THz beam shaping via a single diffractive optical element is used to convert a divergent beam into a focal line segment perpendicular to the optical axis. The novel structure was designed for narrowband applications as a kinoform element and we successfully applied it in active, high-speed, THz linear scanners. The theoretical approach and experimental results are presented.

Physical mechanism responsible for hydrogen sensing by AlGaN/GaN transistors was investigated. Original resistance studies versus magnetic fields (up to 8T) were used to determine precisely hydrogen induced change of carrier mobility and density. Results clearly show that the carrier mobility change is responsible for less than five percent and that the hydrogen sensitivity is mainly related (more than 95 percent) to the carrier density change. These results support the physical model that relates hydrogen sensing with a change of surface charges resulting in modification of the GaN depletion layer.

We report on room temperature electrically-induced terahertz emission from interdigitated GaN quantum well structures. The emission spectrum has been analysed in a Michelson interferometer using a 4K-Si bolometer as a terahertz detector. A resonant peak at the frequency of around 3 THz was observed in emission spectra. A threshold behaviour of the resonance with respect to applied voltage takes place. By using the proposed analytical model the measured/observed experimentally resonant behaviour of emission spectra is interpreted as a result of ungated stream-plasma instability in the channel.

We report on sub-Terahertz photoconductivity of bulk HgxCd1-xTe crystals withcomposition close to semiconductor-to-semimetal topological transition. Low magnetic fieldShubnikov-de Hass like oscillations were observed with a period which does not depend onthe incident frequency. Moreover, anomalous B-periodic oscillations were also observed athigher magnetic fields.

Terahertz power dependence of the photoresponse of field effect transistors, operating at frequencies from 0.1 to 3 THz for incident radiation power density up to 100 kW/cm2 was studied InGaAs high electron mobility transistors. The observed signal saturation behavior is explained by analogy with current saturation in standard direct currents output characteristics. The theoretical model of terahertz field effect transistor photoresponse was developed shows a good description match with experimental data. Our experimental results show that dynamic range of field effect transistors based terahertz detectors is very high and can extend from mW/cm2 up to kW/cm2.

The channel of the field effect transistor can operate as a cavity for plasma waves. For the electrons with high enough mobility, the plasma waves can propagate in field effect transistors (FETs) channel leading to resonant Terahertz (THz) emission or detection. In the low mobility case, plasma oscillations are overdamped, but a plasma density perturbation can be induced by incoming THz radiation. This perturbation can lead to efficient broadband THz detection. We present an overview of the main experimental results concerning the plasma oscillations in field effect transistors for the generation and the detection of terahertz radiations.