Kohei Soga

Upconversion mechanism in Er3+-doped fluorozirconate glasses under 800 nm excitation

The upconversion emission intensities of Er3+ ion around 550 and 660 nm in fluorozirconate glasses were measured under 800 nm excitation. Though energy transfer processes played an important role in upconversion mechanism at high concentration of ErF3, those have not been treated quantitatively. The energy transfer rates were calculated from the optical parameters assuming some distribution of Er3+ ions. We calculated the upconversion intensities around 550 and 660 nm by using rate equations. It was found that the dependence of upconversion emission intensities on the ErF3 concentration could be reproducible and the principal upconversion mechanism could be evaluated.

Calculation and simulation of spectroscopic properties for rare earth ions in chloro-fluorozirconate glasses

Abstract Europium doped fluorozirconate and chloro-fluorozirconate glasses with Zr–Ba–La–Al–F–Cl system were prepared by conventional melting method and their emission spectra were studied. Simulation of emission spectra of these glasses by point charge models of crystal field were performed. We found that the variation of electronic state of rare earth (RE) ions should be taken into account for the simulation of optical spectra in chloro-fluorozirconate mixed anion glasses. We applied electronic state calculations for estimating the electronic state of europium ions in glass structural models that were derived from molecular dynamics simulations. By applying the parameters derived from DV-Xα calculation, point charge model of the crystal field and Judd–Ofelt theory, the simulation of spectra agreed with observed spectra of the europium ions in chloro-fluorozirconate glasses in terms of spectral width and intensity.

Fluorescence properties of fluorozirconate glasses containing Eu3+ ions

Abstract Fluorozirconate glasses containing Eu 3+ ions and chloride ions were prepared and the lifetimes of the 5 D 0 state and the phonon side bands in these glasses were measured. It was found that the addition of chloride ions had a great effect to increase the radiative transition rate. The phonon side bands of the 7 F 0 - 5 D 0 transitions were clearly observed for several glasses. These spectra were compared with the FT-IR spectra and the electron-phonon coupling strengths of total vibrational modes were calculated from the phonon side band spectra.

Temperature dependence of electrical conductance in single-crystalline boron nanobelts

We studied electrical transport in single-crystalline boron nanobelts with α-tetragonal crystalline structure. We obtained ohmic contacts to the boron nanobelts by metal electrodes of Ni∕Au bilayer. From the temperature dependence of electrical conductance, we found that the boron nanobelt is a semiconductor. The electrical conductivity was of the order of 10−3(Ωcm)−1 at 295 K. Fitting the results to variable-range-hopping conduction revealed a high density of localized states at the Fermi level compared with bulk β-rhombohedral boron.

Luminescent properties of nanostructured Dy3+- and Tm3+-doped lanthanum chloride prepared by reactive atmosphere processing of sol-gel derived lanthanum hydroxide

Dy3+- and Tm3+-doped lanthanum chloride was prepared via reactive atmosphere processing (RAP) of sol-gel derived hydroxide powders. The low-phonon energy chloride host facilitated the 1.3-μm emission from Dy3+ and the 1.2- and 1.4-μm emissions from Tm3+. The emission intensities of the Dy3+ and Tm3+ ions increased logarithmically with increasing RAP temperature. The dependence of emission intensities and lifetimes on the rare-earth (RE) concentration was also investigated. The emission intensity was found to increase up to 1 moleu200a% RE for both ions. Luminescence quenching was observed for concentrations exceeding 1 moleu200a% for Tm3+ and 0.1 moleu200a% for Dy3+.

Simulation of the optical properties of Er:ZBLAN glass

Abstract Absorption and emission spectra of Er3+-doped ZBLAN glass at room temperature were estimated from structural models prepared by using molecular dynamics (MD) simulation based on crystal field theory and point-charge approximation. The calculated absorption and emission spectra substantially agreed with the observed ones. Furthermore, it was found that the emission spectra at low temperatures could be also reproduced to some extent. The relation between the splitting of the energy level and the crystal field parameters calculated from the Er3+ ion in each structural model was examined. According to this relation, the energy levels could be classified into several groups. The splitting of the group represented by the 4 I 9/2 level was dominated by the short-range structure around the Er3+ ions. The transition process for the measurement of the well-defined fluorescence line narrowing (FLN) spectrum was estimated from the characteristics of the splitting of the energy levels.

Structure and optical properties of Eu3+-doped fluoroaluminate and fluorophosphate glasses

Abstract Emission spectra of Eu 3+ in fluoroaluminate and fluorophosphate glasses at 80 K were measured as a function of excitation energy by the fluorescence line narrowing (FLN) technique. The magnitudes of the splitting of the 7 F 1 manifolds at lower excitation energies increased with increasing phosphate content. The relative intensity of 5 D 0 → 7 F 2 to 5 D 0 → 7 F 1 transitions increased by adding the phosphate compound. This increase decreased gradually with increasing excitation energy. The atomic structure around Eu 3+ in these glasses was simulated using molecular dynamics techniques. The Stark splitting and the transition probabilities between 5 D 0 and 7 F J energy states at each simulated Eu 3+ ion were calculated on the basis of the point-charge approximation. The predicted energy differences between 5 D 0 and ( 7 F 2 , 7 F 1 and 7 F 0 ) energy levels agree within 100 cm −1 with the observed ones. The transition probabilities are discussed in terms of the crystal field potential and the configuration interaction parameters, Ξ ( k , λ ).

Electron density distributions in derivative crystals of α-rhombohedral boron

Boron carbide (B 12 C 3 ) and boron phosphide (B 12 P 2 ) have the similar structures to α-rhombohedral boron (α-B 12 ) and are considered to be derivative crystals of it. The peculiar bonds in these two derivative crystals are visualized by MEM/Rietveld analysis, which involves a combination of the maximum entropy method (MEM) and Rietveld refinement for powder X-ray diffraction (XRD) data. The distinctive bending of bonds observed in α-B 12 is reversed by an insertion of other atoms. This bending arises from the conflict between the crystal structure of the rhombohedral lattice and the icosahedral structure of the B 12 cluster. Bond strength and interatomic distances are also varied by the insertion. In order to estimate the bond strength, we obtained the electron density height at each bond critical point, and compared it with the force constant, which is derived from the Raman shift.

Effect of Yb3+ doping on upconversion emission intensity and mechanism in Er3+/Yb3+-codoped fluorozirconate glasses under 800 nm excitation

Er3+ singly- and Er3+/Yb3+-codoped fluorozirconate glasses were prepared. The upconversion emission intensities of the Er3+ ion around both 550 and 660 nm were measured under 800 nm excitation. The absorption band of the Yb3+ ion is located around 980 nm. Though the absorption band of Yb3+ does not interact directly with the incident light at 800 nm, both upconversion intensities around 550 and 660 nm emission increased with the increasing YbF3 concentration. We calculated the upconversion intensities around 550 and 660 nm by using rate equations and evaluated the principal upconversion mechanism in Er3+ singly- and Er3+/Yb3+-codoped samples quantitatively. We also investigated the reason for the increase of the upconversion emission intensities with the codoping of Yb3+ under 800 nm excitation.

Dependence of photocurrent in single-crystalline boron nanobelts on atmosphere

This letter describes the dependence of photocurrent of single-crystalline boron nanobelts on the atmosphere. In ambient air, slow photoresponse under blue light illumination was observed. Rise and decay times exceeded three days. The magnitude of photoresponse in ambient air and oxygen was greater than that in hydrogen and argon atmospheres. In vacuum, a photoresistivity effect consisting of the continuous decrease of conductance under blue light illumination was observed. Variation of band bending of the nanobelt surface by adsorption or desorption of oxygen and water molecules appeared to switch the photoconduction on and off by the respective trapping and recombination of photoexcited carriers at the nanobelt core and surface.This letter describes the dependence of photocurrent of single-crystalline boron nanobelts on the atmosphere. In ambient air, slow photoresponse under blue light illumination was observed. Rise and decay times exceeded three days. The magnitude of photoresponse in ambient air and oxygen was greater than that in hydrogen and argon atmospheres. In vacuum, a photoresistivity effect consisting of the continuous decrease of conductance under blue light illumination was observed. Variation of band bending of the nanobelt surface by adsorption or desorption of oxygen and water molecules appeared to switch the photoconduction on and off by the respective trapping and recombination of photoexcited carriers at the nanobelt core and surface.