Circulating cavity photons
Movable mirror (mechanical mode)
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A laser drives a Fabry-Pérot cavity whose second mirror is mounted on a spring, free to move under radiation pressure from the circulating light field. Because the intracavity field takes a finite time (set by the cavity linewidth κ) to respond to the mirror's motion, the radiation-pressure force lags the mirror's displacement — and that time lag either damps or amplifies the mirror's mechanical vibration depending on whether the laser is red- or blue-detuned from the cavity resonance. This dynamical-backaction effect, together with the accompanying optical-spring frequency shift, is the mechanism used to laser-cool micro- and nano-mechanical oscillators toward their quantum ground state, and (in its amplifying regime) to drive optomechanical self-oscillation. Adjust the detuning, drive power and coupling strength and give the mirror a kick to watch the vibration ring down under cooling or grow under heating in real time.