Masaki Takesada

A gigantic photoinduced dielectric constant of quantum paraelectric perovskite oxides observed under a weak DC electric field

A gigantic enhancement of dielectric constant by UV light irradiation is reported for the first time in the quantum paraelectric family of perovskite oxides, such as SrTiO 3 and KTaO 3 , under a weak DC field. The enhancement has not been observed in ferroelectric SrTi 18 O 3 induced by substitution of 18 O for 16 O. The present result suggests a new class of light-switching phenomena controllable externally.

Brillouin scattering study of ZnO

Polarized Brillouin scattering measurements were carried out for a bulk ZnO single crystal. The whole set of elastic stiffness constants was determined to be c11=19.0, c12=11.0, c13=9.0, c33=19.6, c44=3.9, and c66=4.0 in units of 1011 dyn/cm2. The relationships between photoelastic constants at wavelength of 514.5 nm were also obtained: p11, p13, p44, and p66=1.8, 2.3, 0.50, and 0.38, respectively, relative to p12.

Glass cutting by femtosecond pulsed irradiation

We report on quartz and glass cutting by a lateral scanning of femtosecond pulses (150 fs at 1 kHz repetition rate) of 800 nm wavelength at room and low pressure (5 Torr) air ambience. Pulses were focused by a low numerical aperture (NA0.1) objective lens. Optimization of fabrication conditions: pulse energy and scanning speed were carried out to achieve large-scale (millimeter-to-centimeter) cutting free of microcracks of submicron dimensions along the edges and walls of the cut. Cutting through out the samples of 0.1-0.5 mm thickness was successfully achieved without apparent heat affected zone. At low air pressure (5 Torr) ambience, redeposition of ablated material was considerably reduced. It is demonstrated that the damage on the rear surface was induced by the stress waves, which originated from the plasma ablation pressure pulse. The mechanism of femtosecond-laser cutting of transparent materials at high irradiance and the influence of stress waves generated by plasma plume are discussed.

Photoinduced Phenomena in Quantum Paraelectric Oxides by Ultraviolet Laser Irradiation

A photo-induced effect in the quantum dielectric system, such as SrTiO3, KTaO3, and CaTiO3, has been investigated under rather small external DC electric field. The dielectric measurement was performed under ultraviolet (UV) laser excitation combined with DC electric field. The gigantic photo-induced effects of dielectric properties of the quantum paraelectric systems have been firstly observed in the perovskite-type materials. The photoinduced dielectric effect was obtained as a function of DC electric field under constant UV irradiation.

Micro-Character Printing on a Diamond Plate by Femtosecond Infrared Optical Pulses

Processing of less than 400 nm has been performed on the surface of a diamond plate by means of a femtosecond infrared pulse laser. Various characters with a size of about 1 µm were drawn by the femtosecond pulse laser system in conjunction with a microscope equipped with a precisely controlled piezo-stage. The tightly focused laser light on the flat surface of the diamond made it possible to minimize the light-induced graphitization. The surface of the diamond plate after laser machining was analyzed by micro-Raman measurements to estimate the graphitization effect induced by laser irradiation. The obtained results indicate that graphitization increased with the number of irradiated laser pulses.

Monoclinic-orthorhombic transition in K2ZnBr4 and Rb2ZnI4

Abstract The first-order transition from the monoclinic α-phase to the orthorhombic β-phase in K2ZnBr4 and Rb2ZnI4 is investigated by X-ray diffraction. The structure of β-phase is confirmed to be isostructural with disordered β-K2SO4. Although structures of α − and β-phases are different so much with each other, the transformation proceeds without cracking or deforming the crystal.

Photo-induced effect of ferroelastic phase transition in KD3(SeO3)2

Abstract The laser-light refraction method has been performed to investigate the photo-induced effect in an early stage of ferroelasitc domain formation in KD3(SeO3)2 crystal. The refracted light intensity has been observed in the temperature region (298.5K–300K) around the transition point (Tc). The temperature dependence of the refracted intensity under the irradiation 266 nm with 130 fs width is drastically different from that without UV-light irradiation. The discrepancy is clear in the temperature region above Tc. The results clearly demonstrate that the pulsed laser illumination produces a large effect in the embryonic fluctuation near Tc.

Phase Transition in Bi-Layered Oxides with Five Perovskite Layers

The dielectric behavior of the Bi-layered perovskite oxides Sr2Bi4Ti5O18 and Pb2Bi4Ti5O18 with five perovskite-like units (m=5) was studied. A new anomaly was found at T1=428 K for Sr2Bi4Ti5O18 and 480 K for Pb2Bi4Ti5O18, in addition to the previously reported phase transition at Tc. The dielectric constant does not show the Curie–Weiss behavior in both Sr2Bi4Ti5O18 and Pb2Bi4Ti5O18 crystals: the anomaly around Tc is diffuse and broad, whereas those at T1 are clearly shoulderlike. These two isomorphous layered-oxide ferroelectrics undergo a new intermediate phase transition at T1. The transition temperatures show a linear dependence on the ionic radii of A-site ions in these layered perovskites.

HIGH-PRESSURE BRILLOUIN SCATTERING STUDY OF DENSE ARGON AND NITROGEN

We have measured the Brillouin shift as a function of hydrostatic pressure in dense argon and nitrogen at different constant temperatures. We show that for argon there is a linear relation between the index of refraction and the reduced density. A comparison of the sound velocities determined ultrasonically and from our Brillouin data shows a negative velocity dispersion. This is in agreement with theoretical predictions and measurements of the temperature dependence of the sound velocity along the vapor‐pressure equilibrium curve using Brillouin scattering. The fact that the negative velocity dispersion increase with pressure at constant temperature is smaller at lower temperatures is, however, puzzling and cannot be explained within the framework of the above‐mentioned theoretical model.

Giant Photoconductivity in Quantum Paraelectric Oxides

Photoconductivity and photo-magneto-transport properties have been measured in quantum paraelectric SrTiO3 crystals. We have found a large isotope effect in the photocurrent value in this system. This isotope effect suggests an important contribution of the quantum fluctuations for the giant photoconductivity in the quantum paraelectric system.