Terahertz radiation from differently doped n- and p-type InAs crystal surfaces was investigated by time-resolved measurement. Large increase of the emitted terahertz power has been observed for p-InAs samples with the p-doping levels of approximately 1016–1017cm−3. This increase was explained by a large surface depletion layer and an electric-field-induced optical rectification effect in this layer.

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Quality control and non-destructive monitoring are of notable interest of food and pharmaceutical industries. It relies on the ability of non-invasive inspection which can be employed for manufacturing process control. We hereby apply terahertz (THz) time-domain spectroscopy as non-destructive technique to monitor pure and degraded oils as well as hydrocarbon chemicals. Significant differences in the spectra of refractive index (RI) and absorption coefficient arising from the presence of ester linkages in the edible and technical oils were obtained. Explicit increase from 1.38 to 1.5 of the RI in all THz spectrum range was observed in hydrocarbons and mono-functional esters with the increase of molar mass. This fact is in contrast of RI dependence on molar mass in multi-functional esters, such as Adipate or vegetable oils, where it is around 1.54. Degradation products, Oleic Acid (OA) and water in particular, lead only to some changes in absorption coefficient and RI spectra of vegetable oils. We demonstrate that complex colloidal and supramolecular processes, such as dynamics of inverse micelles and oil hydrolysis, take part during oil degradation and are responsible for non-uniform dependence of optical properties on extent of degradation.

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Temperature-dependent effective mass in AlGaN/GaN heterostructures was experimentally observed via THz time domain spectroscopy of 2D plasmons in the range of 80–300 K. Grating couplers with different periods and filling factors were developed in order to monitor the behavior of plasma resonances in transmission spectra in the frequency range of 0.5–3.5 THz. For the grating with a 50% filling factor, the fundamental modes were excited and observed at temperatures below 225 K. The change of the filling factor to 80% led to the excitation of the fundamental and second order plasma harmonics observable up to 300 K and 220 K, respectively. Moreover, with an increase in temperature, the 2D plasmons experienced the red-shift in transmission power and phase spectra of all samples. This phenomenon was explained by the renormalization of effective mass, which started distinctly to deviate at 134 K temperature and at 295 K increased up to 55% of its nominal value. The THz spectroscopy of 2D plasmons further confirms a temperature-dependent effective mass in AlGaN/GaN heterostructures as reported previously in optical Hall effect studies.