Properties of chromogenics materials have been of great interest for more than 50 years till now. Some examples of their practical application are photochromic lenses, electrochromic smart windows or even some examples of sensors devices based on gasochromic thin films have already been commercially available on the market. However, recognition of different physical and chemical processes that influence the optical response of such materials under changes in surrounding atmosphere is still an open subject for discussion. This work presents results of experimental and theoretical investigations of optical response of the two selected gasochromic (Ti-V-Ta-W)Ox and (Ti-V-Ta-Nb)Ox oxide thin films under ethanol vapor stimulations. Based on the measured experimental transmission spectra, the complex refraction index characteristics were plotted using optical models elaborated for the VIS-NIR spectral range. The models were further used for the prediction of optical responses of optical gas sensing structures with observed gasochromic behavior.
Most of the challenges in laser technology can be overcome by using yttrium-aluminum perovskite (YAlO_3, YAP). These crystals are characterized by more advantages than typical Y_3Al_{5}O_{12} (YAG) crystals. However, the creation of microlasers with these materials is just under development. The aim of the work was to theoretically design the input and output cavity mirrors for microlasers on the base of YAlO_3:Nd or YAlO_3:Tm single crystals, and to investigate those resonators obtained according to the theoretical design using electron beam evaporation method.
The optical properties of the semiconducting compounds Zn_{1-x}Co_{x}Se crystallizing in the zinc-blende structure have been investigated. The reflectivity spectra of these materials for different concentration of Co ions, have been taken out in a wide energy range between 4 and 25 eV at room (RT) and liquid nitrogen temperature (LNT) using synchrotron radiation from ADONE Storage Ring in Frascati. The comparisons between the reflectivity spectra of ternary systems and host crystal ZnSe are made. On the basis of the experimental and theoretical results, the changes of the structures of the reflectivity spectra of host crystal ZnSe caused by the influence of transition metal ions are discussed.
Low temperature fluorescence excitation spectroscopy was carried out on single nitrogen vacancy defect centers. At temperatures below 90 K the fluorescence intensity of individual centers drastically diminishes because of the population of a metastable singlet state in near resonance with the optically excited state. Low temperature fluorescence excitation spectroscopy down to 5 K becomes possible via deshelving of this state with a second laser source. Surprisingly individual centers reveal low temperature fluorescence excitation line widths around 0.6 meV.
UV absorption spectra of glycine crystals, i.e. triglycine sulphate, selenate, diglycine sulphate as well as their deuterated analogues in solid state and aqueous solutions are given. Results are compared with the electronic transition energies calculated by LCAO INDO method for glycine molecule and ion, assuming their crystal geometry.
The renewal of the second order nonlinear crystals is very strong in laser materials optics. We are involved in the search for new systems based upon highly nonlinear niobate crystal family. Among these crystals, Ba_{2}NaNb_{5}O_{15} is characterized by the highest nonlinear parameters but, unfortunately, it is difficult to grow crackless samples. The obtention of good quality and crackless Ba_{2}NaNb_{5}O_{15} single crystals doped with different concentrations of Nd_{2}O_{3} is reported. The crystals were grown as monocrystalline fibres by the laser heated pedestal growth technique. A new determination of the crystallographic structure of Ba_{2}NaNb_{5}O_{15} has been performed. The structure of fibres doped with 1 at.% Nd^{3+} was found orthorhombic with the new space group Pba2 instead of Cmm2. The well-known twinning of Ba_{2}NaNb_{5}O_{15} due to the exchange of the a and b axes of the unit cell, which disturbs the optical properties, decreases as the Nd^{3+} content of the fibre increases. Above 3%Nd^{3+} ions, such monocrystalline fibres were found to be of tetragonal structure. Low temperature spectroscopy reveals that Nd^{3+} ions substitute probably both Ba^{2+} and Na^{+} ions. The stimulated emission cross-section near 1060 nm of the ^{4}F_{3/2} → ^{4}I_{11/2} channel and its branching ratios are determined with the help of Judd-Ofelt analysis. This work shows that b-axis grown monocrystalline fibres are potentially self-doubling laser materials.
We present a study of detailed line shapes of photoreflectance spectra for Al_{0.3}Ga_{0.7} As/SI-GaAs epitaxial layers grown by MBE. All measurements were performed at 80 K under UHV conditions with a special care for the samples surface quality. A set of the photoreflectance spectra was collected for photon energies close to the GaAs and Al_{0.3}Ga_{0.7}As band gaps (E_{0}). The photoreflectance spectra originated in the vicinity of the Al_{0.3}Ga_{0.7}As/SI-GaAs interface were analyzed using the complex Airy function model of Franz-Keldysh oscillations. To examine the effect of the epitaxial layer thickness on parameters characterizing the interface, a step-by-step chemical etching was applied for stripping the top layers. The built-in electric field intensity, field inhomogeneity and phenomenological broadening parameter for interface regions were determined as a function of the epilayer thickness.
The paper describes the results of a study of the influence of silver content on the absorption edge and optical band gap of the newly synthesized glasses of the Ag_{x}(As_{40}S_{30}Se_{30})_{100-x} type for x=0, 0.5, 1, 2, 3, 5 at.% and of the corresponding films. The synthesis of bulk samples was performed in a rocking furnace from high-purity elemental components by a melt quenching method. Films were prepared from the synthesized bulk samples by pulsed laser deposition. Transmission spectra of the investigated samples were recorded at room temperature. The absorption edge and the optical band gap were determined by extrapolating the linear parts of the absorption spectra. It was found that the investigated range of Ag doping concentrations has a great influence on the absorption edge and optical band gap. Namely, with the increase of the silver content in the material the optical band gap showed a decrease. For films, it decreased from 2.02 eV, for the glass without silver, to 1.805 eV for the composition with 5 at.% Ag, whereas for the analogous bulk samples this decrease was from 1.84 eV to 1.609 eV.
The coherent population trapping effect in double tunnel-coupled quantum wells is analyzed. One of two quantum wells interacts with the two-frequency laser radiation and low-frequency field, thus forming a closed contour of excitation. It is possible to control the excited level population in such a scheme of excitation by changing relative phases of the fields in the coherent population trapping state. The quantum well is bound to the other quantum well by tunnel coupling of the excited levels, therefore the population and optical properties of the other quantum well depend on the coherent population trapping state in the first quantum well and can be controlled.
Spatial changes of properties of Gd₃Ga₅O₁₂ (GGG) single crystals caused by diffusion of cobalt ions during high-temperature annealing (1200°C, 24 h) in Co₃O₄ powder are investigated. The registration of these changes was carried out by optical spectrophotometry, microscopy and micro-Raman scattering methods. Changes in structure of near-surface layers of the crystal were investigated by X-ray diffraction technique. It was shown that the additional absorption induced by annealing is related to intra-center optical transitions in Co²⁺ ions, which occupy tetrahedral positions in the garnet structure at the distances of 250-500 μm from the crystal surface. The dependence of induced absorption with depth has got a non-monotonous character with a maximum at 400 μm. A comparison of the results obtained by different methods allows to suppose that the thermal treatment of GGG in the presence of cobalt ions leads to formation of the structurally and chemically non-uniform layer with a width about 500 μm.
Thermal stability in terms of characteristic temperature and kinetic parameters of different compositions of glassy Se_{80-x}Te_{20}Sb_{x} (x=0, 2, 4, 6, 8, 10) have been investigated. Differential scanning calorimetry under nonisothermal conditions have been proposed to investigate thermal characteristic of these compositions. The thermal stability of these glasses was obtained in terms of various simple quantitative methods based on the characteristic temperatures, such as the glass transition temperature, T_{g}, the onset temperature of crystallization, T_{in}, the temperature corresponding to the maximum crystallization rate, T_{p}, and the melting temperature, T_{m}. Furthermore, the kinetic parameter K_{r}(T) was achieved as another indicator for thermal stability and its results about stability compared with those evaluated by other criteria. The results of both the criteria and the kinetic parameter K_{r}(T) confirm that the thermal stability increases with increase of Sb content. The results also refer to that: the glass transition T_{g}, activation energy of crystallization E_{p} and the frequency factor K_{0} were increased with the addition of Sb. These results have been discussed in terms of the average coordination number, cohesive energy and average heat of atomization.
Spatial changes of optical properties of bulk LiNbO₃ crystal were investigated after annealing in CuO powder. The incorporation of copper ions into the crystal was confirmed by registration of additional absorption spectra that revealed formation of the absorption bands of both Cu⁺ (400 nm) and Cu²⁺ (1000 nm) ions. The changes of optical absorption caused by thermal treatment were registered along the direction of diffusion by the probe beam perpendicular to this direction. The anisotropy of diffusion was revealed. The maxima were observed on the depth dependences of additional absorption both for the wavelengths of 400 and 1000 nm for all main crystallographic directions. The concentrations of copper ions were calculated in accordance with the Smakula-Dexter formula. The X-ray diffraction study revealed reflexes which probably belong to CuNb₂O₆, CuNbO₃ and CuO. The halo was observed on these diffraction patterns that confirms the formation of the scattering centers (about 1 nm in diameter) in the near-surface region.
We compare theoretically predicted, by means of ab-initio calculations, dielectric tensor and related experimental values obtained from Mueller-ellipsometry measurements of an insulating monoclinic (optically biaxial) crystal of Cu(H₂O)₂(en)SO₄, en=C₂H₈N₂. We concentrate on the static limit, ω→ 0.
Boron doped MnO films were prepared by spray pyrolysis technique at 375°C substrate temperature, which is a low cost and large area technique to be well-suited for the manufacture of solar cells, using boric acid (H₃BO₃) as dopant source, and their properties were investigated as a function of doping concentration. Boron doping was achieved by adding 0.1 M, 0.2 M, 0.3 M, and 0.4 M H₃BO₃ to the starting solution. X-ray analysis showed that the films were polycrystalline fitting well with a cubic structure and have preferred orientation in (111), (220) and (311) directions. Optical band gap of the undoped and B-doped MnO films were found to vary from 2.25 to 2.54 eV. The changes observed in the energy band gap and structural properties of the films related to the boric acid concentration are discussed in detail.
We report on the investigation of the spectral dependence of nonlinear optical susceptibilities, that describe second harmonic generation - circular dichroism, the nonlinear optical analogue of circular dichroism. The design and implementation of a state of the art experimental setup, used for the characterization of a series of organic molecules in the near-infrared range, will be discussed. We emphasize on the main characteristics like wavelength control, polarization and data-acquisition to present the setup. Some preliminary results show its strength to detect chirality at the molecular and supramolecular level.
We report on photoreflectance investigations of strained-layer In_{0.2}Ga_{0.8}As/GaAs/Al_{0.3}Ga_{0.7}As single quantum well laser structures grown by molecular beam epitaxy. All the observed photoreflectance spectral features were assigned to the e-hh transitions with Δn=0. The transition energies were determined and compared to their values calculated within the envelope function approximation. Assuming that one third of the total strain in the central In_{0.2}Ga_{0.8}As layer is relaxed by biaxial deformation of surrounding thin GaAs layers, it is possible to explain reasonably the results of our photoreflectance experiment.
An iterative procedure of calculation of reflectivities and transmission coefficients of light rays for an uniaxial film with a lapse rate of an index of refraction on a uniaxial substrate was gained by orienting the axis of anisotropy along the normal to the boundary. With its help the dependences of ellipsometric anglesΔ andψ for a linear, quadratic, and sine-shaped profiles of refractive indices of a uniaxial immersing film with optic axis oriented along the normal to the boundary were analyzed. The dependences of anglesΔ andψ on quantity of an uptake and anisotropy of the film were also examined. The numerical modelling for ZnO films and Langmuir-Blodgett-like films on a melted quartz was carried out, and this allowed to draw conclusions of practical importance for the ellipsometric investigations of the film structures.
In this work we present the new experimental results of total photoelectric yield as well as energy distribution of photoelectrons excited in a thin carbon film deposited on Ni mirror in the presence of resonance-enhanced X-ray propagation effect. The measurements were performed using conventional X-ray tube as a radiation source for the energy Cu K_{α} (8047 keV). The spectra were recorded using a flow proportional electron counter with energy resolution of about 15%, and multichannel pulse height analyzer. A comparison with the reflectivity spectra recorded at the same time show an excellent correlation of both kinds of spectra, consistently with the theoretical prediction. A map of electron energy distribution is reported. Although the applied electron counter was of low energetic resolution the recorded spectra show characteristic regularities and indicate that the photoelectron yield excited in the presence of resonance-enhanced X-ray propagation effect can provide depth dependent information about impurity distribution and processes in thin layers.
Black porous silicon is a new material with low light reflectance and high light absorbance values. Black porous silicon layers are especially useful and important in solar energy conversion. In this work black porous silicon plates were prepared by wet chemical metal-assisted method. Silver and gold nanoparticles were precipitated from colloidal silver nitrate and chloroauric acid solutions. Obtained black porous silicon samples with precipitated nanoparticles were investigated by scanning electron microscope combined with energy dispersive X-ray spectrometer, ultraviolet and visible light spectrometer, the Fourier transformation infrared spectrometer. Scanning electron microscopy and energy dispersive X-ray analysis have revealed the presence of silver and gold nanoparticles on black porous silicon surface. Silver nanoparticles size varied from 30 to 100 nm, gold - from 20 to 70 nm. During UV-VIS analysis significant changes in reflectance of processed black porous silicon samples were obtained. Reflectance of black porous silicon samples was lower than 10%. The Fourier transform infrared analysis has revealed decrease in reflectance in far infrared region. Changes in the Fourier transform infrared spectra in "fingerprint" zone prove modification of the surface of black porous silicon layers after precipitation of metal nanoparticles.
GaAs samples doped with indium atoms by ion implantation and thermal annealed were studied using a channelling method, Rutherford backscattering, and an ellipsometry. From these measurements it was observed that the layer implanted with 3×10^{16} cm^{-2} indium dose was totally damaged and its optical properties, namely a refraction index n and an extinction coefficient k, corresponded to the amorphous material. Subsequent isobaric heating of the implanted samples resulted in recovery of the crystalline structures with simultaneous change of the n and k index values.
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