahertz range, but generally require metal coatings. The metal coating unfortunately cause bending of mirrors due to thermally induced stress. The novel MEMS-based optical phased array presented here consists of electrostatically driven 8 × 8 HCG pixels fabricated on a silicon-on-insulator (SOI) wafer. The HCG mirror is designed to have 99.9% reflectivity at 1550 nm wavelength without any reflective coating. The size of the HCG mirror is 20 × 20 μm2 and the mass is only 140 pg, much lighter than traditional MEMS mirrors. Our 8 × 8 optical phased array has a total field of view of ±10° × 10° and a beam width of 2°. The maximum phase shift regarding the actuation gap defined by a 2 μm buried oxide layer of a SOI wafer is 1.7π at 20 V.