We performed optical pump-THz probe spectroscopy on bulk GaAs to investigate the nature of exciton Mott transition. The behavior of excitonic correlation in the proximity of the Mott transition density is elucidated through the resonant excitation of 1s excitons with using a nonlinear terahertz spectroscopy technique. We discuss the anomalous charge carrier dynamics of the metallic phase on the verge of Mott transition that appears only at low temperatures.
Recent development of intense terahertz (THz) pulse generation technique has offered novel opportunities to reveal ultrafast phenomena in a variety of materials on tabletop experiments and provided a new pathway toward ultrafast control of quantum phases. Here we present our recent study of nonequilibrium dynamics in metallic superconductors NbN excited by intense THz pulse. Since the superconducting gap energy is located in the THz frequency range, the intense THz pulse excitation makes it possible to instantaneously excite high-density quasiparticles at the gap edge without injecting excess energies. It has also become possible to coherently drive the superconducting ground state without exciting incoherent quasiparticles by tuning the pump frequency below the gap energy. The ultrafast dynamics of the order parameter induced by such an intense low energy excitation is directly probed, and the nature of a collective excitation, namely the Higgs amplitude mode, is revealed. Efficient THz higher-harmonic generation from a superconductor is discovered, manifesting the nonlinear coupling between the THz wave and the Higgs mode. We also report the experimental results in a multi-gap superconductor MgB2.
By using terahertz (THz) pump-THz probe spectroscopy, we investigated ultrafast dynamics of s-wave superconductors NbN. After instantaneous excitation with an intense monocycle THz pulse, transient oscillation of the superconducting order parameter is observed in the transmission of the THz probe pulse, which is interpreted as the collective Higgs amplitude mode associated with spontaneous symmetry breaking. We also found that the Higgs mode can be resonantly excited by a sub-gap multi-cycle THz pump pulse in the nonlinear response regime, resulting in efficient third-order harmonics generation. These results shed new light on the ultrafast optical control of quantum condensates and its application to THz nonlinear optics.
We report on the recent progress of terahertz magneto-optical measurement combined with terahertz time-domain
spectroscopy. The detailed experimental scheme for the observation of optical quantum Hall effect in a two dimensional
electron gas system in the integer-quantum Hall effect regime is described.