Toshihiro Nakamura

Blue-light-emitting ZnSe random laser

We observed blue-light-emitting random laser action with coherent feedback in ZnSe particles. Below the threshold excitation, we observed a broad spontaneous emission band peaking at approximately 470 nm. Above the threshold, several discrete lasing lines appear at the center of the spontaneous emission band (approximately 475 nm). The linewidth of each lasing emission is less than 0.4 nm, which is about 40 times narrower than that of the spontaneous emission band. Although the lasing intensity of each line varies from pulse to pulse, the peak wavelengths do not shift significantly. The lasing emission was also found to radiate in all directions.

Gold-nanoparticle-assisted random lasing from powdered GaN

We demonstrate Au-nanoparticle-assisted random lasing from a powdered GaN sample. In the presence of Au nanoparticles on GaN powder surfaces, several lasing lines are observed in photoexcited luminescence spectra near the center of the GaN band-edge emission peak. The random lasing is considered to arise from a decrease in the lasing threshold due to the suppression of crystal defect loss by surface plasmon excitation on Au. From spatially resolved lasing emission spectra and their FT analysis results, the formation of random lasing cavities at different spatial positions is confirmed. The size of the random lasing spot is determined to be larger than that of the scattered light speckle of the pumping source on a thin powdered GaN sample.

Control of random lasing in ZnO/Al2O3 nanopowders

We demonstrate that the random lasing wavelength of ZnO/Al2O3 nanopowders can be controlled by varying the weight fraction of Al2O3. This controllability is due to the fact that the extent of self-absorption by ZnO nanopowder can be modified by changing the photon-transport mean free path. The lasing threshold excitation power is also dependent on the weight fraction of Al2O3. The random lasing characteristic can be explained well by a theoretical model based on the photon-transport mean free path.

Electron-hole plasma induced band gap renormalization in ZnO microlaser cavities.

We report electron-hole plasma (EHP) lasing in hexagonal ZnO microrods and thin nanobelts. Under the excitation of 325 nm line femtosecond pulsed laser, ultraviolet whispering-gallery mode (WGM) lasing was observed from hexagonal ZnO microrods. When EHP was formed at high excitation energy density, the center wavelength of the WGM lasing band presented a redshift from 387.5 nm to 397.5 nm, and the full width of half maximum (FWHM) of the WGM lasing band increased from 2.5 nm to 7 nm. Each lasing mode showed obvious blueshift and broadening. Such lasing characteristics were attributed to the band gap renormalization (BGR) due to the high carrier concentration at the EHP condition. In addition, EHP Fabry-Perot (F-P) mode lasing from thin ZnO nanobelt was also observed and discussed. According to the phenomenological BGR calculation with including the carrier density dependent screening effect, the values of the band gap of ZnO at different excitation energy densities were obtained, which agree well with the experimental results.

Origins of lasing emission in a resonance-controlled ZnO random laser

We investigate the origins of lasing emission in a scatterer-resonance-controlled random laser made of ZnO nanopowder over a wide temperature range (20–300 K). At higher temperatures ( K), the lasing emission appears around exciton recombination energies and the lasing threshold carrier density is comparable to the Mott density, indicating that the resonance-controlled random laser is going toward showing excitonic lasing; at lower temperatures, random lasing is caused by usual electron–hole plasma recombination because of the threshold carrier density being much larger than the Mott density.

Improved lasing characteristics of ZnO/organic-dye random laser

We investigate the random lasing characteristics of ZnO nanopowder suspended in methanol with the addition of organic-dye molecules. At a certain dye concentration, the random lasing intensity is enhanced, and the excitation threshold power is reduced. These improvements are explained by Forster-type resonant energy transfer from the dye-molecule donor to the ZnO-nanopowder acceptor.

Properties of silver/porous-silicon nanocomposite powders prepared by metal assisted electroless chemical etching

We demonstrate that silver/porous-Si nanocomposite powders are obtained by etching Si powders with metal assisted electroless chemical reaction in an aqueous solution of AgNO3 and HF. From x-ray photoelectron spectroscopy and scanning electron microscope measurements, metal layer consisting of nanometer-sized Ag particles and their aggregates is formed on each Si powder surface. Immoderate etching at higher concentrations of AgNO3 results in the decrease in Si volume and increase in Ag layer thickness. Dips are observed in reflectance spectra of the nanocomposite powders at ∼400u2002nm. By comparing the experimental spectra with theoretically calculated ones, these dips are found to arise from the optical absorption due to the roughness-assisted surface plasmon excitation and to be dependent on the size of the Ag nanostructure. The nanocomposite powders show weaker photoluminescence intensity than that of conventionally stain-etched porous Si powders, however, their photostability is improved. The improvement...

Resonant energy transfer in (Eu3+, Bi3+)-codoped CaZrO3 red-emitting phosphor

(Eu3+, Bi3+)-codoped CaZrO3 phosphor crystallizing in the orthorhombic structure was synthesized from a mixture of CaCO3, ZrO2, Eu2O3, and Bi2O3 using the solid-state reaction method. The structural and optical properties of this phosphor were investigated using X-ray diffraction analysis, photoluminescence (PL) analysis, PL excitation (PLE) spectroscopy, and PL decay measurements. The effects of Bi3+ codoping on the Eu3+ emission properties were discussed in detail. Temperature dependence of the PL intensity was also measured between T = 20 and 450 K and obtained a quenching energy of Eq ∼ 0.1 eV. Remarkably, the red emission intensity of Eu3+ in the (Eu3+, Bi3+)-codoped phosphor was enhanced more than 10 times compared to the Eu3+ singly doped phosphor, thereby suggesting an efficient resonant energy transfer from Bi3+ to Eu3+. The asymmetry ratio (5D0 → 7F2)/(5D0 → 7F1) for the Eu3+ emission had different values with different Eu3+ concentrations, indicating that its local site depends on the Eu3+/Bi3+ concentration. Comparative discussion was also given on the PL spectra for some red-emitting phosphors, CaZrO3:Eu3+, Bi3+, SnO2:Eu3+, and K2SiF6:Mn4+.

Discrete-mode ZnO microparticle random laser

A random laser incorporating irregular-shaped ZnO microparticles exhibits a small number of lasing lines with stable lasing intensities and negligibly low background emission. This unique feature is in direct contrast to that observed for a conventional ZnO nanoparticle film random laser. Gain competition between the discrete laser modes also occurs in this microparticle random laser. These lasing characteristics are because of the intra-particle confinement of the laser cavity modes in each ZnO microparticle.

Properties of magnetic nickel/porous-silicon composite powders

The magnetic and photoluminescence (PL) properties of nickel/porous-silicon (Ni/PSi) composite powders are investigated. Ni/PSi composite powders are prepared by stain etching of Si powder in a HF/HNO3 solution followed by electroless plating of Ni nanoparticles on the stain-etched PSi powder in a NiCl2 solution. The Ni/PSi powders exhibit hydrophillicity, superparamagnetism caused by the deposited Ni nanoparticles, and orange-red PL owing to the nanostructured PSi surface. The degree of magnetization decreases with increasing Ni plating time, indicating its dependence on the size of the Ni nanoparticles. The Ni/PSi composite powders also show a stronger magnetization as compared to that of the Ni-particle-plated Si powder. The stronger magnetization results from the larger surface area of PSi. The PL intensity, peak wavelength, and lifetime of Ni/PSi are strongly dependent on the NiCl2 concentration. This dependence is due to the different thickness of the oxide overlayer on the PSi surface formed during...