Spectral Properties and Tunability of Far-Infrared Difference-Frequency Radiation Produced by Picosecond Laser Pulses

The spectra of far-infrared radiation from ZnTe and LiNbO 3 crystals produced by the optical difference-frequency mixing of mode-locked Nd: glass laser pulses have been measured in the frequency range of 8∼30 cm -1 by using a Fabry-Perot interferometer. Under the conditions where the collinear phase-matching does not occur at any finite difference frequencies, the spectra showed broad-band character with almost the same shape for ZnTe and LiNbO 3 , and with the band-width of about 25 cm -1 which is of the same order as the spectral width of the incident laser. Measurements were made also under the phase-matched condition in LiNbO 3 . For the two particular crystal orientations, sharp spectra centered at 19.2 cm -1 and 15.5 cm -1 have been obtained. This demonstrated the tunable far-infrared generation by means of crystal rotation. These results were interpreted on the basis of the theory assuming collinear propagation of plane waves.