We present numerical modeling and experimental characterization of the photonic bound states in high-contrast Si-based subwavelength grating waveguide structures. The resonant modes in the grating waveguides show some of the unique features of the photonic bound states in the continuum: continuous narrowing of the resonance linewidth and cancellation of radiative waves. The calculated field distributions show strong internal field buildup around resonances. To verify our simulation results, a Si-based subwavelength grating waveguide was fabricated and experimentally characterized. The measured reflection spectra show two resonance peaks around λ0 = 1490 nm and λ0 = 1505 nm. According to the simulated results, these two peaks are located near a BIC condition. The captured infrared microscope images in the reflection measurement reveal the dynamical interaction between the incident light and the subwavelength grating waveguide. The demonstrated Si grating waveguides has potential to be used as highly efficient frequency-selective couplers between free-space optical waves to in-plane guided optical waves in existing Si integrated photonic circuits.
We report our in-house R&D efforts of designing and developing key integrated photonic devices and technologies for a chip-scale optical oscillator and/or clock. This would provide precision sources to RF-photonic systems. It could also be the basic building block for a photonic technology to provide positioning, navigation, and timing as well as 5G networks. Recently, optical frequency comb (OFC)-based timing systems have been demonstrated for ultra-precision time transfer. Our goal is to develop a semiconductor-based, integrated photonic chip to reduce the size, weight, and power consumption, and cost of these systems. Our approach is to use a self-referenced interferometric locking circuit to provide short-term stabilization to a micro-resonator-based OFC. For long-term stabilization, we use an epsilon-near-zero (ENZ) metamaterial to design an environment-insensitive cavity/resonator, thereby enabling a chip-scale optical long-holdover clock.
An RF-Photonic phased array antenna beamformer was previously demonstrated using cascaded fiber Bragg gratings with 1 x 2 couplers for true-time-delay beamforming. This work's focus is to design, build, and test an integrated Si-photonic beamforming circuit to replace the fiber-optics system, allowing for chip-scale beamformers with low size, weight, power, and cost. Several metastructure waveguides were designed to provide a strong slowlight effect near their transmission band edge. By tuning the wavelength near the band edge, tunable optical truetime delay is achieved. We report the design, simulation, fabrication and test of these high-contrast metastructure waveguides to provide group velocity variation against wavelength near the band-edge. Wavelength-tunable delay was verified using both an interferometric approach using an integrated Mach-Zehnder interferometer, and using a direct measurement of the true-time delay of an RF signal modulated onto a C-band optical carrier. We have also designed an integrated photonic beamforming circuit for a small array, including photodetectors, fabricated by AIM Photonics. Experimental test results for those integrated photonic circuits will be discussed. We will continue to improve our integrated photonic circuit to pursue larger array implementation. The goal is to further integrate this photonic circuit with an RF phase array antenna and demonstrate the scan of an RF beam by optical control.
The ability to tune the delay of an optical signal is a key component in photonics-based RF phased-array beamforming applications. Recent work has shown that high-contrast metastructure waveguides can be designed for a wide range of delay tuned by carrier injection or signal wavelength, enabling two-dimensional beam steering. In this work, we further explore the parameter space of these structures to maximize the delay change over optical wavelength while maintaining low insertion loss, with the goal of implementing phased-array beamforming in integrated photonic devices.
In this work, we have designed a novel Si based 1-dimensional high contrast meta-structure waveguide that has slow light effect as well as phase tunability using p-n junction. The goal is to use such waveguide to design active optical devices such as high frequency modulators and tunable filters for analog RF-photonics or data communication applications. The Si ridge waveguide has a pair of high contrast grating wings adhered to the waveguide core in the center. Grating bars at two sides of the waveguide are doped P and N-type respectively, while a p-n junction region is formed in the middle of the waveguide core. By applying a voltage to bias the p-n junction, one can sweep the free carriers to change the effective index of the waveguide as well as the dispersion property of the grating. This metastructure Si waveguide is ideal in the design of high frequency optical modulators since the slow light effect can reduce the modulator waveguide length, increase the modulation efficiency as well as compensate other nonlinearity factors of the modulator for analog applications.