Alexander Bruch

Broadband nanophotonic waveguides and resonators based on epitaxial GaN thin films

We demonstrate broadband, low loss optical waveguiding in single crystalline GaN grown epitaxially on c-plane sapphire wafers through a buffered metal-organic chemical vapor phase deposition process. High Q optical microring resonators are realized in near infrared, infrared, and near visible regimes with intrinsic quality factors exceeding 50 000 at all the wavelengths we studied. TEM analysis of etched waveguide reveals growth and etch-induced defects. Reduction of these defects through improved material and device processing could lead to even lower optical losses and enable a wideband photonic platform based on GaN-on-sapphire material system.

Electrochemically sliced low loss AlGaN optical microresonators

High quality single crystal III-Nitride films are often formed over a thick buffer to reduce growth induced defects on a lattice mismatched substrate. However, it is challenging to fabricate nanophotonic waveguiding structures directly from this very top layer. Here, we demonstrate electrochemical slicing of high quality AlGaN thin films and its subsequent transfer to a lower index oxided silicon substrate for lithographic patterning of photonic waveguide and microresonators. TEM analysis of the nanomembrane waveguide demonstrates an AlGaN layer free of misfit dislocations commonly found in conventional epitaxial AlGaN grown on sapphire or Si. We probe the low material optical loss (1.22 dB/cm) of the nanomembrane by measuring the optical quality (Q) factor at 780 nm. High intrinsic quality factors of 680 000 are achieved after optimizing fabrication process. This versatile, low loss AlGaN device opens applications for nonlinear photonics at visible wavelengths.

17 000%/W second-harmonic conversion efficiency in single-crystalline aluminum nitride microresonators

High quality factor optical microcavities have been employed in a variety of material systems to enhance nonlinear optical interactions. While single-crystalline aluminum nitride microresonators have recently emerged as a low loss platform for integrated nonlinear optics such as four wave mixing and Raman lasing, few studies have investigated this material for second-harmonic generation. In this Letter, we demonstrate an optimized fabrication of dually-resonant phase-matched ring resonators from epitaxial aluminum nitride thin films. An unprecendented second-harmonic generation efficiency of 17,000%/W is obtained in the low power regime and pump depletion is observed at a relatively low input power of 3.5 mW. This poses epitaxial aluminum nitride as the highest efficiency second-harmonic generator among current integrated platforms.

Efficient Generation of a Near-visible Frequency Comb via Cherenkov-like Radiation from a Kerr Microcomb

Optical frequency combs enable state-of-the-art applications including frequency metrology, optical clocks, astronomical measurements, and sensing. Recent demonstrations of microresonator-based Kerr frequency combs or microcombs pave the way to scalable and stable comb sources on a photonic chip. Generating microcombs in the short-wavelength range, however, has been limited by large material dispersion and optical loss. Here we demonstrate a scheme for efficiently generating microcombs near the edge of the visible spectrum in a high-Q aluminum nitride microring resonator. The enhanced Pockels effect strongly couples infrared and near-visible modes into hybrid mode pairs, which participate in the Kerr microcomb generation process and lead to strong Cherenkov-like radiation in the near-visible band an octave apart. A surprisingly high on-chip conversion efficiency of 22% is achieved from a pulsed pump laser to the near-visible comb. As a result of pulse pumping, the generated microcombs are in the chaotic state. We further demonstrate a robust frequency tuning of the near-visible comb by more than one free spectral range and apply it to the absorption spectroscopy of a water-based dye molecule solution. Our work is a step towards high-efficiency visible microcomb generation and its utilization, and it also provides insights into the significance of the Pockels effect and its strong coupling with Kerr nonlinearity in a single microcavity device.