Jeff Blyth
jeff@biotech.cam.ac.uk

About the author
After graduating in 1973 in Applied Chemistry he worked in a company with dichromated gelatin, unrelated to holography. In ’77, he was amazed to see a holographic pendant made using the very material he was researching. His life ‘changed for ever’. He subsequently worked on photopolymer materials for Ilford, which became the subject for an MPhil at Wolverhampton Polytechnic. Since ’91 he has been involved in ‘blue sky’ research at the Institute of Biotechnology in Cambridge, UK. Jeff is the recipient of the Royal Photographic Society’s Saxby award for 2003
(http://www.holography.co.uk/events/saxbyaward/jeffblyth/jeff.htm).

Until recently, I always thought that if a small object reflected laser light well enough, and was kept completely still during any holographic exposure, then you could always make a reflection hologram of it which would reproduce monochromatically the features of that object. Well it turns out I have been deluded for a long time…

My object in question is in the centre of the photo here. It is a well made glass “corner-cube”¹

Anyone who looks at this actual object will see a fascinating sight of a single eye looking back at them from the central section of the cube corner (CC). I also find it interesting to ask people which eye they think it is. Some decide it is their right, others their left, and only a few say it is both eyes equally. It does nicely show that most of us seem to have a dominant eye independently of whether we are right- or left-handed. The fact is that the eye you see is the eye that is looking at it so it can be both eyes simultaneously.

For an experiment I positioned a piece of pre-swollen BB640 holographic plate on the beaker and shot a single beam “in-line” hologram with a 633nm HeNe laser. Now you can see my camera lens in the photo of the CC but not in the hologram of the CC. Therein lies the heart of the problem – you just cannot reproduce in the hologram that same “eye effect” that follows you around the room from a true retro-reflector.

I had a serious use in mind for getting a holographic corner cube to act as a direction tolerant sensor but it was not to be or at least not entirely so. However, it strangely does do a quasi retro-reflection of a torch beam in 3 positions, 120^∘ apart, and each reflection is triangular and is uniformly orange whereas when the light source is almost directly overhead, the result is a green replay, as you can see in the photo. Mini corner cube arrays are familiar to all of us as bicycle reflectors on mud guards and pedals. Even those bright fluorescent sash bands that cyclists wear are seen under a microscope to be made of micro corner cube arrays. If you make a hologram of these arrays, just as with the single CC, this hologram can also replay at specific wide angles but again at a longer wavelength than that seen along the normal. I do not have an explanation for this effect – would anyone like to volunteer one please?

The retro-reflectivity of CCs is caused by a reflection from each wall of the CC in turn. But a flat hologram inspite of all its wonderful properties just cannot cause light to carry out three internal reflections. So that was holographic problem number 1. Now, what happens if you actually make a real holographic CC out of three triangular pieces of flat in-line reflection hologram? Well, I tried doing this using the corner of a common plastic (slide holder) box as a template. It was sprayed black and a side of the box was partially removed as you see in photo. It is then that the inherent holographic problem number 2 crops up…

As you move the light source away from the optimum position lighting up any bright reflection hologram, you get some wavelength shift just before it fails to replay at all. Saxby refers to this as the “Venetian blind effect”. (As you view through a venetian blind, the distance between the slats seem to shorten as you view them more obliquely. This is analogous to what is happening to the holographic fringes). So in my three-dimensional holographic CC you can hardly have three consecutive light bounces because the angle change will cause a wavelength shift each time which will simply not match the fringe spacing on each consecutive face.

I then did the simplest experiment in a dark room with a torch beam shining in my eye. I could just make out the light from my pupil only near the apex of the CC in the box. (The holographic grating material had been particularly efficient at acting as a normal diffracting mirror before it was cut into triangles). I could prove it was a holographic reflection, rather than just a three-fold specular reflection off the smooth surfaces, by allowing breath to condense on the system which was glued down with the emulsion outwards. The feeble spot momentarily changed from green to yellow and slightly brightened up, perhaps because it became more broadband, just enough to increase the tolerance angle for the light between each reflection to reconstruct, but it was still very feeble and restricted to about the first couple of millimetres from the
apex.

So I think I can now put the “narrow-band holographic retro-reflector” in that special box marked “holographic howlers”, wherein lie those wretched Hollywood perennials of giant three-dimensional real images of people reconstructed in mid-air without any holographic plate producing it in sight and also, dare I say it, full 3D colour holograms looming out of a TV, etc, etc.

Footnote

¹The £180 corner cube was purchased from Mr. Eric Frisk (opticalworks@btconnect.com) or cube corner, and it is sitting in a plastic beaker for support.

Jeff Blyth
jeff@biotech.cam.ac.uk

About the author
After graduating in 1973 in Applied Chemistry he worked in a company with dichromated gelatin, unrelated to holography. In ’77, he was amazed to see a holographic pendant made using the very material he was researching. His life ‘changed for ever’. He subsequently worked on photopolymer materials for Ilford, which became the subject for an MPhil at Wolverhampton Polytechnic. Since ’91 he has been involved in ‘blue sky’ research at the Institute of Biotechnology in Cambridge, UK. Jeff is the recipient of the Royal Photographic Society’s Saxby award for 2003
(http://www.holography.co.uk/events/saxbyaward/jeffblyth/jeff.htm).

It is always impressive to see how a green reflection hologram temporarily changes to red when you let breath-moisture condense on the gelatin surface. (This effect is due to the gelatin swelling and the inter-fringe distance in the gelatin film increasing so that it reflects red instead of green light.) Apart from the obvious use of this effect in measuring the relative humidity of gases or liquids [1] our team at the Institute of Biotechnology wanted to use this effect to make many other types of measurements.

It was at the end of August 1995 that I tried an experiment which was to cause a revolution in our Institute and beyond! For over three years I had been experimenting on making so called “Lippmann” silver halide emulsions using basically the traditional method of squirting, alternately, solutions of silver nitrate and potassium bromide into a hot solution of gelatin [2]. The precipitated grains of silver bromide (AgBr) are very much finer in Lippmann emulsions than in emulsions used for normal photography. To be able to record holographic fringes satisfactorily the photosensitive grains need to have diameters at least a factor of 10 smaller than the width of a hologram fringe (around 200 nm, i.e. half the laser wavelength divided by the refractive index of the gelatin layer.) Conventional fine-grained photographic emulsions have grain sizes around 1000 nm or more. The experiment which changed everything for us was to take a just a precoated and hardened gelatin layer and to try and produce those vitally small AgBr grains inside the layer just by using a silver ion diffusion process, while still achieving a sufficient degree of photosensitivity [3]. The moment of success came when I glimpsed a holographic image on a coated microscope slide of a polished penny.—One of those all-too-rare eureka moments!

a. The diagram of a microscope slide coated with a smart polymer layer, which has been impregnated with photosensitive silver bromide, being exposed in a trough of aqueous liquid. b. An actual transmission electron micrograph showing the very fine grain size achieved to make the fringe structure.

So the success had come by sequentially diffusing into the layer, a solution of silver nitrate and following it up by diffusing in a solution of lithium bromide. (I chose lithium rather than the common potassium salt because I thought I might need the extra leeway of its very large solubility in both water and alcohol [3].)

The reason this was such a breakthrough was that we could then use the principle on a whole range of pre-coated hydrophilic polymers which responded to various specific factors in the environment.

The vast majority of tests required in biotechnology are made in physiologically based aqueous liquids, so polymers need to be hydrophilic from this standpoint apart from the fact that the precipitation of AgBr is an ion exchange process which does not lend itself to taking place in a hydrophobic polymer. So an early question was just to see how far the scope of this diffusion process could be taken towards the hydrophobic end between the typical extremes of say polyacrylamide and polypropylene. Using as reactants the two most soluble salts of silver and bromine in organic liquids, namely silver perchlorate and lithium bromide, I tested out the sequential diffusion system on a few different polymers that were to hand. I found I could get reflection holograms in the cellulose based wrappers commonly on so many of our consumer products. But I did not manage to get any result on the backside of a piece of old Agfa holographic film. This meant that cellulose triacetate was too hydrophobic but with the help of a proportion of acetone I did manage to get an ‘OK’ result in cellulose diacetate polymer film.

I also managed to get a hologram in nylon (polyamide) but it was not brilliant. I must say at once for those who instantly jump as I did, to the idea of putting holographic gratings inside the fabric of say nylon tights, I had used nylon film and not mesh. I did, however, carry out a few experiments trying it with nylon mesh where the first requirement was to “index out” the mesh i.e. make it invisible by a suitable liquid to actually record the hologram. Although I managed to make the pure mesh almost disappear in an organic liquid (DMSO), the act of putting the AgBr salt inside the nylon mesh caused some unavoidable refractive index variation and therefore bad light scatter so I only got photographic images in silver, not holographic. A pulsed 532 nm YAG laser was used so movement did not cause the failure.

As for other common plastics, I found “Perspex” or PMMA (polymethylmethacrylate) to be too hydrophobic whereas poly(hydroxy)methylmethacrylate or poly-HEMA is excellent at forming bright holograms. A nice piece of work carried out in 1998 by my colleague Andrew Mayes (now at the University of East Anglia) used a small polyHEMA hologram to make very effective measurements of the alcohol content of various drinks [4]. I show the table of his results in table 1. Strong spirits are not included in the table because they shift the hologram replay wavelength into the infrared—beyond the range of the small reflection spectrometer available for us at the time.

The slide is sliced up and placed in a sample liquid in a cuvette. As the substance in the liquid alters then the replay colour of the hologram may change. In the 3 cuvettes shown, the same polymer material is contracting from left to right.

I remember we had a bit of trouble reclaiming the petty cash from the accounts department for this lot and had to convince them that it really was for a new scientific breakthrough. (Actually we only needed about 5 ml out of each bottle and as for the rest of each bottle… Well, we did not want to bother them with the finer details of the experiment!)

For several years we have been making polymers with certain chemical groups able to respond specifically to specific ions, enzymes and other analytes by swelling or contracting in saline solution, so when we make a reflection hologram of a mirror in these “smart” polymers we obtain a “smart hologram”mirror. (see: the OE magazine report – http://www.oemagazine.com/fromTheMagazine/mar03/diagnostics.html)

In a future article in the Holographer I hope to reveal more about the amazing possibilities opened up by this new holographic ball game.

References

[1]    J.  Blyth et al. 1996 Anal. Chem. 68 1089–94

[2]    H.  Thiry 1987 J. Phot. Sci. 35 150–4

[3]    J.  Blyth et al. 1999 Imaging Sci. J. 47 87–91

[4]    A.  Mayes et al. 1999 Anal. Chem. 71 3390–6