Red edge photophysics of ethanolic rhodamine 101 and the observation of laser cooling in the condensed phase

Laser cooling in fluid solutions of the laser dye rhodamine 101 in acidified ethanol has been observed when the molecule is excited into the low-energy tail of the absorption spectrum, at energies in the region 585-1593 cm^-1 lower in energy than the 0-0 band origin at 590 nm. The most efficient cooling is achieved when the laser is tuned to 615 nm when a net cooling power of 0.37 mW per 100 mW of pumping power is observed. In this paper, details leading up to this unique observation are documented: in particular, the quantum efficiencies in highly concentrated solutions and the characterization of anti-Stokes fluorescence as a nonintrusive temperature sensor.