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Parametric coupling of optical and mechanical degrees of freedom forms thebasis of many ultra-sensitive measurements of both force and mechanicaldisplacement. An optical cavity with a mechanically compliant boundary enhancesthe optomechanical interaction, but also gives rise to qualitatively newcoupled dynamics. As early as 1967, in a pioneering work, V. Braginsky analyzedtheoretically the role of radiation pressure in the interferometric measurementprocess, but it has remained experimentally unexplored for many decades. Here,we use whispering-gallery-mode (WGM) optical microresonators to study theseradiation pressure phenomena. Optical microresonators simultaneously hostoptical and mechanical modes, which are systematically analyzed and optimizedto feature ultra-low mechanical dissipation, photon storage times exceeding themechanical oscillation period (i.e. the "resolved-sideband regime") and largeoptomechanical coupling. It is demonstrated that dynamical backaction can beemployed to cool mechanical modes, i.e., to reduce their thermally excitedrandom motion. Utilizing this novel technique together with cryogenicpre-cooling of the mechanical oscillator, the phonon occupation of mechanicalradial-breathing modes could be reduced to about 60 excitation quanta. Thecorresponding displacement fluctuations are monitored interferometrically witha sensitivity at the level of 10^-18 m/Hz^(1/2), which is below the imprecisionat the standard quantum limit (SQL). It is shown that optical measurementtechniques employed here are operating in a near-ideal manner according to theprinciples of quantum measurement, displaying a backaction-imprecision productclose to the quantum limit. An outlook on optomechanical experiments withcrystalline WGM resonators and the effect of "optomechanically inducedtransparency" (analogous to electromagnetically induced transparency) is given.

Cavity optomechanics with whispering-gallery-mode optical micro-resonators