Time-domain terahertz spectroscopy
Coherent terahertz (THz) electromagnetic pulses can be produced by illuminating biased photo-conductors such as GaAs with femtosecond laser pulses. These broadband THz pulses can be used to probe fast electron dynamics in materials and nanostructures by measuring the complex conductivity as a function of time. Traditionally THz photo-conductor emitters consisted of a single photo-conductive gap between two electrodes. For high-frequency emission smaller gap sizes are needed and a Si lens is employed to efficiently couple. However, for high field THz emission larger gap sizes are needed to avoid saturation effects. In 2005 inter-digitated photo-conductive emitters were introduced that consisted of arrays of photo-conductive gaps. This allowed both high-frequency and high-power operation to be achieved in a single device. We use inter-digitated emitters with a 1.5mm gap spacing that were fabricated at UC Santa Barbara.
In addition to photo-conductive THz emitters, other femtosecond activated THz emitters exist such as electro-optic crystals. Recently quantum cascade lasers have been phase-locked to femtosecond lasers. This allows coherent detection of the QCL emission and presents the possibility of using THz quantum cascade as bright narrow-band sources for time-domain spectroscopy. In addition the gain bandwidth of QCLs can be very large (>1 THz), and terahertz QCLs can be mode-locked to form short pulses.
Left: Terahertz electro-magnetic pulse produced by a 1.5mm inter-digitated photoconductive emitter.
Right: Spectral amplitude of the THz electro-magnetic pulse.