Effects of spectral cross relaxation and collisional dephasing on the absorption of light by organic-dye solutions

The nonlinear absorption of intense laser light is used to obtain information about the times required for spectral cross relaxation (T₃) and collisional dephasing (T₂) in organic-dye solutions. The steady-state transmission experiment of Huff and DeShazer indicates that T₂ is in the range 0.1-1 psec for cryptocyanine-methanol, while T₃ is in the range 1-10 psec. For DDI-glycerin (1, 1{'}-diethyl-2, 2{'}-dicarbocyanine-iodide-glycerin), measurements of ultrashort-pulse transmission as a function of incident energy and optical polarization suggest a value T₂=0.4±0.2 psec for that system. These results are based on a new set of rate equations for absorption by inhomogeneously broadened organic-dye solutions. The equations display the effects of spectral cross relaxation and are not limited to light intensities less than those at which coherent interaction effects become important. For the case of ultrashort incident pulses, the equations are extended to account for excited-state absorption, the dependence of absorption on optical polarization, and the existence of a multiplicity of interacting excited states.