Background: the massive potential for holography

In the entrance hall to the Centre for Modern Optics at the North East Wales Institute of Higher Education, there are some unusual and beautiful objects: one a magnificent vase, another a highly-decorated Russian egg.  Many visitors to the Centre glance at these objects and then walk past; until it is pointed out to them that they are holograms, whereupon they stare in disbelief!  These objects are excellent and rare examples of full-colour holograms, and are so realistic that they are difficult to distinguish from the real object. 

Currently, holograms are generally regarded as a sort of novelty item by the general EU public.  Most are familiar with the mass-produced embossed holograms used as a security device on credit cards or bank notes (known as optically variable devices or OVDs).  Some are familiar with the wonderful high-quality artistic holograms: there are several galleries of holographic art throughout Europe and other parts of the world, e.g. Museum of Holography (Chicago, USA), MIT Museum of Holography (Cambridge, USA) etc.  Few are aware of the numerous more practical uses of holograms, for example holographic optical elements (HOEs) with a wide range of applications from head-up displays (HUDs) in state-of-the-art military aircraft through the applications of HOEs in telecommunications devices to the rotating HOE used to scan barcodes at supermarket checkouts. 

Whilst it is true that holography has found many useful and remarkable applications in many areas of technology, it is also true that it is a technology which has only fulfilled a fraction of its potential.  Holography is the most perfect imaging technique known to science.  In theory, using holography, it is possible to record and replay an image of a scene which is identical in every way to the original scene.  The potential for such a perfect imaging technique is truly phenomenal.  Of course there are the wonderful artistic and portraiture applications, but also there is the possibility of reproducing perfect images of artefacts in museums and art galleries.  The economic implications of holographic advertising are most exciting.  In addition there are the many applications for white light HOEs: full-colour head-up displays in cars and aircraft, energy-saving elements in liquid crystal displays (LCDs), medical 3D imaging applications, “light pipes” for lighting in buildings etc.  Furthermore there are methods of creating almost irreproducible images for security applications which could be used on every passport, credit card and bank note in Europe. 

However, there is currently a barrier to all this progress: there is no commercially available medium capable of recording full-colour holograms. 

The science: overview of holography

A hologram is a recording of the interference pattern formed by the scattered “object” wave from a scene and a “reference” wave in a high-resolution light-sensitive material.  When the hologram is illuminated with a correctly chosen source, an exact copy of the object wave is reproduced with the same spatial frequencies, amplitudes and phases of the original object wave.  If the recording and replay are in full colour, and the light-sensitive medium is capable of recording the interference pattern, then the reproduced three dimensional image will also be in full colour. The observer perceives a perfect image, almost indistinguishable from the original scene. 

In practice the quality of the reproduced field depends on a range of variables: the stability of the recording arrangement, the coherence properties of the recording light, the power ratio of the object and reference, the geometry of the exposure, the recording material used, the processing of the exposed plate, the replay conditions etc.  Most of these variables are controllable and may be adjusted by an experienced holographer in the same way a photographer adjusts his exposure time, lighting, processing etc.   

However, the whole process relies on having a suitable recording material.  In order to accurately reproduce the image wave, the recording material must be able to resolve the highest spatial frequencies of the interference pattern.  In order to record blue laser light for a typical HOE, this requires the material to resolve features with a size of around 10nm or less.  The material also needs to have high sensitivity and a panchromatic response, as well as a suitable dynamic range to support the several superimposed holograms required for colour holography. 

The problem: no suitable recording medium

Currently there are no materials on the market that may be used for recording high quality full-colour holograms or holographic optical elements (HOEs).  Holography as a technology is held back by this lack of a suitable recording medium and this project aims to solve this problem by developing a new nanoparticle (5-10nm), low light-scattering panchromatic silver halide emulsion which could be used for optical information storage and ultra-high quality imaging recording techniques, including full-colour holograms and HOEs. 

Although it has been possible to manufacture monochromatic holograms since the 1960s, there has been surprisingly little improvement in the recording materials when compared with other areas of technology.  The possibilities for recording high-quality full-colour holograms are extremely limited.  Although there are techniques to give the appearance of colour from a monochromatic hologram, these colours are not the original colours of the recorded object.  One of the problems is that the recording material needs to have a very high dynamic range (i.e. a very large change in refractive index when exposed and developed).  Even if there is sufficient dynamic range, when p holograms are recorded in the same material, the diffraction efficiency of each hologram is reduced by approximately p².  Another important problem is that due to shrinkage in the photographic emulsion layer during processing, the colours reproduced on replay of the hologram are completely different to those of the original object  (e.g. a hologram recorded with red light typically replays a green image).  Furthermore even the best current commercially available emulsions do not give good recordings of the blue part of the spectrum, because the grain sizes are too large and lead to scattering of light. 

To obtain the necessary high-quality colour images, a nanoparticle recording material with sufficient light sensitivity is required.  There are a number of candidate materials which are currently used to record high resolution holograms including: silver halide emulsions, dichromated gelatin (DCG) emulsions, photopolymer materials, photoresist materials, thermoplastic materials, bacteriorhodopsin and photorefractive crystals.  The best candidate material for this task is photographic silver halide emulsion (see below for descriptions of all these materials).   

However, to date there are no commercial materials which can record full-colour holograms.  The Russian company SLAVICH located outside Moscow make the best material currently available, but this emulsion does not have the required grain size of (5-10nm) for scatter-free recording in the deep blue part of the spectrum and there are problems with quality control and availability.  Other photographic material producers (e.g. Agfa, Ilford, Kodak et al.) have not been able to manufacture suitable material for colour holograms.  There are several reasons why these major companies have not made a suitable material, the main one being that they did not invest the effort into research because there was no market.  This was, of course, a “chicken and egg” problem, since until there was a suitable material there could be no market.  The chief technical difficulty is the strong scattering from the relatively large silver halide grains embedded within the emulsion.  In full colour holographic recordings, these grains strongly scatter the shorter wavelengths of the illuminating light (i.e. the blue light), resulting in a fuzzy low contrast image.

Objectives of SilverCross

The development of a suitable material for recording high resolution full colour holograms would have remarkable benefits for Europe.  Silver halide is the most suitable candidate material for this purpose.  SilverCross aims to develop a process and prototype apparatus suitable for mass-manufacture of super-resolution silver halide emulsion-coated plates capable of making full-colour holographic recordings.  The research approach has been to break this problem into three parts:

1. Grain preparation: A process for obtaining a silver halide emulsion with uniform nanoparticle size (5-10nm) will be devised.

2. Emulsion sensitisation: This nanoparticle emulsion will be sensitised to provide a material that is both panchromatic and isochromatic without a serious reciprocity failure problem and a sensitivity of <2.0mJ cm^-2.

3. Manufacture: A mass-production technique for uniformly coating large glass plates or film with this emulsion will be devised.

Main innovations

This project will produce a process suitable for mass-manufacture of ultra-high resolution silver halide emulsion-coated plates capable of making full-colour holographic recordings.  In order to achieve this aim, firstly, the necessary silver halide grain size must be attained; secondly, the photo-sensitivity and dynamic range of the material must be increased and the reciprocity failure addressed. Finally the manufacturing processes must be scaled up from the laboratory to mass manufacture.  This suggests that there are three separate areas of innovation:

1. Grain preparation: The emulsion work itself will be based on experience gained using the Russian ultrafine-grain emulsion technology originally developed by Kirillov et al. [[i], [ii]] and other techniques developed at CLOSPI and NEWI.  However these are currently empirical techniques which have not been developed sufficiently to give reliable results, and the particle sizes are not sufficiently uniform.  This project will identify a standard procedure through meticulous targeted iterative experiment to manufacture emulsions with a uniform grain size of 5-10nm. 

2. Emulsion sensitisation: In order to obtain an emulsion that is both panchromatic and isochromatic without a serious reciprocity failure problem, it is necessary to sensitise the emulsion.  This is achieved using synthetic dyes for spectral sensitisation and other novel techniques to increase the general sensitivity of tiny silver halide particles [e.g. [iii]].  Once the emulsion with the required grain size has been achieved, this unique sensitisation process will be devised to achieve a panchromatic sensitivity of <2.0mJ cm^-2. 

3. Manufacture: Techniques for attaining plates or film uniformly coated with these emulsions have to date only been achieved on a small scale and with poor quality control.  Increasing the output to the levels of volume manufacture will take a significant degree of invention. 

If these materials can be made on a large scale, it will represent a major advancement in the state of the art, and it will open up whole new markets for holography in the areas of art, cultural heritage, security, displays and many other important applications.