Dynamical Interaction Of Photon And Electron Spin Investigated By Ultrafast Spectroscopy

Recently, the development of ultrashort pulse laser technology provides aneffective tool for investigating the interaction of light and matter on criticaltimescales. The studies on spin dynamics in semiconductors and ferromagnetic filmshave become a new forefront of condensed matter physics, which promotes theprocesses in spintronics. However, the basic issues related to the interaction ofelectron spin and photon in paramagnetic, multiferroic, and antiferromagneticmaterial systems have not been understood completely yet, more originalexperimental and theoretical investigations are needed. In this thesis, these threekinds of materials are chosen for study the fundamental issues, such as inverseFaraday effect, three temperature mode and coherent control of spins. The mainresearch work is summaried as follows:（1） We report all-optical femtosecond time-resolved measurements of the ultrafastinverse Faraday effect in paramagnetic NaTb（WO₄）₂, terbium gallium garnet （TGG）and magnetic ionic liquid1-butyl-3-methylimidazolium tetrachloroferrate（[bmim]FeCl₄）. The operation of all-optical magnetic switching based on theNaTb（WO₄）₂demonstrates that the switching time can be as fast as⁵00fs at roomtemperature. The switching amplitude is proportional to the magnetization inducedby the circularly polarized light and modulated by tailoring the magneto-opticalproperties; The transient rotation and ellipticity of polarization of the probe pulseinduced by pump pulse allows us perform a systematic study of the full nonlinearoptical response in TGG and [bmim]FeCl₄. The magneto-optical contributionsattributed to the inverse Faraday effect and the optical Kerr effect are then extractedby thoroughly studying the dependence of the signal upon the pump polarization;The observation of about1.8THz coherent superposition between magneticsublevels of the Fe3+-ion’s ground-state multiplet is assigned to the impulsivelystimulated Raman scattering in the magnetic ion liquid. （2） The electronic energy relaxation of polycrystalline BiFeO₃films is studied usingtwo-color pump-probe spectroscopy. The fast relaxation of photo-excited electrons isattributed to scattering of electrons with lattice-vibration modes. Due to thestructural strain and symmetry breaking, the electron-phonon interaction strength oftetragonal BiFeO₃films is larger than that of rhombohedral counterparts; Thegeneration mechanism of coherent longitudinal acoustic （LA） phonons in BiFeO₃films is attributed to the transient photostriction effect, which is strongly connectedwith the ferroelectric polarization and significantly enhanced by La and Nb codoping;The spin-lattice interaction in YMnO₃film is identified from the slow component ofthe transient transmittance change with the excitation energies close to Mn3+ions d-dtransition, which is strongly connected with the type of transition and enhanced byspin ordering.（3） We present the magnetic dipole transition at0.38THz for quasi-ferromagneticmode （FM） and0.52THz for quasi-antiferromagnetic mode （AFM） excited bymagnetic component of terahertz electromagnetic pulse in an antiferromagneticDyFeO₃crystal, without disturbances such as electronic excitation and heating.Magnetic field perpendicular to the magnetization for exciting the FM mode, andmagnetic field parallel to the magnetization for the AFM mode; Coherent control ofthe precession motion of FM and AFM is demonstrated by choosing an appropriateinterval of double terahertz pulses. In addition, we can selectively choose any spinmode when these two magnetic resonances were excited simultaneously; Theintrinsic dielectric anisotropy of YFeO₃crystal in the THz range allows for coherentcontrol of both the amplitude and the phase of the excited spin wave, by varying thepolarization of the incident single THz pulse with respect to the crystal axes.