Characteristics and classification of the hottest

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Characteristics and classification of holographic recording materials (middle)

commonly used holographic recording materials ideal recording materials should have high sensitivity, high resolution, low noise and linear curve to the wavelength used for exposure. From the perspective of production, it should also have the advantages of being able to repeat other environmental experimental facilities, which will have other phenomena in use, low price and so on. In fact, it is impossible for a certain recording material to meet these conditions at the same time

1, silver halide emulsion

silver halide photosensitive material is the most commonly used holographic material. It has the advantages of high sensitivity, wide spectral response, high resolution and strong versatility. It is extremely difficult to decompose in soil, but the diffraction efficiency and signal-to-noise ratio are low. Silver halide emulsion film is divided into film (film) and dry Version (hard film). Its main structure is photosensitive layer and film base. The photosensitive layer is also called emulsion layer. The emulsion made of ultrafine silver halide (mainly composed of AgBr and AgCl) grains suspended in gelatin and certain sensitizer is generally 0.01 ~ 5 thick μ m. Latex is attached to the film base. Traditionally, the film base of glass plate is called holographic dry plate, while the film base of acetate and polyester film is called holographic film. The particle size of silver halide emulsion used for holography is required to be 0.03 ~ 0.09 μ m。 Because the smaller the particle size, the lower the photosensitive sensitivity, so the exposure of holographic dry plate is much larger than that of ordinary photographic dry plate. The standard developer of holographic dry plate is D19, which can get a higher value. The size of silver particles after development is 0.1 ~ 0.2 μ m。 The developer of fine particles is d76, and the developed silver particles are filiform. The finer the silver particle, the higher the diffraction efficiency of the hologram and the lower the noise. If the particle size of silver is smaller than the wavelength, Rayleigh scattering can be applied. The ultrafine particle dry plate is developed with d76 diluted developer. The dry plate is brownish red, and the particle size of silver is 0.005 ~ 0.025 μ m. The diffraction efficiency of hologram can reach 40%. Another way to improve the diffraction efficiency is to use d76 development and F5 fixing. The upgrading of technical standards in the relevant field has brought greater opportunities to the development of composite polyurethane adhesives. Then use R10 (NaCl) bleaching, and then use strong light to irradiate, so that the product of the exposed part on the hologram is photolysis silver, with a particle size of 0.01 μ m。 Holographic diffraction can also be improved

The amplitude hologram is obtained after exposure and development of the holographic plate. If it is bleached, it becomes a phase hologram. Bleaching is the reduction of metallic silver into transparent silver salt with oxidant. The result is that the refractive index of gelatin in the exposed part of the hologram is different. The improvement of diffraction efficiency is related to the difference of refractive index between the above two, but it is not that the greater the difference is, the better, but that there is an optimal value. There are many kinds of oxidants for bleaching, such as mercuric chloride (HgCl2), ferric chloride (FeCl3), potassium ferricyanide [k3fe (CN) 6], ammonium dichromate [(NH4) cr2o7], and copper bromide (CuBr2), etc. See table 5-9 for the products and their refractive indices after the above oxidants interact with metallic silver

table 5-9 silver salt after bleaching and its refractive index

silver salt refractive index generated by oxidant hgcl2









k3fe (CN) 6

(NH4) Cr4 2o7

ag4fe (CN) 6




in order to obtain a high-quality hologram, Some technical problems should be paid attention to when using and processing holographic dry plate. Holographic dry plate often produces stress in the drying process after coating latex. In order to eliminate this stress, the dry plate can be placed in a container with a certain humidity overnight before use. The surface flatness of the sheet based glass of the holographic dry plate is generally not good. During recording, due to the influence of the reflected light on the back, a kind of wood grain interference fringe is produced. The way to eliminate this kind of fringes is to cover a piece of neutral glass on the back of the dry plate base glass during exposure, add a refractive solution with refractive index matching the glass in the middle, and remove it when developing. If the reflection hologram is recorded, the dry plate can be placed in a liquid tank with refractive index matching. During exposure, if the light beam is obliquely incident on the dry plate at a large angle, and the light enters the glass from the side, multiple reflections will occur between the two surfaces, forming small holograms, which will affect the quality of the hologram; The elimination method is to use a light bar to limit the beam, so that its cross section is smaller than the holographic dry plate, or block the edge of the dry plate

2. Dichromate gelatin

dichromate gelatin is a good phase recording medium, and its resolution can reach 5000 lines/mm. Dichromated gelatin has the advantages of high diffraction efficiency and high signal-to-noise ratio. The disadvantage is low sensitivity, only (3 ~ 7) × J/cm2, about one millionth of Kodak 649f. Dichromate gelatin is sensitive to temperature and bacteria, and is sensitive to the temperature and humidity of the environment. It is afraid of humidity, unstable image, and easy to eliminate image, so it should be stored in closed containers. Chromium is poisonous to people

the refractive index of the hardened gelatin of dichromate gelatin can reach 0.08, which is used to record its refractive index when the initiative aims to improve transparency and governance volume holography η- H curve. Gelatin has two kinds of hardening and non hardening. Unhardened gelatin can be used to make relief holograms; Hardened gelatin is suitable for making refractive phase holograms. Because the part of this material irradiated by light is not black and does not absorb light during hologram reconstruction, its diffraction rate is high, and the diffraction efficiency of the volume hologram made with it can reach 90%

dichromate gelatin and other dichromate colloids are the earliest photographic materials. Gelatin is extracted from the natural protein bone glue of animals. After gelatin is mixed with a small amount of dichromate for sensitization, it is irradiated with short wavelength blue violet light, and the exposed place becomes hard, which is less soluble in water than the unexposed place. After washing, it can be changed into thickness, density or both. Therefore, it can be used as holographic material to produce a phase hologram modulated by thickness or refractive index. The wavelength of dichromate gelatin absorption region is about 540nm, so it can only be used for 514.5nm and 488nm of argon ion laser or 412nm of helium cadmium laser. Because the dichromate gelatin dry plate, unlike the silver halide dry plate, has tiny silver particles, its resolution is very high, up to 2000 ~ 5000 lines/mm

if the 632.8nm wavelength of He Ne laser is used, other sensitizing dyes must be added to gelatin. The commonly used dyes are methylene blue and methylene green

(to be continued)

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