Xiaochuan Xu

Recent advances in silicon-based passive and active optical interconnects

Silicon photonics has experienced phenomenal transformations over the last decade. In this paper, we present some of the notable advances in silicon-based passive and active optical interconnect components, and highlight some of our key contributions. Light is also cast on few other parallel technologies that are working in tandem with silicon-based structures, and providing unique functions not achievable with any single system acting alone. With an increasing utilization of CMOS foundries for silicon photonics fabrication, a viable path for realizing extremely low-cost integrated optoelectronics has been paved. These advances are expected to benefit several application domains in the years to come, including communication networks, sensing, and nonlinear systems.

Complementary metal–oxide–semiconductor compatible high efficiency subwavelength grating couplers for silicon integrated photonics

We demonstrate a through-etched grating coupler based on subwavelength nanostructure. The grating consists of arrays of 80 nm × 343 nm rectangular air holes, which can be patterned in a single lithography/etch. A peak coupling efficiency of 59% at 1551.6 nm and a 3 dB bandwidth of 60 nm are achieved utilizing the silicon-on-insulator platform with a 1 μm thick buried-oxide layer for transverse electric mode. The performance is comparable to gratings requiring much more complicated fabrication processes.

Cavity-Waveguide Coupling Engineered High Sensitivity Silicon Photonic Crystal Microcavity Biosensors With High Yield

We present a high yield and high sensitivity on-chip biosensing system by combining subwavelength grating coupling and high sensitivity photonic crystal microcavity side coupled to a photonic crystal waveguide. 80% yield of working devices was experimentally demonstrated for sensitivity engineered L13 photonic crystal microcavities and 70% for L21 photonic crystal microcavities. Subwavelength grating couplers significantly improved the quality of the output transmission spectrum. By engineering the optical loss rate from the cavity to the waveguide, we experimentally detected 1 pM (67 pg/ml) and 50 femto-molar (3.35 pg/ml) concentration of avidin binding to biotin in phosphate buffered saline for L21 and L55 PC microcavities respectively, which represents the highest sensitivity versus other chip-based optical biosensors.

Ultracompact and fabrication-tolerant integrated polarization splitter

Design and fabrication of a 2×2 two-mode interference (TMI) coupler based on-chip polarization splitter is presented. By changing the angle between the access waveguides, one can tune the effective TMI length for the mode with less optical confinement (transverse magnetic, TM) to coincide with the target TMI length for a desired transmission of the mode with higher optical confinement (transverse electric, TE). The fabricated 0.94 μm long 2×2 TMI splits the input power into TM (bar) and TE (cross) outputs with splitting ratio over 15 dB over 50 nm bandwidth. Fabrication tolerance analysis shows that the device is tolerant to fabrication errors as large as 60 nm.

On the role of evanescent modes and group index tapering in slow light photonic crystal waveguide coupling efficiency

We investigate effects of different mechanisms on coupling efficiency between strip waveguides and the slow light mode in photonic crystal waveguides (PCWs). Both numerical simulations and experimental results show that group index (ng) tapering improves strip-PCW butt-coupling efficiency when compared to a direct coupling between a strip waveguide and a high-ng PCW. However, coupling efficiency is even higher when an intermediate low-ng PCW is used to couple from a strip waveguide to a high-ng PCW without an ng tapering. Our results suggest that the role of evanescent mode is more dominant in efficient coupling between two PCWs with large ng mismatch.

Large optical spectral range dispersion engineered silicon-based photonic crystal waveguide modulator.

We present a dispersion engineered slow light silicon-based photonic crystal waveguide PIN modulator. Low-dispersion slow light transmission over 18 nm bandwidth under the silica light line with a group index of 26.5 is experimentally confirmed. We investigate the variations of the modulator figure of merit, V(π) × L, as a function of the optical carrier wavelength over the bandwidth of the fundamental photonic crystal waveguide defect mode. A large signal operation with a record low maximum V(π )× L of 0.0464 V · mm over the low-dispersion optical spectral range is demonstrated. We also report the device operation at 2 GHz.

On-chip silicon optical phased array for two-dimensional beam steering

A 16-element optical phased array integrated on chip is presented for achieving two-dimensional (2D) optical beam steering. The device is fabricated on the silicon-on-insulator platform with a 250 nm silicon device layer. Steering is achieved via a combination of wavelength tuning and thermo-optic phase shifting with a switching power of P(π)=20  mW per channel. Using a silicon waveguide grating with a polycrystalline silicon overlay enables narrow far field beam widths while mitigating the precise etching needed for conventional shallow etch gratings. Using this system, 2D steering across a 20°×15° field of view is achieved with a sidelobe level better than 10 dB and with beam widths of 1.2°×0.5°.

Inkjet-Printed Two-Dimensional Phased-Array Antenna on a Flexible Substrate

In this letter, we present a two-dimensional 2-bit 4 × 4 phased-array antenna on a flexible Kapton substrate fabricated using inkjet printing. Printed carbon nanotube thin-film transistors (CNT-TFTs) form the switching elements in the phase-shifting network. A multilayer interconnection scheme has been utilized to fully package the subsystem and enable convenient access of the control lines to the 64 CNT-TFTs. By appropriately controlling the on and off states of the TFT switches using a computer mainframe, beam steering of a 5-GHz RF signal at four steering angles of θ = 0^°, φ = 0^°; θ = 14.5^°, φ = 0^°; θ = 20.7^°, φ = -45^°; and θ = 34^°, φ = -26.5^° are experimentally demonstrated. The insertion loss and the power consumption by the switch array are measured to be 8.17 dB and 11.2 mW, respectively .

Efficient light coupling into in-plane semiconductor nanomembrane photonic devices utilizing a sub-wavelength grating coupler.

We report a subwavelength grating (SWG) coupler for coupling light efficiently into in-plane semiconductor nanomembrane photonic devices for the first time. The SWG coupler consists of a periodic array of rectangular trenches fabricated on a silicon nanomembrane (SiNM) transferred onto a glass substrate. At a wavelength of 1555.56 nm, the coupling efficiency of the fabricated 10 µm wide, 17.1 µm long SWG is 39.17% (-4.07 dB), with 1 dB and 3 dB bandwidths of 29 nm and 57 nm, respectively. Peak efficiency varies by 0.26 dB when measuring 5 fabricated grating pairs. Coupling efficiency can further be improved with an improved SiNM transfer process. Such high efficiency couplers allow for the successful realization of a plethora of hybrid photonic devices utilizing nanomembrane technology.

Group-index independent coupling to band engineered SOI photonic crystal waveguide with large slow-down factor

Group-index independent coupling to a silicon-on-insulator (SOI) based band-engineered photonic crystal (PCW) waveguide is presented. A single hole size is used for designing both the PCW coupler and the band-engineered PCW to improve fabrication yield. Efficiency of several types of PCW couplers is numerically investigated. An on-chip integrated Fourier transform spectral interferometry device is used to experimentally determine the group-index while excluding the effect of the couplers. A low-loss, low-dispersion slow light transmission over 18 nm bandwidth under the silica light line with a group index of 26.5 is demonstrated, that corresponds to the largest slow-down factor of 0.31 ever demonstrated for a PCW with oxide bottom cladding.