Topologically protected surface states present rich physics and promising spintronic, optoelectronic, and photonic applications that require a proper understanding of their ultrafast carrier dynamics. Here, we investigate these dynamics in topological insulators (TIs) of the bismuth and antimony chalcogenide family, where we isolate the response of Dirac fermions at the surface from the response of bulk carriers by combining photoexcitation with below-bandgap terahertz (THz) photons and TI samples with varying Fermi level, including one sample with the Fermi level located within the bandgap. We identify distinctly faster relaxation of charge carriers in the topologically protected Dirac surface states (few hundred femtoseconds), compared to bulk carriers (few picoseconds). In agreement with such fast cooling dynamics, we observe THz harmonic generation without any saturation effects for increasing incident fields, unlike graphene which exhibits strong saturation. This opens up promising avenues for increased THz nonlinear conversion efficiencies, and high-bandwidth optoelectronic and spintronic information and communication applications.

In this study, the efficient generation of terahertz radiation by a dipole photoconductive antenna, based on a thin island film of a topological insulator, was experimentally demonstrated. The performance of the Bi_1.9Sb_0.1Te₂Se antenna was shown to be no worse than those of a semiconductor photoconductive antenna, which is an order of magnitude thicker. The current–voltage characteristics were studied for the photo and dark currents in Bi_1.9Sb_0.1Te₂Se. The possible mechanisms for generating terahertz waves were analyzed by comparing the characteristics of terahertz radiation of an electrically biased and unbiased topological insulator.

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. The methods are demonstrated depending on whether two single-photon avalanche photo detectors are used in both SPDC channels, or at least one single-photon detector is replaced by a photo-multiplier tube which cannot operate in the photon counting mode.

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. Results of the calibration were verified in an independent experiment; the numbers of incident photons detected by the PMT and a single-photon counter are in good agreement.

The terahertz (THz) generation efficiency in the Cherenkov optical rectification scheme can be improved significantly if the silicon adaptor, mounted at the lateral surface of a nonlinear crystal, has a convex output surface with proper geometry. We demonstrate and compare with the results of direct experiments a method for theoretically modeling the angular distributions of the spectral power of THz radiation in the case of different Si adaptors, constructed by mounting plano-spherical lenses on a conventional flat Si prism. The applied method of theoretical modeling shows its usefulness in choosing the best Si adapter geometry.

The second-order correlation function which characterizes the quantum correlation between the optical and terahertz photons generated under spontaneous parametric down-conversion has been measured for the first time. A scheme of experiment based on the analysis of analog signals from terahertz and optical detectors has been proposed. Various approaches to exclude the effect of electronic and thermal noise on the measured correlation level have been discussed. The results make it possible to expand quantum optical technologies such as quantum calibration of detectors, fabrication of single-photon sources, and imaging with single-pixel detectors to the terahertz range.