Spectroscopy of thermally excited acoustic modes using three-mode opto-acoustic interactions in a thermally tuned Fabry-Pérot cavity

Three-mode opto-acoustic interactions can excite acoustic modes of the mirrors of an optical cavity. This was achieved when the frequency difference between the fundamental and higher order optical mode matches the frequency of appropriate acoustic mode of the mirror. The excitation also critically depends on the spatial overlap between acoustic and optical modes. In this Letter, we use a controlled CO2 laser to thermally change the radius of curvature of one mirror of an 80 m Fabry-Pérot cavity for three-mode interaction. Several acoustic modes of the cavity end mirror were observed with quality factors of ∼105-106 at the thermal noise level.