Kazuyuki Hirao

Bio: Kazuyuki Hirao is an academic researcher from Kyoto University. The author has contributed to research in topics: Femtosecond & Laser. The author has an hindex of 60, co-authored 542 publications receiving 14829 citations. Previous affiliations of Kazuyuki Hirao include Kobe University & Kyocera.

Writing waveguides in glass with a femtosecond laser

Abstract: With the goal of being able to create optical devices for the telecommunications industry, we investigated the effects of 810-nm, femtosecond laser radiation on various glasses. By focusing the laser beam through a microscope objective, we successfully wrote transparent, but visible, round-elliptical damage lines inside high-silica, borate, soda lime silicate, and fluorozirconate (ZBLAN) bulk glasses. Microellipsometer measurements of the damaged region in the pure and Ge-doped silica glasses showed a 0.01–0.035 refractive-index increase, depending on the radiation dose. The formation of several defects, including Si E′ or Ge E′ centers, nonbridging oxygen hole centers, and peroxy radicals, was also detected. These results suggest that multiphoton interactions occur in the glasses and that it may be possible to write three-dimensional optical circuits in bulk glasses with such a focused laser beam technique.

Fabrication of long-period fiber gratings by focused irradiation of infrared femtosecond laser pulses

Abstract: We have fabricated long-period fiber gratings by use of a novel technique using focused irradiation of infrared femtosecond laser pulses. We investigate the thermal stability of the fabricated fiber gratings. The values of the loss peak wavelength and the transmittance of the fiber gratings after heat treatment below 500°C are the same as initial values before heat treatment. The fiber gratings that were fabricated by this technique have a high resistance to thermal decay. We propose that this technique mill be useful for fabrication of fiber gratings with a superior aging characteristic.

Ultrafast dynamics of nonequilibrium electrons in a gold nanoparticle system

Abstract: Transient absorption spectra were measured to investigate the nonlinear response of a gold nanoparticle system by the femtosecond pump-probe method We obtained temporal changes of electron temperatures and effective damping constants by fitting transient absorption spectra with Mie scattering theory In the ultrafast region, the nonlinear response originates mainly from the hot electron system which is heated by the incident pump pulse It is noteworthy that the lattice temperature plays an important role even in the first step of the nonlinear response through the change of the effective damping constant In the long-time-scale region, over 10 ps, both the electron and the lattice temperatures contribute to the nonlinear response comparably The origin of the effective damping constant is also discussed with the surface scattering and the $e\ensuremath{-}p$scattering processes

Synthesis of Monolithic Al2O3 with Well-Defined Macropores and Mesostructured Skeletons via the Sol−Gel Process Accompanied by Phase Separation

Abstract: Alumina (Al2O3) monoliths with well-defined macropores and mesostructured skeletons have been synthesized via a spontaneous route from the aqueous and ethanolic solution of aluminum salts in the presence of propylene oxide and poly(ethylene oxide) (PEO). The addition of propylene oxide to the starting solution controls the gelation, whereas the addition of PEO induces the phase separation. Appropriate choice of the starting composition, by which the phase separation and gelation concur, allows the production of bicontinuous macroporous Al2O3 monoliths in large dimensions (10 × 10 × 10 mm3). The size of macropores is controlled in the range of 400 nm to 1.8 μm, depending on the PEO content in starting solutions. The dried gel is amorphous, whereas heating at temperatures above 800 °C leads to the formation of crystalline phases without spoiling the macroporous morphology; nanocrystalline γ-Al2O3 is precipitated at 800 °C, α-Al2O3 starts to form at 1000 °C, and complete transformation into α-Αl2O3 is achiev...

Gold nanoparticles: assembly, supramolecular chemistry, quantum-size-related properties, and applications toward biology, catalysis, and nanotechnology.

Abstract: Although gold is the subject of one of the most ancient themes of investigation in science, its renaissance now leads to an exponentially increasing number of publications, especially in the context of emerging nanoscience and nanotechnology with nanoparticles and self-assembled monolayers (SAMs). We will limit the present review to gold nanoparticles (AuNPs), also called gold colloids. AuNPs are the most stable metal nanoparticles, and they present fascinating aspects such as their assembly of multiple types involving materials science, the behavior of the individual particles, size-related electronic, magnetic and optical properties (quantum size effect), and their applications to catalysis and biology. Their promises are in these fields as well as in the bottom-up approach of nanotechnology, and they will be key materials and building block in the 21st century. Whereas the extraction of gold started in the 5th millennium B.C. near Varna (Bulgaria) and reached 10 tons per year in Egypt around 1200-1300 B.C. when the marvelous statue of Touthankamon was constructed, it is probable that “soluble” gold appeared around the 5th or 4th century B.C. in Egypt and China. In antiquity, materials were used in an ecological sense for both aesthetic and curative purposes. Colloidal gold was used to make ruby glass 293 Chem. Rev. 2004, 104, 293−346

Chemistry and properties of nanocrystals of different shapes.

Abstract: The interest in nanoscale materials stems from the fact that new properties are acquired at this length scale and, equally important, that these properties * To whom correspondence should be addressed. Phone, 404-8940292; fax, 404-894-0294; e-mail, mostafa.el-sayed@ chemistry.gatech.edu. † Case Western Reserve UniversitysMillis 2258. ‡ Phone, 216-368-5918; fax, 216-368-3006; e-mail, burda@case.edu. § Georgia Institute of Technology. 1025 Chem. Rev. 2005, 105, 1025−1102

Spectral properties and relaxation dynamics of surface plasmon electronic oscillations in gold and silver nanodots and nanorods

Abstract: The field of nanoparticle research has drawn much attention in the past decade as a result of the search for new materials. Size confinement results in new electronic and optical properties, possibly suitable for many electronic and optoelectronic applications. A characteristic feature of noble metal nanoparticles is the strong color of their colloidal solutions, which is caused by the surface plasmon absorption. This article describes our studies of the properties of the surface plasmon absorption in metal nanoparticles that range in size between 10 and 100 nm. The effects of size, shape, and composition on the plasmon absorption maximum and its bandwidth are discussed. Furthermore, the optical response of the surface plasmon absorption due to excitation with femtosecond laser pulses allowed us to follow the electron dynamics (electron−electron and electron−phonon scattering) in these metal nanoparticles. It is found that the electron−phonon relaxation processes in nanoparticles, which are smaller than t...