GHz-THz ultrashort pulse generation and phase matching in crystals

High radiation in GHz-THz range can be obtained both by free electron lasers (FELs) and solid state emitters. The pulsed optical excited emitters are not powerful and efficient yet, however, by means of nonlinear frequency conversion, the greater part of GHz and THz radiation generation techniques were realized without phase matching. In these studies crystals with a high coefficient of nonlinear susceptibility were applied, their length being less than the length of coherence. Among inorganic nonlinear crystals, the semiconductor crystals GaAs, InTe, ZnTe and GaP, possess a high nonlinear coefficient d/sub 14/ of the order 100 pm/V or more, and among organic crystals, DAST and DANPH have the same property. Since the semiconductor crystals are nonbirefringent, they cannot be phase matched by angle tuning. We have demonstrated an efficient ultra-short pulse generation within the range of 4-46 mm in LiNbO/sub 3/ and LiTaO/sub 3/ crystals by a new phase matching method. Choosing the crystal filling degree of a rectangular waveguide provides phase matching of the nonlinear polarization wave and difference frequency radiation (DFR). In this case the crystal acts both as a nonlinear frequency converter and a millimeter wave phase velocity moderator. This technique allows us to remove restrictions on the choice of the nonlinear material; so, the material may possess small birefringence or none at all - such as, for example, the cubic symmetry crystals. Here we present the preliminary results of a collinear phase matching study of nonlinear polarization wave with difference frequency in the millimetre and submillimeter (GHz-THz) range at optical rectification in GaAs, LiNbO/sub 3/ and DAST crystals.