Xianwen Liu

Aluminum nitride-on-sapphire platform for integrated high-Q microresonators

We demonstrate aluminum nitride (AlN) on sapphire as a novel platform for integrated optics. High-confinement AlN microring resonators are realized by adopting a partially etched (pedestal) waveguide to relax the required etching selectivity for exact pattern transfer. A wide taper is employed at the chip end facets to ensure a low fiber-to-chip coupling loss of ~2.8 dB/facet for both transverse-electric (TE) and transverse-magnetic (TM) modes. Furthermore, the intrinsic quality factors (Qint) recorded with a high-resolution linewidth measurement are up to ~2.5 and 1.9 million at telecom band for fundamental TE00 and TM00 modes, corresponding to a low intracavity propagation loss of ~0.14 and 0.2 dB/cm as well as high resonant buildup of 473 and 327, respectively. Such high-Q AlN-on-sapphire microresonators are believed to be very promising for on-chip nonlinear optics.

Broadband tunable microwave photonic phase shifter with low RF power variation in a high-Q AlN microring.

An all-optically tunable microwave photonic phase shifter is demonstrated based on an epitaxial aluminum nitride (AlN) microring with an intrinsic quality factor of 3.2×106. The microring adopts a pedestal structure, which allows overcoupling with 700 nm gap size and facilitates the fabrication process. A phase shift for broadband signals from 4 to 25 GHz is demonstrated by employing the thermo-optic effect and the separate carrier tuning technique. A phase tuning range of 0°-332° is recorded with a 3 dB radio frequency (RF) power variation and 48 mW optical power consumption. In addition, AlN exhibits intrinsic second-order optical nonlinearity. Thus, our work presents a novel platform with a low propagation loss and the capability of electro-optic modulation for applications in integrated microwave photonics.

Generation of multiple near-visible comb lines in an AlN microring via χ(2) and χ(3) optical nonlinearities

On-chip frequency upconversion of a near-infrared (NIR) Kerr comb in a χ(2) and χ(3) system provides a convenient route to extending the comb spectra into the visible band. Yet to date, only limited visible or near-visible comb lines have been obtained using this scheme. In this work, we demonstrate the generation of multiple near-visible comb lines based on spectral translation from a broadband NIR Kerr comb. This physical process is implemented in an aluminum nitride (AlN)-on-sapphire microring, where we achieve a wideband frequency upconversion by incorporating the phase-mismatched fundamental and first-order near-visible modes. Upon tuning the pump into the resonance with sufficient power, we attain a broadband NIR Kerr comb and 153 corresponding near-visible comb lines in 720–840 nm with a reasonable efficiency over 4.1 × 10−5%. The wideband frequency upconversion can be adapted to on-chip frequency stabilization of self-referenced microcombs, as required for precision optical clocks and frequency metrology.On-chip frequency upconversion of a near-infrared (NIR) Kerr comb in a χ(2) and χ(3) system provides a convenient route to extending the comb spectra into the visible band. Yet to date, only limited visible or near-visible comb lines have been obtained using this scheme. In this work, we demonstrate the generation of multiple near-visible comb lines based on spectral translation from a broadband NIR Kerr comb. This physical process is implemented in an aluminum nitride (AlN)-on-sapphire microring, where we achieve a wideband frequency upconversion by incorporating the phase-mismatched fundamental and first-order near-visible modes. Upon tuning the pump into the resonance with sufficient power, we attain a broadband NIR Kerr comb and 153 corresponding near-visible comb lines in 720–840 nm with a reasonable efficiency over 4.1 × 10−5%. The wideband frequency upconversion can be adapted to on-chip frequency stabilization of self-referenced microcombs, as required for precision optical clocks and frequency me...

Tunable microwave phase shifter with low RF-power variation based on high-Q epitaxial AlN ring resonator

A novel optically tunable microwave phase shifter is demonstrated based on high-Q (intrinsic quality factor ~ 3.3×106) epitaxial AlN microring, exhibiting a tuning range of 0-332° and RF-power variation less than 3 dB for a 19 GHz signal.

Improvement of thermal properties in epitaxial aluminum nitride pedestal microring

We report thermal properties improvement of aluminum nitride (AlN) microring resonator by adopting epitaxial material and pedestal structure. With optimized inductively coupled plasma (ICP) dry etching using Cl2/BCl3/Ar mixture, pedestal microring resonators have been fabricated on epitaxial AlN. Transmission measurement reveals a loaded quality factor of ~1×104 at under-coupled condition, corresponding to a propagation loss of 27 dB/cm around 1.55 μm. Thermal effect induced resonant peak shift at different coupled powers is 6.3 pm/mW, and optical bistability can be observed at an input power above ~115 mW.