Dichromated Gelatin Research Articles

Holographic optical elements recorded in silver halide sensitized gelatin emulsions. Part 2. Reflection holographic optical elements.

Silver halide sensitized gelatin (SHSG) holograms are similar to holograms recorded in dichromated gelatin (DCG), the main recording material for holographic optical elements (HOEs). The drawback of DCG is its low energetic sensitivity and limited spectral response. Silver halide materials can be processed in such away that the final hologram will have properties like a DCG hologram. Recently this technique has become more interesting since the introduction of new ultra-fine-grain silver halide (AgHal) emulsions. In particular, high spatial-frequency fringes associated with HOEs of the reflection type are difficult to construct when SHSG processing methods are employed. Therefore an optimized processing technique for reflection HOEs recorded in the new AgHal materials is introduced. Diffraction efficiencies over 90% can be obtained repeatably for reflection diffraction gratings. Understanding the importance of a selective hardening process has made it possible to obtain results similar to conventional DCG processing. The main advantage of the SHSG process is that high-sensitivity recording can be performed with laser wavelengths anywhere within the visible spectrum. This simplifies the manufacturing of high-quality, large-format HOEs, also including high-quality display holograms of the reflection type in both monochrome and full color.

Fabrication of refractive microlens array by etching dichromate gelatin with enzyme solution

A new method is developed to fabricate a refractive microlens array by etching ammonium dichromate gelatin (ADG) with enzyme solution. The light exposes ADG through a high contrast binary mask, then the exposed parts of ADG generate a cross-linking reaction. Usually, the relief achieved by water developing is very shallow (<1 ?m), so we compound a 0.2% concentration protein enzyme solution as the developer to develop ADG and deepen its relief. At 37°C, the biochemical reaction occurs between ADG and the enzyme solution, and because of surface tension, ADG turns to spherical structure after developing. The optimum technique parameters of this process are presented. The principle of etching ADG with an enzyme solution to achieve deep relief and the swelling of ADG are discussed in detail. The process of fabricating microlens array does not require the use of expensive equipment, and it is not sophisticated and time consuming. Results are presented for experiments and evaluated by a profile meter and an interference microscope.

Refractive micro lens array made of dichromate gelatin with gray-tone photolithography

A novel method is proposed to fabricate refractive micro lens array using dichromate gelatin (DCG) and an enzyme. DCG has optical properties similar to glass but is sensitive to UV light. The enzyme solution is used to develop the optically exposed DCG instead of a conventional water–isopropanol developer, which greatly enhances the surface relief depth. The continuous surface relief structure is realized by a coded grey-tone mask. A computer program has been developed to code an arbitrary 3D surface relief shape into a grey-tone mask design. The DCG properties and the lithography process are described. The grey-tone coding principle is presented. Unlike the current processes where patterned photoresist relief structure has to be transferred to a substrate material, the fabricated DCG micro lens array can be directly used as an optical element. The new process opens a new route of micro lens fabrication, which is simple and low cost.

Induced and form birefringence in high-frequency polarization gratings

High-frequency phase polarization gratings are fabricated holographically in dichromated gelatin dyed with malachite green. It is observed that the intensity of the -1 diffracted beam is a sinusoidal function of the incident polarization angle. In addition, we analyze the dependence of the diffracted order polarization on grating frequency. It is evident from our results that form birefringence becomes significant when the grating period is smaller than the illumination wavelength, thus modifying the optically induced birefringence. Then, in polarization hologram reconstruction, it is not possible to obtain the polarization distribution at the recording step for high-frequency objects.

Observation of a fast-formed absorption grating and a slowly formed phase grating in undeveloped dichromated gelatin

Real-time diffraction efficiency is measured during hologram formation in undeveloped dichromated gelatin (DCG) film. The main features of real-time diffraction efficiency of undeveloped DCG reveal double peaks with increasing exposure time. Adopting the grating translation technique, we show that the first peak originates from the absorption grating and that the latter is related to the phase grating. To explain such complicated hologram-formation dynamics, we present a phenomenological model for mixed gratings associated with photoinduced refractive-index changes and absorption changes. We have compared the model results with the experimental results, which show good agreement.

Improving the holographic sensitivity of dichromated gelatin in the blue–green part of the spectrum by sensitization with xanthene dyes

The holographic properties of dichromated gelatin (DCG) sensitized with various xanthene dyes were studied, and results are reported. The sensitivity of dyed DCG in the green part of the spectrum compared with that of pure DCG (215 mJ/cm(2)) was significantly improved by addition of Rhodamine 6G (140 mJ/cm(2)) or Erythrosin B (90 mJ/cm(2)). Diffraction gratings were recorded with a He-Ne laser at 543.5 nm. The maximum diffraction efficiency was found to be 32% at normal incidence of the readout beam; it was 80% at the Bragg angle.

Holographic optical elements recorded in silver halide sensitized gelatin emulsions Part I Transmission holographic optical elements

Silver halide sensitized gelatin (SHSG) holograms are similar to holograms recorded in dichromated gelatin (DCG), the main recording material for holographic optical elements (HOE's). The drawback of DCG is its low sensitivity and limited spectral response. Silver halide materials can be processed in such a way that the final hologram will have properties like a DCG hologram. Recently this technique has become more interesting since the introduction of new ultra-high-resolution silver halide emulsions. An optimized processing technique for transmission HOE's recorded in these materials is introduced. Diffraction efficiencies over 90% can be obtained for transmissive diffraction gratings. Understanding the importance of the selective hardening process has made it possible to obtain results similar to conventional DCG processing. The main advantage of the SHSG process is that high-sensitivity recording can be performed with laser wavelengths anywhere within the visible spectrum. This simplifies the manufacturing of high-quality, large-format HOE's.

Photodarkening and partial photobleaching: application to dichromated gelatin

Photodarkening or partial photobleaching of photochromic solid media is described by means of an approximated analytical equation. The equation shows that the effective rate of the photochemical reaction is proportional to the square root of the final transmittance of the illuminated layer. The model is applied to describe the photodarkening of dichromated gelatin at 488 nm.

Materials for the recording of thin holographic grating couplers

Two types of recording media are investigated for the recording of thin and highly asymmetric holographic grating couplers: (1) commercial photoresists for thin surface relief gratings, and (2) dichromated gelatins (DCG). As a result of the investigations on the photoresists (Shipley S1805), we found that very high diffraction efficiencies (DE) of over 80% are obtained into the first order, when reconstructed at optimal geometrical conditions diffracting the output beam into free space with visible light at 514 nm. However, when readout is at normal incidence and the output beam is diffracted into the substrate, two significant changes occur. Namely, the incident light is now diffracted into both diffracted first orders, with a ratio varying between about 2 and 6 between the two first orders, depending on the exposure energy. The angular bandwidths of these thin blazed gratings were very large (>30°); yet in a double grating substrate-mode configuration, the bandwidth was significantly reduced to about 10°, indicating that the resultant DE is the product of the DEs of the two gratings. The DCG layers and processing procedures, which were optimized for recording at 514 nm in thin layers of 2–3 μm yielded the highest DEs, approaching 90%, indicating that relatively large refractive index modulations, reaching at least 0.05 are stored in these materials. Under substrate-mode conditions, we obtain DEs of over 70% into only one first order. As expected, these thin holographic volume (Bragg) grating couplers exhibited large angular bandwidths. As the bandwidths did not exhibit any significant exposure dependence, this indicates that in the investigated exposure ranges (50–100 mJ/cm 2) the gratings are recorded within the entire depth of the DCG layer.

Thin films: chemical coating as an alternative to vacuum deposition for preparation of wave front transformers

Thin films prepared by wet chemical coating are an alternative to films fabricated by evaporation in vacuum. Nanostructures in such materials can be obtained by holographic or “bit-by-bit” recording with light. Those structures can be used as wave front transformers, for optical storage and processing of information, etc. Our investigations are focused on fabrication of chemically coated thin films for holographic storage. The holographic recording in light-sensitive chemically deposited thin dichromated gelatin films shows that the nature of the chemical baths in isopropanol strongly affects the performance of the wave transformers. The thermal treatment gives an additional degree of freedom for the design of the optical behavior of the recorded structures.

Chemical cleaving method of relief microstructure formation on dichromated gelatin

A chemical cleaving method to improve the resolution of relief modulation in dichromated gelatin is presented. The process (which uses hydroxylamine) yields higher relief resolution and better profile fidelity than a process that uses the enzyme trypsin because it has many fewer cleaving sites. Experiments prove that relief modulation can produce a resolution of as much as 500 lines/mm. With a projection exposure system, a microprism array with a 30-mum-wide, 0.8-mum-high cell is fabricated by this method. A method for forming a continuous relief on dichromated gelatin is also discussed.

Holographically recorded axicon for grazing incidence interferometry

A diffractive axicon for grazing incidence interferometry of cylindrical objects is presented. For industrial measurement, an element with a diameter of 8 cm was manufactured. The axicon was recorded in dichromated gelatine (DCG) with a high-precision rotating stage. The diffraction theory for the design of the element is presented. The axicon was tested interferometrically. Some possible errors are discussed.

Dichromated gelatin hologram using digital image processing technology to optimize processing

In this paper a high-efficiency diffraction hologram fabricated using digital image processing technology in dichromated gelatin (DCG) is described. A charge coupled device (CCD) camera and frame capture card were used to obtain the holographic grating processing parameters using hologram manufacturing process procedures. With this image system the transmittance of the DCG solution can be measured, the uniformity and thickness of the emulsion on the glass plate can be controlled, the hardness of the prepared gelatin layer analysed and the variations in the hologram diffraction efficiency monitored. The experimental results verified that this image system provides an economical and effective method for producing a DCG hologram.

Design of low-haze holographic notch filters

We have studied the optical quality of holographic notch filters and have identified that stray illumination during the exposure stage and substrate scatter are the limiting factors. We propose and analyze a novel holographic recording scheme in which the substrate is rotated during exposure. Using this method, we have produced high-uniformity, flare-free dichromated gelatin notch filters with optical density greater than 5 and with scatter comparable with that from polished substrates.

Dichromated Gelatine as a Material of Optical Element

In the fabrication process of optical elements (OEs) by the laser scanning method using a dichromated gelatin (DCG) photoplate, the expansion and drying stress of gelatine caused by inhomogeneous liquid flow inside the gelatine affects the shape of OEs. The reason this inhomogeneous liquid flow exists in the energy oversaturated parts of OEs is the presence of surplus energy. In order to obtain the OEs of desired spherical lens shape, the drying stress should be reduced and therefore the maximum energy of the illuminating laser should be defined not to cause the surplus energy. The maximum energy is investigated according to the relative concentrations of (NH4)2Cr2O7 to DCG. The use of photoplates with a relative concentration of (NH4)2Cr2O7 to gelatin of more than 20% has some advantages when making the lens raster, especially a short-focal-length lens raster. It is also very important to increase the drying time to reduce the total drying stress by maintaining high humidity during the drying process.

Photochemical Reaction of Aqueous Dichromated Gelatin Solution

The photochemical reaction of aqueous dichromated gelatin solution in the intrinsic sensitive region under normal room conditions as investigated employing a water-soluble gelatin. The sample solution, containing dichromate, gelatin, and pH buffer solution, was exposed, and the decrease in absorbance for the photoreduction of Cr(VI) was monitored by UV-vis spectroscopy. Having compared the behavior of gelatin with that of alcohol at photoreduction of dichromate, we found that both systems are consistent with respect to the reaction order, the kinetic order for each reactant solution. Similar results were also obtained from the standpoint of pH dependence,activation energy, and chemical sensitization by EDTA. On the basis of experimental observations and discussion, a mechanism of photoreduction of dichromated gelatin and its rate determining step were suggested.

New developments using holographic concentration in solar photochemical reactors

This paper presents test results achieving on (20cm×50cm) cylindrical holographic mirrors being recorded in dichromated gelatin for use in solar chemistry applications. The realized spectral reflectivity range 480-620 nm is adapted to the sensitizer Rose Bengal which is the sensitizer frequently used in photo-oxidation reactions. Aluminum mirrors and holographic elements were used as reflecting elements in a lab-scale parabolic trough reactor. The effect of the different elements on the heat transfer into the reaction was compared.

Effect of the content of ammonium dichromate in dichromated gelatin on Cr 2P_3/2 x-ray photoelectron spectroscopy

The effect of the content of (NH(4))(2)Cr(2)O(7) in dichromated gelatin (DCG) on the binding energy of the Cr 2P(3/2) level was studied with x-ray photoelectron spectroscopy. It was found that the binding energy of the Cr 2P(3/2) level of chromium in DCG is lower than that of pure ammonium dichromate. When the content of (NH(4))(2)Cr(2)O(7) is not greater than 1%, the chromium in DCG has only one state, near 577.5 eV; when this content is between 1% and 20%, the chromium in DCG has two states, near 577.5 and 579.1 eV. The relative contents of these two states change with the content of (NH(4))(2)Cr(2)O(7). As the (NH(4))(2)Cr(2)O(7) content increases, the relative content of the state near 577.5 eV decreases almost linearly, but its absolute content first increases, then reaches a maximum at ~10% (NH(4))(2)Cr(2)O(7), and finally decreases. In addition, the absolute content of the state near 577.5 eV changes very slowly between 5% and 15% (NH(4))(2)Cr(2)O(7). According to these experimental results and holography data reported in the literature, it is inferred that the chromium of the state near 577.5 eV is the chromium that forms the latent image center after exposure and then forms the hologram after development. As a result the basis for the optimum content of (NH(4))(2)Cr(2)O(7) is found, and an approach to increasing sensitivity is suggested through this experiment.

CONSTRUCTION OF HIGH-QUALITY NARROW-BAND HOLOGRAPHIC FILTERS

We have studied the factors limiting the quality of holographic notch filters. We have employed a novel recording scheme in which the sample is translated during exposure, eliminating flare due to reflections at the boundaries, and simultaneously reducing the scatter of the final hologram. We have also refined a gravity-settling dichromated gelatine (DCG) deposition technique and incorporated hardening of the DCG layers during the drying stage. With this, we have produced high optical quality, uniform, flare-free DCG filters with an optical density, OD > 5, bandwidth < 20 nm and scatter almost as low as that for state of the art dielectric notch filters.

Electron Phototransfer in the Dichromate - Macromolecule Systems and Photoinduced Changes of Polymer Structure

The Dichromated Gelatin (DCG) known as medium for the phase holograms registration and production of special planographic printing plates as well as the solutions of dichromates in alcohols have been studied for chromium compound reactions and phase-conformational transformations of denatured collagen molecules in their own polymer matrix. One- and multielectron transfer schemes are discussed as well as the delocalization of an electron along the macromolecular chains and polypeptide fragments and phase conformational changes in them.