Under the irradiation of the femtosecond laser pulses, coherent oscillations of Raman active phonons can be excited through electronic excitation via real or virtual intermediate states, which are characterized by displacive and impulsive excitations of coherent phonons, respectively. There have been extensive studies on the generation of coherent phonons and on the dephasing dynamics of coherent phonons, in semimetals, semiconductors, and superconductors in the past decade. Recently, it has been revealed that coherent phonons in solids can be optically controlled by the irradiation of terahertz-rate pulse trains whose time periods match to the oscillation period of the coherent phonons. Thus the coherent phonon has been the focus of much attention because of its wide applications as well as its fundamental interest from the viewpoint of lattice dynamics.

　As the characteristic of the coherent phonon, the reflectance change of 10^-2 to 10^-４ is induced by coherent oscillations at a sample surface. Therefore, by use of this reflectance changes, amplitude modulation of the light can be possible. In addition, phonons in solids are closely related to structural phase transition. In particular, in displacement-type ferroelectrics there is so-called soft mode whose frequency becomes extremely low near the critical point, and it has been accepted that phase transitions are caused by displacement of this soft mode. Therefore, operation of structure phase transition is expected if light pulses can control the amplitude of the coherent soft mode.

　In the present study, I will try to manipulate the phase transitions in solids, the mobility of electrons, and the properties of photons, which are interacting with coherent phonons, by precisely controlling the amplitude, the frequency, and the phase of coherent phonons. Furthermore, in the near future, I would propose completely new phonon devises by using controllability of the coherent phonons.

References:
•  M. Hase , K. Ishioka, J. Demsar, K. Ushida, M. Kitajima,” Ultrafast dynamics of coherent optical phonons and nonequilibrium electrons in transition metals”, Phys. Rev. B. Vol. 71 , 184301 (2005).
•  M. Hase , M. Kitajima, A. M. Constantinescu and H. Petek,”The birth of quasiparticles in Si observed in time-frequency space”, Nature , Vol. 426 , pp.51-54 (2003).
•  M. Hase , M. Kitajima, S. Nakashima and K. Mizoguchi,”Forcibly driven coherent soft phonons in GeTe with intense THz-rate pump fields”, Appl. Phys. Lett., Vol. 83 , No. 24, pp.4921 (2003).
•  M. Hase , M. Kitajima, S. Nakashima, and K. Mizoguchi,”Dynamics of coherent anharmonic phonons in bismuth using high density photoexcitation”, Phys. Rev. Lett., Vol. 88, No. 6, pp. 067401 (2002).