One, two and three photon absorption of two level system in femto-second laser excitation

摘要

Theoretically, we have studied the multi-photon absorption by rate equation approach in the presence of femto-second laser excitation. The time dependent and saturation effects in the nonlinear transmittance are incorporated as they will occur in the absorption process. At low intensity excitation, the transmitted pulse shape has the same shape as incident pulse, but at excitation intensities above the saturation intensity, the shape of the transmitted pulse has got change according to the absorption coefficient variation with intensity. Transmittance in the two and three photon absorption has discussed comparatively in both single beam transmittance and z-scan. In the nonlinear absorption study under femto-second laser excitation, the high peak power of femto-second laser can saturate the absorption in multi-photon absorption even though the process is through virtual levels. We have shown that, at excitation intensities above the saturation intensity, how the absorption coefficient deviates the logarithmic plot of transmittance from its straight line nature.