Light-Pressure Cooling of a Crystal

We investigate laser cooling of phonons in a low-density crystal of ions. If the linewidth of the optical transition covers the range of phonon frequencies, the rate of decrease of mechanical energy per one ion in the crystal is found to be essentially the same as in an ideal gas of two-level systems, but the cooling is concentrated on a subset of the phonon modes. A broad phonon spectrum tends to counteract the cooling.