Values Of Diffraction Efficiency Research Articles

New substituted pentazadienes as initiators of free-radical polymerization: synthesis, photochemical properties and perspectives for holographic media

The series of 4,4′-substituted 3-hydroxyethyl-pentaza-1,4-dienes were synthesized in the reaction of aromatic diazonium salts with an ethanol amine. Photolysis of pentaza-1,4-dienes was investigated by means of UV/Vis spectroscopy. Electron donating methyl and methoxy substituents were found to increase photosensitivity whereas electron withdrawing chloro-substituents cause higher photolytic stability. 3-hydroxyethyl-pentaza-1,4-dienes as a radical polymerization initiator has been successfully investigated in 10% methyl methacrylate solution in DMF. Gel permeation chromatography (GPC) and differential scanning calorimetry (DSC) were used for obtained polymers characteristics evaluation. The highest values of PMMA Mn and Đ can be obtained in the case of chloro-substituted derivative. The holograms of a plane wavefront were recorded in media based on obtained PMMA doped by azo-dye by a 532 nm laser in a ∼1-μm-thick sample. The diffraction efficiency values for parallel polarized acting light increased in Cl→Me→OMe series.

Polymer-Based High Diffraction Efficiency and High Resolution Volume Holographic Transmission Gratings

We report on the optical characterization of very high-efficiency and high-resolution holographic volume phase transmission gratings. The gratings are recorded in a new photo-polymerizable mixture made by epoxy-resin and multi-acrylate. The epoxy-resin used is known to make tenacious acrylate-based films. The holographic mixture contains two photo-initiators, the synergic effect of which enables a reliable photo-polymerization process in the visible region of the electromagnetic spectrum. The recorded holograms are mechanically stable, show long-term temporal stability and very high values of diffraction efficiency, coupled with good angular selectivity due to a relatively narrow band of wavelengths. We measured the intensity of the transmitted beam and calculated the intensity of the diffracted beam at different wavelengths, deriving the refractive index modulation and the grating pitch by fitting the experimental data with a slightly modified theoretical approach. These kind of mixtures can be used in several fields of application, such as chemical or bio-sensors, high resolution optical sensors, high-density optical data storage, encryption and security.

Optimization of Optical Phase Profile in Beam Deflector with Advanced Simulation Method for High Diffraction Efficiency.

Controlling the phase of light with a high efficiency and precision is essential for applications in imaging, tunable devices, and optical systems. Spatial light modulators (SLMs) based on liquid crystals (LCs) have been regarded as one of the best choices for the generation of phase profiles for the steering of light. The upper glass substrate has an unpatterned electrode for a common electrode, while the lower glass substrate has one-dimensional micro-patterned electrodes for controlling the single pixel level by the applied voltages. By applying different voltages to each electrode to create a sawtooth-shaped phase profile, the collimated input beam is deflected to the desired angle. To maximize the diffraction efficiency (DE) values, an advanced simulation method has been developed to find the optimized phase profile through the analysis of LC director distributions. The resulting diffraction patterns are investigated both computationally and experimentally, with a good agreement between the results obtained. Finally, the beam deflector (BD) system with an advanced driving algorithm has a high 1st order DE, about 60%, 37%, and 7.5% at 1°, 2.5°, and a maximum steering angle of 7.5°, respectively. The LC director distributions in relation to various diffraction angles are simulated and an experimental success in realizing enhanced DE for the beam steering device is presented.

Dichromated gelatin, an efficient material for the fabrication of wavelength selective holographic solar concentrators for high-efficiency operation

The use of holographic concentrators for photovoltaic solar concentration is a very attractive proposition as the hologram can be designed to be wavelength selective i.e. it can be designed to disperse, diffract and concentrate specific spectral regions of the solar spectrum with maximum efficiency in a spectrum splitting solar cell system. The practical application of highly efficient holographic solar concentrators requires intensive as well as extensive research work on the choice of the appropriate recording material. Among the available materials for recording holographic concentrators, Dichromated gelatin (DCG) is considered to be one of the best materials because maximum diffraction efficiency and high value of modulation in refractive index (reported value is up to 0.08) can be achievable in DCG film. Also, DCG has high resolution and shows low scattering noise.In present paper theoretical study on refractive index modulation capability of dichromated gelatin (DCG) for same and different thickness of DCG film have been performed to design a holographic concentrator for high-efficiency wavelength selective performance. To validate the theoretical results, holographic concentrators have been recorded in DCG film with different values of index modulation, and the typically recorded holographic concentrators were reconstructed with different wavelengths of the laser source, and corresponding diffracted power is measured. A comparison between experimental results and theoretical results are presented. Experimental and theoretical study reveals that for a constant film thickness low value of modulation in the refractive index gives maximum efficiency in lower wavelength regions and high values of index modulation are desired for maximum efficiency in higher wavelength regions. Further for a constant value of index modulation maximum efficiency gets shifted towards higher wavelength regions with an increase in film thickness. Theoretically simulated curves show that by appropriate selection of index modulation and film thickness, holographic concentrator may be fabricated to use almost entire useful portion of the solar radiation with reasonably high diffraction efficiency.

Temperature‐Sensitive Holograms with Switchable Memory

Herein, the dynamics and reversibility of temperature‐induced changes in volume phase holographic gratings (VPHGs) recorded in the low‐toxicity self‐processing photopolymer poly(N‐isopropyl acrylamide) (poly‐NIPA) are reported on. Transmission and reflection gratings are exposed to temperatures ranging 8–60 °C to study the effect of temperature on their properties. First, the role of the photopolymer chemical composition is investigated by comparing layers containing NIPA with those containing acrylamide (AA) and measuring the normalized diffraction efficiency (DE) at 60 °C; higher thermal sensitivity is observed for poly‐NIPA gratings (24% drop) versus low sensitivity for AA gratings (2% increase). The impact of the physical properties of the sensing layers (e.g., thickness/protective top layer) on their thermal response is studied. The reversibility of thermally induced changes in VPHGs is studied, especially the effect of exposure to the elevated temperature of 60 °C on the subsequent low‐temperature response. The results reveal the significance of water desorption/absorption for the observed changes in poly‐NIPA upon exposure to varied temperatures and indicate a thermally controlled memory effect. This effect is observed from DE values in transmission and from the spectral characteristics of the reflection gratings. A full reversibility of the DE and spectral characteristics is achieved at 8 °C.

Effect of molecular weight of PEG polymer matrix on the diffraction efficiency of Methyl Orange holographic media

Abstract Recording media for diffraction gratings formation based on polymer films of polyethylene glycol doped with the photoisomerisable azo-dye Methyl Orange have been developed. Holographic recording of the plane wave front for parallel and orthogonal orientation of polarization vectors of light beams have been performed at room temperature. In order to optimize the recording conditions and obtain high-efficient polarization holographic gratings, in the present work we study the dependence the average molecular weight of polymer matrix and solvent system for thin layers formation on the diffraction efficiency. Five PEG polymers were used as host for azo-dye incorporation −1500, 4000, 6000, 10,000 and 40,000, and two solvent systems, EtOH–H2O and THF-H2O, were examined to investigate film-forming properties. It was concluded, that structure of films as well as diffraction efficiency value and curves character strongly depend on PEG molecular weight.

Investigation on dynamic holographic display in liquid crystal film doped with carbon dots

In this paper, we studied the holographic properties of liquid crystal (LC) thin film doped with carbon dots (CDs) which can be used as real-time holographic display screen. The maximum value of diffraction efficiency can reach up to 30% by using a low applied electric field 0.2 V/μm. Holograms in the LC film can be dynamically formed and self-erased. The hologram build-up time and the hologram self-erasure time in the material is fast enough to realize video refresh rate. In addition, the forming process of hologram was studied. The holographic diffraction efficiency was measured depending on the intensity of recording light, applied electric field, the intensity of readout light, and readout light polarization direction. Triple enhancement of the diffraction efficiency value by the modulation of voltage under the condition of low recording energy is presented. Therefore, we develop an easy way to obtain real-time dynamic holographic red, green and blue displays with high diffraction efficiency, which allow the LC film doped with CDs to be used as a holographic 3D display screen.

Surface Morphology Formation of Edible Holographic Marker on Potato Starch with Gelatin or Agar Thin Coatings.

Edible films and coatings based on biopolymers to protect and extend the shelf life of food and medicine can be functionalized, by applying a holographic marker on the coating surface for marking products or sensing storage conditions. In this work, holographic markers were prepared on the surface of thin biopolymer coatings based on starch, gelatin, agar and also starch/gelatin and starch/agar blends by the nanoimprint method from a film-forming solution. The morphology of the surface of holographic markers using optical microscopy in reflection mode was examined, as well as the reasons for its formation using an analysis of the flow curves of film-forming solutions. It was found that the surface morphology of the marker strongly depends on the composition, consistency index of film-forming solution and miscibility of the components. It was shown that the starch/agar film-forming solution at the ratio of 70/30 wt.% has a low consistency index value of 21.38 Pa·s0.88, compared to 64.56 Pa·s0.67 for pure starch at a drying temperature of 30 °C, and the components are well compatible. Thus, an isotropic morphology of the holographic marker surface was formed and the value of diffraction efficiency of 3% was achieved, compared to 1.5% for the marker made of pure starch. Coatings without holographic markers were analyzed by tensile strength and water contact angle, and their properties are highly dependent on their composition.

Structural properties of Ag–As–S chalcogenide glasses in phase separation region and their application in holographic grating recording

In this paper we focus on structural properties of As2S3–Ag glasses in phase separation region (Ag content: 0, 5, 7 at. %) using XRD technique and Raman spectroscopy and report on some application of studied glasses. X-ray diffraction experiments confirm amorphous structure of glasses. Pair distribution functions have been obtained and analyzed, and they have testified about changes in the radius of the first coordination sphere after introduction of silver into As2S3. The changes of the structure can be observed from obtained Raman spectra of doped As2S3: Ag. Compositional changes in studied chalcogenide glasses were observed in the evolution of the detected Raman bands. The main effect that is observed at introduction of silver (5, 7 at. %) into nanostructured As2S3 glass is the change of the relative concentration of the main and non-stoichiometric molecular structural units. As2S3–Ag glasses are promising for application in the photonics and optical data storages. Surface relief holographic diffraction gratings were directly recorded using composite As2S3: Ag/Se nano-multilayered structures. Diffraction efficiency values in transmission of recorded gratings is ~22%.

Development of a photopolymer holographic lens for collimation of light from a green light-emitting diode.

A holographic optical element (HOE) was developed to collimate a monochromatic light-emitting diode (LED). The fabrication was achieved by the interference of collimated and diverging beams from a 532nm laser to produce the required photonic structure in a self-developing photopolymer material. The experimental values of diffraction efficiency and spatial period across the HOE were compared with their expected theoretical values. Good agreement was found for the spatial period; however, the diffraction efficiency varied significantly across the lens. In this paper, two approaches have been taken to address this variation: (1)modification of recording geometry and (2)optimization of recording intensity and exposure time at constant energy. The performance of an optimized HOE (cylindrical holographic lens) was then demonstrated using a 532nm laser beam. The optimized conditions for the cylindrical holographic lens were used to fabricate a spherical holographic lens of the same numerical aperture for LED applications. This type of lens has the potential to be used in combination with LED sources.

Reversible Polarization Recording in As2S3–Se Multilayer Nanostructures

Polarization holographic recording of diffraction gratings and their subsequent erasure have been investigated on As2S3–Se multilayer nanostructures. The work investigates how preexposure to actinic laser radiation up to complete photoinduced changes in the optical properties affects the formation of diffraction gratings in the studied structure. It is shown that the preexposure of an As2S3–Se multilayer nanostructure (MNS) leads to photobleaching, and the maximum achievable diffraction efficiency (DE) of 35% does not change; however, the required exposure value is increased. It is also shown that exposure using one laser beam results in complete erasure of the diffraction grating recorded up to the maximum. Seven recording–erasure cycles show that the kinetics of the increase in diffraction efficiency and its maximum value do not change, which indicates that the As2S3–Se multilayer structure is capable of reversible holographic recording under orthogonal circular polarization. Study of the gratings recorded with an atomic-force microscope shows that the main factor determining the diffraction efficiency value is modulation of the relief, the depth of which is greater than 200 nm.

СИНТЕЗ ТА ОПТИЧНІ ВЛАСТИВОСТІ НОВОГО СИМЕТРИЧНОГО ПІРАЗОЛВМІСНОГО ГЕТЕРОЦИКЛУ І ПЕРСПЕКТИВА СТВОРЕННЯ ГОЛОГРАФІЧНИХ СЕРЕДОВИЩ НА ЙОГО ОСНОВІ

By the interaction of a symmetric bis-aldehyde with 1-phenyl-3-methylpyrazolone-5, a new symmetrical dye capable of photoinduced isomerization was obtained. Recording media (RM) based on synthesized dye and soft polymer matrix were prepared. Intensity of absorption of RM rises with increasing of dye percentage in polymer matrix. But, Beer-Lambert law is not fulfilled. Therefore, the partial aggregation of dye molecules is present. As in the case of azo-compounds, information recording proceeds, probably, through spatial changes caused by the photoinduced E,Z double bond isomerization. Recorded holograms have the polarization nature – it was proved by the quenching of diffraction orders during changing the polarization of reference beam from е1⊥e2 to е1║е2. It was shown that compounds containing the –НС=С< bond, where =С< is the part of heterocyclic moiety are able to the isomerization. The diffraction efficiency is expected to be higher due to presence of two photoactive groups in symmetric molecule, comparing with molecules with one photoactive group. Light absorption with λ=532 nm by the dye molecules is proved to be sufficient for holograms recording and relaxation despite the low adsorption intensity at such wavelength. The recording and relaxation speed are high. Fast increasing of η in the beginning of hologram exposition and, after turning off the illumination beam, fast decay, according to the η||(t) and η⊥(t) curves. Hence, this polymer composite is capable for further applications in dynamic holography. It was shown that increasing of dye amount in polymer matrix leads to growth of diffraction efficiency. It has been established that data recording media is characterized by higher values of diffraction efficiency with parallel polarization compared to perpendicular.

Methods of compression of digital holograms, based on 1-level wavelet transform

To reduce the size of memory required for storing information about 3D-scenes and to decrease the rate of hologram transmission, digital hologram compression can be used. Compression of digital holograms by wavelet transforms is among most powerful methods. In the paper the most popular wavelet transforms are considered and applied to the digital hologram compression. Obtained values of reconstruction quality and hologram's diffraction efficiencies are compared.

Requirements to lasers and formfactor of holograms

The paper analyzes the cross effect of nonuniform exposure nature on hologram field and nonlinearity of holographic material response which significantly adjusts the values of achievable aggregate diffraction efficiency and the optimum exposure. The cross effect is supposed to be registered with the help of a parameter named as a hologram formfactor by analogy with the description of complex body interaction. The technique for calculating formfactor is described. It is shown that the key differences when choosing the optimum exposure can be taken into account with the help of the correction coefficients defined with a hologram formfactor. The calculations for actual Gaussian holograms used in Holographic Memories as well as for model holograms being a good illustration to the dynamics of average diffraction efficiency versus exposure characteristic are made. The Gaussian holograms show good correlation with earlier empirical data.

Influence of corona discharge on holographic recording of diffraction gratings in Cu–chalcogenide film structures

The influence of corona discharge on holographic recording of gratings in Cu–As2Se3 and Cu–As2S3 thin film structures was investigated. Applying of negative corona discharge during optical exposure leads to the increase of holographic sensibility of the investigated registration structures, and increases the diffraction efficiency of recorded gratings. Accordingly, the relief of the diffraction gratings formed by chemical etching of such patterns, recorded under action of negative corona discharge, exhibits much higher values of diffraction efficiency in comparison with the case without corona.

光谱宽度与光束截面因子对平面光栅衍射效率测量的影响

To improve the measurement accuracy of a plane grating diffraction efficiency measuring instrument, the affect factors on the diffraction efficiency measurement of a plane gating were researched. According to the measuring principle of relative diffraction efficiency of the plane grating, the concept of beam cross-section factor k() of plane grating was proposed and the correspondent expression was developed. Then, the factors affecting the measurement accuracy of diffraction efficiency, the spectral width and the beam cross-section factor were analyzed, and two independent variables for the regression were defined by combination with the measurement value of diffraction efficiency. Regarding the nonlinear relation between the theoretical value of dependent variable and the independent variable defined, the complete quadratic regression analysis was applied as the methodology to establish a revised equation to improve the measurement accuracy of the diffraction efficiency. The results show that the variation between the revised and the theoretical values is less than 0.5%, which means the measurement accuracy of diffraction efficiency has been improved and the performance of the instrument is enhanced significantly.

Influence of blue and red light illumination on the holographic storage in an azopolyester–PMMA blend

Volume holographic polarization gratings have been stored in thick films of blends of a side-chain azobenzene polyester and a poly(methyl methacrylate) (PMMA) homopolymers. The azobenzene content in the blend is 0.2wt%, and the holograms are recorded by using 2ms 4mJ/cm2 488nm light pulses. The dependence of holographic storage on the recording condition has been studied. No stable gratings can be recorded in thermally quenched films. The presence of cis isomers that can be induced by blue light irradiation seems to be necessary to store stable gratings. The cis isomer content and consequently the grating efficiency can be changed by blue and red light irradiation. By choosing appropriate conditions stable and rewritable polarization gratings have been recorded. Up to 20 gratings with diffraction efficiency values higher than 5×10−5 have been multiplexed.

Photorefractive parameters in GaAs:Cr with externally applied magnetic field

Using band charge transport model, photorefractive (PR) parameters in semiconductors in presence of applied magnetic field have been studied. It is found that external magnetic field significantly affect the photorefractive space charge electric field (ESC), convenient gain coefficient and diffraction efficiency ESC ∼105V/m, normalized gain coefficient ∼0.9m−1 and diffraction efficiency ∼90% observed at B0=2T. This value of gain coefficient and diffraction efficiency is nearly nine times higher than the value obtained in the absence of B0. A typical behavior of recording and erasure of hologram with respect to time has also been investigated. Results manifest that GaAs:Cr is efficient for ultrafast writing and erasing media for PR-grating.

Holistic Method for Evaluating Large Area Transparent Conducting Electrodes

Nowadays, specifying the quality of a transparent conducting electrode (TCE) using a figure of merit (FoM) is considered nearly mandatory. However, not much attention is paid to the local variations in the FoM itself across the large area of the TCE. This calls for the definition of a local FoM (LFoM), particularly relevant with regard to several new generation TCEs which have been and are being proposed recently. A LFoM based on local measurements of transmission and sheet resistance, pixel by pixel, would be a Herculean task. The present article addresses this central issue by defining a LFoM based on the diffraction efficiency (DE) of a calibrated high-resolution transmission grating overlaid with a given TCE. The DE value, which critically depends on the periodic nature of the grating material, is shown to be highly sensitive to the various nonuniformities in the TCE overlaid on the grating, with length scales comparable to the grating period. The effectiveness of the so-defined LFoM was demonstrated using a pointer laser and a photodiode in combination with a transmission grating with ∼μm periodicity by taking ITO/glass and ITO/PET as case examples. A metal grating pattern of Cu deposited on seed Pd grating lines was fabricated as an example of new generation TCE and examined for FoM and LFoM, however, without the aid of the external grating. The LFoM based on DE presented here should serve as an excellent screening method for both conventional and emerging TCEs.

Pd grating obtained by direct micromolding for use in high resolution optical diffraction based sensing

Pd grating patterns have been fabricated using the process of micromolding in capillary employing a Pd alkanethiolate precursor, which could be converted to metal in situ by thermolysis. Thus generated Pd grating were uniform in width (~950 nm) and spacing (~450 nm) over millimeter square areas on glass substrates. Importantly, the pattern when used as an optical grating produced a diffraction pattern with a high resolution (>2000); the intensities of widely separated (diffraction angle, ~26·8°) diffracted spots could be measured using a simple photodiode. By varying the concentration of Pd precursor (2 mM to 25 mM), thickness of the resulting gratings could be adjusted in the range of ~15–115 nm. By adjusting the grating parameters optimally, a maximum diffraction efficiency of 36% has been achieved. Thus fabricated Pd grating was used as seed catalyst to deposit Cu by electroless plating. The Cu deposition process has also been monitored by employing AFM, SEM and EDS analysis. The diffraction efficiency values corroborate well with the changes in the grating thickness due to Cu deposition. The grating structures presented can be reproducibly fabricated for rapidly emerging optical diffraction based sensing applications. This has been demonstrated in the case of aqueous Cu2 + by depositing the latter electrolessly on Pd.