The detector response of YBa2Cu3O7−x Josephson grain-boundary junctions to monochromatic radiation with the frequency f in the range from 60 GHz to 4 THz has been studied. Frequency-selective odd-symmetric resonances in the responses ΔI(V) of these junctions to radiation with different frequencies f have been observed near the voltages V=hf/2e in almost a decade of spectral range for any operating temperature in the range from 30 to 85 K. The spectral range of the selective detection has scaled with the IcRn product of the Josephson junction, reaching the range of 0.16–3.1 THz for a IcRn product of 1.5 mV. A resolving power δf/f of around 10−3 has been demonstrated in the selective detection by Josephson junctions. The high-frequency falldown of the amplitude of the selective response has been found to be proportional to exp[−P/P0], where P=(hf/2e)2/Rn is the power dissipated in the junction at the resonance and P0 is a characteristic power level. The values of P0 for our junctions were around 20 μW at 34 K and 2 μW at 78 K.

A model based on a system of balance equations for localised and continuum states is developed to calculate the current − voltage (I − V) and power characteristics of quantum-cascade lasers (QCLs) operating in the terahertz (THz) range. A method for modifying the eigenbasis of the Schrödinger equation by reducing the dipole moments of tunnel-coupled states is proposed to take into account the effect of dephasing on the carrier transport. The calculated and experimental data on the current − voltage characteristics and the dependence of the integrated radiation intensity on current for the THz QCLs lasing at 2.3 THz are compared. The calculated and measured values of the threshold current, lasing current range, and maximum operating temperature T max are found to be in good agreement. It is shown that T max can be increased by 25 % by reducing the thickness of the top contact layer n +-GaAs of the laser structure under study from 800 to 100 nm.

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