Coherent phonons were observed in SrTiO 3 by using ultrafast polarization spectroscopy. The ultrafast dynamics and softening of phonon modes, which contribute to the structural phase transition at 105 K, are studied. The temperature dependences of the phonon frequency and the relaxation rate are obtained from the observed damped oscillation of coherent phonons. The observed phonon relaxation is explained well by a population decay due to anharmonic phonon-phonon coupling for the soft mode phonons.

Ultrafast lattice and spin dynamics in an antiferromagnet NiO were studied via reflection and transmission experiments using polarization spectroscopy along with the pump-probe technique. The damped oscillations observed in the reflection experiment are not of spin origin but can be attributed to an interference effect caused by the probe pulse reflected by a propagating wave packet of acoustic phonons. Terahertz oscillations of coherent magnons were observed in the transmission experiment. Three antiferromagnetic modes were observed at low temperatures. As the temperature increases, the two higher modes degenerate above ∼250 K and then soften toward the Néel temperature (523 K).Ultrafast spin dynamics has recently been one of the most attractive topics in condensed matter physics, and understanding ultrafast spin dynamics from fundamental and technological viewpoints is greatly desired. Ultrafast spin dynamics and optical spin control have potential applications for the development of high-speed memory devices, spintronics, quantum computing, and optical control of correlated spin systems. As the frequency of antiferromagnetic resonance is considerably higher than that of ferromagnetic resonance, antiferromagnetic spin dynamics has been studied extensively with the aim of realizing faster optical spin control.NiO is a typical transition-metal oxide that exhibits antiferromagnetism. It is a paramagnetic insulator with a cubic rock-salt structure above the Néel temperature T N = 523 K. In the antiferromagnetic phase below T N , the Ni 2+ spins align ferromagnetically in (111) planes and antiferromagnetically between adjacent (111) planes, 1 and a slight lattice deformation from a cubic structure to a rhombohedral structure occurs along the (111) axes owing to the nearest-neighbor and nextnearest-neighbor interactions. Experiments on NiO have been conducted; these experiments have demonstrated terahertz radiation from coherent antiferromagnetic magnons excited by femtosecond laser pulses 2-4 and magnon oscillations induced by circularly polarized light. 5,6 A damped oscillation signal with an oscillation period of 19 ps was observed in NiO through the pump-probe experiment on reflected second harmonic generation (SHG). 7-9 The origin of this signal was interpreted to be the coherent oscillations of Ni 2+ spins between hard-and easy-axis states owing to the change in magnetic anisotropy. In the present study, we investigated the ultrafast lattice and spin dynamics in an antiferromagnet NiO via reflection and transmission experiments using polarization spectroscopy along with the pump-probe technique. In the reflection experiment, damped oscillations of linear birefringence, whose period is ∼20 ps, were observed. The observed dependences on the probe wavelength and the pump power indicate that the observed oscillations are not of spin origin but originate from the coherent acoustic phonons owing to the propagation of strain pulse.In the transmission experiment, terahertz oscillations of circular birefringence were ob...

Optical manipulation of spin coherence in rubidium atoms is studied. The effect of off-resonant and circularly polarized light on optically induced magnetization is investigated. The change in precession frequency caused by the light-shift effect is verified. Absorption-free phase control of spin precession and pure spin rotation about an arbitrary axis are demonstrated. A theory of precession frequency shift that includes the effect of absorption is considered by using the density matrix and the experimental results are in agreement with the predictions of the theory. Thus, we show that it is possible to carry out off-resonant control of spin coherence and all-optical manipulation of spins.

The generation, relaxation, and diffusion dynamics of optically induced lattice distortion in the relaxed excited state of SrTiO 3 are studied by using polarization spectroscopy with the pump-probe technique. The relaxed excited state is generated with a rise time on the order of 100 ps. Three kinds of thermal activation processes of the localized lattice distortion are found, and these processes are considered to be caused by photogenerated carriers in trapped states, which play important roles in photoluminescence or photoconductivity. We observed the lattice distortion induced by a separated pump beam from the probe beam to investigate its itineracy. The lattice-distortion signal appears later as the separation along the [100] axis becomes larger. The temperature dependence of the mobility suggests that the observed diffusive lattice distortion is caused by photogenerated electrons accompanied by lattice distortion, or electron polarons. Thus, the spatial and temporal dynamics of polaron diffusion were observed directly in our experiment.

The doping-induced ferroelectric phase transition in Ca-doped SrTiO 3 is investigated by observing the birefringence and coherent phonons. The structural phase-transition temperature is determined by the birefringence measurement. Coherent phonons of the soft modes are studied by using ultrafast polarization spectroscopy. Two phonon modes are observed to be softened toward the ferroelectric phase-transition point at 28 K and other two phonon modes are observed to be softened toward the structural phase-transition point at 180 K.Another structural deformation at 25 K is found below the ferroelectric phase-transition temperature. From the temperature dependence of phonon frequencies under an ultraviolet ͑UV͒ illumination, a shift of the ferroelectric phase-transition point toward the lower temperature side is found. A decrease in phonon frequencies after the UV illumination is also found.