Observation of photothermal feedback in a stable dual-carrier optical spring

We report on the observation of photothermal feedback in a stable dual-carrier optical spring. The optical spring is realized in a 7 cm Fabry-Perot cavity comprised of a suspended 0.4 g small end mirror and a heavy input coupler, illuminated by two optical fields. The frequency, damping, and stability of the optical spring resonance can be tuned by adjusting the power and detuning of the two optical fields, allowing for a precise measurement of the absorption-induced photothermal feedback. The magnitude and frequency dependence of the observed photothermal effect are consistent with predicted corrections due to transverse thermal diffusion and coating structure. While the observed photothermal feedback tends to destabilize the optical spring, we also propose a small coating modification that would change the sign of the effect, making a single-carrier stable optical spring possible.