Kohji Mizoguchi

Study on dynamical structure in water and heavy water by low‐frequency Raman spectroscopy

Depolarized Raman spectra below 250 cm−1 in water and heavy water were measured and analyzed from 373 K to the supercooled region. The spectral feature of the central component below 20 cm−1 is stressed in the present work. The spectra below 250 cm−1 in water and heavy water are interpreted as a superposition of one Debye‐type relaxation mode and two damped harmonic oscillators. The damped harmonic oscillators (broadbands around 60 cm−1 and 190 cm−1) are interpreted as the restricted translational modes. Analyzing the temperature dependence of the relaxation mode, the reciprocal relaxation time τ−1 in heavy water changes linearly with τ−1 ∝ (T − 240 K) in the whole temperature range. On the other hand, the temperature dependence of the reciprocal relaxation time in water above 303 K deviates from a straight line which holds below 298 K as τ−1 ∝ (T − 225 K).

Optical control of coherent optical phonons in bismuth films

Interference of impulsively excited coherent phonons in semimetals has been studied by using a double‐pulse pump–probe technique. Enhancement of the oscillation amplitude of an A1g mode is observed when the separation time of the double‐pulse is matched to the period of the phonon oscillation, and a cancellation is observed when the separation time is adjusted to half the period of the phonon oscillation. The amplitude after the second pulse shows a sinusoidal dependence as a function of the separation time, and this dependence is explained in terms of a superposition of two coherent phonon oscillations. In addition, not only the A1g mode but also an Eg mode have been observed by electro‐optic sampling.

Determination of crystallographic orientations in silicon films by Raman-microprobe polarization measurements

Local crystallographic orientations of semiconductors have been determined by a scanning Raman microprobe combined with polarization measurements. We present a method of Raman scattering determination of crystallographic orientations in silicon crystals which allows rapid determination with an accuracy of ±2°. It is found that surface morphology affects the Raman polarization analysis. For laser‐annealed silicon layers capped with silicon nitride films, unpolarized scattered light is superimposed on polarized scattered light, because incident light beams entering rough surfaces are directed into random orientations and produce the unpolarized scattered light. The degree of the surface roughness is estimated by assuming that the rough surface consists of periodically arranged spherical segments.

Raman Scattering Measurements of Strains in ZnSe Epitaxial Films on GaAs

Raman spectroscopy has been applied to the evaluation of strains in ZnSe films grown by molecular beam epitaxy on GaAs. The observed variation of the ZnSe LO phonon frequency with thickness is explained well in terms of the conversion from the elastic strain due to accommodation of the lattice mismatch to the thermal strain due to the difference in thermal expansion coefficients of the substrate and epitaxial film. The strains estimated from the Raman measurements are consistent with the results obtained from X-ray analyses by other researchers.

Dynamical structure of water in NaCl aqueous solution

We report on the dynamical structure of water in NaCl aqueous solutions as functions of temperature and concentration by a low-frequency Raman spectroscopy. The spectral profiles in the frequency range from −50 to 250 cm−1 have been analyzed with a superposition of one Cole–Cole type relaxation mode and two damped harmonic oscillator modes. The distributions of the characteristic frequencies of the intermolecular vibrational modes observed around 50 and 180 cm−1 in NaCl aqueous solutions are always wider than those in pure water. In NaCl aqueous solutions with various concentrations, the spectral line width g1=1/(2πcτ) of a central component, which corresponds to the reciprocal relaxation time, linearly changes with temperature from about 255 K to 300 K, while the relaxation time above 300 K holds an Arrhenius-type behavior. The slope of the spectral linewidth against temperature below 300 K decreases with increasing concentration. The distribution parameter of the relaxation time in the Cole–Cole type re...

Effect of oxygen content on phonon Raman spectra of YBa2Cu3O7−δ

Abstract Raman spectra of YBa2Cu3O7−δ have been measured on samples with various oxygen content, which were obtained by quenching from various temperatures. It is found that the Raman band associated with oxygen vibrations observed at ∼ 500 cm−1 is very sensitive to the oxygen content; its frequency shifts continuously from 502 to 484 cm−1 as the quenching temperature is varied from below 500°C up to 1000°C. This result indicates that this band can be used to determine the oxygen content in YBa2Cu3O7−δ.

Forcibly driven coherent soft phonons in GeTe with intense THz-rate pump fields

We propose an experimental technique to generate large amplitude coherent phonons with irradiation of THz-rate pump pulses and to study the dynamics of phase transition in GeTe ferroelectrics. When a single pump pulse irradiates the sample at various pump power densities, the frequency of the soft phonon decreases sublinearly and saturates at higher pump powers. By contrast, when the THz-rate pump pulse sequence irradiates the sample at matched time intervals to forcibly drive oscillation, a large redshift of the phonon frequency is observed without any saturation effects. After excitation with a four pump pulse sequence, the coherent soft phonon becomes strongly damped and leads to a near critical damping condition. This condition indicates that the lattice is driven to a precursor state of the phase transition.

Growth and characterization of 70Gen/74Gen isotope superlattices

Abstract We report on the molecular beam epitaxial (MBE) growth of 70 Ge n / 74 Ge n isotope superlattices composed of alternating layers of the stable isotopes 70 Ge and 74 Ge. Samples prepared in this work have atomic layers n =4, 8, 16, and 32. All superlattices are p-type having a net-impurity concentration of ∼10 16 cm −3 . Zone-folding of optical phonons due to the mass periodicity in the growth direction has been observed clearly for all samples using high resolution Raman spectroscopy. The corresponding phonon mode of each Raman peak has been indexed according to theoretical calculations using the linear-chain model and the planar bond-charge model. The frequency of the Raman peaks found by the experiment agree very well with those of the phonon modes calculated for each superlattice structure. A detailed analysis of the Raman spectra concludes that the degree of interface mixing between 70 Ge and 74 Ge layers for our typical growth condition is less than two monolayers.

Characterization of Silicon Implanted with Focused Ion Beam by Raman Microprobe

Damage to silicon implanted with Si++, Au++ and Be++ ions using focused ion beams has been evaluated by a Raman microprobe technique. A quantitative evaluation of damage to ion-implanted silicon was made by comparing the intensities of Raman scattering from implanted and unimplanted areas. The minimum dose at which damage is detectable by Raman measurements is 9×1011 ions/cm2 for Au++, 7.5×1012 ions/cm2 for Si++ and 7.5×1013 ions/cm2 for Be++. From the depth distribution of the damage measured using laser lines of different wavelengths, it is inferred that damage at the near-surface region in Be++ implanted silicon is reduced by self annealing during high-dose-rate implantation and that a channeling effect causes the saturation dose in Be++ implantation to be higher than that in other ion implantations.

Selective Enhancement of Coherent Optical Phonons Using THz-rate Pulse Train

Selective enhancement of coherent optical phonons in Bi-Sb mixed crystals has been performed using terahertz (THz) rate pulse trains generated by spatial phase modulation of femtosecond pulse with a liquid-crystal spatial light modulator. The profile of the shaped pulse train has a flat-toped shape. The Bi-Bi, Bi-Sb, and Sb-Sb vibrations can be enhanced and canceled selectively for pulse trains with a suitable repetition rate.