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.
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.
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.
Spectral dependence of terahertz emission is a sensitive tool to analyze the structure of conduction band of semiconductors. In this work, we investigate the excitation spectra of THz pulses emitted from MOCVD-grown InN and InGaN epitaxial layers with indium content of 16%, 68%, and 80%. In InN and indium-rich InGaN layers we observe a gradual saturation of THz emission efficiency with increasing photon energy. This is in stark contrast to other III-V semiconductors where an abrupt drop of THz efficiency occurs at certain photon energy due to inter-valley electron scattering. From these results, we set a lower limit of the intervalley energy separation in the conduction band of InN as 2.4 eV. In terms of THz emission efficiency, the largest optical-to-THz energy conversion rate was obtained in 75 nm thick In
0.16
Ga
0.84
N layer, while lower THz emission efficiency was observed from InN and indium-rich InGaN layers due to the screening of built-in field by a high-density electron gas in these materials.
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