Using thermo-optical nonlinearity to robustly separate absorption and radiation losses in nanophotonic resonators

TL;DR

This paper introduces a novel all-optical spectroscopic method leveraging thermo-optical effects to accurately separate absorption and radiation losses in nanophotonic resonators, enhancing resonator engineering precision.

Contribution

The authors develop and experimentally validate a technique that uses thermal time constants to distinguish absorption from radiation losses using only linear spectroscopy.

Findings

Absorption losses are consistent across modes despite varying radiation losses.

The method accurately quantifies absorption, radiation, and coupling losses in microdisk resonators.

It enables differentiation between scattering and leakage components of radiation loss.

Abstract

Low-loss nanophotonic resonators have been widely used in fundamental science and applications thanks to their ability to concentrate optical energy. Key for resonator engineering, the total intrinsic loss is easily determined by spectroscopy, however, quantitatively separating absorption and radiative losses is challenging. While the concentrated heat generated by absorption within the small mode volume results in generally unwanted thermo-optical effects, they can provide a way for quantifying absorption. Here, we propose and experimentally demonstrate a technique for separating the loss mechanisms with high confidence using only linear spectroscopic measurements. We use the optically measured resonator thermal time constant to experimentally connect the easily-calculable heat capacity to the thermal impedance, needed to calculate the absorbed power from the temperature change. We report the absorption, radiation, and coupling losses for ten whispering-gallery modes of three different radial orders on a Si microdisk. Similar absorptive loss rates are found for all the modes, despite order-of-magnitude differences in the total dissipation rate due to widely differing radiation losses. Measuring radiation losses of many modes enables distinguishing the two major components of radiation loss originating from scattering and leakage. The all-optical characterization technique is applicable to any nanophotonic resonators subject to thermo-optical effects.