The unique features of nanowires (NW), such as the high aspect ratio and extensive surface area, are expected to play a key role in the development of very efficient semiconductor surface emitters in the terahertz (THz) spectral range. Here, we report on optically excited THz emission from catalyst-free grown arrays of intrinsically n-type InAs NWs using THz time-domain spectroscopy. Depending on the aspect ratio, the THz emission efficiency of the n-type InAs NWs is found to be up to ∼3 times stronger than that of bulk p-type InAs, known as currently the most efficient semiconductor-based THz surface emitter. Characteristic differences from bulk p-type InAs are particularly revealed from excitation wavelength-dependent measurements, showing monotonously increasing THz pulse amplitude in the NW arrays with increasing photon energy. Further polarization-dependent and two-color pump-probe experiments elucidate the physical mechanism of the THz emission: In contrast to bulk p-type InAs, where the anisotropic photoconductivity in the surface electric field is the dominant cause for THz pulse generation, the origin of the intrinsic THz emission in the NWs is based on the photo-Dember effect. The strong THz emission from high aspect ratio NW arrays further suggests an improved out-coupling of the radiation, while further enhancements in efficiency using core-shell NW geometries are discussed.

The dependence of terahertz (THz) radiation on the excitation wavelength of femtosecond pulses (from 640 to 2600 nm) was investigated. Four different materials, InAs, InSb, InN and GaAs, were analyzed. The generated THz amplitude increases linearly with the photon energy due to the reduced absorption length and the increased quasi-ballistic transit distance. When the electron excess energy reaches the position of the subsidiary conduction band valleys, the intervalley scattering sets in and the THz amplitude drops. Thus, this method allows us to determine intervalley separation of various semiconductors. THz generation due to subsurface band bending in GaAs was observed as well.

We report the terahertz frequency radiation detection using photoconductive antennas fabricated from molecular-beam-epitaxy-grown GaAsBi. We have estimated that the detector has the highest sensitivity when illuminated with 1.1- to 1.3-µm-wavelength femtosecond pulses. Electron scattering to higher conduction valleys takes place at shorter wavelengths; thus, we have determined the intervalley separation in the conduction band (0.26–0.29 eV). Though the sensitivity of GaAsBi detector at 1.55 µm decreases by about 40% compared with its maximum value, we have demonstrated a terahertz time domain spectroscopy system based on the femtosecond Er-doped fiber laser using this detector.

Charge carrier transport and trapping was investigated in organic solar cell structures consisting of poly-3-hexylthiophene blended with the fullerene derivative [6,6]-phenyl-C61-butyric acid methyl ester in 6:5 weight ratio. The analysed devices having solar efficiency of 3.7 per cent were produced in the inverted layer sequence. The fill factor of the IV characteristics was as high as 68 per cent. It was demonstrated that despite of such relatively high fill factor carrier trapping is effectively involved in the charge transport phenomena. The density of the trapping states was evaluated to be up to 10(20) division by 7 x 10(21) cm(-3) and their activation energy was about 0.18 eV. At such high densities these states may probably act as transport states, limiting carrier mobility. The results were analyzed by taking into account carrier thermal generation from traps as well their mobility variation according to the Gaussian disorder model. The mobility parameters obtained by both methods demonstrated good coincidence.

We report pulsed terahertz (THz) emission from solution-processed metal halide perovskite films with electric field one order of magnitude lower than p-InAs, an effi-cient THz emitter. Such emission is enabled by a unique combination of efficient charge separation, high carrier mobilities, and more importantly surface defects. The mecha-nism of generation was identified by investigating the dependence of the THz electric field amplitude on surface defect densities, excess charge carriers, excitation intensity and energy, temperature and external electric field. We also show for the first time THz emission from a curved surface, which is not possible for any crystalline semiconductor and paves the way to focus high-intensity sources. These results represent a possible new direction for perovskite optoelectronics, and for THz emission spectroscopy as a complementary tool in investigating surface defects on metal halide perovskites, of fundamental importance in the optimization of solar cells and light-emitting diodes.