Properties of CdF2:Ga as a medium for real-time holography

The metastable nature of the excited state of bistable Ga centers in semiconducting CdF₂ crystals allows for the use of CdF₂:Ga and CdF₂:Ga,Y crystals as materials for real-time holography over a wide range of response times (1–1000 ms). The characteristics of these materials and optimal conditions for their use are discussed in the framework of a model that describes the decay of photo-induced gratings written in them. Received: 20 November 2000 / Published online: 20 April 2001     

Holographic recording by excitation of metastable electronic states in Na2[Fe(CN)5NO]·2H2O: a new photorefractive effect

Elementary holographic phase gratings can be written in single crystals of Na₂[Fe(CN)₅NO]·2H₂O, sodiumnitroprusside, by excitation of metastable electronic states in the blue–green spectral range. For light polarized parallel to the crystallographic a and b axes of the orthorhombic crystal the light-induced modulation of the refractive index reaches Δn≈2×10^-3 at λ=514.5 nm. Although the largest population of the metastable states is reached for light polarized parallel to the crystallographic c axis, a photorefractive response is not observed. In contrast to electro-optic photorefractive materials the photorefractive effect depends mono-exponentially on the exposure and on the modulation of the incident light interference pattern. Beam-coupling experiments demonstrate that written gratings are in phase with the interference pattern in correspondence with the fact that the excitation of the metastable electronic states is local. The width of the rocking curve shows that the holographic gratings are written completely over the volume of the crystal. Variations of the wavelengths within the excitation range as well as of the crystal thickness do not influence the maximum photorefractive response. Investigations on the grating vector of the written gratings show unambiguously that charge migration is not responsible for the photorefractive effect. Received: 18 November 1998 / Revised version: 26 January 1999 / Published online: 12 April 1999     

Photorefractive properties of Cr-doped Sr0.61Ba0.39Nb2O6 related to crystal purity and doping concentration

The purity and the concentrations of the constituents Sr, Ba, Nb and Cr were determined in single crystals of chromium-doped Sr_0.61Ba_0.39Nb₂O₆ (SBN) by instrumental neutron activation analysis (INAA) and X-ray fluorescence analysis (XRF). Experiments with different Cr concentrations C_Cr reveal constant Sr and Ba concentrations as well as a decreasing Nb concentration with an increasing C_Cr. Therefore, Cr is incorporated at Nb lattice sites. The distribution coefficient of Cr is 1.2 between 100 and about 7000 ppm Cr and 1 above. From measurements of the holographic two-beam coupling gain Γ for different C_Cr, we deduced response times for the buildup of holographic gratings. The saturation value of Γ depends in a nonlinear manner on C_Cr and reaches its maximum at about 6000 ppm Cr. The inverse photorefractive response time of the grating erasure depends linearly on the erasure intensity for all doping concentrations. Thus a one-center model can be assumed for the charge transport in SBN:Cr. Received: 8 December 2000 / Revised version: 23 January 2001 / Published online: 21 March 2001     

High-resolution photorefractive gratings in nematic liquid crystals sandwiched with photoconductive polymer film

Photorefractive gratings with high grating resolution were observed in the 20 μm thick low-molar-mass nematic liquid crystal (NLC) cell with a separate photoconductive (PC) poly(N-vinylcarbazole) layer. An orientational grating with a grating spacing of 1.9 μm was produced. It is believed that a space–charge field with small fringe spacing forms in the PC layer and its evanescent component penetrates into the NLC layer. The penetrated evanescent field drives the NLC to reorient, and consequently the orientational grating forms. The model indicates that the modulated field exists in several hundred nanometers near the surface, and thus the orientational grating is not full of the NLC film, which is consistent with the observed phenomena of the multiple diffractions. Besides, asymmetric two-beam coupling of 11.2% was achieved for the grating with a grating spacing of 1.9 μm, and a net gain coefficient of larger than 62 cm⁻¹ was obtained.     

Orbital correlations in doped manganites

We review our recent X-ray scattering studies of charge and orbital order in doped manganites, with specific emphasis on the role of orbital correlations in Pr_1-xCa_xMnO₃. For x=0.25, we find an orbital structure indistinguishable from the undoped structure and long-range orbital order at low temperatures. For dopings 0.3≤x≤0.5, we find scattering consistent with a charge and orbitally ordered CE-type structure. While in each case the charge order peaks are resolution limited, the orbital order exhibits only short-range correlations. We report the doping dependence of the correlation length and discuss the connection between the orbital correlations and the finite magnetic correlation length observed on the Mn³⁺ sublattice with neutron-scattering techniques. The physical origin of these domains, which appear to be isotropic, remains unclear. We find that weak orbital correlations persist well above the phase transition, with a correlation length of 1–2 lattice constants at high temperatures. Significantly, we observe similar correlations at high temperatures in La_0.7Ca_0.3MnO₃, which does not have an orbitally ordered ground state, and we conclude that such correlations are robust to variations in the relative strength of the electron–phonon coupling. Received: 22 May 2001 / Accepted: 4 July 2001 / Published online: 5 October 2001     

Single-frequency Ti:Er:LiNbO3 distributed Bragg reflector waveguide laser with thermally fixed photorefractive cavity

The first single-frequency Ti:Er:LiNbO₃ distributed Bragg reflector waveguide laser with two thermally fixed photorefractive gratings as resonator mirrors is reported. The optically pumped (λ_p=1480 nm,120-mW incident power) laser emits up to 1.1 mW at λ_s=1561.1 nm. The threshold pump power is 70 mW. Received: 7 June 2001 / Published online: 30 October 2001     

Experiment to Test Whether We Live in a Four-Dimensional Physical Space–Time

For quantum systems, whose energy ratios E_n/E₀ are integers, and |E₀| is the smallest energy, the time dependent wavefunctions and expectation values of time independent operators have time periodicitiy with a time period T equal to T = h/|E₀|, where h is the Planck constant. This periodicity is imposed on the wavefunctions due to undersampling in energy, but following a similarity with aliasing in signal analysis, it may allow to probe future and past events under the condition that our world is in reality a true four-dimensional, “static” space–time. We suggest an experiment to test that possibility. A positive result will indicate that we live in a four-dimensional space–time. A negative result (not getting signals from the future) will indicate that four-dimensional space–time is not physical one and that we live in a three-dimensional space with a time.     

Measurement of the space–time interval between two events using the retarded and advanced times of each event with respect to a time-like world-line

Several recent studies have been devoted to investigating the limitations that standard quantum mechanics and/or quantum gravity might impose on the measurability of space–time observables. These analyses are often confined to the simplified context of 2D flat space–time and rely on a simple procedure for the measurement of space-like distances based on the exchange of light signals. We present a generalization of this measurement procedure applicable to all three types of space–time intervals between two events in space–times of any number of dimensions. We also present a preliminary account of an alternative measurement procedure that can be applied taking into account the gravitational field of the macroscopic measuring apparatus.     

Nonlinear refraction and absorption in hetero- thiometallic planar clusters

The nonlinear absorption and refraction of the clusters [MoS₄Cu₄Br₂(py)₆] and [Et₄N]₂[MoS₄Cu₄(SCN)₄(2-pic)₄] have been investigated using the z-scan technique with a ns laser at 532 nm wavelength. They have the same planar ‘open’ structures and the same skeleton metal atoms; the only difference is that the former has halogen ligands while the latter possesses pseudo-halogen groups – SCN – as ligands. Alteration of nonlinear refractive index and enhancement of nonlinear absorption were found in these two clusters. A steady state model of excited state nonlinear refraction was proposed to explain this phenomenon. Received: 12 June 2001 / Revised version: 4 September 2001 / Published online: 29 November 2001     

Computer Simulations for Some One-Dimensional Models of Random Walks in Fluctuating Random Environment

We report some results of computer simulations for two models of random walks in random environment (rwre) on the one-dimensional lattice for fixed space–time configuration of the environment (“quenched rwre”): a “Markov model” with Markov dependence in time, and a “quasi stationary” model with long range space–time correlations. We compare with the corresponding results for a model with i.i.d. (in space time) environment. In the range of times available to us the quenched distributions of the random walk displacement are far from gaussian, but as the behavior is similar for all three models one cannot exclude asymptotic gaussianity, which is proved for the model with i.i.d. environment. We also report results on the random drift and on some time correlations which show a clear power decay     

On Hot Bangs and the Arrow of Time in Relativistic Quantum Field Theory

 A recently proposed method for the characterization and analysis of local equilibrium states in relativistic quantum field theory is applied to a simple model. Within this model states are identified which are locally (but not globally) in thermal equilibrium and it is shown that their local thermal properties evolve according to macroscopic equations. The largest space–time regions in which local equilibrium states can exist are timelike cones. Thus, although the model does not describe dissipative effects, such states fix in a natural manner a time direction. Moreover, generically they determine a distinguished space–time point where a singularity in the temperature (a hot bang) must have occurred if local equilibrium prevailed thereafter. The results illustrate how the breaking of the time reflection symmetry at macroscopic scales manifests itself in a microscopic setting. Received: 17 January 2003 / Accepted: 5 March 2003 Published online: 17 April 2003 Communicated by H. Araki and K. Fredenhagen     

Laser-induced periodic surface structures on different poly-carbonate films

Laser-induced periodic surface structures (LIPSS) were generated on oriented and amorphous thick, as well as on spin-coated thin, poly-carbonate films by polarized ArF excimer laser light. The influence of the film structure and thickness on the LIPSS formation was demonstrated. Below a critical thickness of the spin-coated films the line-shaped structures transformed into droplets. This droplet formation was explained by the laser-induced melting across the whole film thickness and subsequent de-wetting on the substrate. The thickness of the layer melted by laser illumination was computed by a heat-conduction model. Very good agreement with the critical thickness for spin-coated films was found. The original polymer film structure influences the index of refraction of the thin upper layer modified by the laser treatment, as was proven by the dependence of the structure’s period on the angle of incidence both for ‘s’- and ‘p’-polarized beams. The effect of the original surface roughness – grains in thick films or holes in thin films – was studied using atomic force microscopy. It was shown that the oblique incidence of ‘s’-polarized beams results in an intensity confinement in the direction of the forward scattering and in asymmetrical interference pattern formation around these irregularities. A new, two-dimensional grating-like structure was generated on spin-coated films. These gratings might be used as a special kind of mask. Received: 10 July 2001 / Accepted: 23 July 2001 / Published online: 30 August 2001     

Analysis of THz-wave surface-emitted difference-frequency generation in periodically poled lithium niobate waveguide

It was shown that the periodically poled LiNbO₃-waveguide with period of poling λ≈λ/n_g (λ is the wavelength of emitted THz-wave, n_g is a refractive index corresponding to optical group velocity) emits THz-wave difference-frequency generation (DFG) in the direction normal to the surface of the planar waveguide. The 5% distinction between the manufactured and required periods of gratings results only in a small deflection (∼6°) of the output THz-beam from the normal direction. The dependence of DFG efficiency on mode size is analyzed. The output THz power at λ=150 μm is estimated as 2 mW, taking into account imperfections in coupling incident beams with guided modes. It was shown that the efficiency of THz-wave DFG in surface-emitting geometry is more than for collinear geometry in bulk crystal, especially in the high-absorption wavelength region. Received: 16 May 2001 / Revised version: 13 August 2001 / Published online: 2 November 2001     

Photoluminescence of terbium-organic complex in aerosol–gel-derived planar waveguides

This work deals with the low-temperature preparation of optically active silica-based materials. Tb³⁺-doped silica monolithic gels were elaborated at 40 °C. Tb³⁺–SSA-doped SiO₂–TiO₂ thin films were deposited by an original sol–gel approach, the aerosol–gel process, and heat-treated at 150 °C. Organic complexation of terbium ions was used to improve the active properties of doped silica gels and thin-film samples. Spectroscopic characterisations are reported for these samples. Photoluminescence increase by a factor two was observed for complexation by sulphosalicylic acid. Received: 16 May 2001 / Revised version: 31 August 2001 / Published online: 23 October 2001     

The Isometry Groups of Asymptotically Flat, Asymptotically Empty Space–Times with Timelike ADM Four–Momentum

We give a complete classification of all connected isometry groups, together with their actions in the asymptotic region, in asymptotically flat, asymptotically vacuum space–times with timelike ADM four–momentum. Received: 25 October 1996 / Accepted: 14 February 1997     

Mid-infrared laser-induced grating experiments of C2H4 and NH3 from 0.1–2 MPa and 300–800 K

Laser-induced thermal gratings (LITG) were generated in mixtures of ethylene and ammonia in nitrogen using mid-infrared laser radiation from a grating-tuned, low-pressure, pulsed (5 ms pulse width) CO₂ laser, and read out with a continuous wave Nd:YLF laser. The LITG signal intensity was measured as a function of pressure (0.1–2 MPa) and temperature (300–800 K, at 0.1 and 1 MPa) by tuning the laser to the accidental coincidences of the 10P(14) and 10R(6) emission lines with molecular absorption transitions of C₂H₄ and NH₃, respectively. Comparisons are made with theoretical predictions for the grating efficiency from a simple thermalization model. A theoretical comparison of the temporal LITG signal response for three excitation pulse shapes – a delta function, a realistic pulse, and a square wave is presented. Furthermore, it is shown that for NH₃, most of the decrease of the LITG signal intensity with increasing temperature is due to the corresponding decrease in fractional molecular absorption of the pump beam radiation. The diagnostic capabilities of the mid-infrared LITG experiment is demonstrated for spatially resolved ethylene measurements with long laser pulses in a premixed stoichiometric CH₄–air flame at atmospheric pressure. Received: 17 March 2000 / Revised version: 23 March 2000 / Published online: 13 September 2000     

The Perturbation of the Quantum¶Calogero–Moser–Sutherland System¶and Related Results

The Hamiltonian of the trigonometric Calogero–Sutherland model coincides with a certain limit of the Hamiltonian of the elliptic Calogero–Moser model. In other words the elliptic Hamiltonian is a perturbed operator of the trigonometric one. In this article we show the essential self-adjointness of the Hamiltonian of the elliptic Calogero–Moser model and the regularity (convergence) of the perturbation for the arbitrary root system. We also show the holomorphy of the joint eigenfunctions of the commuting Hamiltonians w.r.t the variables (x ₁, …,x _N ) for the A _{N -1}-case. As a result, the algebraic calculation of the perturbation is justified. Received: 30 May 2001 / Accepted: 27 November 2001     

An arena for model building in the Cohen—Glashow very special relativity

The Cohen—Glashow Very Special Relativity (VSR) algebra is defined as the part of the Lorentz algebra which upon addition of CP or T invariance enhances to the full Lorentz group, plus the space—time translations. We show that noncommutative space—time, in particular noncommutative Moyal plane, with light- like noncommutativity provides a robust mathematical setting for quantum field theories which are VSR invariant and hence set the stage for building VSR invariant particle physics models. In our setting the VSR invariant theories are specified with a single deformation parameter, the noncommutativity scale ╕_NC. Preliminary analysis with the available data leads to ╕_NC ≳ 1–10 TeV.     

Surface relief diffraction gratings written on <Emphasis Type="BoldItalic">β</Emphasis> -BaB<Subscript>2</Subscript>O<Subscript>4</Subscript> crystal by femtosecond pulses

Surface relief diffraction gratings were written at the entrance surface of a β-BaB₂O₄ (BBO) crystal under irradiation with femtosecond laser pulses. Probe–beam diffraction and atomic force microscopy (AFM) were employed to characterize the diffraction properties and the microstructures of the gratings. The maximal first-order diffraction efficiency of these gratings is approximately 63%. The surface periodic structure of the material induces noncollinear propagation of the fundamental input beam in the crystal. Using this diffractive optical element, efficient generation of multiple second-harmonic beams was achieved. PACS 42.79.Dj; 42.40.Lx; 42.65.–k     

γ-ray irradiation effect on hysteresis symmetry and data retention of Pt/SrBi2Ta2O9/Pt thin-film capacitors

The γ-ray irradiation effect on hysteresis symmetry and data retention of Pt/SrBi₂Ta₂O₉/Pt ferroelectric thin-film capacitors has been investigated as a function of irradiation dose. A horizontal shift of polarization–voltage curves along the voltage axis, known as imprint, was observed after the polarized capacitors had been irradiated. The voltage shift increased with increasing irradiation dose and increasing remanent polarization written before irradiation. After irradiation, severe data loss was observed after these capacitors had been written to the state opposite to the one stored during irradiation. Possible functional failure due to such a data loss was discussed. Received: 17 January 2001 / Revised version: 27 April 2001 / Published online: 20 June 2001