GaSe and InSe are important members of a class of 2D materials, the III-VI metal monochalcogenides, which are attracting considerable attention due to their promising electronic and optoelectronic properties. Here an investigation of point and extended atomic defects formed in mono-, bi-, and few-layer GaSe and InSe crystals is presented. Using state-of-the-art scanning transmission electron microscopy (STEM), it is observed that these materials can form both metal and selenium vacancies under the action of the electron beam. Selenium vacancies are observed to be healable; recovering the perfect lattice structure in the presence of selenium or enabling incorporation of dopant atoms in the presence of impurities. Under prolonged imaging, multiple point defects are observed to coalesce to form extended defect structures, with GaSe generally developing trigonal defects and InSe primarily forming line defects. These insights into atomic behavior could be harnessed to synthesize and tune the properties of 2D post transition metal monochalcogenide materials for optoelectronic applications.

Optical properties of 4H-SiC were measured using time-domain and Fourier transform spectroscopy in the range of 0.1-20 THz. A high-transparency region was found between <0.1-10 THz. Based on the obtained data and published results, the refractive indices for o-wave and e-wave were approximated in the form of Sellmeier equations for the entire transparency range. Phase matched frequency conversion was found to be possible at wavelengths from the visible through the mid-IR and further into the far-IR (THz) region beyond 17 μm. Extremely low absorption coefficient, high damage threshold, and the possibility of phase matching make this material highly suited for high power THz optics and generation.

The article presents a source producing high-power ultrawideband electromagnetic pulses. The source includes a generator of monopolar pulses, a bipolar pulse former, and a combined ultrawideband transmitting antenna. Monopolar 150-kV, 4.5-ns pulses are transformed into bipolar 120-kV, 1-ns pulses, which are emitted by the antenna. The pulse repetition rate of the setup is up to 100 Hz. The peak power of the source is 170 MW as measured with a TEM-type receiving antenna having 0.2–2 GHz passband. The pattern width of the transmitting antenna at a half-level of peak power is 90° and 105° for the H- and E-planes, respectively. The electric field strength measured 4 m from the transmitting antenna in the direction of the main radiation maximum is 34 kV/m.

High nonlinearity, wide transparency range and optical quality allowed potassium titanyl phosphate (KTiOPO4, KTP) crystals to be used in a wide range of nonlinear applications. The success of KTP usage in the visible and infrared (IR) ranges drives interest in applying it at longer wavelengths, that is, in the terahertz (THz) range. We use THz optical properties of KTP crystals measured by terahertz time-domain spectrometer (THz-TDS) and refractive index approximated in the form of Sellmeier equations to investigate KTP application possibilities for IR-to-THz and THz-to-THz wave conversion. As a result, phase matching for s − f → f and s − f → s types of difference frequency generation (DFG) of Ti:Sapphire laser (at the wavelengths of 0.65, 0.8 and 1.1 µm) is found possible, as well as second harmonic generation (SHG) of THz waves by f + s→f type of interaction in the XZ principle plane of the crystal. Terahertz wave generation by phase-matched parametric processes in KTP demonstrates evident advantages in comparison with that of widely used MgO-doped LiNbO3 crystals.