The carrier lifetimes and electron mobility values were estimated for 2 μm thick GaAs films grown on Si (100) substrates by means of optical pump terahertz probe (OPTP) technique. The GaAs/Si films measured were epitaxial grown at different substrate temperatures (T S =520 °C or T S =630 °C). From x-ray diffraction measurements and Raman spectroscopy, the GaAs/Si films were shown to experience minimum strain at room temperature, and crystal misorientation in the (111) or (110) direction. With no measureable photoluminescence at room temperature, carrier lifetimes were measured via OPTP and found to be ∼20 and ∼35 ps for a fluence of ∼4 μJ cm −2 , which is in the same order of magnitude as a reference bulk GaAs grown on SI-GaAs (T S =630 °C) having a lifetime of ∼70 ps. From OPTP photoconductivity measurements, the estimated GaAs/Si films' electron mobility are ∼2900 cm 2 V −1 s −1 (T S =520 °C) and ∼3500 cm 2 V −1 s −1 (T S =630 °C) at a pump-probe delay time of Δt=50 ps, in which the bulk GaAs electron mobility is ∼5200 cm 2 V −1 s −1 .

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A comparative experimental study of terahertz generation by optical rectification of femtosecond laser pulses with a wavelength of 1560 nm in a 100 μm thick slab of GaAs sandwiched between the walls of a tapered metallic waveguide and in a bulk GaAs crystal has been performed. In both cases, the velocity mismatch between the optical and terahertz beams propagating in GaAs was overcome by using the Cherenkov phase-matching method. The dependence of the terahertz time-domain signal amplitude, spectral bandwidth, and dynamic range on the focusing conditions were measured and compared. For both samples, the most efficient generation was observed for the 50 mm focal length lens. A significant improvement in the directionality of the terahertz emission from the thin GaAs slab due to the tapered waveguide was predicted by FDTD simulations and this was also confirmed experimentally.

We present the use of a "double optical pump" technique in terahertz time-domain emission spectroscopy as an alternative method to investigate the lifetime of photo-excited carriers in semiconductors. Compared to the commonly employed optical pump-probe transient photo-reflectance, this non-contact and room temperature characterization technique allows relative ease in achieving optical alignment. The technique was implemented to evaluate the carrier lifetime in low temperature-grown gallium arsenide (LT-GaAs). The carrier lifetime values deduced from "double optical pump" THz emission decay curves show good agreement with data obtained from standard transient photo-reflectance measurements on the same LT-GaAs samples grown at 250 °C and 310 °C.

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