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We characterize superconducting antenna-coupled hot-electron bolometers for direct detection of terahertz radiation operating at a temperature of 9.0 K. The estimated value of responsivity obtained from lumped-element theory is strongly different from the measured one. A numerical calculation of the detector responsivity is developed, using the Euler method, applied to the system of heat balance equations written in recurrent form. This distributed element model takes into account the effect of nonuniform heating of the detector along its length and provides results that are in better agreement with the experiment. At a signal frequency of 2.5 THz, the measured value of the optical detector noise equivalent power is 2.0×10 -13 W·Hz -0.5 . The value of the bolometer time constant is 35 ps. The corresponding energy resolution is about 3 aJ. This detector has a sensitivity similar to that of the state-of-the-art sub-millimeter detectors operating at accessible cryogenic temperatures, but with a response time several orders of magnitude shorter.  Index Terms-Radiation detectors, hot electron bolometer, frequency response.

We report a study of the relaxation time of the restoration of the resistive superconducting state in single crystalline boron-doped diamond using amplitude-modulated absorption of (sub-)THz radiation (AMAR). The films grown on an insulating diamond substrate have a low carrier density of about 2.5 × 10 21 cm −3 and a critical temperature of about 2 K. By changing the modulation frequency we find a high-frequency rolloff which we associate with the characteristic time of energy relaxation between the electron and the phonon systems or the relaxation time for nonequilibrium superconductivity. Our main result is that the electron-phonon scattering time varies clearly as T −2 , over the accessible temperature range of 1.7 to 2.2 K. In addition, we find, upon approaching the critical temperature T c , evidence for an increasing relaxation time on both sides of T c .

An overview is presented of experiments on ballistic electrical transport in inhomogeneous superconducting systems which are controlled by the process of Andreev reflection. The initial experiments based on the coexistence of a normal phase and a superconducting phase in the intermediate state led to the concept itself. It was followed by a focus on geometrically inhomogeneous systems like point contacts, which provided a very clear manifestation of the energy- and directional dependence of the Andreev reflection process. The point contacts have in recent years evolved towards the atomic scale by using mechanical break-junctions, revealing in a very detailed way the dependence of Andreev reflection on the macroscopic phase of the superconducting state. In present day research the superconducting inhomogeneity is constructed by clean room technology and combines superconducting materials with, for example, low-dimensional materials and topological insulators. Alternatively the superconductor is combined with nano-objects, such as graphene, carbon nanotubes, or semiconducting nanowires. Each of these 'inhomogeneous systems' provides a very interesting range of properties, all rooted in some manifestation of Andreev reflection.Comment: 22 pages, 14 figures. Contribution to the special issue of JETP to honor A.F. Andreev's 75th birthda