Optical frequency comb generation using a microtoroid resonator in an aqueous environment

Detecting single molecules without labels or capture probes is of great interest for both medical applications and scientific research. Frequency-locked microtoroid optical resonators are capable of label-free single molecule detection, however, this approach requires a priori knowledge of the molecule to be detected as well as surface functionalization of the cavity. Optical frequency microcombs can be a precise source of spectral information on molecules, however, microcombs have not been generated in an aqueous biological sensing environments due to altered dispersion, coupling instability, and reduced quality factor of the resonator. Here we suggest a way toward single-molecule spectroscopy by demonstrating frequency comb generation in water and air at visible wavelengths using a microtoroid optical resonator. Local anomalous dispersion is achieved because of the interaction between different transverse mode families in an overall normal dispersion region. With this approach, the advantageous structure and material of the microtoroid resonator for biosensing is preserved. We believe that in the future this will enable single molecule detection and identification simultaneously in both air and liquid at any wavelength with no labels or capture probes.