Ryusuke Matsunaga

Superconductors host collective modes that can be manipulated with light. We show that a strong terahertz light field can induce oscillations of the superconducting order parameter in NbN with twice the frequency of the terahertz field. The result can be captured as a collective precession of Anderson's pseudospins in ac driving fields. A resonance between the field and the Higgs amplitude mode of the superconductor then results in large terahertz third-harmonic generation. The method we present

Ultrafast responses of BCS superconductor Nb(1-x)Ti(x)N films in a nonadiabatic excitation regime were investigated by using terahertz (THz) pump-THz probe spectroscopy. After an instantaneous excitation with the monocycle THz pump pulse, a transient oscillation emerges in the electromagnetic response in the BCS gap energy region. The oscillation frequency coincides with the asymptotic value of the BCS gap energy, indicating the appearance of the theoretically anticipated collective amplitude mo

We report the first observation of trions (charged excitons), three-particle bound states consisting of one electron and two holes, in hole-doped carbon nanotubes at room temperature. When p-type dopants are added to carbon nanotube solutions, the photoluminescence and absorption peaks of the trions appear far below the E11 bright exciton peak, regardless of the dopant species. The unexpectedly large energy separation between the bright excitons and the trions is attributed to the strong electro

Using terahertz (THz) pump-THz probe spectroscopy, we have investigated the dynamics of the nonequilibrium BCS state in a superconducting NbN film after the impulsive photoinjection of high-density Bogoliubov quasiparticles. The superconducting state rapidly changes within the duration of the monocycle THz pump pulse (1.6 ps). The complex optical conductivity spectrum in the nonequilibrium BCS state significantly deviates from that in the equilibrium state. The observed spectral features are qua

Recent advances in time-domain terahertz (THz) spectroscopy have unveiled that resonantly enhanced strong THz third-harmonic generation (THG) mediated by the collective Higgs amplitude mode occurs in $s$-wave superconductors, where charge-density fluctuations (CDFs) have been shown to also contribute to the nonlinear third-order susceptibility. It has been theoretically proposed that the nonlinear responses of Higgs and CDF exhibit essentially different polarization dependences. Here we experime

We studied exciton structures and the Aharonov-Bohm effect in a single carbon nanotube using micro-photoluminescence (PL) spectroscopy under a magnetic field at low temperatures. A single sharp PL peak from the bright exciton state of a single carbon nanotube was observed under zero magnetic field, and the additional PL of dark exciton state appeared below the bright exciton peak under high magnetic fields. It was found that the split between the bright and dark exciton states is several milliel

We performed photoluminescence (PL) spectroscopy on single carbon nanotubes to investigate the satellite PL peaks, which are much lower in energy than the lowest $({E}_{11})$ bright exciton peak. From the temperature and tube-diameter dependences of the PL spectra, we clarified two origins of the low-energy PL peaks. The weak peak, lying about 130 meV below the ${E}_{11}$ bright exciton state, is well explained by the phonon sideband of the $K$-momentum dark exciton states above the lowest-brigh

We investigated the magnetic and temperature dependence of the bright and dark exciton luminescence spectra in single carbon nanotubes. We found that the phonon-induced exciton scattering rate from the bright to the dark state is only one order of magnitude larger than the dark exciton recombination rate below 10 K at zero magnetic field. Our results indicate that excitons are nonequilibriumly distributed between the bright and dark states due to the different parities of the wave functions and

The electromagnetic response of Dirac semimetals in the infrared and terahertz frequency ranges is attracting growing interest for potential applications in optoelectronics and nonlinear optics. The interplay between the free-carrier response and interband transitions in the gapless, linear dispersion relation plays a key role in enabling novel functionalities. Here we investigate ultrafast dynamics in thin films of a photoexcited Dirac semimetal Cd₃As₂ by probing the broad

We review our recent experiments of ultrafast dynamics in s-wave superconductors Nb1-xTixN by using nonlinear terahertz (THz) spectroscopy. The free oscillation of the Higgs mode, i.e. the amplitude mode of the superconducting order parameter, is observed after instantaneous injection of quasiparticles at the superconducting gap edge by an intense monocycle THz pulse. The ultrafast nonequilibrium dynamics of the order parameter under the strong AC driving field with the photon energy tuned below

We studied exciton structures and the Aharonov-Bohm effect in a single carbon nanotube using microphotoluminescence (PL) spectroscopy under a magnetic field at low temperatures. A single sharp PL peak from the bright exciton state of a single carbon nanotube was observed under zero magnetic field, and the additional PL of dark exciton state appeared below the bright exciton peak under high magnetic fields. It was found that the split between the bright and dark exciton states is several milliele

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

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 poss

We perform terahertz(THz)-pump-THz-probe spectroscopy to study ultrafast dynamics of BCS superconductor NbN. Resonant photo-injection of high density quasiparticles realized by the intense THz pump pulses results in a nonequilibrium BCS state.