Wavelength dependence of the nonlinear absorption properties of laser dyes in solid and liquid solutions Original Research Article

Nonlinear transmission properties of laser dyes rhodamine 6G in solid and liquid solution and pyrromethene 557 and 556 in liquid solution in the ultraviolet and visible spectral regions have been studied using nanosecond laser pulses. The dyes behave as saturable absorbers when irradiated a 534 nm, but their transmittance exhibits no intensity dependent behavior or decreases with increasing laser intensity at 337 and 308 nm irradiation wavelengths. A five-level theoretical model incorporating excited-state absorption and stimulated emission was used to compute the dependence of transmittance with intensity in the studied solutions and to estimate the absorption cross sections of the first excited single state of the dye molecules. The different nonlinear behavior of the studied dyes depending on the irradiation wavelength can be understood in terms of the relative values of the absorption cross sections of the ground and first excited singlet state at the irradiation wavelength.