Propagation of optical pulses in a resonantly absorbing medium is studied. Propagation time of nanosecond pulses was measured for the Rb D 1 transition. At the center of two absorption lines, delay of the pulse peak which is about ten times as large as the pulse width was observed, where zero delay is defined for the propagation with the light velocity in vacuum. On the other hand, at the peak of an absorption line, negative delay was observed for large absorption, where the advance time is as large as 25% of the pulse width. Simulation including the effect of absorption and phase shift reproduced well the experimental results.

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.

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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...

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