Spectral modification of femtosecond laser pulses in the process of highly efficient generation of terahertz radiation via optical rectification

The narrowing and “red” shift of the spectrum of a femtosecond laser pulse due to the high-efficiency transformation to terahertz radiation by means of optical rectification in a LiNbO₃ crystal are experimentally observed. These changes are in good agreement with the description of the appearance of the terahertz radiation as the difference-frequency generation and make it possible to determine the energy of terahertz pulses. A photon-number transformation coefficient larger than 50% is obtained.