Characteristics of fourier phase holograms recorded on photopolymers

The results of the investigation of recording Fourier phase holograms on a self-developing photopolymer photosensitive in the range λ=400–515 nm are presented. It has been found that, due to the transient energy transfer between the beams, noise gratings are recorded, and a corresponding sharp reduction in the signal-to-noise ratio occurs, while the diffraction efficiency of the hologram as a whole remains relatively high (above 50%). It has been found that the noise-grating recording can be substantially suppressed by increasing the intensity of the reference beam relative to the intensity of the object beam. In this way, the signal-to-noise ratio has been considerably improved for Fourier holograms of binary phase masks: at a reference to object beam intensity ratio R=26, Fourier phase holograms are recorded with a diffraction efficiency η=15% and signal-to-noise ratio N=20 dB.     

Recording volume holograms on color centers in a CaF2 crystal

The conditions for preparing photochromic CaF₂ crystals, recording holograms with 532-nm radiation in them, and postexposure photothermal treatment of samples with holograms that provide the formation of holographic elements of significant thickness (about 1 cm) are considered. It is shown that the diffraction efficiency of volume holograms in such crystals in the case of reading out by radiation with wave-lengths of 532 and 632.8 nm can be as high as 10–15%. The role of the recording mechanism and the Borrmann effect in the fabrication of these holograms is demonstrated.     

Effect of light polarization on holographic recording in glassy azocompounds and chalcogenides

Light polarization effects on a holographic grating recording in a glassy chalcogenide a-As₄₀S₁₅Se₄₅ film has been experimentally studied and compared with previously studied glassy molecular azobenzene film 8a at 633, using s − s,p − p, CE-1 and CE-2 circular-elliptic recording-beam polarizations (differing by light electric field rotation directions). The azocompound exhibited much higher self-diffraction efficiency (SDE) and diffraction efficiency whereas chalcogenide was more sensitive. Their recording efficiency polarization dependences also were different. SDE up to 45% was achieved in 8a with p − p and up to 2.6% in a-As₄₀S₁₅Se₄₅ with CE-2 polarized recording beams. The polarization changes in the diffraction process were studied as well in these and other materials (11, 16, 19 and a-As₂S₃ film, LiTaO₃:Fe crystal). It was found that light polarization changes in the process of diffraction from gratings recorded vectorially by s−p polarizations depended on chemical composition, wavelength, and exposure time. Vector gratings with SDE up to 25% were recorded in 8a, rotating a linear polarization by 90°. No light polarization changes were found in azobenzene 19 and chalcogenide films and in LiTaO₃:Fe crystal, thus showing a vector recording of scalar holograms. The recording mechanisms in azocompounds and chalcogenides are discussed and compared.     

Holograms in CaF2 crystals with high content of alkaline impurities

Similarities and dissimilarities of holograms recorded in fluorite crystals with color centers that are pure or contain relatively high concentrations of an alkaline impurity (sodium) are considered. It is shown that, in these two cases, the mechanism of recording is the same, but formation of the impurity-vacancy complexes in heavily doped crystals causes certain peculiarities in the types of color centers in the initial crystals and those with holograms and determines specific features of spatial redistribution of centers in the process of recording. These features reduce the diffraction efficiency of holograms in heavily doped crystals compared to pure and weakly activated crystals.     

Postexposure sensitization of holograms on the basis of color centers in CaF2 crystals by incoherent radiation

It has been reported about recording highly stable holograms on the basis of color centers in calcium fluoride crystals at a temperature of 200°C by UV light. In this study, holograms were recorded by visible light (4880 Å). The recorded holograms have a low diffraction efficiency, which can be significantly increased using postexposure irradiation of a sample by incoherent UV light at a temperature of 200°C. This increase demonstrates an important role of the drift component in the diffusion-drift mechanism of recording holograms based on color centers in ionic crystals.     

Narrow-band holographic interference LiNbO3-based filters

New experimental data on antiparallel-beam holograms recorded in LiNbO₃, temperature fixation of the recorded holographic gratings, and the spectral characteristic of a narrow-band (0.01 nm) interference filter are reported. The effects of asymmetry in the diffraction efficiency and the emergence of satellites in the transmission spectrum of the filter are observed. These effects are explained using the birefringence properties of the crystal and the ability of holograms to transfer power between the beams in the course of recording.     

Investigations of As-S-Se thin films for use as inorganic photoresist for digital image-matrix holography

As-S-Se chalcogenide thin films are successfully employed in classical and dot-matrix holography as inorganic photoresists for obtaining a relief-phase hologram. However using these films for image-matrix hologram recording has not been studied due to some features of image-matrix technology. For the applied research of the optical properties of As-S-Se films an experimental device of digital image-matrix holographic recording based on 100 mW 405 nm semi-conductor laser and Spatial Light Modulator (SLM) has been created. The device has the following main parameters: 140 × 105 μm frame size; laser intensity during exposure 10 W/cm². With the help of this device diffraction grating and security holograms were recorded on As-S-Se thin films. The work reported herein presents results of an experimental study of how diffraction efficiency (DE) of the received relief-phase holographic gratings depends on an exposure and period. Diffraction grating profiles and speed of etching corresponding to different exposure doses are shown. Hologram samples with DE = 65% have been received which allows for using chalcogenide film as alternative to organic photoresists in applied dot-matrix and image-matrix holography.     

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.     

Holographic recording on the basis of forbidden singlet-triplet electronic transitions

The possibility of recording holograms in thick layers of a photopolymeric material on the basis of a forbidden singlet-triplet transition of a sensitizing dye, erythrosine, is theoretically substantiated and experimentally confirmed. A single hologram and 16 superimposed holograms are recorded with a low-power He-Ne laser (632 nm) at high concentrations of the sensitizing dye and high optical densities in the range of its main absorption. The diffraction efficiency of the single hologram is ～50%. The dependence of the increase in the diffraction efficiency of a transmission grating in samples of photopolymeric material on the intensity of incident radiation of a Kr⁺ laser (647 nm) is studied. The observed linear dependence of the maximal rate of increase in the diffraction efficiency in the photopolymer on the incident light intensity is explained by the manifestation of a two-step excitation of the dye (T ₁←S ₀, T ₂←T ₁) in the samples studied.     

Effective room-temperature broad-band recording of 3D dynamic holograms in CdF<Subscript>2</Subscript>:Ga crystals

Photochromic CdF₂:Ga crystals with bistable impurity centers were effectively used for the dynamic recording of holograms and readout over the visible and near IR spectral regions at spatial frequencies of up to 5000 mm^?1 at room temperature. The diffraction efficiency of the dynamic holograms was as high as 60% at maximum and exceeded 1% when the beams’ intensities were in the ratio 1:100. As one goes from the low temperatures (≤200 K) to 300 K, the peak diffraction efficiency of the dynamic holograms decreases approximately by a factor of 1.5, while the speed of their response and photosensitivity in the long-wavelength spectral region increases by more than an order of magnitude. For the sake of comparison, the dynamic holograms were recorded under the same conditions as the widely used electrooptical SBN crystals. Comparative analysis ascertained a unique combination of the useful features offered by CdF₂:Ga crystals in holography.     

Films made of substituted azobenzene polycomplexes with cobalt for polarization holography

We have studied the possibility of recording holograms in films of the 4-methacroyloxy-(4′-carboxy-3′-hydroxy)-2-chloroazobenzene polycomplex with cobalt, with parallel and orthogonal orientation of the light beam polarization. We have shown that these films can be used as recording media for polarization holography. The characteristic features of relaxation of the diffraction efficiency of the holograms are connected with cistrans isomerization of the azobenzene groups and the characteristic features of structural rearrangement in the polymer matrix. We hypothesize that the information-related characteristics of the studied recording media can be controlled by external electric or magnetic fields, due to the presence of magnetic metal ions within the polymer film. __________ Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 74, No. 6, pp. 830–832, November–December, 2007.     

Optimization of the recording conditions for holograms recorded in additively colored KCl crystals

The dependences of the characteristics of volume holograms recorded in additively colored crystals of potassium chloride on the spatial frequency and the recording temperature are studied. It is found that these dependences are nonmonotonic, which indicates the presence of nonlocal mechanisms of holographic recording in the crystals. These mechanisms involve diffusion and drift processes of redistribution of color centers between the nodes and antinodes of a recorded interference pattern. The results obtained are used to determine the recording conditions providing the maximum possible diffraction efficiency of the holograms.     

Diffusion of photoexcited azo dye in a liquid-crystal host

Dynamic holograms are created and studied in a D2 azo-dye-doped nematic liquid crystal. The dependence of diffraction on the excitation and probe polarizations is observed. The efficiency of excitation and the corresponding relaxation time depend also on the period and orientation of the light-interference pattern with respect to the initial molecular orientation. The excitation and diffusion of dye molecules are suggested to be the origin of these phenomena.     

Holographic polymer materials with diffusion development: Principles, arrangement, investigation, and applications

We present a review of information that has been published in the scientific literature on the mechanism of formation (development, enhancement) of holograms at the expense of the photochemical attachment of phenanthrenequinone molecules and other compounds to polymer chains and diffusion mixing of unreacted molecules, on different types of light-sensitive materials that realize this mechanism, on the application of these materials to create volume holographic elements, on recording of information, and for research purposes.     

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.     

Hologram multiplexing in acrylamide hydrophilic photopolymers

Unslanted diffraction gratings are recorded in a 900 μm thick acrylamide photopolymer by means of peristrophic multiplexing. A solid state Nd:YAG (λ = 532 nm) laser is used as the recording beam, with a total incident intensity of 5 mW/cm², and a He-Ne laser as the reconstruction beam. The dye concentration in the photopolymer is optimized so that it does not limit the dynamic range. Nine holograms are recorded using constant exposure time scheduling and variable exposure time scheduling. From the results obtained it may be deduced that optimization of the dye allows us to work in the linear response region of the photopolymer and at the same time obtain high values of diffraction efficiency for each hologram. An exponential increase in exposure time as the number of holograms increases enables the values of diffraction efficiency to be homogenized with regard to the case of constant exposure scheduling. In this way, better use is made of the dynamic range of acrylamide hydrophilic photopolymer.     

Angular Selectivity of Amplitude-Phase Holographic Gratings in Polymer Material with Phenanthrenequinone

The values of the amplitudes of the refractive index and absorption coefficient modulations in volume hologram gratings, which were recorded in a polymer material with phenanthrenequinone (PQ), have been determined by the results of the approximation of the angular selectivity contours, which were measured at wavelengths of 450, 473, 532, and 633 nm before and after the development of holograms, resulting from the diffusion of PQ molecules in a glassy polymer. Immediately after recording, before the diffusion development, the holograms are mixed amplitude-phase in the PQ absorption spectral region and phase for the red light, which is not absorbed by it (633 nm), and become purely phase during the development regardless of the wavelength. The shape of the selectivity contour of the undeveloped hologram indicates the counterphase between the modulations of the absorption coefficient and refractive index, illustrating the concept of complementary gratings, on which the principle of hologram formation in the material under consideration is based.

     

The dynamic characteristics of self-diffraction efficiency of azo polymer film

When two coherent beams illuminate into the azo-dye-doped polymer film with small angle and equal intensity, the phenomena of self-diffraction occur. We study the dynamic characteristic of self-diffraction efficiency experimentally and theoretically by the model of population grating, which is formed by the molecules populations of trans and cis state modulated by the coherence optical field.     

Film carriers for super-high-density magnetic storage

The problem of increasing the recording density in magnetic storage devices is considered. It is shown that nanograined magnetic film media are candidate materials for magnetic data carriers. For these materials to completely meet the requirements for super-high-density magnetic carriers, appropriate structure ordering must be set in the films. To this end, it is suggested to take advantage of the high adsorbability of 3d metal nanoparticles on high-molecular compounds. To produce the carriers based on these materials with a recording density of as high as 10¹⁰ bit/cm², nanoparticles of size ≤5 nm should be embedded in a polymer matrix. To do this, it is necessary to combine chemical and physical methods for nanocomposite production.     

Spatial-frequency version theory of three-dimensional holograms

A spatial-frequency version of high efficiency three-dimensional holograms of complex wave fields has been achieved. It has been shown that a three-dimensional hologram recorded by a plane reference wave may be described by means of a complex transfer function, which is found here analytically. Analysis of amplitudephase wave field distortions, under different recording and reconstruction conditions, has been carried out.