Extended Data Fig. 9: Photothermally distorted cavity resonance scan.

Photothermal-induced broadening of the transmitted cavity resonance in water, collected under cavity-length tuning at two different pump wavelengths. The blueshift in pump wavelength shifts the resonance position to lower piezo ramp voltage, demonstrating that increased piezo ramp voltage corresponds to increased cavity length. Furthermore, the direction of the broadening is indicative of a negative thermo-optic coefficient of the medium, as is expected in water. Despite the low circulating power (5.5 mW), photothermal broadening was apparent and enabled the high sensitivity of this measurement. The smaller peaks originate from polarization splitting owing to the birefringence of the cavity mode. Data collected with cavity four.