Spectral modifications of femtosecond laser pulses induced by phase-matched optical rectification in LiNbO3

In this paper, we report experimental observations of the spectral modifications of ultrashort laser pulses caused by high-efficiency optical rectification. Moreover, we assert that the observed spectral modifications can be used to evaluate the absolute energy of generated THz pulses without calibrated THz detector. Abilities to obtain more than 100% optical-to-THz quantum conversion efficiency by phase-matched optical rectification are discussed. THz radiation in our experiment was generated in a LiNbO₃ crystal using a tilted pulse front phase-matching.