Applied Physics Seminar: "New Techniques for Laser Cooling of Solids"

In some environments, like outer space, keeping your cool is difficult because convection cannot take place and the only mechanism for heat transfer is through radiation. Since the radiation of heat into vacuum is a slow process, it is hard to improve the operation of electronic devices like solid-state cameras by lowering their temperature to eliminate sources of thermal noise. For example thermo-electric coolers do not work well in space. They are limited to a lowest attainable temperature in the neighborhood of 150 K. So there is great interest in the possibility of laser cooling of solids as an alternative. Unfortunately the Doppler effect which makes it possible to cool vapors to cryogenic temperatures is absent in solids. Thus a new challenge has emerged.
To date laser cooling of solids has explored a single method called anti-Stokes fluorescence and has encountered a fundamental limit imposed by impurity absorption. Although experimental efforts have successfully cooled rare earth solids to liquid nitrogen temperature (77K), the cooling rate by anti-Stokes fluorescence (ASF) is proportional to phonon occupation probability. The net cooling rate therefore falls with temperature and drops to zero when the cooling rate equals the rate of heating from unavoidable impurity absorption. Thus there is a minimum attainable temperature at which refrigeration stops. In this talk single mode cooling in opto-mechanical systems and optical refrigeration by ASF will be discussed, together with a novel method called Raman laser cooling that offers the prospect of steady cooling from room temperature to the realm of liquid helium in solids, overcoming the limitations of existing cooling methods and opening the door to applications in optically-cooled focal plane arrays and radiatively-balanced high power lasers.