Nondestructive read-out holograms recording in photochromic salicylideneaniline

The plane wave holograms were recorded with the 488 nm line in salicylideneaniline and nondestructive read-out was by the 632.8 nm line. The 0.8% diffraction efficiency was achieved.     

Diffraction efficiency of volume holograms

In this paper we give an exact solution of the coupled differential equation system as developed by Kogelnik [1] for the case of symmetrical recording without neglecting the second differentials. This generalized solution is applicable to dielectric, absorption and so-called mixed volume holograms. It is shown that under special assumptions and approximations we get the same equations for the diffraction efficiency as derived by Kogelnik. In this first part of the paper transmission holograms are only investigated and in the second part we will give the solution for reflection holograms.     

Improvement of the diffraction efficiency and kinetics of holographic gratings in photochromic media by auxiliary light

Holographic gratings recorded in photochromic media often do not obtain the maximally achievable diffraction efficiency because of diminishing the fringe contrast caused, e.g., by a photochemically active readout beam or unequal intensities of object and reference waves. For nonreversible materials this problem causes a decrease in diffraction efficiency that is proportional to the signal-to-noise ratio (SNR). However, in nonlinear materials such as photochromic media, for which saturation effects need to be considered, an out-of-proportion decrease in the SNR results. It is shown that an overshooting peak during hologram growth, which then decays to a lower permanent level of diffraction, is an indicator for such a situation. Even a weak readout beam may cause such effects, which significantly affect the hologram kinetics. The observed overshooting diffraction efficiency may even be misinterpreted to be dependent on material properties. Experimental and theoretical proof that with low levels of auxiliary light this type of problem can be eliminated completely is presented. Throughout this research bacteriorhodopsin films were used, but the results are valid for photochromic media in general.     

Simulation of diffraction from volume holograms

Coupled wave equations that consider two-dimensional distribution and phases of gratings are derived by using weak grating and slow varying approximation. The result of angular selectivity on rand binary data page shows that each minimum is nonzero and each maximum that is not the middle maximum is a little bit smaller than that on plane wave volume hologram. Angular multiplexing is also modeled using the same method, and the middle page is read out in two different angular spacings. The relationship about PSNR versus angular spacing at different multiplexing pages is analyzed to reflect crosstalk between pages. All simulative results show this method is effective for volume holograms simulation.     

Signal distortion noise in volume phase holograms

Signal distortion noise in volume phase holograms is analysed for the case in which the object beam consists of two plane waves. It is shown that, for a given (on-Bragg) signal diffraction efficiency, the magnitude of the signal distortion noise is inversely proportional to the square of crystal thickness. Furthermore, as a function of the angular position of the reconstruction beam, the signal distortion noise has two peaks: one at the Bragg angle and the other at a deviation ₀ from the Bragg angle, where ₀ is a function of relative angles between the components of the object beam and the reference beam. The on-Bragg peak of the signal distortion noise is experimentally located for volume phase holograms recorded in Fe-doped LiNbO₃. The measured angular position is found to be in accord with the calculated value.     

Noise gratings in silver halide volume holograms

Diffraction efficiencies of 70% have been reported for planar gratings in silver halide emulsion. The main obstacle in increasing the efficiency is the granular structure of the recording material which causes scattering of the input light both at recording and at reconstruction. This study concentrates on the effect of noise gratings recorded when the incident beam interferes with its own scattered radiation. The experimental evaluation is divided into two parts. Firstly, one single beam is used for recording a holographic plate, and the transmitted intensity of the reconstructing beam illuminating the developed hologram is measured as a function of incident angle and wavelength. Dips in the measured intensity are associated with the reconstruction of a strong scattered beam. Secondly, a main grating is recorded by two incident beams, which also interfere with the field scattered by the grains and give rise to noise gratings. When a beam is incident upon the developed hologram at the same angle and wavelength as one of the recording beams then besides the desired beam the scattered radiation is also reconstructed. This may reduce the otherwise available efficiency by several percent.     

Localized one-dimensional theory for volume holograms

Coupled partial differential equations are derived, consistent with localized use of one-dimensional theory in a thin volume hologram, from modifications to both one-dimensional and two-dimensional theory. The approximations inherent in the model and the range of validity of the solution are discussed.     

Diffraction efficiency of a dynamic hologram in a reversible photochromic medium with allowance for diffusion of photochromic particles

Spatial resolution of a dynamic hologram in a reversible photochromic medium is analyzed, with allowance made for diffusion of photochromic particles. It is shown that the bandwidth of spatial frequencies of a thin hologram decreases over time, approaching the value determined by the diffusion coefficient and the hologram recording/erasing time. The optimum relationship between the intensity of the beam that records the volume hologram and the parameters of the photochromic particles is found for which the difference between the limiting value of the bandwidth of spatial frequencies of the volume hologram and the value of the band-width of spatial frequencies of a thin hologram located in the plane z=0 is the greatest.     

Photoinduced reversible changes in the electronic structure of photochromic diarylethene films

The change in the valence electronic structure upon switching of two photochromic diarylethene derivatives, i.e. 1,2-bis(2-methyl-5-p-tolylthiophen-3-yl)cyclopent-1-ene (DAE1) and 1,2-bis(5-(4-hexyloxycarbonylphenyl)-2-methylthiophen-3-yl)cyclopent-1-ene (DAE2), was measured by photoelectron spectroscopy. Switching between open and closed forms was followed in situ upon illumination. The increase of the ionization energy from the closed to the open form was 0.85 eV for DAE1 and 0.80 eV for DAE2. For DAE1, the work function also decreased by 0.25 eV upon switching, which is explained by a decrease of the intrinsic molecular dipole moment and a preferential orientation of molecules in thin films.     

Polarization volume holograms in layers of polymethylmethacrylate with phenanthrenequinone

Polarization volume holograms are recorded in the polymethylmethacrylate layers that contain phenanthrenequinone at a molar content of 2.5–3%. The effect of the polarization of recording beams on the kinetics of diffraction efficiency and properties of holograms is analyzed. Polarization hologram recording in the polymethylmethacrylate layers with phenanthrenequinone and a relatively high optical stability of the holograms are demonstrated.     

A general solution for two-dimensional volume holograms

We inv estigate the problem in which a volume hologram is produced by two waves of arbitrary amplitude and phase distribution (the only restriction being that they must obey the rules of geometrical optics), and the developed hologram is illuminated by one of the waves having the same phase distribution but not necessarily the same amplitude distribution. Coupled partial differential equations are derived and solved in the form of an infinite series valid for both transmission and reflection holograms. For transmission holograms a simple approximate solution is given for the amplitude of the reconstructed wave in the immediate vicinity of the input surface.     

Selective properties of volume phase holograms in photorefractive crystals

The paper deals with the selective properties (both angular and spectral) of volume phase holograms in birefringent and cubic optically active photorefractive crystals. It is shown that in birefringent crystals of LiNbO₃ type, the spectral selectivity of the grating depends remarkably on the type of diffraction (isotropic or anisotropic) and the polarization of the reconstructing plane light wave. In cubic optically active crystals the peculiarities of diffraction reveal themselves in a fine structure of differently polarized peaks in the Bragg maximum that can be easily observed, in particular in Bi₁₂SiO₂₀ samples with a thickness d ≈ 8 mm.     

High longitudinal selectivity of shifting multiplexing in volume holograms

High longitudinal selectivity of the shifting multiplexing with spherical reference wave is proposed and demonstrated. A simplified method based on wave optics is used for calculating the selectivity, and the result fits well the experimental measurement. Under the paraxial condition, a simple formula for the longitudinal selectivity is introduced. With use of an object lens with effective NA=0.817, we obtain that an FWHM of selectivity is as small as 1 μm.     

Rotation selectivity of random phase encoding in volume holograms

We present the study of angular selectivity for holographic multiplexing based on random phase encoding by a ground glass. The rotational selectivity of the volume hologram is calculated theoretically and coincides with the experimental measurement. By controlling the parameters including rotational center, effective numerical aperture of both volume hologram and the ground glass, we can obtain different rotational selectivity applied to random phase encoding in volume holographic storage.     

A power conservation theorem for volume holograms

The two-beam coupled wave analysis of volume holograms given by Solymar, which is a two-dimensional generalisation of Kogelnik's formulation, is extended further to provide an alternative treatment of ‘d.c.’ dephasing effects, and the resulting coupled wave equations are shown to contain an implicit power conservation theorem.     

Dynamic holograms and phase conjugation in multilevel resonant media

The generation of dynamic holograms and four-wave phase conjugation in resonant media has been investigated under conditions of interaction between radiation and excited singlet and triplet states. Two mechanisms of optical control of resonant media non-linearities using independent pump-up to increase the diffraction efficiency of dynamic holograms have been considered. The peculiarities of non-linear recording of holograms, and a variant of quadratic recording, have been investigated. The dependence of the efficiency of diffraction by dynamic holograms on the intensity of interacting waves and the spectroscopic characteristics of the medium has been analysed.     

Non linear recording in silver halide planar volume holograms

A series of bromine-vapour bleached, planar, volume holograms were formed in silver halide emulsion. The holograms were characterised by measuring the variation in intensity of the transmitted replaying beam with angles at several wavelengths. The results were investigated using coupled-wave theory, allowing for the existence of grating harmonics and higher diffraction orders. The effects of first, second and third grating harmonics were found in the experimental results. Modulation profiles synthesised using the results suggested the presence of a small fourth harmonic in the higher exposure holograms.     

Diffraction efficiency of sequentially stored gratings in reflection volume holograms

On the basis of the theory of coupled waves the amplitudes of the first-order diffracted waves and the diffraction efficiency is calculated for sequentially superposed phase gratings stored in dielectric reflection holograms. The investigations are restricted to the case of a fixed reference wave during recording and Bragg-angle incidence.By analogy to the transmission volume grating the coupling effects which appear between incident beams and diffracted waves give the possibility of building up beam-couplers and -splitters with certain coupling parameters determined by the recording process. Moreover, reflection gratings allow a favourable combination between coupling and reflecting properties and avoid the diffraction efficiency oscillations that are characteristic of transmission gratings. The performance of the reconstruction process with several waves simultaneously allows the tuning of the intensities of diffracted waves with dependence on amplitude or phase differences between incident beams.