Influence of pulsed and cw pumping on optical nonlinear parameters of laser dyes probed by a closed-aperture Z-scan technique

Abstract

Two dye pairs suitable for energy transfer along with a photostable dye have been studied by using the closed-aperture (CA) Z-scan technique under pulsed and cw pumping. Here we use a theoretical model to elucidate the refractive and absorptive nonlinearity present simultaneously in the CA Z-scan profile. A separate open-aperture (OA) Z-scan study has been carried out to compare the nonlinear absorption parameters obtained from the CA Z-scan technique. At a fixed pumping wavelength, values of optical nonlinear parameters increase with the absorbance of dyes. It is found that the sign of refractive nonlinearity is dependent on the irradiance and the pulse width of the pump beam. In addition to the contribution of the third-order optical nonlinearity, various other mechanisms such as fifth-order nonlinearity, population relaxation to triplet states, and thermal effects are discussed here.