Intense tunable ir generation by four-photon mixing in germanium

We have considered the method of noncollinearly phase-matched four-photon mixing of CO2 laser beams in germanium at room temperature for the generation of intense tunable or quasitunable pulses of infrared radiation. Our theoretical analysis shows that it should be possible to obtained output intensities of multi-MW/cm2 in the 8-μm region with conversion efficiencies as high as 37%. In the initial experiments reported here, we have observed as much as 33 mJ of energy per pulse (corresponding to a peak pulse intensity of ∼0.3 MW/cm2) at 8.6 μm from a 10-cm-long antireflection(AR) -coated crystal using CO2 laser beams of ∼10-MW/cm2 intensity. This corresponds to a conversion efficiency of ∼1%. We have also examined several other practical aspects of this four-photon mixing technique. Use of single-mode CO2 lasers should provide a step-tunable ir radiation with linewidth much less than 0.01 cm−1. A simple analysis of the heat dissipation from the germanium crystal into a copper block indicates that the temperature rise is only a few degrees even at pulse repetition rates of several hundred Hz. Thus, the noncollinear four-photon mixing scheme should find important applications in laser photochemistry.