T-RAY SPECTROSCOPY AT NIST. A. G. Markelz and E. J. Heilweil, Physics Laboratory, Building 221, Room B58, NIST, Gaithersburg, MD, USA. (301-975-2358, email: andrea.markelz@nist.gov)

We have recently completed the devlopment of a pulsed terahertz system, (T-ray), at NIST. The system generates short pulse, (~ 1 ps) broadband far-infrared light (10 Ghz - 2 Thz) enabling time resolved studies of semiconductors, superconductors and biomolecular systems as well as efficient measurements of optical constants in the terahertz range. The T-ray system consists of a DC biased hertzian dipole generating antenna with a photoconductive gap on semi_insulating GaAs or LTGaAs. The gap is illuminated with ~20 fs pulses at 796 nm from a mode locked Ti:Sapphire laser. A gated receiver antenna is used for direct mapping of the Thz pulse electric field . By measuring the transmitted field through a sample we can immediately determine the real and imaginary parts of the dielectric constant. The terahertz spectral content and integrated power output are measured as a function of temperature, carrier mobility and carrier lifetime. As the mobility and carrier lifetime of the generating antenna's substrate are continuously varied, a 3 fold increase in the integrated output power and shifting of the spectral output occur. These results facilitate optimization of the T-ray system for use in time_resolved spectroscopy.