Self-sustained photothermal oscillations in high-finesse Fabry-Perot microcavities

We report the experimental investigation of a regime of microscopic Fabry-Perot resonators in which competing light-induced forces---photothermal expansion and photothermal refraction---acting oppositely and on different timescales lead to self-sustained persistent oscillations. Previously concealed as ordinary thermo-optic bistability---a common feature in low-loss resonator physics---these dynamics are visible as fast pulsations in cavity transmission or reflection measurements at sufficiently high time resolution. Their underlying mathematical description is shared by many slow-fast phenomena in chemistry, biology, and neuroscience. Our observations are relevant in particular to microcavity applications in atom optics and cavity quantum electrodynamics, even in nominally rigid structures that have not undergone lithography.