Knowledge Bank

The powerful new technique of THz time-domain spectroscopy $(THz-TDS)1$ is based on the optoelectronic generation and measurement of freely-propagating, subpsec pulses of THz (far-infrared) electromagnetic $radiation.2$ The unique THz beam system can generate and detect THz pulses with a time-resolution better than 65 fsec and signal-to-noise ratio of more than 3000. The THz pulses consist of a single oscillation and have a phase-coherent spectrum extending from 0.1 to more than 6 THz. With the THz-TDS technique two electromagnetic pulse shapes are measured, the input (reference) pulse and the propaged pulse, which has changed shape due to its passage through the sample under study. Consequently, via Fourier analyses of the input and propagated pulses, the frequency dependent absorption and dispersion of the sample can be obtained. Within spectral range of THz-TDS the precision of the measurements can excede that of comparable measurements made by Fourier transform spectroscopy (FTS). These two techniques are closely related. Both THz-TDS and FTS are based on scanning interferometric delay lines, and the resulting data are comparable objects. Both allow for spectroscopic determinations, using the sample-in, sample-out sequence to generated the reference and signal scans, which are subsequently Fourier analyzed. The two techniques will be compared in detail. A series of THz-TDS measurements on water vapor, nitrous oxide, and methyl chloride will be described. Recent $measurements3$ showing a line-shape transition on the far wings of collision broadened lines will be discussed. The observations extending out to frequency offsets as much as 5x the resonant frequency show that a transition occurs from the regime of the van Vleck-Weisskopf theory to the regime of the Lorentz theory. New measurements will be presented providing the first comprehensive absorption spectra of flames. We have recently extended to the THz frequency range the experimental and theoretical study of coherent transient effects from resonant $systems.4$ The propagation of a THz excitation pulse through nitrous oxide or methyl chloride vapors simultaneously excites a manifold of rotational transitions, thereby causing the molecules to radiate a free-induction signal consisting of a series of uniformly spaced subpsec THz pulses. These pulses carry almost the complete information about the molecular system. Such observations will be described in detail.