Topological Insulator Films for Terahertz Photonics.

We discuss experimental and theoretical studies of the generation of the third terahertz (THz) frequency harmonic in thin films of BiSe and BiSbTeSe (BSTS) topological insulators (TIs) and the generation of THz radiation in photoconductive antennas based on the TI films. The experimental results, supported by the developed kinetic theory of third harmonic generation, show that the frequency conversion in TIs is highly efficient because of the linear energy spectrum of the surface carriers and fast energy dissipation. In particular, the dependence of the third harmonic field on the pump field remains cubic up to the pump fields of 100 kV/cm.

Modified Klyshko method for an analog detector calibration.

We extend the absolute quantum efficiency (QE) calibration method, previously used only for photon counters, to analog detectors with a high dispersion of single-photon responses. Our approach is demonstrated on a reference-free measurement of a photomultiplier tube (PMT) cathode QE with 4% relative uncertainty. It involves not only measuring the biphoton field correlation function, but also a special approximation of the distribution of detector readings in order to determine the average number of photoelectrons and the average charge corresponding to a single photoelectron.

Measurement of the biphoton second-order correlation function with analog detectors.

An experimental scheme and data processing approaches are proposed for measuring by analog photo detectors the normalized second-order correlation function of the biphoton field generated under spontaneous parametric down-conversion. Obtained results are especially important for quantum SPDC-based technologies in the long-wave spectral ranges, where it is difficult to use the single-photon detector at least in one of the two biphoton channels. The methods of discrimination of analog detection samples are developed to eliminate the negative influence of the detection noises and get quantitatively true values of both the correlation function and the detector quantum efficiency.

Towards Measuring Terahertz Photon Statistics by a Superconducting Bolometer.

Statistical distributions of the analog readings of an antenna-coupled THz superconducting bolometer were measured and analyzed under a special type of irradiation by low-energy fluxes of THz photons with Poisson photon statistics and controllable mean photon numbers. The photons were generated via low-gain parametric down-conversion in pulse-pumped Mg:LiNbO crystal placed to a cooled cryostat together with the bolometer NbN film. Results of theoretical approximation of experimental histograms reveal the discrete nature of THz detection by superconducting bolometers and open a way for studying their quantum characteristics.

Direct detection of the idler THz radiation generated by spontaneous parametric down-conversion.

We study parametric down-conversion (PDC) of optical laser radiation in the strongly frequency non-degenerate regime which is promising for the generation of quantum-correlated pairs of extremely different spectral ranges, the optical and the terahertz (THz) ones. The possibility to detect tenuous THz-frequency photon fluxes generated under low-gain spontaneous PDC is demonstrated using a hot electron bolometer. Then experimental dependences of the THz radiation power on the detection angle and on the pump intensity are analyzed.