A thermoelastic limit on the focal intensity in Fabry-Pérot cavities

Light in the mode of a Fabry-Pérot cavity heats the mirror surfaces via optical absorption, causing thermoelastic deformation of the mirror substrates, which in turn dictates the shape of the mode. We develop an analytical model which predicts that this effect limits the maximum focal intensity of the mode. Using two near-concentric Fabry-Pérot cavities -- one with 4.5-fold higher mirror absorption than the other -- we measure the thermoelastic properties of the cavity mirrors and demonstrate that it is possible to achieve at least 70% of this predicted limit (in the high-absorption cavity), and that the predicted limit is 2.9 TW/cm^2 (in the low-absorption cavity).