Etch-Tuning and Design of Silicon Nitride Photonic Crystal Reflectors

TL;DR

This paper introduces a gentle hydrofluoric acid etching method to finely tune silicon nitride photonic crystal reflectors, achieving near-target resonance wavelengths with high precision and analyzing factors affecting their optical performance.

Contribution

It presents a novel etching technique for precise resonance tuning of SiN photonic crystal reflectors and offers design insights to improve their robustness and reflectivity.

Findings

Achieved 57-nm tuning of the resonance wavelength within 0.15 nm of the target

Identified divergence angle effects as a cause for transmission dip discrepancies

Provided design considerations for high-reflectivity, robust photonic crystal devices

Abstract

By patterning a freestanding dielectric membrane into a photonic crystal reflector (PCR), it is possible to resonantly enhance its normal-incidence reflectivity, thereby realizing a thin, single-material mirror. In many PCR applications, the operating wavelength (e.g. that of a low-noise laser or emitter) is not tunable, imposing tolerances on crystal geometry that are not reliably achieved with standard nanolithography. Here we present a gentle technique to finely tune the resonant wavelength of a SiN PCR using iterative hydrofluoric acid etches. With little optimization, we achieve a 57-nm-thin photonic crystal having an operating wavelength within 0.15 nm (0.04 resonance linewidths) of our target (1550 nm). Our thin structure exhibits a broader and less pronounced transmission dip than is predicted by plane wave simulations, and we identify two effects leading to these discrepancies, both related to the divergence angle of a collimated laser beam. To overcome this limitation in future devices, we distill a series of simulations into a set of general design considerations for realizing robust, high-reflectivity resonances.