Mode mapping Q > 500 000 photonic crystal nanocavities using free carrier absorption

TL;DR

This paper introduces a nonlinear photomodulation spectroscopy technique to image high-Q photonic crystal resonator modes with near-diffraction-limited resolution by leveraging free carrier absorption for localized heating.

Contribution

The study demonstrates a novel high-resolution mode mapping method for ultrahigh-Q photonic crystal cavities using free carrier absorption to surpass thermal resolution limits.

Findings

Achieved near-diffraction-limited spatial resolution in mode imaging.

Demonstrated effective mode mapping at surprisingly low carrier densities.

Validated the method with a thermo-optical model matching experimental results.

Abstract

We demonstrate a nonlinear photomodulation spectroscopy method to image the mode profile of a high-Q photonic crystal resonator (PhCR). This far-field imaging method is suitable for ultrahigh-Q cavities which we demonstrate on a Q = 619000 PhCR. We scan the PhCR surface with a 405 nm pump beam that modulates the refractive index by local thermal tuning, while probing the response of the resonance. We enhance resolution by probing at high power, using the thermo-optical nonlinearity of the PhCR. Spatial resolution of the thermo-optical effect is typically constrained by the broad thermal profile of the optical pump. Here we go beyond the thermal limit and show that we can approach the diffraction limit of the pump light. This is due to free carrier absorption that heats up the PhCR only when there is overlap between the optical pump spot and the optical mode profile. This is supported with a thermo-optical model that reproduces the high-resolution mode mapping. Results reveal that the observed enhanced resolution is reached for surprisingly low carrier density.