TECHNICAL
Requirements For Holographic Recording Materials
This page does not specifically addresses the requirement for holographic storage (or holographic data storage), but considers the material requirement for recording holograms in general.
| Holographic Films, Silver Halide Emulsion | Commercial Holographic Folms & Plates | Hardened Dichromated Gelatin (DCG) | Photoresists | Photothermoplastics | Photochromatics | Photodichroics | Photorefractive Materials (Electro-Optic Materials) | Health Hazards of Processing Chemicals |A hologram may be recorded in a medium as a variation of absorption or phase or both. The recording material must respond to incident light pattern causing a change in its optical properties. In the absorption or amplitude modulating materials, the absorption constant changes as a result of exposure, while the thickness or the refractive index changes due to the exposure in phase modulating materials.
In the phase modulating materials there is no absorption of light and all the incident light is available for image formation, while the incident light is significantly absorbed in an amplitude modulating medium. Thus a phase material can produce a higher efficiency than an amplitude material. Also in phase modulating media the amount of phase modulation can be made as large as desired.
The resolution capability of a recording material depends on its modulation transfer function. The nonlinear effects of the recording material are minimized for obtaining high quality holographic images.
No single material possesses all the requirements of a holographic material. A material is yet to be discovered which will have the high sensitivity of silver halides, high diffraction efficiency and index modulation capability of DCG and photopolymers, recyclability of photorefractive crystals, and useful at all laser wavelengths.
Different Hologram Recording Materials
Silver halide emulsion
The silver halide materials have been the most popular choice of the holographers for obvious reasons of high exposure sensitivity over a wide range of spectral regions and high resolving power. These materials are suitable for transmission as well as reflection holograms, both of amplitude and phase type. A large number of developers, bleaches and processes have been reported for silver halide materials. Most of the work has been on improving the diffraction efficiency and SNR, particularly for reflection holograms.Dichromated gelatin material
The recording sensitivity of DCG has been extended to red wavelength making it possible to record multicolour reflection holograms. This material is very difficult to handle and the recorded holograms are sensitive to environmental conditions, yet this is a very good material suitable for very high efficiency and low noise holograms. Photopolymers are expected to replace DCG as these are also capable of producing large index modulation and high diffraction efficiencies and are free from the disadvantages of DCG. Photopolymers do not require lengthy controlled processing techniques and can be processed in situ. The photopolymer holograms are insensitive to environmental changes.Photoresist material
Photoresists are suitable for producing surface relief holograms for making masters needed for replication by embossing techniques. This material is most sensitive to ultravioletlue light only. Efforts have recently been made to make photoresist material sensitive at red wavelength.Photorefractive crystals
Photorefractive crystals are very promising materials for real-time holography. They can be recycled. Photothermoplastics can also be recycled several hundred times and are most suitable for holographic interferometry.Copyright © 2005, www.hololight.net, All Rights Reserved.