Optomechanical Cavities for All-Optical Photothermal Sensing

Abstract

Cavity optomechanics enables strong coupling of optics and mechanics. Although remarkable progress has been made, practical applications of cavity optomechanics are only recently being realized. In this work we propose an all-optical sensing technique enabling the measurement of photothermally induced strains with ultrahigh-resolution. We demonstrate an optomechanical sensor consisting of a silicon nitride (Si3N4) microring cavity that is evanescently coupled to a suspended SiNx micromechanical (MEMS) oscillator. Experiments show that MEMS resonances are excited purely via cavity-enhanced gradient optical forces. However, small levels of absorption in the oscillator result in photothermally induced strains that shift the mechanical resonance frequencies. By measuring absorption-induced frequency shifts our technique enables high-resolution with nanostrain sensitivity corresponding to fJ-levels of absorption. As a demonstration, we perform absorption spectroscopy on the MEMS device and measure the known Si–H absorption feature of deposited silicon nitride. The unprecedented sensitivity enabled by absorption-induced frequency shifts enables entirely new sensors in fields ranging from materials and chemical sensing to bolometers and imaging arrays.