On the microscopic origin of the photochromic and photorefractive behavior of doped Bi4Ge3O12 single crystals

Doping Bi₄Ge₃O₁₂ single crystals with Co impurities enables the presence of a photochromic behavior which is induced at room temperature (rt) with ultraviolet (uv) light. As a consequence of the uv illumination a broad optical absorption from 1.0 to 4.1 eV is formed. This absorption is attributed to the creation and trapping of holes. The spectral efficiency of the damage and its thermal and optical stability have been characterized. Optically induced holographic gratings have been observed. They have been attributed mainly to a photorefractive effect. Some of those gratings exhibit a transient behavior that has been related to the simultaneous diffusion of electrons and holes. A microscopic model to account for the photochromic and photorefractive effects is discussed.     

Synthesis and photochromic properties of EDTA-induced MoO3 powder

In this study, the photochromic MoO₃ powder with novel morphology has been synthesized via hydrothermal method, using ethylene diamine tetraacetice acid (EDTA) as organic inducing agent. The influence of EDTA on the morphology, structure and photochromic properties of MoO₃ powder has been characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), as well as ultraviolet and visible spectroscopy (UV-vis) and color difference meter. When the molar ratio of EDTA/Mo⁶⁺ is 0.05:1, the EDTA-induced MoO₃ powder is found to have 3D flower-like morphologies and excellent photochromic properties. Furthermore, the possible growth mechanism of the flower-like structure and the photochromic mechanism of MoO₃ powder are also discussed in detail.     

Self-assembled PMo12 3-DODA+ composite superlattice thin films with photochromic properties

A layered superlattice PMo₁₂ ^3-DODA⁺ self-assembled composite film has been successfully prepared. Infrared (IR) spectra revealed that the Keggin structure characteristic of the PMo₁₂O₄₀ ^3- anion was preserved in the composite film. A well-ordered superlattice structure, d spacing of 2.945 nm, was identified by X-ray diffraction (XRD). The superlattice film showed good photochromic properties. On irradiation with UV light, the transparent film changed from light yellow to blue. Then, bleaching occurred when the film was in contact with ambient air or O₂ in the dark. The photochromic mechanism was studied by electron spin resonance (ESR), IR spectra, and UV-Vis spectra.     

XPS study on BiI3–nylon 11 nanocomposites

The chemical states of Bi in BiI₃–nylon 11 nanocomposites during X-ray irradiation were investigated by X-ray photoelectron spectroscopy (XPS). It has been found that with prolonging exposure to soft X-rays, Bi 4f peak of Bi metallic state emerged at lower binding energy side and the peak intensity continuously increased, while Bi 4f peak intensity of BiI₃ gradually decreased. The valence band spectra of BiI₃–nylon 11 nanocomposites show that the density of states near Fermi level increases with increasing the X-ray irradiation time due to the formation of metallic Bi, which is favorable to the conductivity. Besides, we have observed that BiI₃–nylon 11 nanocomposites show a new photochromic effect, changing color from orange-red to black after prolonged exposure to the soft X-rays. The color turns to orange-red after air exposure. It has been found that the color change is intimately related to redox behavior of Bi atoms in BiI₃–nylon 11 nanocomposites. We have also observed that the XPS charging shift and the resistance of BiI₃–nylon 11 nanocomposites decrease with increasing X-ray exposure time. This phenomenon may be used to inspect the X-ray photoconductivity preliminary.     

Optical nutation in GaAs/GaxA11−xAs quantum well structures

Abstract

The possibility of occurrence of the coherent optical transient effect known as optical nutation has been analytically established in the semiconductor quantum well (QW) structure, namely GaAs/Ga_xA1_1?xAs most extensively used in optical electronics. Ultra-short-pulse low-intensity band-to-band excitation of electrons to the 1s Wannier-Mott exciton state of the crystal has been considered to play an important role in the coherent radiation—QW interaction. Numerical estimations of the complex optical susceptibility and the transmitted intensity under the transient regime reveal ringing behaviour confirming the occurrence of optical nutation in III-V semiconducting QW structures.     

Photochromism enhancement in thin RbAg4I5 films using simultaneous hydrogen-induced sensitization

A new method of enhancement of the photochromic effect in thin RbAg₄I₅ films has been developed, which is based on the injection of hydrogen atoms on the film surface simultaneously with its exposure to the exciting light. Hydrogen is supplied from the molecules of a specially selected organic compound, which are adsorbed on the RbAg₄I₅ film surface. The hydrogen-induced sensitization leads to a deep photolysis of RbAg₄I₅ with the formation of silver clusters and colloids, which results in a sharp change in the optical properties of the film material.     

Features of accelerated electron beam formation in LHCD experiments on FT-2 tokamak

In experiments with lower hybrid current drive (LHCD) on the FT-2 tokamak, lower hybrid (LH) waves have been successfully used for the first time to ensure effective additional heating of plasma electrons from 450 to 600 eV (I _Pl = 32 kA, Δt _RF = 14 ms, P _RF = 100 kW, F = 920 MHz). Several factors influencing the efficiency of plasma heating have been discovered. In particular, significant growth of radiation losses in the LHCD regime has been found, which is probably related to an increase in the intensity of synchrotron radiation from accelerated electrons. The increase in this intensity in the 53–156 GHz frequency range was accompanied by short spikes of microwave radiation, which were observed only in a narrower frequency range (53–78 GHz) and apparently resulted from interaction of a runaway electron beam with significant local mirrors of toroidal magnetic field. A model of the additional heating of plasma electrons due to absorption of the microwave radiation generated by a beam of accelerated electrons is proposed.     

Flow type barrier-discharge UV photoreactor for irradiating liquids and gases

A flow type barrier-discharge UV photoreactor intended for irradiation of liquids and gases has been developed. In the proposed reactor design, both the discharge region and processed medium occur inside the bulb of an excilamp and the electrodes are made of an UV-reflecting material. The UV radiation intensity in the photoreactor is determined using the photochemical reaction of acetic acid decomposition (CH₃COOH + h?? ?? CH₄ + CO₂) that takes place under the action of KrCl exciplex emission at 222 nm, and the UV exposure dose is evaluated by measuring the evolved gas volume. The experiments showed that the UV radiation intensity in the proposed photoreactor is higher by an order of magnitude than that at the surface of an excilamp with analogous geometry.     

Cellulose acetate nanofibers with photochromic property: Fabrication and characterization

Photochromic nanofibers were fabricated by electrospinning the complex of cellulose acetate (CA)-1′,3′,3′-trimethyl-6-nitrospiro (2H-1-benzopyran-2,2′-indoline) (NO₂SP) and their photochromic properties were investigated. The as-spun nanofibers were characterized by infrared spectra (IR) and the results showed a significant hydrogen bonding reaction between CA and NO₂SP. Incorporated NO₂SP was found to exert no effect on the nanofiber morphology. Through monitoring the color change and the water contact angle of the CA-NO₂SP nanofibrous mat, the reversible photochromic property of the nanofibers was testified. The results of ultraviolet-visible (UV-vis) spectrophotometry and fluorescence microscopy indicated that NO₂SP could endow electrospun fibers with good photochromic and fluorescent properties. Photochromic nanofibers with excellent photosensitivity have great potentials for the applications in optical devices and/or biosensors.     

Synthesis and characterization of polymetalate based photochromic inorganic–organic nanocomposites

A novel reversible photochromic nanocomposite film based on a hybrid inorganic–organic matrix in which heteropolyacid H₃PW₁₂O₄₀ (PWA) was entrapped was prepared. The structure, photochromic behaviors and mechanism of the film were investigated by means of infrared (IR) spectroscopy, UV-Vis absorption spectra and electron spin resonance (ESR). The results showed that heteropolyanion, i.e. PW₁₂O₄₀³⁻ (PW₁₂), maintained a Keggin structure in the film and there was a strong interaction between anion PW₁₂ and cation R–NH₃⁺ (R=link of hybrid composite). The photochromic properties of the composite film originated from the reversible charge transfer between the anions and cations. Under ultraviolet (UV) irradiation, the anion would be reduced via one-electron step with simultaneous oxidation of the cation, accompanied by a color change from colorless to blue. Then the bleaching could occur when the film was in contact with ambient air or O₂ in the dark.     

A dramatic increase of photochromic sensitivity in the case of hydrogen photochromism in WO3

The use of two hydrogen-containing molecules with different functions (donor of hydrogen atoms and provider of proton surface diffusion) enables one to dramatically increase the photochromic sensitivity at hydrogen photoinjection in both films and bilayer structures of WO₃. Application of a bilayer structure of WO₃ films enables one to unambiguously prove the effect of proton diffusion on the photochromic sensitivity at hydrogen photochromism.     

Theoretical study on influence of electron beam irradiation on the first-order hyperpolarizability of BBO crystals

A mechanism not based on the traditional thermodynamics effect but on the microscopic origin of the nonlinear effect was investigated for full consideration of the correlation between the e-beam irradiation electric field (IEF) and the properties of the nonlinear material BBO (β-BaB₂O₄). Firstly, the IEF magnitude is computed under different conditions of beam intensity, ray energy and radiation dose. Secondly, the relationship model of IEF and nonlinear coefficients is built according to quantum mechanics principle. The results show that low-energy irradiation of 0–400?MeV ray energy (corresponding to IEFs of 0–12?×?10^?4?a.u.) has no obvious effect on the nonlinear optical properties of BBO crystals. When the energy is higher than 420?MeV (corresponding to an IEF of 13?×?10^?4?a.u.), the hyperpolarizability changed significantly, indicating permanent damage of the BBO crystals. With an increase of the ray energy, βx and βy tend to decrease, but βz increases markedly especially when the ray energy is more than 450?MeV (corresponding to an IEF of 16?×?10^?4?a.u.).     

Green emission from Eu/Dy codoped SrO-Al2O3-B2O3 glass-ceramic by ultraviolet light and femtosecond laser irradiation

A spectroscopic investigation of Eu²⁺/Dy³⁺ codoped SrO-Al₂O₃-B₂O₃ glass-ceramic is presented. The sample exhibits green emission excited by ultraviolet (UV) light and near-IR femtosecond (fs) laser. The emission profile obtained by near-IR fs laser irradiation is similar to that by UV excitation, indicating that both of the emissions come from 5d → 4f transition of the Eu²⁺ ions. The relationship between the upconversion luminescence (UCL) intensity and pump power reveals a two-photon process in the conversion of near-IR radiation to the green emission. The possible mechanism of UCL from such glass-ceramic is proposed.     

Absorption Cross-section and Spin Analysis in the Three-photon Process

The mean cross-section and the relationship between the intensity of fluorescence to the incident laser light have been obtained theoretically. A comparison of theoretical and experimental values of cross-sections for naphthalene are 4 2 10⁵ and 1 2 10⁵ barn, respectively, while the scattering parameters are 1·64 2 10 m83 and 1·9 2 10 m⁸³, respectively, after proper modification of the intensity term. Spin factor is used to decide the nature of the final state and its effect on the magnitude of intensity, cross section, etc. It is found that the terminal state in a three photon process is a triplet while it is a singlet in the two photon process which are ¹B_2u+ and ³A_1g m, respectively. There is good agreement between experiment and theory and the discrepancy between frequencies of emission for theoretical and experimental values is explained.     

Electrochromic and photoelectrochemical behavior of thin WO3 films prepared from quantized colloidal particles

The electrochromic, photochromic and photoelectrochemical properties of thin semiconductor WO₃ films, prepared from quantum-size colloids, have been examined. These films have been found to exhibit reversible electrochromic and photochromic behavior (blue coloration). The trapped electrons in the particulate films are thought to be the major species responsible for the blue coloration. The photoelectrochemical measurements have shown a very low conversion efficiency of WO₃ particulate films. This is most probably due to the efficient trapping of electrons by the lattice defects in the film.     

Selective soft mode heating in a ferroelectric film

The effect of electromagnetic radiation with a frequency of f ～ 0.3 THz on the temperature of a soft-mode phonon subsystem in a (Ba,Sr)TiO₃ film has been studied. Features of the temperature regimes in a ferroelectric capacitor structure were revealed using a capacitive thermometer and a thermocouple. It is established that, at a pumping radiation intensity of ～6 mW/mm², the overheating of soft-mode phonons, as evaluated from a change in the capacitance of a planar capacitor, exceeds the integral overheating of this capacitor measured using a thermocouple. Conditions determining the nonequilibrium state of the soft-mode phonon subsystem in a ferroelectric film are analyzed.     

Effect of ZnSe doping on the photochromic and thermochromic properties of MoO3 thin films

The photochromic and thermochromic properties of molybdenum oxide (MoO₃) thin films doped with zinc selenide (ZnSe) were studied. The films were deposited on glass substrates by thermal evaporation of MoO₃-ZnSe powder mixtures. The characteristic optical absorption band attributed to color center formation indicated an enhanced photochromic effect for MoO₃ films doped with ZnSe 5 mol% relative to undoped MoO₃ samples. In addition, the thermochromic effect was most pronounced for ZnSe 5 mol% samples annealed at 23-125 °C, and for undoped samples annealed at 125-225 °C. The chromogenic properties were essentially suppressed for MoO₃ doped with ZnSe 10 mol%.     

Theoretical calculation of the angular distribution of radiation at the output of a flat nanodimensional X-ray waveguide

A model is proposed for theoretical calculations of the angular distribution of radiation intensity at the output of a flat X-ray waveguide representing an air-filled gap between two quartz plates. Good agreement between the theoretical and experimental angular profiles of CuK_α fluorescence intensity is obtained for planar waveguides with the gap widths ranging from 40 to 3000 nm. It is concluded that the Kirchhoff method can be applied to calculations of the spatial distribution of radiation intensity at the X-ray waveguide output.     

Phase relationship,magnetic properties and Mössbauer studies in as cast and directionally solidified Tb0.3Dy0.7Fe1.95

Tb_0.3Dy_0.7Fe_1.95 alloy was cast and directionally solidified by modified Bridgman technique at different solidification rates and investigated for microstructural features and magnetic properties. The phase relationship between the co-existing phases viz., (Tb,Dy)Fe₂, (Tb,Dy)Fe₃ and (Tb,Dy)-rich are studied as a function of solidification rate to evolve a structure–property correlation. Higher solidification rate improves <110> grain texture and favour large magnetostriction. The magnetization and Mössbauer studies indicate no perceptible change in Fe magnetic moment, with a slight deviation in the 1:3 intensity ratio of the two sextets. Comparing the Mössbauer and microstructural results a relationship has been established between the intensity ratio and the volume fraction of the deleterious minor phase (Tb,Dy)Fe₃ in (Tb,Dy)Fe₂ microstructure.     

Effect of neutron irradiation on the structure and optical properties of fused SiO<Subscript>2</Subscript>

The effect of fast-neutron irradiation on the structure of fused silica has been studied by IR reflection spectroscopy in a wide range of neutron fluences, from 10¹⁷ to 10²¹ n/cm². The spectral characteristics of the bending and stretching modes of the bridge bonds in silica have been shown to be nonlinear functions of neutron fluence. The kinetics of radiation-induced changes in the optical properties of silica in the UV through visible spectral region have been analyzed in relation to those in its structure and microscopic characteristics. A strong correlation has been found between the dose dependences of the optical and structural properties of silica, such as the 465-nm luminescence intensity, the intensity and position of the amorphous halo, the reflectivity and frequency of the IR bands at 1125 and 480 cm^?1, the density of the material, and its ionic polarizability. We assume that there is a threshold dose in the range 10¹⁹ to 10²⁰ n/cm² which produces sharp changes in the optical and structural properties of SiO₂ due to a transition to a metamict-like state. An analytical expression has been derived for the radiation-induced changes in the intensity of the first halo. The driving force and mechanism of the radiation-induced structural changes in fused SiO₂ have been tentatively identified.