M. Kunitomo

Experimental verification of oscillatory free induction decay

Abstract Oscillatory behavior in the free induction decay, predicted by Schenzle, Wong and Brewer, was observed and examined quantitatively for the first time by using inhomogeneously broadened proton nuclear magnetic resonance.

Production and detection of axions by using optical resonators

Abstract We propose a new scheme of experiments for detecting the invisible axion, which is required by the standard theory of elementary particle. In the experiment, photons from a laser are converted to axions in a magnetic field, and photons reproduced from these axions in another magnetic field are to be detected. By introducing optical resonators in both conversion processes, the signal photons are enhanced by a factor of F 1 F 2 , where F i (∼ 10 4 ) is the finesse of the i -th resonator. A preliminary experiment is described.

Raman-induced rotational coherence in gaseous molecules observed by the ultrafast polarization spectroscopy

Abstract Raman-induced rotational coherence in the ground vibronic state of gaseous molecules, air, nitrogen, and oxygen, was studied by using the femtosecond polarization spectroscopy. Coherence-burst signals due to the rotational coherences were observed in picosecond region. Their Fourier spectra give the frequencies of the induced coherences between the rotational levels.

Domain-wall pairing in the 3D-ordered phase of the Ising-like antiferromagnet CsCoCl3 studied by 133Cs transverse relaxation

Abstract The transverse relaxation time T2 of 133Cs NMR was measured in the 3D-ordered phase of the quasi-one-dimensional S = 1 2 Ising-like antiferromagnet CsCoCl3. A pulse-like fluctuation due to spin inversion in the Co2+ chains well explains the experimental results. This strongly suggests a domain-wall pairing in the chain.

Rotational coherence in gaseous molecules observed by the femtosecond Raman-induced polarization spectroscopy

Femtosecond coherence-burst signals due to the rotational coherences in the ground vibronic state of gaseous molecules, air, nitrogen, and oxygen, were observed by using the Raman-induced polarization spectroscopy. The frequencies of the induced coherences between the rotational levels are obtained from the Fourier spectrum of the coherence-burst signal. Calculations using a theory of coherent beats due to the Raman-induced rotational coherence reproduced well the observed signal and spectrum.

Optical-pumping model of synchronized quantum-beat echoes

We develop a new theoretical treatment that can explain, for the first time to our knowledge, unexpectedly strong synchronized-quantum-beat-echo (SQBE) signals that we previously observed in the ground state of sodium atoms. The SQBE is obtained by successive application of two light-pulse trains tuned to an optical transition under the condition that the pulse repetition frequency or that of one of its harmonics is equal to the separation of a ground-state sublevel pair. The theory is developed for weak optical pumping for which the duration of the light-pulse trains is much longer than the spontaneous decay time of the excited state. We show that the echo is formed effectively as a result of nonuniform depopulation from the sublevel pair to the other levels because of frequency-selective optical pumping during the rapid spontaneous decay of the excited state. Details of the experiment with sodium atoms are described also.

Adiabatic demagnetization in the rotating frame by non-resonant oscillating field

Abstract The adiabatic demagnetization in the rotating frame is carried out by slowly turning on a non-resonant oscillating field in this frame and it is shown that Zeeman order along an rf field H 1 is transformed into dipolar order.

Noise excitation spectroscopy in Rb atoms

Abstract The signal shapes appeared in the noise excitation spectroscopy are studied. This spectroscopy is based on broad-band excitation with noise light, wave-form correlation, and Fourier analysis. The Zeeman signals of the excited state of the D1 transition in Rb atoms were observed. The Fourier spectra obtained from the auto- and cross-correlations of excitation and emission wave forms are compared and are analyzed by a linear response theory.

Ultrafast dynamics and softening of a phonon mode in strontium titanate

Coherent phonons were observed in the low temperature phase of SrTiO3. Ultrafast dynamics and softening of phonon modes, which contribute to the structural phase transition at 105 K, are studied by using ultrafast polarization spectroscopy. Strontium titanate (SrTiO3) is a well-known quantum paraelectric material. Its dielectric constant increases extraordinarily with decreasing temperature, while the paraelectric phase is stabilized by quantum fluctuations without a ferroelectric phase transition. The dielectric property and the lattice dynamics have been extensively studied. Recently, ferroelectricity induced by the isotopic substitution of oxygen 18 for oxygen 16 [1] and strong enhancement of dielectric constant by UV-light irradiation [2] were discovered, and deeper interest has been taken in SrTiO3 again. This crystal is a typical dielectric material with perovskite structure. The crystal structure is cubic (Oh) above TC=105K, where all phonon modes are Raman-inactive. At TC, SrTiO3 undergoes structural phase transition at zone boundary. The crystal structure changes into tetragonal (D4h) below TC, where Raman-active modes appear. Coherent phonons and their softening observed in the time domain is studied by the ultrafast polarization spectroscopy with the pump-probe technique. Coherent phonons are created by a linearly polarized pump pulse (790 nm, 0.2 ps) through the process of impulsive stimulated Raman scattering, and induce time-dependent anisotropy of refractive index. The change of the refractive index after the pulse excitation is detected by a polarimeter with a quarter-wave plate as the change of the polarization of a linearly polarized probe pulse (690 nm, 0.2 ps), whose plane of polarization is tilted by 45 from that of the pump pulse. The polarimeter measures elipticity of the probe pulse due to the birefringence induced by pump pulse. The thickness of the sample is 0.5 mm, and the direction of the laser beams is perpendicular to the (100) surface. At zero delay, a large signal due to the Kerr effect, whose width is determined by the pulse width, appears. After that, dumped oscillation of coherent phonons is observed as shown in Fig. 1. The change of the polarization corresponding to the oscillation amplitude of the coherent phonon signal is about 10 of the electric-field amplitude of the probe pulse. The oscillation period and the relaxation time of coherent phonons at 6 K in Fig. 1(a) are 0.7 ps and 12 ps. As the temperature is increased, the oscillation period becomes longer and the relaxation time becomes shorter. At TC=105 K in Fig. 1(c), the phase transition point, the oscillation disappears. Above TC no signal of coherent phonons are observed. Fig. 1. Observed signals of coherent phonons in SrTiO3 at (a) 6 K, (b) 90 K, and (c) 105 K. 0 2 4 6 8 Delay Time (ps) Si gn al In te ns ity (a rb . u ni t) (a) 6 K

Coherent phonon spectroscopy in lanthanum aluminate

Coherent phonons were observed in LaAlO3 by ultrafast polarization spectroscopy. Softening of a phonon mode related to the structural phase transition at TC=~750K is studied. Observed beat signal suggests violation of axial symmetry below TC. Lanthanum aluminate (LaAlO3) is known to undergo a structural phase transition at TC = ~750K from a cubic perovskite structure (Oh) to a rhombohedral structure (D3d) [1]. This phase transition is characterized by the softening of phonons at the R corner of the cubic Brillouin zone and concomitant doubling of the unit cell. The triply degenerate phonon mode belongs to the R25 irreducible representation, and its components can be thought of as alternate librations of AlO6 octahedra around the cubic axis. In the low temperature phase, the distortion from cubic symmetry corresponds to a condensation of a linear combination of all three cubic components of the R25 mode, while other materials with perovskite structure such as SrTiO3 (TC = 105 K) have a tetragonal distortion in the low temperature phase resulting from the condensation of only one of the cubic components of the R25 mode. Below TC, the R point becomes the Γ point of the D3d phase, where the soft phonon splits into Raman-active modes of symmetries A1g (singlet) and Eg (doublet). Coherent phonons and their softening observed in the time domain is studied by the ultrafast polarization spectroscopy with the pump-probe technique. Coherent phonons are created by a linearly polarized pump pulse, through the process of impulsive stimulated Raman scattering, and induce time-dependent anisotropy of refractive index. The change of the refractive index after the pulse excitation is detected by a polarimeter with a quarter-wave plate as the change of the polarization of a linearly polarized probe pulse, whose plane of polarization is tilted by 45 from that of the pump pulse. The polarimeter measures elipticity of the probe pulse due to the birefringence induced by pump pulse. The pulse width, repetition rate, and waist size at the sample are 0.2 ps, 1 kHz, and 1 mm, respectively. The wavelength and pulse energy of the pump (probe) pulse are 790 nm and ~1μJ (690 nm and ~0.1μJ). The thickness of the sample is 0.5 mm, and the direction of the laser beams is perpendicular to the (100) surface. Figure 1 shows observed dumped oscillation of coherent phonons. The oscillation period and the relaxation time of coherent phonons at 6 K in Fig. 1(a) are 0.9 ps and > 100 ps. As the temperature is increased, the oscillation period becomes longer and the relaxation time becomes shorter. Near TC the oscillation disappears. Fig. 1. Observed signals of coherent phonons in LaAlO3 at (a) 6 K, (b) 100 K, (c) 300 K, and (d) 600 K. 0 10 20 Delay Time (ps) (a) 6 K