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.

We report on the observation of broadband (40 THz) bright twin beams through high-gain parametric downconversion in an aperiodically poled lithium niobate crystal. The output photon number is shown to scale exponentially with the pump power and not with the pump amplitude, as in homogeneous crystals. Photon number correlations and the number of frequency/temporal modes are assessed by spectral covariance measurements. By using sum-frequency generation on the surface of a non-phase-matched crystal, we measure a cross-correlation peak with the temporal width of 90 fs.

Using a canonical pump–probe experimental technique, we studied terahertz (THz) wave generation and detection by optical rectification and mixing in Czochralski-grown periodically poled $\mathrm{Mg}:\mathrm{Y}:{\mathrm{LiNbO}}_3$ (PPLN) crystals. THz waves with frequencies at 1.37 and 0.68 THz as well as 1.8 THz were obtained for PPLN with nonlinear grating periods of 30 and $60\mu \mathrm{m}$, respectively. A general theoretical model was developed by considering the dispersion and damping of a low-frequency phonon-polariton mode. Our results show that THz waves are generated in forward and backward directions by pumping pulse rectification. The generated THz waves depend on the spectral shape of the laser pulses, quasi-phase mismatches, and dispersion characteristics of a crystal.

We show that broadband biphoton wave packets produced via sponateous parametric down-conversion in crystals with linearly aperiodic poling can be easily compressed in time using the effect of group-velocity dispersion in optical fibers. This result could foster important developments in quantum metrology and lithography.