Interaction of a tunable probe beam with a high-intensity pulsed CO2-laser beam inp-type germanium

Abstract

A theoretical study is presented of the interaction between a high-intensity pump beam with a wavelength of 10.6 ..mu..m and a low-intensity probe beam which is tunable in the (9--11)-..mu..m region. The dominant absorption mechanism of the optical beams in p-type Ge is direct intervalence-band transitions between states in the heavy- and light-hole bands. The hole states in resonance with the pump tend to saturate at sufficiently high intensities due to changes in the occupation probabilities of the initial and final states. When the material is simultaneously irradiated by the pump and probe beams, there is a nonlinear response which produces a component in the hole population difference between the heavy- and light-hole bands which oscillates at the beat frequency. This oscillating component in the population difference acts as a spatial and temporal grating which mixes the two beams. The absorption of the probe beam is modified by the presence of the pump beam by both the saturation of the intervalence-band transitions and by the oscillating population difference, which can scatter pump photons into the direction of the probe beam with the frequency shifted to that of the probe. Calculations of the probe absorption as a function of the frequencymore » difference between the probe and pump beams are presented. These calculations of the absorption spectrum are compared with experimental results and good agreement is found. In addition to modifying the probe absorption spectrum, the oscillating population difference leads to a current-density component which oscillates at the pump frequency plus the beat frequency. If the pump and probe beams are nearly phase matched, this current-density component can generate a new optical beam that oscillates at the pump frequency plus the beat frequency.« less