Quaternary GaInAsBi alloy epitaxial layers were grown on InP substrates with 6% Bi. It was found that the thick layers remain fully strained. The measured carrier lifetimes were of the order of a few picoseconds. The terahertz (THz) emission was investigated using a GaInAsBi layer as an unbiased surface emitter and as a substrate for photoconductive antenna. It was observed that fabricated THz emitters were sensitive to the optical pulses with wavelengths longer than 2 μm. The demonstrated spectral characteristics of THz pulses obtained when using an Er-doped fiber laser for photoexcitation were comparable with those observed in other emitters used for THz-time-domain spectroscopy systems.

Surfaces of semiconducting materials excited by femtosecond laser pulses emit electromagnetic waves in the terahertz (THz) frequency range, which by definition is the 0.1–10 THz region. The nature of terahertz radiation pulses is, in the majority of cases, explained by the appearance of ultrafast photocurrents. THz pulse duration is comparable with the photocarrier momentum relaxation time, thus such hot-carrier effects as the velocity overshoot, ballistic carrier motion, and optical carrier alignment must be taken into consideration when explaining experimental observations of terahertz emission. Novel commercially available tools such as optical parametric amplifiers that are capable of generating femtosecond optical pulses within a wide spectral range allow performing new unique experiments. By exciting semiconductor surfaces with various photon energies, it is possible to look into the ultrafast processes taking place at different electron energy levels of the investigated materials. The experimental technique known as the THz excitation spectroscopy (TES) can be used as a contactless method to study the band structure and investigate the ultrafast processes of various technologically important materials. A recent decade of investigations with the THz excitation spectroscopy method is reviewed in this article. TES experiments performed on the common bulk A3B5 compounds such as the wide-gap GaAs, and narrow-gap InAs and InSb, as well as Ge, Te, GaSe and other bulk semiconductors are reviewed. Finally, the results obtained by this non-contact technique on low-dimensional materials such as ultrathin mono-elemental Bi films, InAs, InGaAs, and GaAs nanowires are also presented.

Terahertz excitation spectroscopy was used for the determination of energy separation between the main (Γ) and subsidiary ( L and X ) conduction band valleys of GaAs _{1− x} Bi _x . The samples used in this study were 1 µ m–1.5 µ m thick bismide layers grown by Molecular Beam Epitaxy on GaAs substrates. They contained up to 8% of bismuth as determined by high resolution x-ray diffraction (HR-XRD) and reciprocal space mapping (RSM), taking into account the layer relaxation. It was found that both subsidiary conduction band valleys at L and X points of the Brillouin zone move away from the conduction band minimum at rates of 18 meV/%Bi and 25 meV/%Bi, respectively, with increasing Bi content in the alloy.

Pulsed terahertz emission excitation spectra from germanium crystals are being presented. The most intense terahertz pulses from germanium crystals are emitted at quanta energies coinciding with technologically significant telecommunication wavelengths. The terahertz generation mechanisms are an interplay of the photocurrent surge in the surface electric field and the photo-Dember effect. Remarkably, the terahertz emission is also observed at quanta energies below the direct bandgap of this material even when photoexcited at a surface normal. This is the result of a broken symmetry of effective electron mass in the L valleys.

Spectral dependences of the amplitudes of terahertz (THz) transients radiated from a GaSe surface after its excitation by femtosecond optical pulses with photon energies in the range from 1.8 eV to 3.8 eV were used for the study of electron energy band structure of this layered crystal. The energy separation of 0.21 eV between the main Γ valleys and the satellite K valleys in the conduction band was determined from the maximum position of THz excitation spectrum; the polarity of the THz transients became inverted at photon energies higher than 3 eV due to the onset of electron transitions from the second, lower lying valence band.

Characterizing subcells in two-terminal multi-junction (M-J) solar cells is challenging due to the lack of direct electrical access. This work presents a novel contactless spectral characterization technique for analysing individual subcells. The technique involves probing terahertz (THz) radiation generated by femtosecond laser pulse excitation and varying the exciting wavelength to selectively absorb light in the desired subcell. The registered THz pulse integral is then proportional to the induced photocurrent in that subcell. The THz photocurrent spectroscopy technique is demonstrated on GaAs and AlGaAs single-junction solar cells, as well as on the triple-junction AlGaAs/GaAs/GaAsBi solar cell. The results show that the recently developed GaAsBi-based subcell, with a nominal energy bandgap of 1.0 eV, exhibits improved electron–hole separation efficiency and can enhance energy harvesting by M-J solar cells.