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%.     

Optically rewritable Bragg gratings in Mn2+ doped RbCdF3

Optical absorption, optically stimulated luminescence, and thermo-luminescence measurements have been made on RbCdF₃:Mn²⁺ single crystals with the aim of characterizing this compound for holographic storage. We show that there are stable traps that are filled after deep ultra-violet irradiation (<270 nm) and they can be emptied after ∼365 nm irradiation. The photo-induced changes in the optical properties can be used to create optically rewritable Bragg gratings in these crystals.     

Far-infrared dispersion of complex dielectric constant in the ferroelectric near-stoichiometric LiNbO3:Fe

The dielectric properties of near-stoichiometric LiNbO₃:Fe single crystal have been investigated by using a terahertz time domain spectroscopy (THz-TDS) in a frequency range of 0.7-1.6 THz at room temperature. When coupled with an applied external optical field, an obvious photorefractive effect was observed, resulting in the modulation of the complex dielectric constant for near-SLN:Fe. The variation of refractive index |Δn| has a linear relationship on scale with the applied light intensity accompanied with a steplike decrease. These findings were attributed to the internal space charge field of photorefraction and the light-induced domain reversal in the crystal.     

The Finite-conjugate Self-imaging and the Generalized Lau Effect Due to Complex Gratings

Abstract

Laser induced gratings in nonlinear optical media, which are complex or phase gratings, are the key structures in many applications involving laser beam-nonlinear material interactions. The concept of coded imaging of self-imaging structures, developed elsewhere, has been utilized to make a comprehensive study of the self-imaging properties, at finite conjugates, of model complex or phase gratings. The same approach has been used to extend our previous work on the generalized Lau effect. An interesting application of the self-imaging process has been made to visualize, at a certain primary magnification, laser induced thin photorefractive grating in a BSO crystal. Possible applications of this technique include visualization of dynamic or developed laser induced gratings in thin nonlinear optical materials.     

Band-edge luminescence from SrTiO3: No polaron effect

We studied the band-edge photoluminescence (PL) of electron-doped SrTiO₃ and strongly photoexcited SrTiO₃ crystals. Two band-edge PL peaks are clearly observed at low temperatures. The PL peak energies coincide with the high- and low-temperature onsets of the optical absorption, corresponding to the phonon-assisted band-to-band optical transition. This result is the direct evidence that the free electrons and holes exist in the conduction and valence bands, and radiative recombination of free electrons and free holes causes the band-edge PL.     

Z-scan measurement for the nonlinear absorption of Bi2.55La0.45TiNbO9 thin films

Bi_2.55La_0.45TiNbO₉ (BLTN-0.45) thin films with layered aurivillius structure were fabricated on fused silica substrates by pulsed laser deposition technique. Their structure, fundamental optical constants, and nonlinear absorption characteristics have been studied. The film exhibits a high transmittance (> 60%) in visible-infrared region. The optical band gap energy was found to be 3.44 eV. The optical constant and thickness of the films were characterized using spectroscopic ellipsometric (SE) method. The nonlinear optical absorption properties of the films were investigated by the single-beam Z-scan method at a wavelength of 800 nm laser with a duration of 80 fs. We obtained the nonlinear absorption coefficient β = 4.64 × 10^− 8 m/W. The results show that the BLTN-0.45 thin film is a promising material for applications in absorbing-type optical device.     

Influence of ultraviolet irradiation on the optical properties of amorphous Sb10Se90 thin films

Amorphous Sb₁₀Se₉₀ thin films were prepared by thermal evaporation of the bulk glass. The changes in the optical properties (transmittance, optical gap, absorption coefficient, refractive index and extinction coefficient) have been measured in the wavelength range 500-900 nm of virgin and ultraviolet (UV) illuminated films. Analysis of the optical absorption data shows that the rule of non-direct transitions predominates. It is found that the optical energy gap decreases (photo-darkening) and the refractive index increases with the increase of UV exposure time. The dispersion of the refractive index (n) has been discussed in terms of Wemple-Didomenico single oscillator model. The oscillator energy E₀ and the dispersion energy E_d have been determined and discussed in terms of UV exposure time. The photo-darkening was discussed in terms of some of the current literature models.     

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.     

Dual-use chromophores for photorefractive and irreversible photochromic applications

Holographic experiments are performed on a series of dual-use chromophore molecules wherein both irreversible photochromic and erasable photorefractive holographic gratings can be written in the same storage volume. At 675 nm, the chromophore undergoes a photochemical reaction leading to the creation of irreversible holographic gratings. Alternatively, at longer wavelengths, application of an electric field during grating formation allows the storage of erasable photorefractive holograms in the same location as previously stored permanent photochemical holograms. Photochemical gratings (eta > 60%) can be written in less than 1 min, whereas photorefractive gratings (eta > 50%) can be written in less than 1 s. The photochemical gratings have a diffusion-limited dark half-life of as long as two weeks depending on the glass transition temperature of the composite.     

Sol-gel processing of SiO2-B2O3 glasses

Glasses from the SiO₂-B₂O₃ binary system containing up to 25 mole percent B₂O₃ have been synthesized. These glasses have been characterized by x-ray diffraction, optical transmission measurement, and I.R. absorption spectroscopy. In order to obtain a homogeneous glass, it was determined that a certain degree of hydrolysis of Si(OC₂H₅)₄ was essential before B(OC₂H₅)₃ could be incorporated. Optical transmission data exhibit characteristic absorption bonds related to Si, B, and OH fundamental and combination frequencies. I.R. absorption spectra in the 620 – 1700 cm^?1 range revealed presence of absorption bands related to vibrations of O-Si-O, Si-O-Si, B-O, and Si-O-B bonds.     

Preparation and photocatalytic activity of WO3/TiO2 nanocomposite particles

A novel photocatalyst WO₃/TiO₂ nanocomposite was prepared through a hydrothermal method by using cetyltrimethylammonium bromide (CTAB) as surfactant. The obtained WO₃/TiO₂ was characterized by X-ray diffraction (XRD), field emission scanning electron microscope (FESEM), transmission electron microscope (TEM) and diffused reflectance spectroscopy (DRS). Photocatalytic experiments indicate that the nanocomposites show much higher photoactivity than that of pure TiO₂ in the photodegradation reaction of Rhodamine B (RhB). The increased photoactivity of WO₃/TO₂ may be attributed to the improvement of the light absorption properties and the slow down of the recombination between the photoexcited electrons and holes during the photoreaction.     

First principles study on the properties of p-type conducting In:SnO2

Detailed theoretical investigations on the structural, electronic and optical properties of p-type conducting In:SnO₂ have been conducted by first principle calculations. Analysis on the thermal stability via standard enthalpy of formation calculations shows that In:SnO₂ remains stable at very high In concentration, although the lattice constant expands in a distorted rutile structure with the increase of indium content. This can be attributed to the larger ionic radii and the one less 5p electron of In³⁺. Due to the differences in thermal stabilities of the structures with the same indium concentration, the preferred In³⁺ distribution is to occupy the Sn sites in different (110) slabs, followed by occupying the location in the same (110) slab with a maximized distance between indium ions. Indium element in SnO₂ introduces a band in the low energy region originated from the In 4d orbitals and an acceptor energy level slightly above the Fermi energy. While the large effective mass of the electron holes in the valence band results in the small p-type conductivity of In:SnO₂. The tiny changes in the conduction band and band gap lead to the invariability of the optical spectra in the ultraviolet-visible region. On the contrary, the dramatic enhancement of dielectric function, reflectivity and absorption in infrared region can be interpreted by the transition from the occupied states to the empty bands near E_f as well as the exciton effect. These features make In:SnO₂ a good candidate for applications such as transparent conducting materials, infrared reflecting materials and gas sensors.     

Spectral dependence of the diffraction efficiency of light-induced gratings in YAlO3:Mn crystals

The results of measurements of spectral dependence of diffraction efficiency of light-induced holographic gratings in YAlO₃:Mn(0.5%) crystal are reported. The experimental data are interpreted as an effect of interaction of both refractive index changes and light-induced absorption associated with optical transition within photo-induced Mn⁵⁺ ions and intrinsic defect centers. Nature of the intrinsic traps involved in the process is discussed.     

Preparation, magnetic and microwave absorption properties of La0.5Sr0.5MnO3/La(OH)3 composites

La_0.5Sr_0.5MnO₃/La(OH)₃ composites with different weight ratio of La_0.5Sr_0.5MnO₃ particles and La(OH)₃ nanowires have been prepared by tuning the reaction time under hydrothermal conditions. The structure, morphology and magnetic properties have been investigated. Additionally, by the measurements of the complex permittivity, permeability and microwave absorption properties in the frequency range of 1-12 GHz, the results shown that the weight ratio of La(OH)₃ nanowires has great influence on reflection loss. Excellent absorption property can be obtained when the ratio is 1.4%, which is attributed to the enhanced electromagnetic match as well as the proper dielectric loss and magnetic loss. The enhanced electromagnetic match is originated from the improved frequency dispersion of the complex permittivity and permeability due to the presence of dielectric La(OH)₃ nanowires.     

Magneto-optical properties of Tb0.81Ca0.19F2.81 and Tb0.76Sr0.24F2.76 single crystals

High optical quality terbium calcium fluoride (Tb_0.81Ca_0.19F_2.81) and terbium strontium fluoride (Tb_0.76Sr_0.24F_2.76) single crystals have been grown by the Czochralski method under controlled atmosphere. While both compounds present similar optical and magneto-optical properties (determined by the high concentration of Tb³⁺ ions), the lower absorption losses and high Faraday rotation exhibited by terbium calcium fluoride, compared to those of terbium strontium fluoride, lead to a notable figure of merit for Faraday isolators operating in the visible wavelength region. In particular at short wavelengths, below 500 nm, the figure of merit of terbium calcium fluoride is remarkably higher than that of the industrial standard terbium gallium garnet.     

Determination of the optical constants of HfO2–SiO2 composite thin films through reverse fitting of transmission spectra

Composite thin films of HfO₂:SiO₂ with wide range of relative composition from 100:0 (pure HfO₂) to 10:90 have been deposited on fused silica substrates by co-evaporation technique and the optical properties of the films have been studied by measuring the transmission spectra of the samples by spectrophotometer. Different important optical parameters viz., band gap, refractive index and absorption coefficients of the samples have been obtained by fitting the measured optical spectra with theoretically generated spectra and the variation of the optical constants as a function of SiO₂ content in the films have been obtained. Two different dispersion models viz., the single effective oscillator model and the Tauc–Lorentz model have been used to generate the theoretical spectra in the above fitting procedure. X-ray reflectivity (XRR) measurement technique has been used to find the densities of the films in order to explain the observed variation in optical properties of the films with increase in SiO₂ content.     

Effects of annealing temperatures on optical and electrical properties of vacuum evaporated Ga15Se77In8 chalcogenide thin films

The optical constants (absorption coefficient, optical band gap, refractive index, extinction coefficient, real and imaginary parts of dielectric constants) of amorphous and thermally annealed thin films of Ga₁₅Se₇₇In₈ chalcogenide glasses with thickness 4000 Å have been investigated from absorption and reflection spectra as a function of photon energy in the wave length region 400-800 nm. Thin films of Ga₁₅Se₇₇In₈ chalcogenide glasses were thermally annealed for 2 h at three different annealing temperatures 333 K, 348 K and 363 K, which are in between the glass transition and crystallization temperature of Ga₁₅Se₇₇In₈ glasses. Analysis of the optical absorption data shows that the rule of non-direct transitions predominates. It was found that the optical band gap decreases with increasing annealing temperature. It has been observed that the value of absorption coefficient and extinction coefficient increases while the values of refractive index decrease with increasing annealing temperature. The decrease in optical band gap is explained on the basis of the change in nature of films, from amorphous to crystalline state. The dc conductivity of amorphous and thermally annealed thin films of Ga₁₅Se₇₇In₈ chalcogenide glasses is also reported for the temperature range 298-393 K. It has been observed that the conduction is due to thermally assisted tunneling of the carriers in the localized states near the band edges. The dc conductivity was observed to increase with the corresponding decrease in activation energy on increasing annealing temperature in the present system. These results were analyzed in terms of the Davis-Mott model.     

Optical properties of the c-axis oriented LiNbO3 thin film

C-axis oriented Lithium Niobate (LiNbO₃) thin films have been deposited onto epitaxially matched (001) sapphire substrate using pulsed laser deposition technique. Structural and optical properties of the thin films have been studied using the X-ray diffraction (XRD) and UV-Visible spectroscopy respectively. Raman spectroscopy has been used to study the optical phonon modes and defects in the c-axis oriented LiNbO₃ thin films. XRD analysis indicates the presence of stress in the as-grown LiNbO₃ thin films and is attributed to the small lattice mismatch between LiNbO₃ and sapphire. Refractive index (n = 2.13 at 640 nm) of the (006) LiNbO₃ thin films was found to be slightly lower from the corresponding bulk value (n = 2.28). Various factors responsible for the deviation in the refractive index of (006) LiNbO₃ thin films from the corresponding bulk value are discussed and the deviation is mainly attributed to the lattice contraction due to the presence of stress in deposited film.     

Photoelectrical properties of In/n-CuIn5Se8 Schottky barriers

CuIn₅Se₈ homogeneous crystals of n-type conductivity have been grown. Donor centers activation energy has been estimated. In/n-CuIn₅Se₈ Schottky barriers have been created and the first spectral dependencies of quantum efficiency of photoconversion of these structures have been derived. The nature of interband optical transitions has been interpreted and the band gap values for direct and indirect transitions in CuIn₅Se₈ crystals have been determined on the results of analysis of the Schottky barriers photoactive edge absorption. A possibility of utilization of CuIn₅Se₈ crystals in wide-band photoconverters of the optical radiation has been established.     

Mode Conversion by Photorefractive Damage in Planar Optical Waveguides: Application to Fully Confined Modes

Abstract

A theoretical analysis of mode conversion induced by photorefractive damage in optical waveguides is presented. The model applies to any geometry and uses the well known coupled-wave approach which has been extended to anisotropic media. The light-induced coupling coefficients have been expressed in terms of the photorefractive space charge field. The formulae have been, then, applied to calculate the initial conversion rate of a fully confined mode in a planar LiNbO₃ waveguide perpendicular to the c axis. The photorefractive field developed by illumination has been obtained under a short-time (initial) approximation, taking into account all transport processes. Some numerical estimates on mode conversion rates are also given and compared to available experimental data.