Overcoming Thermo-Optical Dynamics in Broadband Nanophotonic Sensing

TL;DR

This paper develops a frequency-dependent transfer function to model thermo-optical effects in nanophotonic sensors, enabling operation at higher powers and sensitivities, and revealing that lower quality factor modes can enhance low-frequency sensitivity.

Contribution

It introduces a novel transfer function that accounts for thermo-optical dynamics, allowing improved sensor performance and broad applicability to photonic sensors.

Findings

Achieved 0.4 fm/Hz$^{1/2}$ sensitivity at high optical power.

Discovered lower Q modes improve low-frequency sensitivity.

Validated the transfer function for broadband optomechanical signals.

Abstract

Advances in integrated photonics open exciting opportunities for batch-fabricated optical sensors using high quality factor nanophotonic cavities to achieve ultra-high sensitivities and bandwidths. The sensitivity improves with higher optical power, however, localized absorption and heating within a micrometer-scale mode volume prominently distorts the cavity resonances and strongly couples the sensor response to thermal dynamics, limiting the sensitivity and hindering the measurement of broadband time-dependent signals. Here, we derive a frequency-dependent photonic sensor transfer function that accounts for thermo-optical dynamics and quantitatively describes the measured broadband optomechanical signal from an integrated photonic atomic-force-microscopy nanomechanical probe. Using this transfer function, the probe can be operated in the high optical power, strongly thermo-optically nonlinear regime, reaching a sensitivity of $\approx$ 0.4 fm/Hz$^{1/2}$, an improvement of $\approx 10\times$ relative to the best performance in the linear regime. Counterintuitively, we discover that higher transduction gain and sensitivity are obtained with lower quality factor optical modes for low signal frequencies. Not limited to optomechanical transducers, the derived transfer function is generally valid for describing small-signal dynamic response of a broad range of technologically important photonic sensors subject to the thermo-optical effect.