The optical properties of $\mathrm{Ga}{\mathrm{Bi}}_{\mathrm{x}}{\mathrm{As}}_{1-x}(0.04<x<0.08)$ grown by molecular beam epitaxy have been studied by photomodulated reflectance spectroscopy. The alloys exhibit a strong reduction in the band gap as well as an increase in the spin-orbit splitting energy with increasing Bi concentration. These observations are explained by a valence band anticrossing model, which shows that a restructuring of the valence band occurs as the result of an anticrossing interaction between the extended states of the GaAs valence band and the resonant $T_2$ states of the Bi atoms.

GaBiAs layers have been grown by molecular beam epitaxy at low $(270–330°\mathrm{C})$ temperatures and were characterized by several experimental techniques. It was shown that the spectral photosensitivity cutoff wavelength reaches $\sim 1.4\mu \mathrm{m}$ when the growth temperature is as low as $280°\mathrm{C}$ . Optical pump–terahertz probe measurements made on these layers have evidenced that the electron trapping time decreases with decreasing growth temperature from 20 to about $1\mathrm{ps}$ . GaBiAs layers were used for manufacturing photoconductive terahertz emitters and detectors, which, when excited with Ti:sapphire laser pulses, have demonstrated a signal bandwidth of $3\mathrm{THz}$ .

Generation and measurement of ultrashort, subpicosecond pulses of electromagnetic radiation with their characteristic Fourier spectra that reach far into terahertz (THz) frequency range has recently become a versatile tool of far-infrared spectroscopy and imaging. This technique, THz time-domain spectroscopy, in addition to a femtosecond pulse laser, requires semiconductor components manufactured from materials with a short photoexcited carrier lifetime, high carrier mobility and large dark resistivity. Here we will review the most important developments in the field of investigation of such materials. The main characteristics of low-temperature-grown or ion-implanted GaAs and semiconducting compounds sensitive in the wavelength ranges around 1 µm and 1.5 µm will be surveyed. The second part of the paper is devoted to the effect of surface emission of THz transients from semiconductors illuminated by femtosecond laser pulses. The main physical mechanisms leading to this emission as well as their manifestation in various crystals will be described.

Nonstoichiometric GaAs obtained by implantation with 2 MeV arsenic ions at 10¹⁵ cm⁻² dose is studied. As‐implanted samples show a <200 fs lifetime of photocarriers and low resistivity due to hopping, with mobility less than 1 cm²/V s. Annealing of the samples at 600 °C leads to substantial recovery of postimplant damage, as seen from Rutherford backscattering channeling spectra and mobility increase to about 2000 cm²/V s, but photocarrier lifetime is still about 1 ps. These parameters are similar to those of low‐temperature GaAs annealed at 600 °C, and make arsenic implanted GaAs an interesting material for optoelectronic applications.

We describe a terahertz time-domain-spectroscopy system that is based on photoconductive components fabricated from low-temperature-grown GaBiAs epitaxial layers and activated by femtosecond $1.03\mu \text{m}$ pulses emitted by an Yb:fiber laser. Experiments performed with the laser pulses of different durations have evidenced that the spectral range of the system is limited by the photoexcited carrier lifetime. The optical to terahertz conversion efficiency of the system exceeds ${10}^{-4}$ with the usable spectral range of about 3 THz in the case of 170 fs excitation pulses.