Introduction to color holographic recording materials

Holographic recording materials can be divided into inorganic and organic categories. There are roughly silver halide latex, dichromate gelatin, photosensitive polymer, photoconductive thermoplastic, photoinduced anisotropic material, photorefractive material, etc. Among them, the most widely used are silver halide emulsion and dichromate gelatin Photosensitive material. According to the types of changes in the material properties after the recording material absorbs light, it can be roughly divided into four types: transmission type, refraction type, relief type and hybrid type. In holographic printing, transmissive silver halide, refraction dichromate gelatin, and relief photoresist are mainly used. They are mixed refraction and relief silver halide materials used in bleaching treatment.

The choice of recording materials should be considered from two aspects. One is the recording wavelength, there are two cases of single wavelength recording and multi-wavelength recording. The other is the type of hologram, with the difference between transmission hologram and reflection hologram. The former needs to consider the sensitive wavelength of the recording material, and the latter considers the thickness of the recording material. Single-wavelength recording transmission hologram, multi-use red sensitive recording materials, such as domestic Tianjin holographic I and II, Agfa8E75, Ilford SP696T, SP673 and Russian PRG-03 and other silver salt dry plates, and thermally conductive plastic films, etc. ; Lan Min uses Tianjin holographic III type, 8E56 silver salt for plate and photoresist, etc.

Single-wavelength recording reflection holograms are mostly used DCG version (blue sensitive), photopolymer (red sensitive or blue sensitive) and 8E56HD, 8E75HD silver salt dry plate, etc. Multi-wavelength recording transmission holograms are commonly used in 649F, 8E56 silver salt dry plate. Reflective holograms are commonly used in 8E56HD and 8E75HD recording sandwich holograms and Russian PRG-03 full-color silver salt dry version; two-color DCG version; emerging photopolymers include DuPont HRF series and Ommi Dex holographic film series. In order to facilitate the detailed introduction of the characteristics of commonly used holographic recording materials, the following briefly introduces several main characteristic parameters that reflect the performance of recording materials.

Characteristic parameters of holographic recording materials

1, sensitivity

Sensitivity is the sensitivity of the recording material's response after receiving light.

Because the recording process is a photochemical effect, the energy of the photon is related to the wavelength. The longer the wavelength, the smaller the energy of the photon, and each recording material has a red limit for the wavelength. Light with a wavelength greater than the red limit cannot play a role in the material. Photochemical reaction. In addition, each material has its own absorption band, and only wavelengths within the absorption band can play a role in photochemical reactions. This is why the sensitivity of the recording medium to the spectrum is different.

2. Spectral sensitivity

Because the recording process is a photochemical action process, the energy of the photon is related to the wavelength. The longer the wavelength, the smaller the energy of the photon. Generally, each recording medium has a red limit for the wavelength. Light with a wavelength greater than the red limit cannot play a photochemical role. In addition, each recording medium has its absorption band, and the wavelength in the absorption band can play a photochemical role, which is the spectral sensitivity. There must be an absorption band in this band. Figure 5-8 shows the characteristic curves of the domestic I-type and I-type holographic dry plates Kador 649F, Ilford and Agfa holographic dry plates.

3, diffraction efficiency

The diffraction efficiency is defined as the ratio of the luminous flux of hologram diffraction imaging to the total luminous flux of reproduced illumination light. The diffraction efficiency is not only related to the nature of the recording medium, but also to the type of hologram and the degree of modulation of the fringes. Generally speaking, the diffraction efficiency of the phase type recording material is higher than that of the amplitude type. The degree of fringe modulation is related to the ratio of the parameter and object beams. For the same recording material, the diffraction efficiency is also related to the spatial frequency of the hologram. The table lists the theoretical diffraction efficiency of different types of ideal holograms.

Table Theoretical value of the diffraction efficiency of an ideal recording medium

4, resolution

The resolution of the recording material is usually expressed in the number of line pairs that can be recognized in a length of 1 mm, that is, the maximum spatial frequency of the spatial modulation of the exposure intensity that can be recorded by the recording material, and the unit is 1 / mm. The resolution of ordinary photographic film is only 200 lines / mm. When recording holograms, the requirements for the resolution of the recording material are related to the angle between the parameter and object beams.

5, characteristic curve

The characteristic curve represents the relationship between some parameters related to the recording material. The curve for ordinary photography is optical density, also known as black density, which is equal to the logarithm of the reciprocal of the transmittance, is the amplitude transmission coefficient, and H is the exposure of the recording material . In holography, the more appropriate H curve is often used. The curve of an ideal amplitude recording material should be linear.

For the phase recording material, the phase change in the transmission function changes with the exposure amount. The ideal phase recording material-H curve should be linear. The actual phase recording material has a linear area within the appropriate exposure range.

In the holographic plate making, the diffraction efficiency of the hologram is a very important parameter. The diffraction efficiency is related to the recording material, the degree of modulation of the exposure intensity, and the size of the exposure. In order to improve the quality of the holographic original plate and optimize the plate-making process, the relationship between the diffraction efficiency of the recording material under different exposure conditions and the modulation degree V of the exposure intensity, that is, the curve with H0 as the parameter; The relationship of exposure, that is, the curve where V is a parameter.

6, noise

The noise of the recording material refers to the scattered light caused by the unevenness of the particle size of the material itself and the roughness of the surface. The noise will also increase after the exposure and development process, such as the noise generated by nonlinear recording, the interference fringes or speckles generated between the reference light and the scattered light of the object light wave, etc. For the silver salt dry plate, the development speed and bleaching process will increase the particle size and generate noise.

Commonly used holographic recording materials

The ideal recording material should have high sensitivity, high resolution, low noise, and a linear curve for the wavelength used for exposure. From the perspective of production, it should also have the advantages of being able to be reused and cheap. In fact, it is impossible for a certain type of recording material to satisfy these conditions at the same time.

1, silver halide latex

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, but the diffraction efficiency and signal-to-noise ratio are low. Silver halide latex film is divided into film (flexible film) and dry plate (hard film). Its main structure is the photosensitive layer and the base. The photosensitive layer is also called the latex layer. It is made of ultrafine silver halide (the main components are AgBr and AgCl) The grains are suspended in gelatin, plus a certain sensitizer made of latex, the thickness is generally 0.01 ~ 5μm, the latex is attached to the base, it is customary to call the base a glass plate called holographic dry Version, and the film base is acetate and polyester film and other films are called holographic film. The particle size of silver halide latex used for holography is required to be 0.03 ~ 0.09μm.

Since the smaller the particle size, the lower the sensitivity, the exposure of the holographic dry plate is much larger than that of the ordinary photo dry plate. The standard developer for holographic dry plates is D19, which can achieve higher values. The size of the silver particles after development is 0.1 to 0.2 μm. The developer of fine particles is D76, and the silver particles developed are filamentous. The finer the silver particles, 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 developing solution. The dry plate is brownish red, the particle size of silver is 0.005 to 0.025 μm, and the diffraction efficiency of the hologram can reach 40%. Another method to improve the diffraction efficiency is to develop with D76 and fix with F5, then bleach with R10 (NaCl), and then irradiate with strong light, so that the product of the exposed part of the hologram is photolytic silver with a particle size of 0. 01μm. Holographic diffraction can also be improved.

The holographic dry plate is subjected to exposure and development to obtain an amplitude hologram. If it is bleached, it becomes a phase hologram. Bleaching is the reduction of metallic silver to a transparent silver salt with an oxidizing agent. As a result, the refractive index of the gelatin on the exposed part of the hologram is different. The improvement of diffraction efficiency is related to the difference between the above two refractive indexes, but it is not that the larger the difference, the better, but there is an optimal value. There are many kinds of oxidants used for bleaching, commonly used mercury chloride (HgCl2), ferric chloride (FeCl3), potassium ferricyanide [K3Fe (CN) 6], ammonium dichromate [(NH4) Cr2O7], and bromination Copper (CuBr2), etc. The product and refractive index after the above oxidant interacts with metallic silver are shown in the table.

Surface bleached silver salt and its refractive index

Oxidant

Silver salt generated

Refractive index

Oxidant

Silver salt generated

Refractive index

HgCl2

FeCl3

CuBr2

AgCl

AgCl

AgBr

2.07

2.07

2.25

K3Fe (CN) 6

(NH4) Cr2O7

Ag4Fe (CN) 6

AgCl

1.56

2.07

In order to obtain a high-quality hologram, we must pay attention to some technical issues when using and processing holographic dry plates. Holographic dry plates tend to generate stress during the drying process after being coated with latex. To eliminate this stress, the dry plates can be placed in a container with a certain humidity overnight before use. The flatness of the surface of the glass substrate of the holographic dry plate is generally not good. During the recording, due to the influence of the reflected light from the back, a wood-like interference fringe is generated. The way to eliminate this streak is to cover the back of the dry plate base glass with a piece of neutral glass during exposure, add a refracting liquid whose refractive index matches the glass, and remove it when developing. If recording a reflection hologram, the dry plate can be placed in a liquid tank with a matching refractive index. During exposure, if the light beam is obliquely incident on the dry plate at a large angle, the light enters the glass from the side, and multiple reflections will occur between the two surfaces, forming a series of small holograms. The way is to limit the beam with a light barrier, making its cross section smaller than the holographic dry plate, or blocking the edge of the dry plate.

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2. Dichromate gelatin

Dichromate gelatin is a good phase-type recording medium, and its resolution can reach 5000 lines / mm. Dichromate gelatin has the advantages of high diffraction efficiency and high signal-to-noise ratio. The disadvantage is that the sensitivity is low, only (3 ~ 7) × 10-3J / cm2, which is about one millionth of Kodak's 649F. Dichromate gelatin is sensitive to temperature and bacterial reaction, and is sensitive to the temperature and humidity of the environment. It is afraid of humidity, the image is unstable, and it is easy to eliminate the image, so it should be stored in a closed container. Chromium is poisonous to people.

The refractive index of hardened gelatin of dichromate gelatin can reach 0.08, which is used to record the η-H curve of volume hologram. There are two types of gelatin, hardened and hardened. Unhardened gelatin can make relief hologram; hardened gelatin is suitable for making refraction phase hologram. Since the part of this material irradiated by light does not become black, and does not absorb light when the hologram is reproduced, 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 used photographic materials. Gelatin is extracted from the animal's natural protein bone glue. After a small amount of dichromate for sensitization is added to gelatin, it is irradiated with short-wavelength blue-violet light. The exposed area becomes hard, and the unexposed area is difficult to dissolve in water. There are changes in both, so it can be used as a holographic material to produce a phase-type hologram modulated by thickness or refractive index. The wavelength of the dichromate gelatin absorption region is about 540nm, so it can only be used for the 514nm and 488nm of the argon ion laser, or the 412nm wavelength of the 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 wavelength of 632.8nm of He-Ne laser is used, other sensitizing dyes have to be added to the gelatin. The commonly used dyes are methylene blue and methylene green.

3. Photoresist

Photoresist is a photosensitive organic recording material that is very suitable for recording thin relief phase holograms. This material is exposed and developed to form a relief. It has two types, positive and negative.

Positive-type photoresist, where the exposed area absorbs light to produce organic acids that break down the cross-linking into soluble organic materials, which are dissolved by development; negative-type photoresist, where exposed, forms cross-links between molecules due to absorbed light The organic material hardens and becomes insoluble. After development, only the unexposed parts are dissolved away, leaving a hologram with uneven surface. As a holographic recording material, the photoresist can be made into a micron-level film by centrifugal spin-off method, and the shrinkage and deformation are very small, and the diffraction efficiency is high. 3000 lines / mm. However, there are two shortcomings in using it as a relief holographic material. One is the low sensitivity, and the other is the narrow spectral response range. The photoresist is sensitive to blue light, and it is usually exposed with a 441.6 nm wavelength of a He-Cd laser. Photoresist is used to record holograms, and it must be well bonded to the substrate. For this reason, only positive photoresist can be used.

The photoresist can produce a strong acid under the action of light, and the main resin can undergo photolysis and photocrosslinking reaction by heating. Since the strong acid generated can be recycled, high sensitivity can be obtained. The currently widely used photo-acid generators are Jin Weng salts, such as iodide Weng salt, sulfur gold Weng salt, iron gold Weng salt, carbon gold Weng salt and so on. For example, the photosensitive system composed of cresol resin-tert-butyl naphthalate-triphenylthiohexafluorophosphoryl aurantium salt is a positive resist with a sensitivity of several mJ / cm2.

Foreign countries generally use Shipley's MICROPOSIT-1350 (AZ-1350) positive photoresist. There are BP-212 positive photoresists produced by Beijing No. 2 Chemical Factory in China.

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4. Light guide thermoplastic

Photoconductive thermoplastic is a relief-type phase recording material, and its structure is shown in Figure 1. The manufacturing method of the photoconductive thermoplastic dry plate is to coat a transparent conductive film, such as tin oxide, on the glass substrate with a vacuum coating method, and then use a chemical method to coat a photoconductor with a thickness of 2 to 3 μm and a thickness of about 1μm thermoplastic. The working principle of the light guide thermoplastic is shown in Figure 1.

In use, the light-guiding thermoplastic is first sensitized in a dark room. The so-called sensitization is to charge with a power grid with a high voltage of 4 to 8 kV, and establish a uniform potential difference between the thermoplastic and the transparent conductor to make the dry plate charged and photosensitive, where d is the total thickness of the thermoplastic and the photoconductor layer, ε is the corresponding dielectric constant, and σ is the surface charge density, as shown in Figure 1 (a). The second step is to expose the dry plate, and the exposed part of the photoconductor discharges, so that the potential V? Of the thermoplastic layer at the irradiation place drops to. The third step is to make the surface charge of the thermoplastic plastic uniformly distributed again, that is, recharge. Since the thickness d? Of the capacitor at the light exposure becomes smaller, the surface charge density will inevitably increase at the same potential, which is formed on the surface of the thermoplastic layer The latent image is shown in Figure 1 (c). The fourth step is development and fixing. The development process here is to first heat the thermoplastic plastic to about 60oC to soften it, and it will deform under the action of an electric field. The setting process is cooling to form a relief-type hologram, as shown in figure (d). Due to the thermal conductivity of thermoplastics, light-guiding thermoplastics can be used repeatedly after erasing. The method of erasing is to heat it to an appropriate temperature, and all the charges on the surface of the wafer disappear, so that the plastic is restored to its original flatness and cooled. As shown in (e).

Figure Structure and working principle of light guide thermoplastic dry plate

The advantages of the light guide thermoplastic as a holographic recording material are sensitivity to visible light and dry development. It is suitable for real-time observation, has a high diffraction rate, can be reused, and has a very small Bragg effect. The disadvantage is that the resolution is low, less than 2000 lines / mm.

5. Photopolymerized material

Photopolymers for holographic recording include four main types of basic components: photoinitiators, vinyl monomers, sensitizing materials, and carriers. There are multiple materials for each component. According to different requirements, different optimization combinations can be made. Photopolymerization is a photochemical process. Under the irradiation of light, active free radicals are generated in the polymerization system, and the polymerization effect is initiated so that small molecules or monomers in the system are combined into large molecules or polymers. Photopolymerizable materials can be used to make refractive or embossed phase holograms. The hologram itself has good geometric fidelity. Photopolymerization materials are used as holographic recording materials, and the mechanism of image formation can be shown in Figure 5-20. In the exposure stage, the photopolymer system generates free radicals under the action of laser energy of a specific wavelength and polymerizes to form large molecules. The distribution of the light intensity of the recording stripes at different positions forms the spatial distribution of the degree of polymerization of the recording material, which is then developed and fixed to form a hologram. Substances with different degrees of polymerization reflect the difference in refractive index in terms of optical properties.

Photosensitive polymer materials, which have the advantages of high sensitivity, high diffraction efficiency, wide spectral response range, easy processing, stable storage, etc., are the focus of current research and development. The photosensitive polymer material can be divided into photopolymerization type, photocrosslinking type and decomposition type from the reaction mechanism. They can be used to make relief-type or refraction-type phase holograms.

In addition to the several commonly used holographic recording materials introduced above, there are photochromic materials, photorefractive crystals, and ferroelectric materials. For details, please refer to the relevant works.

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