Dichromated Gelatin Films Research Articles

Surface Profile Studies of Photoinduced Gratings Made with DCG Films with Optional Papain Development.

The use of surface relief structures is increasing in the field of optics. A study of photoinduced relief using dichromated gelatin films with different thickness is described in this paper. Two light sources were used: a laser (λ = 468 nm) and an ultraviolet mercury-metal halide lamp. Gratings with low spatial frequencies were contact-copied on the DCG (dichromated gelatin) films. Two development processes were used, one included washing the plates with just water and the other with a mixture of water and papain. This enzyme is used to improve the gratings’ relief which was studied with a profilometer. For the development process with just water, it was found that when gratings were recorded using visible or UV light, the height profile inversely correlated to spatial frequencies. For short exposure times, the reliefs showed a sinusoidal profile. When visible light was used, the DCG areas where the Ronchi grating had transparent slits showed a flat relief and the areas where the Ronchi grating had opaque slits showed a round peak, with the peak being taller than the flat surface. In contrast, when UV light was used, the flat surfaces were taller than the peaks. The relief height increased up to seven times when papain was used.

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.

Observation of modulated spontaneous emission of Rhodamine 6G in low refractive index contrast 1D-periodic gelatin film

The modulation of the spontaneous emission of Rhodamine 6G has been observed in one-dimensional periodic dielectric structure of dichromated gelatin film with refractive index contrast as low as 0.01. The spontaneous emission is enhanced at the band edges and inhibits in the band gap, which agree well with the theoretical analysis on the redistribution of the fractional local density of optical states.

Spectropolarimetric investigation of the photoinduced dichroism and birefringence in malachite green/dichromated gelatin films

We report a spectropolarimetric investigation of the photoinduced anisotropy in dichromated gelatin films containing the dye malachite green. The investigation is done with a novel Spectral Stokesmeter which measures simultaneously and in real time (20 ms) the spectra of all the Stokes parameters of light in the spectral range 500–750 nm. This made it possible to measure not only the spectrum of the dichroism but also, for the first time to our knowledge, the spectrum of the photoinduced birefringence in these films. The results show that we can measure trustworthy dichroism larger than 0.02 and birefringence larger than 2 × 10−4.

Non-destructive thickness measurement of dichromated gelatin films deposited on glass plates

A quick estimation of the thickness of thin films deposited on glass plates is described in this paper. The principle of the method is based on the measurement of the Haidinger fringes generated by the film. For ease of observation and measurement, a commercial Fizeau-type interferometer such as a Zygo interferometer using a large angle of illumination has been used. The simple modification to observe the Haidinger fringes with the Fizeau-type interferometer is also described. The thickness of the film is related to the diameters of the Haidinger fringes. It is possible to estimate the thickness quickly by counting the number of fringes. A more accurate estimate can be obtained by the measurement of the diameters of the fringes and using these data in a formula that relates these two to the thickness of the film. The method is found to be useful for measuring the thickness of dichromated gelatin films (DCG) coated on a substrate to make holographic optical elements. The accuracy in estimating the thickness of the film is of the order ± 1 μ m . The method is entirely non-destructive and works well in the thickness range of 5– 150 μ m .

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.

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.

Primary photochemical process in films of dichromated gelatin: a quantitative approach

The photochemical behavior of dichromated gelatin films is investigated on irradiation at 365 nm and 491.6 nm. Fourier transform infrared (FTIR) spectroscopy confirms the two-step reduction of chromi- um(VI) into chromium(III). Prior to the irradiation, the molar extinction coefficient ? of chromium(VI) in the film is measured as a function of the pH of the starting solution with the result that e appears to be pH independent in the range 2 to 10 contrary to what is observed in solution. Chromium(V) resulting from a charge transfer between the polymeric matrix and chromium(Vi) species in the excited state appears to be surprisingly stable in the gelatin matrix. Accordingly, the subsequent slow reduction of chromium(V) into the final chromium(III) does not interfer in a significant way in chromium(VI) reduction. As a result, it is possible to determine the UV-visible spectrum of chromium(V) and to quantify its formation. So far no quantitative results were reported in the literature concerning the chromium(V) spectrum and its formation in a film of dichromated gelatin. FTIR spectroscopy confirms the two-step reduction of chromium(VI) into chromium(III).

Self-enhancement of holographic gratings in dichromated gelatin and polyvinyl alcohol films

New experimental results concerning real-time hologram recording in dichromated sensitized gelatin and polyvinyl alcohol are presented. Self-enhancement of the holographic gratings observed in the dark in both of these dye-polymer systems is shown.

Dual-functional polymeric waveguide with optical amplification and electro-optic modulation

Optical amplification and first-order electro-optic effect have been observed simultaneously in one polymeric material photolime gel which has been used widely as a volume holographic material to produce dichromated gelatin films. In this letter, the dual functions were achieved by doping neodymium chloride hexahydrate and chlorophenol red. The optimized doping concentration of Nd+3 is 6.7×1019 cm−3. The weight percentage of chlorophenol red is 23%. We observed a 3.8 dB of optical gain at 1.06 μm and an electro-optic coefficient of 22 pm/V at 633 nm.

Diffraction gratings in dry developed dichromated gelatin films

A new method for using dichromated gelatin films in photolithography has been recently developed in our laboratory. In this paper, we describe the experiments carried out for making efficient surface-relief diffraction gratings using that method. Basically, gratings are obtained by developing, in an oxygen electron cyclotron resonance stream, the interference of two coherent waves recorded on a dichromated gelatin film. Diffraction efficiency and incident angle selectivity of 0.5 μm period gratings are studied as a function of development time, and results are compared with theoretical predictions.

Oxygen ECR stream etching of dichromated gelatin films

An experimental study of the development of dichromated gelatin (DCG) films using an ECR source with oxygen is presented in order to evaluate DCG as a photoresist material in microlithography. The kinetics of the etching process shows saturation behavior for high exposure levels and long development times. From the initial linear dependence, values for development rate are obtained ranging from 300 nm/min for an unexposed film to 25 nm/min for exposed films, depending on ammonium dichromate concentration of the gelatin film, exposure energy and the operation variables of the etching tool—an ECR source—mainly microwave power and oxygen pressure/flow. The etching behavior can be explained using a model based on the formation of a stop layer on the surface of the DCG film, as a result of the modification produced by the O 2 stream. The analysis of the characteristic curve of DCG films gives a threshold energy of 10 mJ/cm 2 and a contrast of 0.65, at 365 nm.

Use of a pulsed neodymium laser in the fabrication of relief phase holographic optical elements

The surface of a recording medium was illuminated with a nonuniformity of less than 10% using second-harmonic radiation from a neodymium laser and the characteristic features of the illumination were considered. A laboratory technology was developed for fabricating 10-cm diameter holographic optical elements. This was based on optimizing the characteristics of a dichromated gelatin film, while obtaining a surface relief by recording with a 0.53-μm radiation pulse. The high image quality formed by a holographic optical element, with a low light scattering level, provides grounds for predicting extensive use of solid-state lasers in the fabrication of holographic optical elements.

Simplified processing method of dichromated gelatin holographic recording material

We are concerned with new simplified procedures for obtaining sufficient initial bias hardness of dichromated gelatin film by inserting the dehydration process before exposure.

Mechanism of Index Modulation in Developed Dichromated Gelatin Films

AbstractWide angle (WAXS) and small angle X-ray scattering (SAXS) studies were performed on the structural changes in a recording dichromated gelatin (DCG) layer produced by exposure and development. The phase diagram of the system gelatin-water-isopropanol was determined. The data obtained enable volume hologram recording to be related to photocrosslink concentration- modulated microphase separation in a water-swollen DCG layer developed with isopropanol.

Wettability and adhesion studies of plasma-treated plastic substrates for gelatin holograms

The feasibility study of using polycarbonate and poly(methyl methacrylate) substrates treated with the oxygen plasma for dichromated gelatin holograms is reported. The contact angles of water on both treated and untreated substrates were measured to indicate the wettability of substrate surfaces. An appreciable increase in adhesion between the dichromated gelatin film and the polycarbonate substrate after the oxygen plasma treatment was demonstrated.

Reduction of sensitiser concentration gradients in dichromated gelatin films

Concentration gradients of the sensitiser in dichromated gelatin films as used in volume holography can produce undesirable variations in the reconstruction wavelength and reconstruction waveband at a fixed angle. These gradients are related to the draining time allowed for the films following sensitisation and a 5 min time limit is recommended to minimise such effects.

Holographic selector telescope for grazing diffraction

A study was made of holographic recording of volume phase gratings in dichromated gelatin films designed for operation in the grazing incidence case. The special properties of grazing diffraction by volume phase gratings were analyzed. The diffraction efficiency achieved in this way was ~30% when the angle of incidence was 88°.

Red sensitivity of dichromated gelatin films

Dichromated gelatin films present sensitivity to red light without any special sensitization. This sensitivity is due to an absorption appearing in dichromated gelatin films and attributed mainly to a charge–transfer transition which results in an electron transfer from gelatin to chromium. To characterize holographically this recording material, both reflection and transmission gratings have been obtained using the 632.8-nm radiation from a He–Ne laser.

Secondary Holograms in Dichromated Gelatin Filters

Reflective volume gratings in dichromated gelatin film have been studied under conditions in which secondary holograms are produced, where, during exposure, the holographic film is placed in optical contact with a prism. Optimum reconstruction angles and diffraction efficiencies are determined for the secondary holograms that are recorded as a result of internal reflection.