Laser cooling in the condensed phase by frequency up-conversion

Frequency up-conversion has been observed in fluid solutions of the laser dye, rhodamine 101, when excited at 1.96 eV into the low energy absorption tail 0.15 eV from the 0-0 band origin. The emission intensity is temperature dependent and is demonstrated to be an excellent measurement of the temperature of the sample. We show for the first time how intense laser irradiation in this spectral region leads to the removal of thermal energy by the frequency up-conversion process, and thus lowers the temperature of the sample. A decrease in temperature at a rate of 0.7 K h(-1) is demonstrated for a sample with a 3 J K-1 heat capacity, when irradiated with 350 mW of laser light at 1.96 eV.