Hologram recording

Parallelism is an intrinsic feature of many physical systems. The design of high-performance computers is increasingly making use of this concept by employing many processors working co-operatively to carry out a single computation. Consequently, the computer simulation of physical systems can rather naturally exploit the latest computer architectures to test theoretical models and to make measurements that are inaccessible to real experiments. In this article we review the design and use of parallel computers. In a subsequent article we will describe how they have been used to obtain insight into some fundamental problems in physics.     

Electric properties of chemically deposited lithium doped cadmium sulphide films

Electrical properties of chemically deposited CdS and lithium doped CdS films have been investigated by using techniques of Photo-thermoelectric and Photo-Hall effects. Dark electron densities are independent of temperature between 100 and 330°K because of shallow donors and quasi-intrinsic behaviour is apparent above 330°K. Electron density and mobility in these films have been measured as a function of temperature under strong photoexcitation. The electron mobility is found to be thermally activated with energies 0.2 and 0.22 eV for CdS:Li and CdS films respectively. The Hall mobility and electron density in these films have also been measured as a function of temperature under strong photoexcitation and found that both the free carrier density and mobility are reduced by the adsorption of oxygen, former by larger factor than the latter. The resulting very high electron density and very low electron mobility in the temperature region studied also indicate that most of the photoconductivity in chemically deposited films is caused by an increase in electron density due to photoexcitation.     

The origin of the 2.0 eV emissions in pure and doped cadmium sulphide

This paper reports on the results of a study of the emission in the 2.0eV region of pure and silver doped cadmium sulphide. The results show that in the pure samples a single band peaking at 2.05eV was observed while in the silver doped samples up to three bands peaking at 2.05eV, 1.94eV were observed. The origin of the emission is analysed in terms of transitions of the interlevel type with donor levels 0.35eV below the conduction band and acceptor levels 0.15eV and 0.25eV above the valence band.     

Observation of memory behaviour in cadmium sulphide nanorods doped ferroelectric liquid crystal mixture

In this study, cadmium sulphide (CdS) nanorods doped ferroelectric liquid crystal (FLC) sample cells have been prepared and studied. A memory effect has been observed in CdS nanorods (≤0.3?wt%) doped FLC mixture and confirmed by textures, dielectric and optical studies. The addition of nanorods increases the memory behaviour and efficiency. The occurrence of memory behaviour has been explained due to charge transfer from liquid crystal molecules to CdS nanorods and exists there for 5–15?min in 0.1–0.3?wt% CdS nanorods doped samples. An improvement in polarization, switching time, threshold voltage and rise time parameters has also been noticed in CdS nanorods doped FLC samples.     

Hologram recording and reading in discrete resonance ring cavity

Holograms are recorded and restored in a resonant ring cavity. The experimental data shows enhanced diffraction efficiencies off the holograms. To maintain the optical resonance of the cavity during writing or reading the hologram we propose discrete resonance ring cavity system. The experimental data is compared with those of the conventional system with no cavity resonance.     

Effect of corona discharge on cadmium sulphide and lead sulphide films

This paper describes the effect of corona discharge on cadmium sulphide (CdS) and lead sulphide (PbS) films prepared using the chemical route. The property of films before and after exposure to corona has been described in detail. The electronic properties of the CdS and PbS films have been studied by current-voltage (I-V), capacitance-voltage (C-V) measurements. The structural properties and surface morphology were studied by using X-ray diffraction and scanning electron microscopy before and after exposing to Corona discharge. The films displayed the change in surface morphology after exposure to the corona discharge. It has been found that the films showed an increase in resistivity after exposure. This change in property has been attributed to modification in surface states. Time-dependent recovery indicated that room temperature annealing is sufficient to regain the normal resistivity of the films. The experiment was carried with the aim of studying the effect of the interaction of corona discharge on the semiconductor films and its subsequent effects.     

Hologram recording in glass and direct reconstruction using visible-wavelength laser beam

We propose a method of hologram recording in a glass plate by corona charging. The holographic recording materials used in this study are conventional soda-lime glass and azobenzene polymer film. A Fourier transform hologram on an azobenzene polymer film coated on the glass plate is recorded in the glass plate by corona charging. After removing the polymer film, the hologram recorded in the plate can be reconstructed using a visible-wavelength laser beam. The first-order diffraction efficiency of the hologram at a wavelength of 532 nm is 0.03%; the efficiency depends on the depth of the surface relief structure on the azobenzene polymer film and the corona-charging time. The hologram recorded in the glass plate has high environmental resistance.     

Disordered structure of cadmium sulphide nanoparticles

Based on X-ray diffraction data obtained for thin polycrystalline CdS films and computer simulation using the Debye formula in the framework of the DISCUS program, it has been established that the location of close-packed Cd planes in CdS nanoparticles is disordered. The absence of strict periodicity of the atomic structure in the presence of close packing, with the mutual tetrahedral environment of Cd and S atoms, is a special feature of CdS in the nanostate. The average lattice of a disordered close-packed structure is described by space group P6 with parameters of the unit cell a = 0.236 and c =0.334 nm and a degree of occupation of crystal-lattice sites of 1/3.     

Diffusion of indium into cadmium sulphide

The diffusion of In into CdS was studied using optical, microprobe and radiotracer methods over a temperature range from 729 to 1411 K. When diffusion occurred with a large excess of In metal, the diffusion coefficient was found to be linearly proportional to In concentration in the CdS and a sharp, optically observable diffusion front existed. Varying the Cd/In atomic ratio in the metal produced a transition to normal, concentration independent diffusion for Cd/In ratios greater than unity. The diffusion is markedly anisotropic, with fast diffusion perpendicular to the crystal c-direction. Very high values were measured for the diffusion coefficient in the concentration dependent regime (up to 2 × 10^?11m²s^?1) and the mobile specie is thought to be the (InV) complex. The total apparent activation energy was found to be 2.03 and 2.30 eV (perpendicular and parallel to the c-direction respectively) and the anisotropy of the activation energy measured directly was 0.250 eV. On converting the concentration dependent diffusion coefficients to a standard concentration a straight line Arrhenius plot was obtained below 1200 K, which gave an activation energy for defect motion of 1.56 ± 0.12eV perpendicular to the c-direction and 1.81 ± 0.12 eV parallel to the c-direction.     

Diffusion of gallium into cadmium sulphide

Measurements of the Ga diffusion into CdS, in the presence of exces Ga metal, using optical and mixroprobe analyser techniques are reported. A reaction layer of CdGa₂S₄ forms on the CdS crystals below 1240±20 K. Above this temperature the reaction layer is liquid. The Ga diffusion is concentration dependent and also orientation dependent with the faster diffusion perpendicular to the hexagonal c-axis of CdS. The anisotropy of the activation energy was calculated to be 0.20±0.06 eV. In the temperature range 940–1240K the linearly concentration dependent diffusion yielded activation energy values for defect motion of 2.39±0.13 eV perpendicular to the c-direction and 2.21 ±0.13 eV parallel to the c-direction.     

Reactively evaporated copper sulphide films

Thin films of covellite (CuS) have been prepared for the first time. The films were prepared by reactively evaporating copper and sulphur. X-Ray diffraction measurements indicate that the grains are oriented with (001) planes perpendicular to the substrate surface. Optical studies show that CuS is a semiconductor, contrary to what has been reported earlier. The films have a resistivity of ～10^?4 Ω cm and show p-type conductivity.     

An electrochemical study of copper diffusion in non-stoichiometric copper sulphide

The diffusion of copper in hexagonal chalcocite (Cu₂S) is not very well-known. In this work electrochemical cells with natural polycrystalline chalcocite working electrodes have been studied by electrochemical impedance spectroscopy (EIS) over a large frequency range at equilibrium potential. To study this phenomenon between 120 and 160°C a solid electrolyte RbCu₄Cl₅ has been used. The impedance spectra present two distinct regions. At high frequencies the general shape of the diagram in the Nyquist plane is a depleted are of a circle. Changes in electrolyte resistance, interfacial capacitance and transfer resistance have been studied as a function of temperature. At low frequencies, a diffusion impedance is observed attributed to the mobility of copper vacancies. Diffusion coefficients, with an activation energy of 1.7 eV, have been deduced from the impedance diagrams (4.7·10⁻⁵cm² s⁻¹ at 130°C). These results are compared with those obtained with orthorhombic chalcocite between 30 and 60 °C by EIS and using an electrochemical cell with a cupric liquid electrolyte. Paper presented at the 3rd Euroconference on Solid State Ionics, Teulada, Sardinia, Italy, Sept. 15–22, 1996     

Hologram recording in DuPont photopolymer films by use of pulse exposure

Reflection hologram recording in DuPont HRF-800X071-20 photopolymer films has been demonstrated by use of pulsed laser exposure with pulse lengths of 25 ns. An expected weak reflectance of the recorded mirror holograms could be significantly increased by preillumination. Although pulsed preillumination enhanced only the reflectance, continuous incoherent preillumination significantly increased both the diffraction efficiency (which reached ~80%) and the sensitivity (which reached an increase of ~100 times, thus approaching the sensitivity of cw recording). The results are compared with those for hologram recordings obtained with cw exposure under the same processing conditions. Spectral absorbance curves of recorded holograms are presented, and possible mechanisms of hologram formation are discussed.     

Hologram recording and read-out with the evanescent field of guided waves

We investigate hologram recording and read-out where the evanescent field of a guided mode propagating in a planar waveguide is used as reference and illuminating wave, respectively. A special case of such a hologram is a new type of grating coupler promising beam to mode coupling with high coupling efficiency. First experiments with dichromated gelatin as the recording medium are reported as feasibility proof.