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The efficiency of electric pumping of THz quantum-cascade lasers (QCLs) with strip geometry is studied depending on the number and position of contact pads. The numerical simulation of the electric potential distribution in the THz QCL active region is used to determine the required thicknesses of the upper metallisation layers of the THz QCLs to minimize the voltage drop along the laser structure in the case of nonuniform current supply. It is found that the efficiency of electric pumping in the case of a centrally located contact is significantly higher than when the contact pads are located near the laser structure edges. From the calculated dependence of the THz QCL integral power on the thickness of the upper metal layer, it is shown that for effective current injection, the contact pads must be located at a distance of less than 0.5 mm from each other.

The results of examination of the terahertz radiation spectra of multilayer GaAs/AlGaAs heterostructures synthesized on GaAs substrates are presented. The dependence of the radiation spectrum on the amplitude of the excitation current pulse and temperature dependences of the threshold lasing current and the radiation power of terahertz quantum cascade lasers with a double metallic waveguide, which were built on the basis of these structures, have been obtained. The maximum amplitude of the total radiation power is estimated at 28 μW in the range 3.25–3.32 THz at a temperature of 15 K. The spectral radiation density of the oscillator is measured. Changes in the mode content of radiation induced by the bias-current variation have been observed.

The design of a compact ultrahigh-vacuum nitrogen-free helium cryostat is described. The volume of the helium vessel of the cryostat is 2.7 l, and the time of operation after filling it with liquid helium is >65 h. The cryostat satisfies the requirements imposed on ultrahigh-vacuum systems, allows heating to 200°C, and ensures operation at an evacuation level of 10−10 Torr.

The evolution of a monostable pulsed response with a distinctive profile and 'cold' optical pulses was observed for the first time experimentally as a reaction to a small perturbation in the pumping of a semiconductor resonator cavity with competing electronic and thermal nonlinear refraction mechanisms. The dependences of the pulse parameters on the pump power were determined.