Ultrahigh-Q chalcogenide micro-racetrack resonators

TL;DR

The paper addresses the challenge of achieving high quality factors and low propagation losses in planar chalcogenide microresonators for nonlinear photonics. It introduces a micro-racetrack design using Euler curves to suppress bend and mode-mismatch losses, and employs a coupled-mode theory model to extract material absorption and nonlinear index directly from resonance spectra. The authors report a record-in-class intrinsic quality factor $Q_{int}=4.55\times10^6$, absorption loss $\alpha_{abs}=0.098\ \mathrm{m^{-1}}$ (≈0.43 dB/m), and nonlinear index $n_2=1.28\times10^{-18}\ \mathrm{m^2/W}$ for Ge$_{23}$Sb$_{7}$S$_{70}$, with a cross-section $<1\ \mu\mathrm{m}^2$. This work establishes Ge$_{23}$Sb$_{7}$S$_{70}$ as a low-loss, high-nonlinearity PIC platform and demonstrates a pathway to ultra-efficient on-chip nonlinear optics, including four-wave mixing and stimulated Brillouin/Raman processes.

Abstract

High-quality factor microresonators are an attractive platform for the study of nonlinear photonics, with diverse applications in communications, sensing, and quantum metrology. The characterization of loss mechanisms and nonlinear properties in a microresonator is a necessity for the development of photonic integrated circuits. Here, we demonstrate a high-quality chalcogenide ($Ge_{23}Sb_{7}S_{70}$) micro-racetrack resonator utilizing Euler curves. The racetrack geometry is studied to minimize loss at both the straight-curved waveguide junction and through the waveguide curve. The material absorption, intrinsic quality factor, and nonlinear index are extracted by a comprehensive model fit to laser wavelength resonance scans. The micro-racetrack resonator possesses an absorption loss of $0.43 dB/m$, an intrinsic quality factor of $4.5 \times 10^6$, and nonlinear index of $1.28 \times 10^{-18} m^2/W$, in a waveguide cross-section less than $1 μm^2$. Our results yield state-of-the-art nonlinear microresonators and establish $Ge_{23}Sb_{7}S_{70}$ as a low-loss PIC platform.

Figures (5)

• Figure 1: (a) Proposed micro-racetrack resonator schematic. (b) Chalcogenide waveguide cross-section. The inset shows the simulated fundamental TE mode in a representative waveguide. (c) Scanning electron micrograph of a fabricated 0.6 $\times$ 1.4 µm chalcogenide waveguide cross-section.
• Figure 2: (a) Calculated mode mismatch loss of the fundamental TE mode as it enters Euler curves of varied maximum radii $R\textsubscript{max}$. (b) Calculated excitation of the TE1 mode as the TE0 mode enters Euler curves of varied $R\textsubscript{max}$.
• Figure 3: Calculated light propagation through a 180$^\circ$ Euler curve with $R\textsubscript{max}$ = 1000 µm and (a) $R\textsubscript{min}$ = 20 µm, (b) $R\textsubscript{min}$ = 35 µm, and (c) $R\textsubscript{min}$ = 50 µm. (d) Corresponding bend loss of the fundamental TE mode for varied $R\textsubscript{min}$.
• Figure 4: Experimental transmission spectra for scans in both the red (up) and blue (down) directions at two different input powers. (a) Low power up-scan (b) Low power down-scan (c) High power up-scan (d) High power down-scan. Spectra are fit simultaneously using a genetic algorithm to prevent overfitting and give more accurate fitting parameters.
• Figure 5: (a) Effective nonlinear performance of high-Q chalcogenide-based PICs. (b) Estimated micro-racetrack scattering loss mitigation using wider waveguide geometries.