From membrane-in-the-middle to mirror-in-the-middle with a high-reflectivity sub-wavelength grating

TL;DR

This paper presents a high-reflectivity sub-wavelength grating membrane in an optomechanical system, enabling the creation of coupled sub-cavities with tunable resonance features and detailed spectral analysis.

Contribution

It introduces a silicon nitride membrane with a subwavelength grating achieving over 99.8% reflectivity, demonstrating coupled cavity modes and providing a theoretical framework for spectral lineshape analysis.

Findings

Achieved over 99.8% reflectivity with the grating

Observed an avoided crossing with a 54 MHz splitting

Derived spectral lineshape expressions for coupled modes

Abstract

We demonstrate a "membrane in the middle" optomechanical system using a silicon nitride membrane patterned as a subwavelength grating. The grating has a reflectivity of over 99.8%, effectively creating two sub-cavities, with free spectral ranges of 6 GHz, optically coupled via photon tunneling. Measurements of the transmission and reflection spectra show an avoided crossing where the two sub-cavities simultaneously come into resonance, with a frequency splitting of 54 MHz. We derive expressions for the lineshapes of the symmetric and antisymmetric modes at the avoided crossing, and infer the grating reflection, transmission, absorption, and scattering through comparison with the experimental data.