Complex conductivity of the topological insulator (TI) Bi2 − xSbxTe3 − ySey samples of various thicknesses and chemical compositions is studied by terahertz time-domain spectroscopy method in the range 0.5 to 2.5 THz. For the first time, a decrease in conductivity in the terahertz range has been observed as the chemical composition approaches the Ren’s curve. The generalized approximate expressions are obtained for complex conductivity with account of the lowest Eu1-phonon mode. Calculations of the Fermi energy and concentration of bulk carriers are performed. Based on the experimental data, an estimate of conductance of the topological states is obtained. The results can be useful in developing terahertz devices based on the specific surface transport in TIs.
We have investigated the influence of indium content (x) increase on spectral characteristics of InxGa1-xAs photoconductor. To avoid the mismatch between crystalline parameters of InxGa1-xAs and GaAs wafer we proposed to incorporate a step-graded metamorphic buffer layer. We showed that x increase strongly enhances THz emission and broadens THz spectrum of InxGa1-xAs. Since no polarity rehearsal of the THz waveform occurs and electron diffusion mobility increases up to 90% with x increase we attribute the increase of THz intensity to photo-Dember effect contribution. The maximum efficiency of optical-to-THz conversion was obtained for In0.72Ga0.28 As at optical fluence ~0.01 μJ=cm2. The fabricated photoconductors can be used as promising photo-Dember or lateral photo-Dember THz emitters in pulsed THz spectroscopy and imaging, in particular, operating with long wave optical pump.