Interferometric cavity ring-down technique for ultra-high Q-factor microresonators

TL;DR

This paper introduces an interferometric cavity ring-down technique to accurately measure the ultra-high Q-factor of microresonators, overcoming challenges posed by mode splitting due to surface roughness.

Contribution

It proposes a novel interferometric excitation method that enables Q measurement in high-Q microresonators despite mode splitting issues.

Findings

Resonant doublet merges into a single Lorentzian under certain conditions.

The method allows exponential decay measurement for Q estimation.

It provides a way to measure the ultimate Q of microresonators.

Abstract

Microresonators (MRs) are key components in integrated optics. As a result, the estimation of their energy storage capacity as measured by the quality factor (Q) is crucial. However, in MR with high/ultra-high Q, the surface-wall roughness dominates the intrinsic Q and generates a coupling between counter-propagating modes. This splits the usual sharp single resonance and makes difficult the use of classical methods to assess Q. Here, we theoretically show that an interferometric excitation can be exploited in a Cavity Ring-Down (CRD) method to measure the ultimate Q of a MR. In fact, under suitable conditions, the resonant doublet merges into a single Lorentzian and the time dynamics of the MR assumes the usual behavior of a single-mode resonator unaffected by backscattering. This allows obtaining a typical exponential decay in the charging and discharging time of the MR, and thus, estimating its ultimate Q by measuring the photon lifetime.