Spectral modifications of femtosecond laser pulses induced by phase-matched optical rectification in lithium niobate

We report experimental observations of the spectral modifications of ultrashort laser pulses caused by the generation of terahertz (THz) radiation via phase-matched optical rectification in LiNbO₃. The measured red-shift and narrowing of the laser pulse spectra are in good agreement with the description of the THz generation in terms of difference frequency mixing. Moreover, we propose and demonstrate a new method for evaluating the absolute energy of the THz pulse generated via high-efficiency optical rectification. The method is based on analyzing the observed laser pulse spectral modifications. Abilities to obtain >100% optical-to-THz quantum conversion efficiency are discussed.