Recording Of Gratings Research Articles

Linear response deviations during recording of diffraction gratings in photopolymers

In a holographic recording is advisable that the diffraction efficiency increases linearly with the exposure in a wide zone of the curve of diffraction efficiency versus energetic exposure and the slope of the curve must be approximately constant before saturation in order to improve the energetic sensitivity and to get reproducibility in different recordings with the same kind of photopolymer, although to find examples of deviations to this behavior it is usual. The more important deviation experimentally observed in photopolymers with high thickness happen when the first maximum in the curve is lower than the second one. This effect is opposed to an overmodulation. We present a main hypothesis related to the dye concentration into the layer and with the molecular weight of the polymer chains generated in the polymerization process in order to explain this effect.

Charging of glassy chalcogenide semiconductors in corona discharge and its effect on holographic grating formation

Recording of optical holographic gratings based on photostructural transformations in thin (≈ 1 μm) As2S3 and As2S3 semiconductor layers in the presence and absence of a corona discharge and also chemical etching of these gratings are studied. Initiation of a corona at the stage of interference grating recording is shown to improve the exposure contrast of metal-glassy chalcogenide semiconductor thin-film structures. The holographic sensitivity, diffraction efficiency, dynamic range, and contrast are also improved severalfold. When phase relief gratings formed in these layers are selectively etched in a chemical etchant in the presence of a corona, their profile becomes more regular and deeper by 25–30% and the diffraction efficiency increases by 30–50%.

Dynamic grating recording in lyotropic ionic smectics of metal alkanoates doped with electrochromic impurities

In this work, we investigated nonlinear-optical properties in lyotropic ionic liquid crystals (LILC) of potassium caprilate doped with electrochromic impurities (viologens). It is known that after applying a direct electric field to the LILC-viologen cells colouration of samples is observed [1]. Dynamic grating recording under the action of nanosecond pulse laser radiation was obtained for the coloured samples. We analyzed the origin of the optical nonlinearity in the investigated materials taking into account their electrochemical and electrooptical properties. To explain grating recording in LILC doped with viologens, we proposed the mechanism of photoconversion in viologen molecules.

Light-induced redistribution of Si-MFI zeolite nanoparticles in acrylamide-based photopolymer holographic gratings

Results of studies of the redistribution of Si-MFI zeolite nanoparticles within a photopolymer layer during hologram recording are presented. Light-induced mass transfer studies provide valuable information leading to ways of controlling the spatial distribution of nanoparticles. Nanoparticle-doped photopolymers are very promising materials for development of new holographic-based sensing techniques and for holographic data storage applications. In our studies, diffraction gratings were recorded with a spatial frequency of 200 lines mm−1 at different recording intensities in an acrylamide-based photopolymer system containing 4 wt.% MFI zeolite nanoparticles. Maps of Si-MFI nanoparticle concentration as a function of position in the samples after recording were prepared by Raman spectroscopy and AFM. It has been proven that mass transfer of MFI zeolite nanoparticles takes place during the recording of holographic gratings.

Fabrication of high-precision silver halide layers for nanoscale linear-and angular-translation measurement systems

A silver halide emulsion casting technique was designed and studied for the fabrication of thin holographic layers of 3 ± 0.5 μm thickness free of phase distortions and with the required resolving power depending on the optical-range wavelength in the holographic recording of metrological gratings.

Pulsed recording of anisotropy and holographic polarization gratings in azo-polymethacrylates with different molecular architectures

Recording of anisotropy and holographic polarization gratings using 532nm, 4ns light pulses has been carried out in thin films of polymers with the same azobenzene content (20wt%) and different molecular architectures. Random and block copolymers comprising azobenzene and methylmethacrylate (MMA) moieties as well as statistical terpolymers with azobenzene, biphenyl, and MMA units have been compared in terms of recording sensitivity and stability upon pulsed excitation. Photoinduced anisotropy just after the pulse was significantly higher in the case of the block copolymers than in the two statistical copolymers. The stability of the recorded anisotropy has also been studied. While a stationary value of the photoinduced anisotropy (approximately 50% of the initial photoinduced value) is reached for the block copolymer, photoinduced anisotropy almost vanished after a few hours in the statistical copolymers. Polarization holographic gratings have been registered using two orthogonally circularly polarized light beams. The results are qualitatively similar to those of photoinduced anisotropy, that is, stability of the registered grating and larger values of diffraction efficiency for the block copolymer as compared with the random copolymers. The recording of holographic gratings with submicron period in films several microns thick, showing both polarization and angular selectivity, has also been demonstrated. Block copolymers showed a lamellar block nanosegregated morphology. The interaction among azo chromophores within the nanosegregated azo blocks seems to be the reason for the stability and the photoresponse enhancement in the block copolymer as compared with the statistical ones.

Direct recording of thin dynamic diffraction gratings in optically addressed spatial light modulators with a diffraction efficiency of more than 50%

The subject of investigation is conditions under which thin asymmetric-groove dynamic diffraction gratings with a diffraction efficiency of more than 50% can be recorded in liquid-crystal optically addressed spatial light modulators.

Holographic recording of surface relief gratings in tolyle-based azobenzene oligomers

Holographic recording of surface relief gratings (SRG) in tolyle-based azobenzene oligomer films have been carried out at 514.5 nm. It was experimentally studied by spectroscopic and AFM methods. The maximum surface modulation amplitude was 35 nm. SRG formation is explained by the mean-field model and by the photodegradation of chromophores enabling the directional mass transport as the result of trans–cis transformations. The conclusion is made that SRG are stable but the diffraction efficiency (DE) changes are due to the complementary decaying amplitude-phase gratings. Strong polarization dependences of transmitted and reflected DE are found. They are explained by the diffraction anisotropy of gratings and by the photoinduced anisotropy of the recording medium. Different more efficient but less stable refractive index grating recording at 632.8 nm in the same samples is performed and studied as well.

High performance holographic gratings formed with novel photopolymer films containing hyper-branched silsesquioxane

A transparent photopolymer film was prepared by embedding the epoxy cured octaaminophenylsilsesquioxane (hyper-branched POSS) into poly(vinyl chloride) (PVC) matrix containing vinyl monomers. Holographic gratings were successfully generated in that films by irradiation of laser light with 532 nm wavelength. The diffraction efficiency was increased with increasing the hyper-branched POSS concentration up to 15 wt%. DSC and TG analysis indicated the plasticizing effect of POSS in photopolymer that loosened the polymer matrix. This plasticizing effect made the monomer diffusion much more effective providing higher diffraction efficiency (D.E. ∼ 80%) compared to the film without hyper-branched POSS (D.E. ∼ 50%). The diffraction efficiency started to decrease with further higher concentration of POSS, because the polymer film became less transparent. Low volume shrinkage of 1.64% could be also obtained in recording of slanted holographic gratings.

TPA-induced long-period gratings in a photonic crystal fiber: inscription and temperature sensing properties

We report on the photochemical recording of long-period fiber gratings (LPFGs) in a photonic crystal fiber made of pure fused silica. Such inscription is based on two-photon absorption (TPA) of high-intensity (∼300 GW/cm2) 264 nm 220 fs pulses and brings about LPFGs of high strength and narrow peak width. The characteristic fluence value for the inscription is 1 order of magnitude less than that for a standard telecom fiber irradiated under similar conditions. The temperature sensitivity of TPA-induced LPFGs is ∼300 pm/°C and overcomes that of LPFGs inscribed by other nonphotochemical methods by 2 orders of magnitude.

Pulse record of holographic transmission and reflection diffraction gratings in absorbent photopolymers. 2. Numerical modeling and experiment

The temporal dynamics of spatial profiles of the first grating harmonic is modeled numerically with allowance for the absorption in the transmission and reflection record geometry. The modeling is carried out for a theoretical model of record of spatially inhomogeneous holographic diffraction gratings by an optical pulse train constructed from a solution of the kinetic photopolymerization equations [1]. A dependence of the post-exposure gain on the recording conditions is analyzed numerically. The influence of the pulse train characteristics on the diffraction grating efficiency is also investigated. The model parameters of the experimentally investigated photopolymer material are estimated from a comparison of the experimental data on the kinetics of the diffraction efficiency with the results calculated for the model presented in [1].

Implementation of diffractive optical element in four-wave mixing scheme for ex situ characterization of hydride vapor phase epitaxy-grown GaN layers

A holographic beam splitter has been integrated into a picosecond four-wave mixing (FWM) scheme. This modification significantly simplified the procedure of dynamic grating recording, thus making the FWM technique an easy-to-use tool for the holographic characterization of wide band gap materials. The novel FWM scheme was applied for characterization of hydride vapor phase epitaxy-grown undoped GaN layers of different thickness. It allowed the determination of carrier lifetime, diffusion coefficient, and carrier diffusion length by optical means, as well as the study of carrier recombination peculiarities with respect to dislocation and excess carrier density.

Holographic diffraction gratings with enhanced sensitivity based on epoxy-resin photopolymers

Photopolymers are interesting materials to obtain high-quality performance for the volume holographic data storage with a low noise and high diffraction efficiency. In this paper, the recording of holographic diffraction gratings with a spatial frequency of 1285lines/mm in photopolymerizable epoxy resin materials is experimentally demonstrated. Diffraction efficiency near 92% and an energetic sensitivity of 11.7 x 10-3cm2/J are achieved by designing the proper structure of matrix and also optimizing photopolymer compositions. The effect of photopolymer compositions on the fundamental optical properties is also discussed.

Femtosecond recording and time-resolved readout of spatial gratings in lithium niobate crystals

Experimental and theoretical results on recording of spatial gratings with interfering femtosecond pulses at 388 nm and time-resolved Bragg-matched readout at 776 nm are presented for LiNbO3 crystals. These include dependences of the diffraction intensity on the time delay between probe and pump pulses, on the pump intensity and angle, and on the crystal composition. The grating buildup involves instantaneous and quasi-permanent changes of the refractive index and the absorption coefficient. These changes are due to Kerr and two-photon absorption effects and the modulation of photoexcited carriers, respectively. A good agreement between theory and experiment is achieved. Our analysis provides an understanding of nonlinear optical phenomena in LiNbO3 crystals on the femtosecond time scale. The theory is applicable to a large range of optical materials with modestly wide, ≈3to5 eV, bandgap.

Analysis of the Kinetics of Diffraction Efficiency during the Holographic Grating Recording in Azobenzene Functionalized Polymers

The laser-assisted holographic grating recording process in films of azobenzene functionalized polymers is usually studied by observation of the efficiency of light scattering on a developing in time diffraction grating. Various possible mechanisms contributing to grating formation as well as the bulk or surface origin (bulk refractive index and/or relief grating) of light scattering make the analysis of kinetics of grating recording, from the light scattering data only, difficult and ambiguous. To fully explain experimentally observed various and complex (frequently nonexponential) kinetics of the first-order light diffraction intensity, we considered a simple single-exponential growth of the two phase gratings in the same polymer film. In modeling we assumed that the bulk refractive index grating Deltan(t) and the surface relief grating Deltad(t) differ considerably in their growth rates and we allowed for a nonstationary phase shift Deltaphi(t) between them which was experimentally observed during the recording process. The origin of the nonstationary phase shift is a result of a slow shift of interference pattern due to delicate symmetry breaking in illumination conditions (e.g., difference in beam intensities and deviation of exact symmetrical beam incidence angles on the sample). Changing only such parameters as stationary amplitudes of refractive index and relief gratings for a span of phase shifts (0-pi) between them, we obtained a series of kinetic responses which we discuss and interpret. The various examples of temporal evolution of diffraction efficiency for the same grating formation kinetics, modeled in our work, supply evidence that great care must be taken to properly interpret the experimental results.

Modified polymethylmethacrylate as a base for thermostable optical recording media

A possibility to improve the thermal properties of holographic gratings in a photosensitive system based on polymethylmethacrylate (PMMA) and to enhance simultaneously the adhesion of the photopolymer to soda-lime glass is demonstrated. The modified PMMA was prepared by radical copolymerisation of methylmethacrylate (MMA) with acrylic acid (AA). Polymer films deposited from samples of the copolymer of MMA with AA containing 9,10-phenanthrenequinone additives were used as a photosensitive material for the recording of holographic gratings. It is possible to generate gratings that are thermally stable up to 200 masculineC using this modified PMMA. Dynamic thermogravimetry, differential thermal analysis and thermal mechanic analyses were used to determine the dependence of the thermal stability of the modified PMMA on the composition and the structure of its macromolecules.

Pulse record of holographic transmission and reflection diffraction gratings in absorbent photopolymers. 1. Theoretical analysis

In the present paper, the process of formation of holographic diffraction gratings (HDG) in photopolymer materials in transmission and reflection geometry of record by an optical pulse train with duration comparable with characteristic times of the polymerization and diffusion processes is investigated with allowance for light-induced absorption. A solution of the problem of forming HDG with 2D spatially non-uniform amplitude profiles by light beams is derived in the approximation of zero and first harmonics of the monomer concentration and refractive index. 3D spatial distributions of the HDG amplitude profiles are presented in an analytical form of recursion relations.

Effective recording of dynamic phase gratings in Yb-doped fibers with saturable absorption at 1064nm

Results of investigation of transient two-wave mixing via dynamic population gratings in Yb-doped fibers with saturable absorption are reported. The recorded gratings are characterized by submillisecond formation times and at wavelength lambda = 1064 nm need cw recording light power of 1 - 10 mW scale. What is important for different applications, the dynamic gratings were found to be predominantly of a phase type with an admixture of a significantly weaker amplitude component only. As in Er-doped fibers, the amplitude grating component proved to be essentially inferior to the theoretical estimation based on the fiber optical density and saturation power.

Energy transfer between laser beams due to recording of optical axis gratings in liquid crystals

We have studied two-beam-coupling processes related to the generation of optical axis gratings in liquid crystals (LCs) due to the electromagnetic torque acting on the orientation of LCs in superimposed laser beams with spatially modulated polarization. Such gratings result in efficient coupling between the laser beams controlled by the frequency shift introduced between the beams at infrasound frequencies. Larger than 80% efficiency of energy transfer from a pump to a signal beam is reported at power density levels of the laser beams at 1-10 kW/cm2. The distinction between pump and signal beams is relative, since the sign of the frequency shift rather than the power ratio between the beams determines the direction of energy transfer. The experiments were performed for radiation at a 1.06 μm wavelength; high optical nonlinearity and low absorption make LCs suitable for use in visible and near-IR spectra, particularly in high-power laser systems. We have characterized the main fundamental features of the process of recording dynamic orientation gratings, plotting the gain spectra for several LCs and verifying the good quality of the amplified signal beam even for a strongly distorted pump. The obtained results set the groundwork for a variety of novel opportunities and applications that include combining high-power laser beams and wavelength division multiplexing at an ultranarrow interval of frequencies of the order of 1-100 Hz.

Response Times and Their Temperature Dependence Measured for a Series of Liquid Crystals Using Gain Spectra of Stimulated Light Scattering

In the present work we have shown the opportunity of using the process of stimulated light scattering due to recording of dynamic orientation gratings for directly measuring the characteristic orientation relaxation time of liquid crystals (LCs) and related material parameters, the elastic constants and the coefficients of orientational viscosity. The measurement consists in obtaining the gain spectra of LCs and determining the magnitude of the gain coefficient and the optimum frequency shift between interfering laser beams. We have designed a setup where the gain spectra of stimulated light scattering process in LCs are obtained within seconds. These spectra are highly specific to LCs serving as a precise and fast tool for studying LC materials and their mixtures with the purpose of optimizing their linear and nonlinear optical properties. Using this technique we have found that certain azo LC materials are faster compared to several well-known Merck's formulations for LC displays.