Optical refrigerator using reflectivity tuned dielectric mirrors

An apparatus for cooling an object, which comprises in combination:

• (a) means for generating substantially monochromatic light having a chosen wavelength;
• (b) a working material having atoms with energy levels such that at most a small number of energy levels in an excited state thereof are directly excited by the chosen wavelength of light from the ground energy level of the atoms, such that the energy levels in the excited state which are not directly excited have an energy spacing whereby they thermally redistribute, and such that fluorescence to the ground state can preferentially occur from the thermally redistributed energy levels, whereby the average fluorescence wavelength to the ground state level is shorter than the chosen wavelength of light, said working material further having at least one first dielectric-coated, substantially flat face and a second dielectric-coated, substantially flat face, the second dielectric-coated face being spaced apart from and substantially parallel to one first face of the at least one first faces, wherein the distance between the at least one first face and the second face parallel thereto is chosen such that the optical depth for the fluorescence therebetween is small, and wherein the dielectric coating on the at least one first face is highly reflective for the chosen wavelength of light while having significant transmission for the shorter fluorescence wavelengths and high reflectivity for the longer wavelengths thereof, while the dielectric coating on the second face is highly reflective at wavelengths which include that of the chosen wavelength of light and the wavelengths of the fluorescence;
• (c) means for directing the light into said working material through an uncoated portion of the one first face of the at least one first faces which is substantially parallel to the second face such that the optical pathlength of the light within said working material between the one first face of the at least one first faces and the second face is large as a result of multiple reflections of the light between the dielectric coatings of the one first face of the at least one first faces and the second face;
• and
• (d) means for bringing the object to be cooled into thermal contact with the dielectric coating on the second face of said working material.