超伝導体の超高速光・テラヘルツ波制御

With recent progress in terahertz technology, it has become possible to generate the intense THz pulse with the peak electric field as much as 1MV/cm, which opens an avenue to control the material properties by optical means, in a non-thermal way. To find a new pathway for the optical control of material phases in various materials, we are studying nonlinear light-matter interactions in the THz range using such an intense THz pulse source.

Here we investigate the ultrafast dynamics of nonequilibrium BCS superconducting states induced by the monocycle THz pulse. The irradiation of the strong THz pulse with its photon energy close to the BCS superconducting gap can break the Cooper pairs, resulting in the high-density creation of Bogoliubov quasiparticles, which in turn causes the collapse of BCS gap. Recently, we have observed such an ultrafast switching from the BCS superconducting state to the normal state in a NbN film induced by the irradiation of an intense THz pulse (Ref.1). With developing a new laser spectroscopy technique such as multi-dimensional time-domain THz spectroscopy, we are studying the coherent transient phenomena in macroscopic quantum phases (Ref.2).

When a physical system undergoes spontaneous symmetry breaking, collective modes associated with the order parameter emerge. Observation of such collective modes provides a clue to identify the underlying nature of physical system, like the Higgs boson in elementary particles. Recently we have observed for the first time the collective amplitude mode of the order parameter, termed as the Higgs mode, in s-wave BCS superconductors Nb_{1-x}Ti_{x}N (Ref.2). A strong monocycle THz pulse drives the BCS state into far from equilibrium in a nonadiabatic way without heating the lattice system. In response to such an instantaneous perturbation, a transient oscillation of the order parameter is clearly identified in the time-domain THz electromagnetic response. Our result provides a substantial platform for investigating nonequilibrium dynamics of fermionic condensates and opens a new avenue for ultrafast coherent control of superconductivity by light.

- "Nonequilibrium BCS State Dynamics Induced by Intense Terahertz Pulses in a Superconducting NbN Film", R. Matsunaga and R. Shimano, Phys. Rev. Lett. 109, 187002 (2012).
- "Higgs Amplitude Mode in the BCS Superconductors Nb1-xTixN Induced by Terahertz Pulse Excitation", R. Matsunaga, Y. I. Hamada, K. Makise, Y. Uzawa, H. Terai, Z. Wang, and R. Shimano, Phys. Rev. Lett. 111, 057002 (2013).
- "Light-induced collective pseudospin precession resonating with Higgs mode in a superconductor", R. Matsunaga, N. Tsuji, H. Fujita, A. Sugioka, K. Makise, Y. Uzawa, H. Terai, Z. Wang, H. Aoki, and R. Shimano, Science 345, 1145 (2014).

半導体の高密度電子正孔系

Photo-excited electron-hole (e-h) system in semiconductors exhibits various phases depending on the e-h density and temperature, such as exciton gas, electron-hole plasma, and electron-hole liquid (droplets). Since the density of e-h pair can be easily controlled by changing the light intensity, the system provides an unique playground to study the phase transition dynamics in which many body Coulomb interaction plays an essential role. Theoretically, Bose Einstein condensation or e-h BCS state have been anticipated to exist at very low temperature. By using laser spectroscopy technique exemplified by terahertz spectroscopy, we are studying the dynamics of metal-insulator transition (exciton Mott transition) and liquid-gas transition in e-h system in the ultrafast temporal resolution of several picoseconds.

- "Time-resolved formation of excitons and electron-hole droplets in Si studied using terahertz spectroscopy", T. Suzuki and R. Shimano, Phys. Rev. Lett. 103, 057401 (2009).
- "Cooling dynamics of photoexcited carriers in Si studied using optical pump and terahertz probe spectroscopy", T. Suzuki and R. Shimano, Phys. Rev. B 83, 085207 (2011).
- "Exciton Mott transition in Si revealed by terahertz spectroscopy", T. Suzuki and R. Shimano, Phys. Rev. Lett. 109, 046402 (2012).

テラヘルツホール効果とベリーの位相

When linearly polarized light is incident on a sample which is under external magnetic field, the polarization of the transmitted (reflected) light rotates, which phenomenon is called magneto-optical Faraday (Kerr) effect. When the materials are conducting ,the phenomenon is nothing but the high frequency version of Hall effect, arising from the off-diagonal (or Hall) conductivity $\sigma_{xy}$. Inversely, one can obtain the frequency dependent Hall conductivity $\sigma_{xy}(\omega)$, from the measured rotation spectrum. This technique allows one to determine the carrier density and the mobility in non-contact way.

The similar phenomenon occurs in ferromagnets, known as anomalous Hall effect (AHE) , in which the effect is caused by the magnetization instead of the external magnetic field. The origin can be classified into extrinsic and intrinsic one and the intrinsic mechanism of the AHE is related to Berry phase curvature of Bloch electrons.

By using the THz-spectrscopic technique, we are trying to measure the energy structure of the Hall conductivity in meV range, to reveal the topological nature of the AHE.

- "Characterization of doped silicon in low carrier density region by terahertz frequency Faraday effect", Y. Ikebe, R. Shimano, Appl. Phys. Lett. 92(1), 012111-1-3 (2008).
- "Optical Hall effect in the integer quantum Hall regime", Y. Ikebe, T. Morimoto, R. Masutomi, T. Okamoto, H. Aoki, and R. Shimano, Phys. Rev. Lett. 104, 256802 (2010).
- "Terahertz Faraday rotation induced by an anomalous Hall effect in the itinerant ferromagnet SrRuO3", R. Shimano, Y. Ikebe, K. S. Takahashi, M. Kawasaki, N. Nagaosa and Y.Tokura, EPL 95, 17002 (2011).
- Quantum Faraday and Kerr rotations in graphene", R. Shimano, G. Yumoto, J. Y. Yoo, R. Matsunaga, S. Tanabe, H. Hibino, T. Morimoto, and H. Aoki, Nature Commun. 4, 1841 (2013).