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'Rainbow' holograms can now be inkjet printed

25 November 2015

Vivid holographic images and text can now be produced by means of an ordinary inkjet printer, thanks to the work of scientists based in Saint Petersburg.

An example of an inkjet printed rainbow hologram (photo: ITMO University)

The new method, developed by a team of scientists from ITMO University in Saint Petersburg, is expected to significantly reduce the cost and time needed to create the so-called rainbow holograms, commonly used for security purposes - to protect valuable items, such as credit cards and paper currency, from piracy and falsification. The results of the study are published the journal, Advanced Functional Materials.

The team, led by Alexander Vinogradov, senior research associate at the International Laboratory of Solution Chemistry of Advanced Materials and Technologies (SCAMT) at ITMO University, developed colourless ink made of nanocrystalline titania, which can be loaded into an inkjet printer and then deposited on special micro-embossed paper, resulting in unique patterned images. The ink makes it possible to print custom holographic images on transparent film in a matter of minutes, instead of days as with the use of conventional methods.

"The conventional way of preparing a hologram is incredibly time-consuming and consists of several stages," says Aleksandr Yakovlev, first author of the study and researcher at the SCAMT laboratory. "First, one needs to create a master hologram, which is usually laser recorded on a thin layer of photosensitive polymer. The polymer is then dried and wash-out to get rid of unexposed parts,"  "The resulting stencil is then transferred to a metallic matrix, which eventually serves to emboss holographic micro-relief on the surface of a transparent polymer film."

According to the scientist, in total, the whole process of obtaining a holographic image may take up to several days. Even more, to prepare a master hologram, a set of very rigid requirements must be met - temperature control and vibration isolation among them. "Printing separate holographic images in a quick and effective manner is a challenge that, until now, has been unresolved," adds Aleksandr Yakovlev.

The new nanocrystalline ink makes it possible to cut the expenditures related to the production of rainbow holograms by several times. The ink is applied with a simple inkjet printer on a micro-embossed surface, which is afterwards covered by varnish. As a result, the holographic image is exclusively seen in those areas, where the protective ink was deposited.

"The peculiarity of our ink lies in its high refractive index in all visible range of light," says project supervisor Alexander Vinogradov, "The use of nanocrystalline ink forms a layer with high refractive index that helps preserve the rainbow holographic effect after the varnish or a polymer layer is applied on top."

The new technique opens the possibility of obtaining holograms of practically any size, makes the process easier and cheaper and, for the first time, leads to the creation of unique custom holograms in a matter of minutes.



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