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A better way to make holograms

A better way to make holograms

Is by copying butterflies’ wings

HOLOGRAPHY is a useful technology, but somehow faintly disappointing. The fantasy is of a “Star Trek” style holodeck, or even the less ambitious idea of three-dimensional television pictures. The reality, for the man or woman in the street, is smudgy images that act as security features on credit cards, passports and an increasing number of banknotes.

Holography does have many uses beyond this. These include projecting 3D art displays in museums, enabling measurements to be made with great precision using a technique called holographic interferometry, and accurately assessing the three dimensions of packages for shipping companies. But the difference between the high-quality holograms required for those applications and the quotidian credit-card variety is that a laser and special equipment are needed to project them. Indeed, if the hologram is in colour, three lasers are needed, one for each primary: red, green and blue. The result is not always persuasive. Getting the primary holograms to overlap perfectly is hard. And to see the picture usually requires a darkened room.

All this led Rajesh Menon, an engineer at the University of Utah, to start eyeing up butterflies—notably the bright blue morphos found in Central and South America. The striking colour of a morpho’s wings (see picture) is the product not of pigment, but of the structure and arrangement of the scales on those wings. These scales refract light, splitting it into its component wavelengths, and also diffract it, causing those various wavelengths to interfere with one another. As a result, blue wavelengths are intensified and reflected back to the onlooker while those of other colours either cancel each other out or are scattered, and thus minimised. Moreover, unlike today’s holograms, the colour and appearance of a morpho’s wings remain the same, regardless of the angle they are viewed from.

Dr Menon and his team thought mimicking the way morphos refract and diffract light might thus let them create more realistic and usable holograms than today’s.

In a paper just published in Scientific Reports, they describe how they have done this.

A conventional hologram is made by splitting a laser beam in two, scanning one of the half beams over the object to be holographed, recombining the half beams and then capturing an image created by the recombined beams on a photographic film. The result is an interference pattern imprinted on the film by the interaction between the out-of-kilter half beams. Shine light (ideally of the same frequency as the original laser) on this pattern and the process is, in essence, reversed. That produces a 3D representation of the original object.

Dr Menon’s approach differs from this established method in several ways. First, it dispenses with the laser. Second, the film on which the hologram is captured is not a smooth one but, rather, a sheet of transparent plastic with microscopic bumps and grooves in it. Third, the pattern of those bumps and grooves is created not photographically but as the product of calculations by a computer.

Instead of the laser, Dr Menon starts with multiple images, taken from different directions, of the object to be holographed. These can come either from a special, stereoscopic camera or, more prosaically, from a single camera moved around to different vantage points.

These images are then fed into a computer. Here, a special algorithm calculates how to shape the topography of the plastic sheet so that it will manipulate the light eventually used to illuminate that sheet in a way which creates the desired 3D image. In essence, the sheet’s bumps and grooves act like the scales of a morpho’s wings, refracting and diffracting the incident light to produce the desired effect.

Once the computer has calculated the topography needed to do this, that topography (or, rather, its inverse) is inscribed onto a master version using photolithography—a technique also employed to make computer chips. This master may then be used to stamp multiple copies of the hologram, in a similar fashion to that employed to make vinyl records.

Crucially, the result—having been created using ordinary light rather than special laser beams—does not require lasers to recreate the image. A beam of white light will do the trick. Even a torch will work. Using one, Dr Menon can generate holograms with a full spectrum of colours and with a richness which he estimates is up to ten times that of today’s most sophisticated holograms. The new holograms may also be viewed from all angles without distortion. And they cost a fraction of those produced by existing techniques.

For now, Dr Menon and his colleagues are focusing on the kind of holograms used as security features, although they have also created holographic images of 3D objects in free space. Eventually, they hope to make holographic movies, using devices called phase spatial light modulators controlled directly by the output from the hologram-generating algorithm. Such modulators deploy liquid crystals instead of bumps on a surface to manipulate light.

If that idea can be made to work, then fantasies such as holographic television might indeed be brought into being. A more immediate market, though, is replacing existing security holograms with ones that are clearer, harder to forge and viewable from any angle. Perhaps, if Dr Menon has his way, the portraits of heads of state and other worthies on banknotes will soon pop up to greet the user as they are pulled from his wallet.

Water Technologies Canada Inc.

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