Changes in fluorescence lifetimes induced by variation of of the radiating molecules' optical environment

We report results of experiments with about 30 nm thick evaporated layers of an europium-chelate fluorescing at λ = 612 nm: i.) For layers on different substrates S₁ in air the lifetime decreases with increasing refractive index n₁ of S₁. ii.) For a layer on a substrate S₁ the lifetime decreases when the width d of an air gap between the layer and a second plate S₂ is reduced from d å λ to d = 0. Theoretically, we derived analytic expressions for the spontaneous emission rates of electric and magnetic dipole transitions taking place in an atomic system embedded in a very thin layer of refractive index n₀ and optical thickness $\text{n}{}_0\text{d}{}_0\text{« λ/8}$, which is sandwiched between two dielectric loss-free media of refractive indices n₁ and n₂. Comparison of theory and experiment allows to determine the absolute quantum efficiency of the emitting state.     

Coloring of silica glass with silver nanoparticles

The possibility of doping of surface layers of optical fused silica with silver nanogranules upon heating of a thin Ag film on the silica surface by a CO₂ laser beam (P ≈ 30 W, λ = 10.6 μm) is established. The effect of exposure time on the doped layer structure has been investigated. The absorption band of the colloidal solution of Ag in silica has been studied. It is shown that this band is homogeneously broadened and its peak (at 420 nm) corresponds to the small volume filling factor (q < 0.1) at an average granule radius of about 3 nm. Based on the measurements of the radial distributions of the reflectance and refractive index over the doped region at λ = 633 nm, it is revealed that the doped layer is an area-irregular asymmetric step waveguide.     

Multiphoton transitions and the resonant optical Stark effect in AgBr nanocrystals

Nonlinear absorption of 30-ps light pulses with λ = 560 nm in AgBr nanocrystals is experimentally studied in the range of intensities 10⁸–10¹⁰ W/cm². The results of a theoretical analysis show that the absorption is related to direct interband n-photon transitions. With increasing light intensity j, the number n increases and the region of the k-space changes for the transitions predominantly contributing to the absorption. It is shown that, due to specific features of the AgBr electronic band structure, the probabilities of two-photon transitions for the light at λ = 560 nm are anomalously low, while those of four-photon transitions are anomalously high. In addition, the increase in the two-photon transition rate with increasing intensity is blocked at j ? 10⁸ W/cm² due to the resonant optical Stark effect and due to a gap arising in the band spectrum, rearranged because of the interaction with light.     

Dependence of the cesium absorption D1 linewidth on the thickness of atomic vapor column

A new nanocell has been elaborated, where the thickness of the atomic vapor column varies smoothly in the range L = 350–5100 nm. The cell allows studying the behavior of the resonance absorption at the D₁ line of cesium atoms by varying the thickness from L = λ / 2 to L = 5 λ with the step λ / 2 (λ being the resonant wavelength of the laser, 894 nm) and the laser intensity. It is shown that at low laser intensities a narrowing of the resonance absorption spectrum is observed for L = (2n + 1)λ/2 (with an integer n) up to L = (7/2)λ, whereas for L = nλ the spectrum broadens. The developed theoretical model well describes the experiment.     

Planar quarter wave stacks prepared from chalcogenide Ge-Se and polymer polystyrene thin films

Planar quarter wave stacks based on amorphous chalcogenide Ge-Se alternating with polymer polystyrene (PS) thin films are reported as Bragg reflectors for near-infrared region. Chalcogenide films were prepared using a thermal evaporation (TE) while polymer films were deposited using a spin-coating technique. The film thicknesses, d∼165 nm for Ge₂₅Se₇₅ (n=2.35) and d∼250 nm for polymer film (n=1.53), were calculated to center the reflection band round 1550 nm, whose wavelengths are used in telecommunication. Optical properties of prepared multilayer stacks were determined in the range 400-2200 nm using spectral ellipsometry, optical transmission and reflection measurements. Total reflection for normal incidence of unpolarized light was observed from 1530 to 1740 nm for 8 Ge-Se+7 PS thin film stacks prepared on silicon wafer. In addition to total reflection of light with normal incidence, the omnidirectional total reflection of TE-polarized light from 8 Ge-Se+7 PS thin film stacks was observed. Reflection band maxima shifted with varying incident angles, i.e., 1420-1680 nm for 45° deflection from the normal and 1300-1630 nm for 70° deflection from the normal.     

<Emphasis Type="Italic">T</Emphasis>-odd asymmetries for evaporation γ-rays in nuclear fission

T-odd asymmetries in angular distributions of evaporation γ-rays emitted by thermalized fragments resulting from the fission of axially symmetric deformed nuclei induced by cold polarized neutrons are investigated within the quantum theory of fission. The asymmetries in question are due to the anisotropy of the angular distributions of evaporation γ-rays, caused by zero wriggling vibrations of the fissioning nucleus and associated with the orientation of large fission fragment spins in the direction perpendicular to the direction n ₀ of the symmetry axis of the fissioning nucleus at the time of its separation into fragments. The angle of rotation of the vector n ₀ with respect to the asymptotic direction of the light fission fragment emission is calculated with allowance for the interference of fission amplitudes of neutron resonances excited in the fissioning nucleus as it captures the incident neutron. It is shown that the angular and energy characteristics of the T-odd asymmetry calculated for evaporation γ-rays agree with the characteristics of the experimentally investigated T-odd asymmetry in angular distributions of all γ-rays emitted by a fissioning ²³⁶U nucleus.     

Interaction of loosely bound radioactive 7Be and stable 7Li with 9Be

Quasielastic scattering angular distributions have been measured for the ⁷Be + ⁹Be system at E _lab = 17 , 19 and 21MeV in the angular range $ \theta_{{cm}}^{}$ = 24^° - 57^° . An optical model (OM) analysis of these data has been carried out in order to extract optical potential parameters and reaction cross-sections. One-proton stripping cross-sections were also measured for this system at E _lab = 19 and 21MeV. These transfer angular-distribution data were compared with the finite-range distorted-wave Born approximation (FRDWBA) calculations. For the ⁷Li + ⁹Be system quasielastic scattering angular distributions were measured and emitted light charged particles were detected at E _lab = 15.75 , 24.00 and 30.00MeV in the angular range $ \theta_{{cm}}^{}$ = 7^° - 70^° . Fusion cross-sections were obtained by reproducing the measured $ \alpha$ -evaporation spectra from the compound nucleus at backward angles with the statistical model calculations. The ratios of the experimental fusion cross-sections to the total reaction cross-sections (obtained from OM analysis) were found to be small. This result suggests that the break-up process has a strong influence on the fusion process leading to a reduction in the fusion cross-section.     

Optical and photoluminescent properties of plasma polymer films used in electroluminescent display structures

The optical and photoluminescent properties of plasma polymer layers synthesized from hexamethyldisiloxane are examined. The value of the polymer layer transparence is in the limits from 55% at 400 nm to 88% at 800 nm. Photoluminescence is stimulated by using the spectral line λ=365 nm emitted by a Hg spectral lamp.The organosilicon plasma polymers are included as protective and capsulating layers in electroluminescent (EL) structures. The structure obtained is characterized by a significant increase in emission brightness, compared to inorganic protective layers. For EL structures with a chalcogenide protective layer the increase is more than 6 times and for structures with heterogeneous matrix on the base of TiO₂ it is more than 20 times. As a stable covering the organosilicon plasma polymer increases the lifetime of the EL structures too.     

Optical and electrical properties of thermally oxidized bismuth thin films

Bismuth trioxide (Bi₂O₃) thin films were prepared by dry thermal oxidation of metallic bismuth films deposited by vacuum evaporation. The oxidation process of Bi films consists of a heating from the room temperature to an oxidation temperature (T_o = 673 K), with a temperature rate of 8 K/min; an annealing for 1 h at oxidation temperature and, finally, a cooling to room temperature. The optical transmission and reflection spectra of the films were studied in spectral domains ranged between 300 nm and 1700 nm, for the transmission coefficient, and between 380 nm and 1050 nm for the reflection coefficient, respectively. The thin-film surface structures of the metal/oxide/metal type were used for the study of the static current-voltage (I-U) characteristics. The temperature of the substrate during bismuth deposition strongly influences both the optical and the electrical properties of the oxidized films. For lower values of intensity of electric field (ξ < 5 × 10⁴V/cm), I-U characteristics are ohmic.     

Errata

Angular distributions of protons emitted in the (e, e'p) reaction on ⁴⁰Ca at an incident electron energy of 710 MeV and for missing energies e_m = 39 MeV and E_M = 81 MeV are presented. The interpretation of these angular distributions in terms of momentum distributions for the struck protons in the original target nucleus indicates l ≠ 0 protons at E_M = 39 MeV and l = 0 protons at E_M = 81 MeV. Finally (p, 2p) and (e, e'p) results in calcium are compared.     

Effect of excitation intensity and electric field on the photoconductivity relaxation in CdxHg1−x Te/GaAs polycrystalline layers

The photoconductivity relaxation and the stationary photoconductivity in the n-Cd_0.8Hg_0.2Te compensated polycrystalline layers at T=300 K have been investigated as a function of the light intensity and the strength of applied electric field E. It is demonstrated that, at low excitation intensities, the saturation of stationary photoconductivity and a decrease in the relaxation time with an increase in E are caused by the minority carrier extraction. The effect of minority carrier extraction is analyzed with due regard for the internal electric field of potential barriers in intergranular layers. It is assumed that the features of nonequilibrium-carrier recombination, which proceeds through several channels and depends on the excitation intensity and extraction electric field strength, can stem from the polycrystalline structure of the Cd_0.8Hg_0.2Te layers.     

Spontaneous periodic structures induced in thin AgCl-Ag films by the continuum in the case of optical rotation of the plane of polarization

The formation of spontaneous periodic structures in thin AgCl-Ag films subject to the continuum (λ≈450–700 nm) in the case of rotation of the plane of polarization with the help of crystalline quartz is studied. The periodic structures are formed due to the excitation of waveguide TE₀ modes in spontaneous Rayleigh scattering of radiation with a wavelength λ incident on the film. The structure periods are smaller than λ and vary in proportion with λ. After special treatment of the irradiated sample (fixing and deposition of an Al layer), periodic structures form a thin reflection hologram capable of reproducing the direction of linear polarization, the angle of optical rotation, and the color of the spectrum recorded. The study of diffraction patterns and small-angle scattering from small portions of a hologram, which were exposed to light with various wavelengths in the recording process, with the help of a probing UV beam (λ_p=337 nm), made it possible to reveal special features of the manifestation of Rayleigh and Wood anomalies associated with the domain structure of PS.     

Nonlinear refraction in photo-induced anisotropic states of azo-dye containing materials

The optical anisotropic states in the azo-dye Disperse Red 13 doped poly(methyl methacrylate) polymeric film are induced by polarized light and the nonlinear refractive index in these states are studied by the Z-scan technique. It is found that the n₂ of the sample can be controlled by changing both the polarization direction and the intensity of the 514 nm light. Photo-induced isomerization and reorientation of azo chromophores in polymer matrices are used to explain the results.     

A TiO2 Film/K+ Ion-Exchanged Glass Composite Optical Waveguide and Its Application to a Refractive Index Sensor

A composite optical waveguide for biological and chemical sensors was successfully developed by sputtering a thin TiO₂ film onto the surface of a low loss potassium ion-exchanged optical waveguide. The electric field of the evanescent wave at the film surface was made strong through adiabatic transition of the guided light. The attenuation of the guided light was sufficiently small in the air, and the guided light intensity was changed sensitively with the refractive index of the cladding layer (n_c). Thus, n_c can be sensitively monitored with a detecting sensitivity of about 10^-4.     

Visible photocurrent response of TiO2 anode

The photoelectrochemical response to the electromagnetic radiation over the visible range is particularly sought for from the point of view of the efficiency of hydrogen generation by water photolysis in a photoelectrochemical solar cell, PEC. The PEC used in this work comprises thin film TiO₂ - based photoanode, Pt foil covered with Pt black as a cathode and SCE as a reference electrode, immersed in an electrolyte solution. Titanium dioxide thin films are deposited by means of rf reactive sputtering and modified, when necessary, by Au or Ag ultra-thin overcoatings. Here we show that even unmodified TiO₂ photoanode, shows a photocurrent peak over the visible range of the light spectrum (λ = 500-650 nm). The effect of the surface modification by noble metals and properties of the aqueous electrolyte on the visible photocurrent are studied. The optical spectra indicate an increased absorption due to noble metal deposits at 410 nm for Ag and at 600 nm for Au. In contrast, the photocurrent peak over the visible range (500 nm < λ < 650 nm) changes its symmetry and decreases in intensity with the increasing thickness of noble metals layers. The visible photoresponse is explained in terms of OH formation at the interface between TiO₂ electrode and aqueous electrolyte.     

Photorefractive polymer composites for the IR region based on carbon nanotubes

The photorefractive properties of polymer composites based on aromatic polyimide and single-wall carbon nanotubes are studied using radiation at a wavelength of 1064 nm. It is found that the nanotubes possess photoelectric sensitivity in this spectral region and that the kinetic photorefractive characteristics of the polymer composites are entirely determined by the photogeneration and charge transport characteristics of the layers. The two-beam gain coefficient of the signal beam measured for a composite consisting of aromatic polyimide and 0.25 wt % of single-wall carbon nanotubes in a constant electric field E₀ = 79 V/μm is equal to 84 cm^?1 and exceeds the optical absorption coefficient by 59 cm^?1. The refractive index modulation is equal to Δn = 0.004 at E₀ = 54 V/μm.     

Determination of optical constants and nonlinear optical coefficients of Violet 1-doped polyvinyl alcohol thin film

The optical properties of Violet 1-doped polyvinyl alcohol (PVA) have been investigated using Wemble and Didomenico (WD) method. The optical constants such as refractive index n, the dispersion energy E _d, the oscillation energy E ₀, the lattice dielectric constant \(\varepsilon _{\infty } \), light frequency dielectric constant ε ₀ and the ratio of carrier concentration to the effective mass N/m* have been determined using reflection spectra in the wavelength range 300–900 nm. The single- beam Z-scan technique was used to determine the nonlinear optical properties of Violet 1:polyvinylalcohol (PVA) thin film. The experiments were performed using continuous wave (cw) laser with a wavelength of 635 nm. The calculated nonlinear refractive index of the film, n ₂ = ?2.79×10^?7 cm²/W and nonlinear absorption coefficient, β = 6.31×10^?3 cm /W. Optical limiting characteristics of the dye-doped polymer film was studied. The result reveals that Violet 1 can be a promising material for optical limiting applications.     

Photocurrent oscillations in luminescent epitaxial thin films of ZnSe (Cu,Ga)

Photoluminescent epitaxial thin films of ZnSe (Cu, Ga), n-type, vacuum-deposited onto CaF₂ crystals, or air cleaved mica substrates, have been studied. At temperatures between 80 K and 220 K, slow photocurrent oscillations can be observed if ZnSe (Cu, Ga) thin films are illuminated with light (200 nm ? λ ? 530 nm) and an electric field is simultaneously applied, whose value is above a definite threshold. These oscillations, with amplitude and frequency varying as a function of temperature, applied electric field and light intensity, are attributed to moving high-field domains similar to those found by Boer and co-workers on CdS single crystals. Field quenching of luminescence is observed, which visualizes moving high-field domains. Anode-adjacent high-field domains occur at applied voltages above the range at which moving domains are observed and a simultaneous electroluminescence is initiated near the anode.     

Probing electric field distribution in organic double-layer diode by electric field induced optical second harmonic generation

Analyzing spectroscopic optical properties of an organic double-layer diode comprised of α-NPD and Alq₃ layers, we studied the selectively probing of electric field distribution in one of the two layers by using the microscopic electric field induced optical second harmonic generation (EFISHG) measurement. Spectroscopic SHGs from Indium–Zinc-Oxide/N,N-Di(naphthalene-1-yl)-N,N′-diphenyl-benzidine/tris(8-quinolinolato) aluminium/Al (IZO/α-NPD/Alq₃/Al) diodes were measured. Results showed that the SHG peaks were generated at 940 and 1050 nm from the α-NPD and Alq₃ layers, respectively, due to the EFISHG process, and the electric field in each layer can be selectively probed. The contribution of the accumulated charge at the double-layer α-NPD and Alq₃ interface was also identified by the d.c. voltage dependence on the EFISHG intensity.     

The ferroelectric and ferromagnetic characterization of CoFe2O4/Pb(Mg1/3Nb2/3)O3-PbTiO3 multilayered thin films

The multiferroic (PMN-PT/CFO)_n (n = 1,2) multilayered thin films have been prepared on SiO₂/Si(1 0 0) substrate with LNO as buffer layer via a rf magnetron sputtering method. The structure and surface morphology of multilayered thin films were determined by X-ray diffraction (XRD) and atom force microscopy (AFM), respectively. The smooth, dense and crack-free surface shows the excellent crystal quality with root-mean-square (RMS) roughness only 2.9 nm, and average grain size of CFO thin films on the surface is about 44 nm. The influence of the thin films thickness size, periodicity n and crystallite orientation on their properties including ferroelectric, ferromagnetic properties in the (PMN-PT/CFO)_n multilayered thin films were investigated. For multilayered thin films with n = 1 and n = 2, the remanent polarization Pr are 17.9 μC/cm² and 9.9 μC/cm²; the coercivity H_c are 1044 Oe and 660 Oe, respectively. In addition, the relative mechanism are also discussed.