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.

The possibility to manage and control spectral and spatial properties of optical-terahertz nonclassical fields generated in strongly nondegenerate parametric down-conversion is demonstrated experimentally using Mach-Zehnder nonlinear interferometer and analyzed theoretically in terms of Schmidt modes. The interferometer can be used for measuring the dispersion properties of different materials in the terahertz frequency range. The key point is that the information about these properties is extracted from the measurements in the optical channel due to strong frequency-angular correlations between optical and terahertz photons. The developed theoretical approach is used to predict properties of the optical-terahertz fields and photon correlations in different regimes of nondegenerate parametric down-conversion. The considered interferometer is shown to be a promising source of terahertz fields with new features and frequency-filtered terahertz Schmidt modes.

We have measured refractive indices and absorption coefficients for ${\mathrm{M}\mathrm{g}:\mathrm{L}\mathrm{i}\mathrm{N}\mathrm{b}\mathrm{O}}_3$ single crystals of different Mg contents (0–7 mol %) in the visible and IR ranges up to the phonon absorption edge. The obtained dependencies of optical characteristics on Mg concentration in an IR region of 4–5 μm indicate the four-step character of structural changes under subsequent Mg doping. Spectral dependencies of optical characteristics reveal the existence of two polaron resonance bands, centered at 1.3 and 3.2 μm, which are not observed in the pure ${\mathrm{LiNbO}}_3.$ The theory approximation of the band at 1.3 μm, enhanced after chemical reduction, enabled us to determine the main polaron parameters for the 1.3-μm band, and to make a possible assignment of the 3.2 μm band. We assign this to direct transitions between energy levels of polarons trapped at ${\mathrm{Nb}}_{\mathrm{Nb}}$ sites and those of polarons trapped at ${\mathrm{Nb}}_{\mathrm{Li}}$ antisites in a lattice. A number of peculiarities are revealed in the absorption behavior at the low-energy edge from 1000 to 3000 ${\mathrm{cm}}^{\mathrm{-}1}$. The values of the uninduced part of absorption are sufficiently less than those predicted by a phonon oscillator model. Absorption spectral dependencies are modulated at a rather stable period, being of the order of frequencies of the lowest-energy longitudinal optical phonons, excited at a room temperature.

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.