Observation of optomechanical coupling in crystalline whispering gallery mode resonators

In this paper, the authors report on the first experimental detection of optomechanical interactions in a crystalline CaF₂ whispering gallery mode (WGM) resonator, possessing both, high optical and mechanical quality factor. Using tapered optical fibers, coupling close to the condition of critical coupling is achieved. In these first measurements, the highest optical quality factor was 10⁸. In order to reduce the influence of clamping losses and gas friction, the resonator was mounted with a special suspension in a low pressure environment. To detect the mechanical modes´ Brownian motion, a Ti:Sapphire laser was locked to the side of an optical resonance´s fringe. By spectral analysis of the transmitted power, more than ten mechanical modes were observed with frequencies ranging from about 500 kHz to 2 MHz and quality factors of up to 136,000 at low pressure. As an example, the calibrated measurement of a mechanical mode with an eigenfrequency of 856.1 kHz and a quality factor of 24,700 was shown.