No abstract

Pulsed terahertz emission excitation spectra from germanium crystals are being presented. The most intense terahertz pulses from germanium crystals are emitted at quanta energies coinciding with technologically significant telecommunication wavelengths. The terahertz generation mechanisms are an interplay of the photocurrent surge in the surface electric field and the photo-Dember effect. Remarkably, the terahertz emission is also observed at quanta energies below the direct bandgap of this material even when photoexcited at a surface normal. This is the result of a broken symmetry of effective electron mass in the L valleys.

We demonstrate that the rectifying field effect transistor, biased to the subthreshold regime, in a large signal regime exhibits a super-linear response to the incident terahertz (THz) power. This phenomenon can be exploited in a variety of experiments which exploit a nonlinear response, such as nonlinear autocorrelation measurements, for direct assessment of intrinsic response time using a pump-probe configuration or for indirect calibration of the oscillating voltage amplitude, which is delivered to the device. For these purposes, we employ a broadband bow-tie antenna coupled Si CMOS field-effect-transistor-based THz detector (TeraFET) in a nonlinear autocorrelation experiment performed with picoseconds-scale pulsed THz radiation. We have found that, in a wide range of gate bias (above the threshold voltage Vth=445 mV), the detected signal follows linearly to the emitted THz power. For gate bias below the threshold voltage (at 350 mV and below), the detected signal increases in a super-linear manner. A combination of these response regimes allows for performing nonlinear autocorrelation measurements with a single device and avoiding cryogenic cooling.

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.

Terahertz (THz) pulse generation from p-InAs, p-InSb, and n-InSb epitaxial layers are investigated using 1.55-μm wavelength femtosecond laser pulses for photoexcitation. The samples are of (111) crystallographic orientation resulting in anisotropic photoconductivity. Experiments have shown that THz generation in InAs is mainly due to anisotropic photocurrent in the surface electric field while a dominant mechanism in InSb is optical rectification. At high optical excitation fluencies, InSb is more efficient than p-InAs. In the presence of an external magnetic field, (111) InSb has exhibited promising viability as an alternative to the photoconductive antenna emitter in a THz time-domain-spectroscopy (THz-TDS) system.

Spectral dependences of the amplitudes of terahertz (THz) transients radiated from a GaSe surface after its excitation by femtosecond optical pulses with photon energies in the range from 1.8 eV to 3.8 eV were used for the study of electron energy band structure of this layered crystal. The energy separation of 0.21 eV between the main Γ valleys and the satellite K valleys in the conduction band was determined from the maximum position of THz excitation spectrum; the polarity of the THz transients became inverted at photon energies higher than 3 eV due to the onset of electron transitions from the second, lower lying valence band.

GaIn)(AsBi) layers were grown on a GaAs substrate. Their alloy composition, structural characteristics as well as the optical and electrical parameters were determined. It was found that by incorporating Bi and In into the lattice of GaAs, the energy bandgaps can be as narrow as 0.6 eV. These epitaxial layers of quaternary bismide alloys have shorter than a one picosecond carrier lifetime and a relatively large dark resistivity, demonstrating that this material is a good candidate for ultrafast optoelectronics applications. Thick quaternary bismide layers were used for the fabrication of photoconductive antenna type THz radiation detectors activated by femtosecond laser pulses. The performance of THz detectors manufactured from (GaIn)(AsBi) layers was comparable to that of previously reported Ga(AsBi) devices, but the range of optical wavelengths at which the detectors can be activated was considerably wider, covering the technologically important 1.55 μm wavelength range.

Terahertz radiation pulses emitted after exciting semiconductor heterostructures by femtosecond optical pulses were used to determine the electron energy band offsets between different constituent materials. It has been shown that when the photon energy is sufficient enough to excite electrons in the narrower bandgap layer with an energy greater than the conduction band offset, the terahertz pulse changes its polarity. Theoretical analysis performed both analytically and by numerical Monte Carlo simulation has shown that the polarity inversion is caused by the electrons that are excited in the narrow bandgap layer with energies sufficient to surmount the band offset with the wide bandgap substrate. This effect is used to evaluate the energy band offsets in GaInAs/InP and GaInAsBi/InP heterostructures.