Efficient generation of 380 fs pulses of THz radiation by ultrafast laser pulse excitation of a biased metal-semiconductor interface

We have demonstrated a new method of generating pulses of freely propagating THz electromagnetic radiation. The resulting 380 fs pulses are the shortest directly measured THz pulses in free space to date and are more powerful than those generated by Hertzian dipoles or by resonant dipole antennas. Temporal features as short as 190 fs were observed on these THz radiation pulses and thereby, illustrate an ultrafast receiver response time.Peer reviewedElectrical and Computer Engineerin

Line-shape transition of collision broadened lines

Using the newly developed technique of THz time-domain spectroscopy, we have measured the far-wing absorption line profile of the ensemble of collision broadened ground state rotational lines of methylchloride vapor out to more than 200 linewidths from resonance, corresponding to frequency offsets as much as 5X the resonant frequency. On these far wings the measured absorption is approximately an order of magnitude less than that predicted by the van Vleck-Weisskopf theory. Our observations show that at higher frequencies a transition occurs from the regime of the van Vleck- Weisskopf theory to the regime of the Lorentz theory.Peer reviewedElectrical and Computer Engineerin

Carrier dynamics of electrons and holes in moderately doped silicon

A time-domain spectroscopic technique, based on the generation and detection of a collimated beam of subpicosecond broadband terahertz pulses, is used to measure the absorption and dispersion of n- and p-type silicon, with resistivities of 0.1, 1, and 10 Ohm-cm in the submillimeter range of 0.1-2 THz. From the transmission measurements performed at room temperature and at 80 K, the absorption and dispersion, and concomitantly the full complex conductivity, of the doped silicon could be obtained. The results provide an accurate view on the dynamics of the electrons and the holes. Although the simple Drude model, with an energy-independent relaxation time, gives a surprisingly accurate description of the observed carrier dynamics, the measurements do show that some refinements are needed. An extended model, with an energy-dependent carrier-relaxation rate, can explain most of the observed deviations from the simple Drude model.Peer reviewedElectrical and Computer Engineerin

Far-infrared time-domain spectroscopy with terahertz beams of dielectrics and semiconductors

Using the method of time-domain spectroscopy, we measure the far-infrared absorption and dispersion from 0.2 to 2 THz of the crystalline dielectrics sapphire and quartz, fused silica, and the semiconductors silicon, gallium, arsenide, and germanium. For sapphire and quartz, the measured absorptions are consisted with the earlier work below 0.5 THz. Above 1 THz we measure significantly more absorption for sapphire, while for quartz our values are consistent with those on the most transpired fused silica measured to date. For the semiconductors, we show that many of the previous measurements on silicon were dominated by the effects of carriers due to impurities. For high-resistivity, 10-kOhm-cm silicon, we measure a remarkable transparency together with an exceptionally nondispersive index of refraction. For GaAs our measurements extend the precision of the previous work, and we resolve two weak absorption features at 0.4 and 0.7 THz. Our measurements on germanium demonstrate the dominant role of intrinsic carriers; the measured absorption and dispersion are well fitted by the simple Drude theory.Peer reviewedElectrical and Computer Engineerin

Point source terahertz optics

We demonstrate an ultrafast 10 micro-m sized, electric dipole source of terahertz radiation closely coupled to a 1 cm spherical mirror. This optical approach has the advantages of subpicosecond response times with essentially complete collection efficiency. Using this technique, we have generated and detected subpicosecond freely propagating electrical pulses.Peer reviewedElectrical and Computer Engineerin

High-brightness terahertz beams characterized with an ultrafast detector

We have significantly improved the emission and detection of electromagnetic beams of single-cycle 0.5 THz pulses, through the use of new dipolar antenna structures. The frequency response was extended to well beyond the 1THz, and the beam power was increased by more than 15 times. The antennas were located at the foci of sapphire lenses and were photoconductively driven by ultrafast laser pulses. An additional collimation by a paraboloidal mirror produced a beam with a 25 mrad divergence, and a subsequent focusing by a second identical mirror improved the coupling between the transmitting and receiving antenna by orders of magnitude.Peer reviewedElectrical and Computer Engineerin

Observation of electromagnetic shock waves from propagating surface-dipole distributions

We have directly measured the time-dependent electromagnetic shock wave generated by an electric surface-dipole distribution propagating faster than the phase velocity in the underlying dielectric substrate. The results are in good agreement with a theory describing the moving volume-dipole distribution with the inclusion of an equal strength surface reflection.Peer reviewedElectrical and Computer Engineerin