Light-induced thermal gratings in LiNbO3: Fe

Holographic gratings are recorded in LiNbO³: Fe using frequency-doubled pulses of aQ-switched Nd : YAG laser (light wavelength = 532 nm, pulse durationt _p = 20ns). We monitor diffraction efficiencies of the holograms during and several milliseconds after exposure. Variations of Fe²⁺ concentration, light intensity, and fringe spacing show that different types of gratings are involved: photochromic, thermooptic, pyroelectric, and space-charge gratings. The influence of the internal pyroelectric field on the light-induced charge transport is discussed in terms of a two-center model. Pyroelectric and bulk photovoltaic effect are shown to partially compensate each other. In LiNbO₃: Fe, the photovoltaic effect is the dominant charge-driving force for our recording conditions.     

Photorefractive response and optical damage of LiNbO<Subscript>3</Subscript> optical waveguides produced by swift heavy ion irradiation

The photorefractive behaviour of a novel type of optical waveguides fabricated in LiNbO₃ by swift heavy ion irradiation is investigated. First, the electro-optic coefficient r ₃₃ of these guides that is crucial in the photorefractive effect is measured. Second, two complementary aspects of the photorefractive response are studied: (i) recording and light-induced and dark erasure of holographic gratings; (ii) optical beam degradation in single-beam configuration. The main photorefractive parameters, recording and erasing time constants, maximum refractive-index change and optical damage thresholds are determined.     

Cd13P4S22I2 — a new holographic recording material

Holographic recording and erasure experiments in Cd₁₃P₄S₂₂I₂ crystals are reported. The refractive index change of the volume phase hologram is larger than n=1.5×10^–4, whereas the static dielectric constant of this reversible storage medium is only about 10. These are rather unusual properties if compared with electrooptic KNbO₃.     

Strong permanent reversible diffraction gratings in copper-doped lithium niobate crystals caused by a zero-electric-field photorefractive effect

Permanent reversible diffraction gratings with refractive-index modulations of up to 10^-4 and with a grating vector perpendicular to the polar axis are realized in LiNbO₃:Cu crystals by making use of high temperature recording and charge compensation. The index changes do not result from light-induced space-charge fields and the linear electro-optic effect. They are linked to fixed high-contrast narrow-band absorption gratings of Cu⁺/Cu²⁺ ions via the Kramers–Kronig relations. PACS 42.65; 42.70; 72.40; 78.20     

UV laser-induced grating formation in PDMS thin films

Excimer laser (193 nm and 157 nm) induced ablation and structure formation in poly-dimethylsiloxane (PDMS) thin films is demonstrated. Ellipsometric measurements provide values of the optical constants of the films as well as their thicknesses, which are below 1 m. At fluences above 160 mJ/cm² two pulses of UV light induce gratings with at minimum 1-m periods and crossed gratings with 4-m periods. The structure heights are between 10 nm and 20 nm with ridge widths of several hundred nanometres. The ablation occurs after a single incubation pulse with a threshold that increases logarithmically with the ablation wavelength increasing from 157 nm to 1064 nm. At 193 nm the ablation rate for 2 J/cm² is 127 nm/pulse. PACS 79.20.La; 34.50.Dy; 68.55.Jk     

Polarization holographic gratings with high diffraction efficiency recorded in azopolymer PAZO

Azopolymers are one of the most efficient materials able to record the polarization state of light. They have numerous applications, such as data storage and diffractive optical elements with unique polarization properties. An essential parameter for each diffractive element is its diffraction efficiency η. In order to optimize the recording conditions and obtain high-efficient polarization holographic gratings, in the present work we study the dependence of the diffraction efficiency on the recording angle and thickness of a series of azopolymer layers. Three recording angles are used ??10°, 20° and 30° and three series of thin films with thicknesses 470, 850 and 2400 nm from the water-soluble azopolymer PAZO. The gratings are inscribed by two plain waves with left and right circular polarization from a He-Cd gas laser (442 nm). The diffraction efficiency of the gratings is probed with a right hand circularly polarized beam from a probe laser with wavelength 635 nm. The kinetics of diffraction efficiency η(t) in the +?1 diffraction order are presented and compared. Our experimental results indicate that highest diffraction efficiency (more than 40%) is obtained for the sample with thickness 2400 nm and for recording angle 10°. As the holographic recording in azopolymers is usually accompanied by formation of surface relief gratings, the surface topography of the recorded samples is also investigated by atomic force microscopy.     

Spectroscopic study of potential magneto-optic storage layers

We present an experimental investigation of magneto-optic (MO) thin film materials of interest in optical storage. XPS valence band spectra of ultrahigh vacuum deposited rare earth — Fe₅₀Co₅₀ alloys are correlated to magneto-optic and reflectivity spectra measured ex-situ in the photon energy range 0.5 E _ph 5.5 eV. It is found that 4f ⁿ 4f ^n–15d ¹ optical transitions contribute to room temperature Kerr spectra in Pr-FeCo and Nd-FeCo, but only in the UV spectral region (300 nm). Eu is found to enter Eu-FeCo intermetallic alloys with a 4f ⁷ ground state at –2 eV binding energy. However, no substantial contribution to the Kerr effect from excitations of this electronic state has been observed n either at room temperaturen or at 100 K. We present a general argument that 4f excitations of RE ions in typical intermetallic alloys can never be expected to contribute more than some tenths of a degree to _k in contrast to the well known huge rotations in RE-chalcogenides.For Yb-FeCo we have observed in XPS a mixed valence behavior of Yb showing a discontinuous transition from divalent Yb²⁺ in the pure metal to mixed valent Yb^2.55+ in amorphous Yb-FeCo films. Nofd contributions to the room temperature MO spectra have been detected. Strong optical enhancement effects are presented for metallic bilayer and multilayer structures of the type MO-layer/reflector. The total thickness of an optimized structure corresponds to about two times the penetration depth of light. Enhancement occurs predominantly at photon energies below the plasma frequency of the reflector material and is caused by low values of the optical constantsn andk of the reflector. In storage materials like Tb-FeCo the read-out performance can be improved by about a factor of 2 with a bilayer of 30 nm Tb-FeCo on Cu. Quantitative modelling of the optical and magneto-optic properties of these films is discussed.     

Diffraction investigations of the radiation strength of vanadium dioxide interference structures

Optical breakdown of vanadium oxide film interferometers is investigated by the method of recording irreversible diffraction gratings, and the optical damage threshold of the gratings is measured and found to be 76±4 mJ/cm². Zh. Tekh. Fiz. 67, 88–91 (April 1997)     

Phase gratings in the visible and near-infrared spectral range realized by metastable electronic states in Na2[Fe(CN)5NO] · 2H2O

Phase and amplitude gratings in the visible and near-infrared spectral range can be written in SodiumNitro-Prusside (SNP), Na₂[Fe(CN)₅NO] · 2H₂O, single crystals by optical excitation of infinitely long-living metastable electronic states, localized in the [Fe(CN)₅NO]^2– anions. Hence, its photorefractive effect does not depend on dopants or defects. The refractive index is modulated by more thann = 1 × 10^–3 in the red (632.8 nm) andn = 5 × 10^–4 in the near-infrared region (1047 nm). The absorption coefficient is modulated by about = 100 m^–1 at 632.8 nm and 40 m^–1 at 1047 nm. The wavelength dependence ofn can be explained by strong absorption bands in the ultraviolet considering Kramers-Kronig dispersive analysis. The time constant of the write-read-erase processes and the diffraction efficiency depend on light intensity, wavelength and polarization of the light with respect to the crystallographic axes. After excitation of the metastable states the indicatrix is modulated only along thea- andb-axis of the orthorhombic system.     

Spatio-temporal structure formation and self-diffraction in CdS using laser-induced thermal gratings

The semiconductor CdS is well known to show various kinds of photo-thermal and photo-electric optical nonlinearities. We present here to our knowledge first results of spatiotemporal structure formation using laser-induced thermal gratings in CdS. By the means of an optical multi-channel analyzer the spatial resolution of transverse dynamic switching processes was observed directly. Spatially and time resolved self-diffracted signals were measured in the far-field. The experimental results agree very well with calculations obtained by solving the heat-flow equation and using a fast Fourier transformation. The calculations verify the experimental parameters, in particular the thermal diffusivity D0.1 cm²/s at room temperature. To obtain a better transverse structuring, CdS samples were also investigated which were fixed on a sapphire substrate for longitudinal heat sinking.     

Nonlinear optical properties of composites based on conductive metal-alkanoate liquid crystals

The dynamic holography in new composite materials based on a novel class of metal-alkanoate ionic liquid crystals (ILCs) is studied experimentally and theoretically. The composites are formed as a dielectric dye film covered by lyotropic metal-alkanoate ILC and ionic smectic glasses with doped dye molecules. The dynamic gratings are created by nanosecond pulses of double frequency Nd:YAP laser, the recording demonstrates fast erasure time of residual thermal gratings. The nonlinear optical properties are determined by the resonance nonlinearity in photosensitive centres of ILC. Note, that permanent relief gratings will be formed on a dielectric dye film only as well as in composite cells either with nematic LC or with polymers under action of pulsed laser radiation. Lyotropic ILC layer applied over the dye film provides the dynamic regime of grating recording in composite cells. We found a secondary thermal grating is much smaller, the conductive ILC matrix provides effective heat dissipation and erasure of this thermal grating. A theory of Raman-Nath self-diffraction holography on thin films followed from the wave equation and the nonlinear mechanism of absorption saturation is developed to explain experimental results.     

On the equilibrium statistical mechanics of isothermal classical self-gravitating matter

The canonical ensemble is investigated for classical self-gravitating matter in a finite container ^{[d] d} ,d=3 and 2. Starting with modified gravitational interactions (smoothed-out singularity), it is proven by explicit construction that, in thew ^*-topology, the canonical equilibrium measure converges to a superposition of Dirac measures when the limit of exact Newtonian gravitational interactions between classical point particles is taken. The consequences of this result for more realistic classical systems are evaluated, and the existence of a gravitational phase transition is proven. The results are discussed with view toward applications in astrophysics and space science. Some attention is paid also to the problem of founding thermodynamics by means of statistical mechanics.     

Structural, optical and thermal investigations on Dy doped NaF–Li2O–B2O3 glasses

Structural, optical and thermal properties of Dy³⁺ doped lithium fluoroborate glasses have been studied for various concentrations of Dy³⁺ from 0.5 to 5 wt%. The XRD studies confirm the amorphous nature of the glasses while the FTIR spectra reveal the presence of BO₃ and BO₄ local structural units. The UV–VIS–NIR absorption studies were carried out to calculate the bonding parameters ( and δ), to identify the ionic/covalent nature of the glasses. The JO parameters, experimental and theoretical oscillator strengths were also determined and reported. The luminescence spectra have been studied to determine the radiative transition probability (A), stimulated emission cross section () and the experimental and calculated branching ratios (β_R) for the excited levels that include ⁴F_9/2→⁶H_11/2, ⁶H_13/2, and ⁶H_15/2 transitions. The variation of optical properties with varying concentrations of dysprosium oxide content in the glasses are reported and discussed. The thermal behavior of Dy³⁺ doped lithium fluoroborate glasses have been reported by recording DSC thermograms.     

Thermodynamic Cost of Reliability and Low Temperatures: Tightening Landauer's Principle and the Second Law

Landauer's principle states that the erasure of one bit of information requires thefree energy kT ln 2. We argue that the reliability of the bit erasure process isbounded by the accuracy inherent in the statistical state of the energy source(the resources) driving the process. We develop a general framework describingthe thermodynamic worth of the resources with respect to reliable bit erasureor good cooling. This worth turns out to be given by the distinguishability of theresource's state from its equilibrium state in the sense of a statistical inferenceproblem. Accordingly, Kullback—Leibler relative information is a decisivequantity for the worth of the resource's state. Due to the asymmetry of relativeinformation, the reliability of the erasure process is bounded rather by the relativeinformation of the equilibrium state with respect to the actual state than by therelative information of the actual state with respect to the equilibrium state (whichis the free energy up to constants).     

Magnetic Study of Doped Ba Ferrites Prepared by Citrate Method

The M-type barium ferrite is doped with either Co²⁺-Ti⁴⁺ or Co²⁺-Zr⁴⁺ pairs to reduce its intrinsically high uniaxial magnetic anisotropy in order to make fine particles for magnetic recording. The magnetic parameters were investigated by magnetic measurements and the Mössbauer spectroscopy. Compounds (BaF) obtained from BaCo _x (Ti,Zr) _x Fe_12-2x O₁₉ with 0x1.0 have been prepared by the modified citrate method with the initial ratio of Ba:Fe equal to 1:10.8. The substitution of Co - Ti or Co - Zr for Fe³⁺ ions affects mainly the positions 2a and 4f ₂. Ba ferrite with the substitution x 0.8 is promising for perpendicular recording media applications.     

Generation regimes of a pulsed Nd:YAG laser with transverse LED pumping and multiloop self-pumped phase-conjugate cavity

The generation regimes in a pulsed Nd:YAG laser with transverse LED pumping and multiloop self-pumped phase-conjugate cavity on the gain gratings are studied. The differential efficiency of laser is 27% in the free-running regime at a pulse energy of up to 1 J and quality parameter M ² of no greater than 1.5. The pulse energy under passive Q-switching is no less than 60% of the pulse energy in the free-running regime at the same beam quality. The generation of the narrow-band radiation is demonstrated. A generation band of no greater than 1.2 GHz corresponds to the primary single-frequency high-power laser pulse in the free-running mode under conditions for self-Q-switching on the gain gratings. When additional elements (F ₂ ^? :LiF and Cr⁴⁺:YAG crystals) are introduced in the optical scheme of the phase-conjugate cavity, similar narrowband single-mode generation is observed in the passive Q-switching regime as a pulse train or monopulse. The laser pulse power is up to 2 MW at a pulse duration of 20 ns.     

Laser evaporation as a source of small free radicals

It is shown that laser evaporation of a solid target followed by adiabatic expansion can be used to produce cold beams of neutral small open shell molecules. LIF is applied to detect CuH (X ¹), CH (X ²), SiH (X ²), CuO (X ²), and FeO (X ⁵). A production of 10^9±1 molecules/sr in a single pulse is obtained for SiH. For a copper target, the optical emission of the jet plume is discussed.     

The Camassa-Holm Hierarchy, N-Dimensional Integrable Systems, and Algebro-Geometric Solution on a Symplectic Submanifold

This paper shows that the Camassa-Holm (CH) spectral problem yields two different integrable hierarchies of nonlinear evolution equations (NLEEs), one is of negative order CH hierachy while the other one is of positive order CH hierarchy. The two CH hierarchies possess the zero curvature representations through solving a key matrix equation. We see that the well-known CH equation is included in the negative order CH hierarchy while the Dym type equation is included in the positive order CH hierarchy. Furthermore, under two constraint conditions between the potentials and the eigenfunctions, the CH spectral problem is cast in: 1. a new Neumann-like N-dimensional system when it is restricted into a symplectic submanifold of ^2N which is proven to be integrable by using the Dirac-Poisson bracket and the r-matrix process; and 2. a new Bargmann-like N-dimensional system when it is considered in the whole ^2N which is proven to be integrable by using the standard Poisson bracket and the r-matrix process.     

On Bianchi type-I vacuum solutions inR+R 2 theories of gravitation. II. The axially symmetric anisotropic case

The field equations following from a LagrangianL(1/L)(–g)^1/2[(1/2)R+l ²(R _lk R ^lk +R ²)] will be considered for Bianchi type-I homogeneous models. Thereby the special case,+3=0, is considered qualitatively for axially symmetric anisotropic metrics. Generically, the solutions have both past and future singularities, but it will be proven by topological arguments that the two-dimensional space of solutions possesses a one-parameter subspace of solutions with a behavior similar to the Kasner solution.