Second Optical Harmonic Generation in the Interaction of Femtosecond Laser and Picosecond Terahertz Pulses in Sapphire

The generation of the second harmonic of laser radiation is studied in the nonlinear interaction of femtosecond infrared and picosecond terahertz pulses propagating collinearly in a sapphire crystal plate. It has been experimentally discovered that the second harmonic is generated not only when the laser and terahertz pulses overlap near the material boundaries, as was previously observed in a fused silica plate but also in the case of their temporary overlap in the volume of the material during counter propagation. The discovered effect is explained by the fact that the length of the divergence of the laser and terahertz pulses turns out to be comparable to the coherence length of the first and second harmonics of laser radiation in a sapphire crystal. Theoretical calculations are carried out, fully confirming the experimental results.