Michihiko Kitao

Linear and nonlinear optical properties of Ag-As-Se chalcogenide glasses for all-optical switching

We prepared Ag(x)(As0.4Se0.6)(100-x) chalcogenide glasses by a melt-quenching method and measured their linear and nonlinear optical properties to evaluate their potential applications to all-optical ultrafast switching devices. Their nonlinear refraction and absorption were measured by the Z-scan method at 1.05 microm. The addition of Ag to As2Se3 glass led to an increase in the nonlinear refractive index without introducing an increase in the nonlinear absorption coefficient. The glass with a Ag content of x = 20 at. % revealed high nonlinearity ranging from 2000 to 27,000 times that of fused silica, depending on the incident optical intensity.

Preparation and Electrochromic Properties of RF-Sputtered NiOx Films Prepared in Ar/O2/H2 Atmosphere

The preparation of electrochromic nickel oxide ( NiOx ) films, which are colorless in an as-deposited state, has been carried out in Ar/O2/H2 mixed gas by the rf-sputtering method. Highly transparent films of as-deposited nickel oxide could be prepared in the sputtering atmosphere consisting of Ar:O2:H2=50:10:40. From IR and XPS measurements, the as-deposited films were found to be hydrated and to have the component of Ni(OH)2. When these films were electrochemically colored, NiOOH was known to remain in the films. The coloration efficiency of the films at the wavelength of 633 nm was nearly independent of hydrogen content in sputtering atmosphere up to 40%, but it depended on sputtering pressure. The maximum value of coloration efficiency was 36 cm2/C for films prepared at hydrogen content of 40% and total pressure of 8 Pa.

Electrochromic properties of sputtered nickel‐oxide films

Electrochromic properties of nickel‐oxide films have been investigated. Films were prepared by rf sputtering from a compressed powder NiO target in an atmosphere of an Ar‐O2 mixture. The deposition rate of the nickel oxide decreased when the total pressure of the sputtering atmosphere was increased, but was independent of the oxygen content. The films obtained can be electrochemically colored and bleached in a 1‐N NaOH electrolyte solution, and are colored anodically. The colored film is dark brown, and the bleached one is transparent and colorless. The maximum coloration efficiency is obtained with the films prepared at the total pressure of 0.04 Torr, and is 36 cm2/C at a wavelength of 640 nm. Coloration efficiency is independent of oxygen content in the sputtering atmosphere.

Ac Conductivity of Amorphous As2Se3

Electrical conduction in amorphous As2Se3 is investigated at frequencies from 30 Hz to 10.3 GHz in the temperature range -180° to 500°C. The observed conductivity σ can be expressed as a sum of dc conductivity σdc and ac conductivity σac, namely σ=σdc+σac. The quantity σdc depends exponentially on reciprocal temperature. The contribution of σac to σ is negligible at high temperature. At low temperature, σac is nearly independent of temperature but strongly depends on the frequency. The frequency dependence of σac is linear and quadratic in the frequency range up to 106 Hz and between 106 and 108 Hz, respectively, and above 108 Hz, σac has no frequency dependence. The experimental results of the conductivity of amorphous As2Se3 cannot be interpreted by the hopping conduction only, and the effects of spatial inhomogeneity may be responsible for the conduction mechanisms in amorphous As2Se3.

Absorption bands of electrochemically-colored films of WO3, MoO3 and MocW1−cO3

Optical absorption bands of electrochemically-colored films of amorphous WO3, MoO3 and MocW1-cO3 were investigated. The peak photon energy of the bands depends not only on the amount of injected charges but also on the Mo content. In the WO3 films, the absorption band originates from the intervalence trasfer of electrons. In MoO3 and mixed films, however, the absorption bands are considered to consist of two different components. The first is the intervalence trasfer band and the second is a component of absorption at higher energies. The second component is considered to be related to Mo atoms. The area under the absorption curve increases linearly with increasing density of injected charges in the WO3 film. In the film with large c, the absorption tends to saturate.

Brillouin-gain coefficients of chalcogenide glasses

We derived the relation between a figure of merit, M1, for the diffraction efficiency of acousto-optical devices and the Brillouin-gain coefficient, gB. We then estimated the values of gB and the phonon lifetime for typical As2S3 and As2Se3 chalcogenide glasses from known acousto-optical parameters. We found that the magnitude of gB for these two glasses is greater than 20 times that of fused silica.

Preparation conditions of sputtered electrochromic WO3 films and their infrared absorption spectra

Abstract Tungsten oxide films were prepared by rf sputtering in an argon-oxygen atmosphere from W and WO3 targets. To bring about reversible electrochromic (EC) characteristics, as-deposited films require an aging process (i.e. cycles of injection/ejection of charge carriers). The infrared absorption band at around 3300 cm−1 increases during the aging process, and it is assigned as OH stretching vibrations of absorbed water. By coloration after aging, the 3300 cm−1 band decreases, and a new band appears at 2400 cm −1. The latter band is considered to be to the stretching mode of radicals incorporated in the WO3 matrix. At low coloration levels, the 2400 cm−1 band increases slightly with injected charge, and a coloration mechanism other than the usual double injection model may be considered. The coloration efficiency depends on the preparation conditions. Its maximum value is the same for films prepared from W and WO3 targets, and is 60 cm2/C at a wavelength of 600 nm. When a tungsten target is used, the substrate temperature is low and the deposition rate is high compared with a WO3 target.

Recombination Radiation as Possible Mechanism of Light Emission from Reverse-Biased p-n Junctions under Breakdown Condition

There are two models, recombination and bremsstrahlung models, proposed for the interpretation of visible light emission from reverse-biased semiconductor junctions under breakdown conditions. The emission spectra are calculated on the basis of these two models and compared with the published experimental results on Si and Ge p-n junctions. Experimental spectra can be interpreted well using the model of recombination between conduction-band electrons and valence-band holes. The spectra calculated on the basis of the bremsstrahlung model cannot be fitted to the experimental ones.

Preparation and aging of sputtered tungstic oxide films

Conditions for the preparation of electrochromic tungstic oxide films with high efficiency for coloration have been investigated. Tungstic oxide films were deposited on glass substrates by rf sputtering in Ar‐O2 mixture from a compressed powder WO3 target. As‐deposited films require an ‘‘aging process,’’ in which the density of charges extracted in bleaching is smaller than that injected in coloring. It is considered that a part of the protons introduced during the aging combine with bonds of unstable oxygen contained in the as‐deposited films. After the aging period, sputtered films have reversible coloration and bleaching cycles. Maximum electrochromic coloration efficiencies are 0.09 and 0.04 cm2/mC at wavelengths of 800 and 500 nm, respectively. They are obtained when the sputtering atmosphere is 0.04–0.05‐Torr total pressure and 5% oxygen content. At any rate, substrate temperature should be kept at 100 °C or less.

Preparation and Electrochromism of RF-Sputtered TiO2 Films

Titanium oxide ( TiO2) thin films were prepared by rf-sputtering in an Ar/O2 atmosphere from a compressed powder TiO2 target and their electrochromic (EC) phenomena were investigated. Films prepared under the sputtering pressures of 1.3, 4.0 and 6.7 Pa were confirmed by X-ray diffraction to be in amorphous state, anatase form and rutile form, respectively. Amorphous and anatase films showed cathodic EC properties. The maximum value of coloration efficiency was 14 cm2/C for the amorphous films in an electrolyte solution of 1N HCl. AC conductance measurements were measured from 110 Hz to 100 kHz for various oxygen contents in the sputtering atmosphere. The ac conductance gradually decreased with increasing oxygen content in the region of up to 30% and was independent of oxygen content in the region of more than 30%. In the films prepared in the sputtering atmospheres of oxygen contents of less than 30%, electronic conduction is interpreted to occur due to lack of oxygen. In the films prepared in oxygen contents of 30% and more, on the other hand, only ionic conduction is considered to exist.