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We report on the generation of tunable, ultrashort THz radiation between 2.0 and 5.3 THz by optical rectification of femtosecond laser pulse trains in the highly nonlinear crystal 4-dimethylamino-N-methyl-4-stilbazolium tosylate ( DAST) . Femtosecond pulse trains with a THz repetition rate are successfully synthesized from a single laser pulse using a zone plate which consists of a stack of thin glass plates. Using four glass slides we could experimentally demonstrate a substantial narrowing of the spectral bandwidth of the THz radiation. The spectral characterization of the THz pulses is accomplished using a Michelson interferometer. By simply tilting the zone plate the peak frequency is tuned from 2.0 to 5.3 THz. Even though higher and lower frequencies can be generated, the reported range is limited by the spectral characteristics of our detector. To our knowledge, our work demonstrates the first application of a zone plate for the generation of ultrashort THz pulses, tunable over a very large spectral range.

We studied the lifetimes and the luminescence spectra of photoexcited carriers in H bombarded InP and GaAs for different damage doses by means of femtosecond luminescence spectroscopy. For InP the lifetime decreases down to 95 fs for the highest dose, whereas for GaAs no shorter lifetime than 650 fs could be observed. With decreasing lifetime we observe an increase of the high energy tail of the timeintegrated luminescence spectrum which is even inverted for the 95 fs InP sample.inverted -the luminescence intensity is higher at higher energies. With estimations for the hole distribution it will be shown that the time average of the electron distribution is inverted. O-8194-0838-7/92/$4.OO SPIE Vol. 1677 (1992)! 9 Downloaded From: http://proceedings.spiedigitallibrary.org/ on 06/15/2016 Terms of Use: http://spiedigitallibrary.org/ss/TermsOfUse.aspx

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