12 Apr 2016
A tamper-proof hologram is being developed that could replace serial numbers and barcodes, reducing the trade in counterfeit goods.
A team of scientists led by Professor Duncan Hand at Heriot-Watt University in Edinburgh, Scotland, are using an ultra-violet nanosecond-pulsed laser to sculpt unique holograms with micro-sized features directly onto the surface of metals, including alloys such as steel and inconel, or glass, making them tamperproof, they claim.
Individual laser pulses delivered at a rate of a few kilohertz melt the surface in a precise, localised way to produce optically-smooth impressions on the metal. By manipulating the laser beam to create specific patterns, holographic structures are produced that can act as security markings.
Dr Krystian Wlodarczyk, a researcher working on the project, said, “The holograms are visible to the naked eye and appear as smooth, shiny textures. They’re robust to local damage and readable by using a collimated beam from a low-cost, commercially-available laser pointer, so border agencies or consumers won’t need expensive technology to verify an item’s authenticity. Actually, the holograms can also be read even using a ’flashlight’ app from a smart phone.”
“We’ve established that we can create the holograms on a variety of metals. We’re now investigating how to make them even smaller and more efficient and whether we can apply them to other materials. Recently, for instance, we have extended the process for use of such holograms on glass.”
There are already two manufacturing partners, including a jewellery manufacturer, involved in refining and adapting the process to protect their intellectual property. Dr Wlodarczyk told optic.org, "The key feature of our holograms is that they need to be optically smooth, which means their pixels need to no larger than 1µm. This project commenced when we were asked to develop holographic sinusoidal gratings on metal substrates for various commercial clients."
The holograms produced by Dr Wlodarczyk can generate diffractive images containing alphanumeric characters or logos: the structure of the hologram is generated by either melting or a combination of melting and evaporation, with sub-micron depth control of the hologram’s individual pixels.
The shape and geometry of the hologram pixels are very important because they affect the optical performance of the holographic structure. To obtain the maximum efficiency (contrast) of the diffractive image produced by the hologram, the pixels must have a certain depth and ideally a flat ‘optically-smooth’ base.UK backs laser production with new £5.6M development center
Dr Krystian Wlodarczyk’s research was funded by the ESPRC and initial findings have been published in the Journal of Material Processing Technology. The journal article concludes:
”A novel laser-based approach for unique security marking of the high value products and components made of metals [shows] that UV nanosecond laser pulses are able to locally and controllably melt the metal surface in order to generate optically smooth surface features. By arranging the laser-induced surface deformations in an appropriate pattern, it is possible to generate holographic structures that can act as security markings.
Cutting hologram build time
”Although the time required to produce the holograms was very long (approximately 2h 30 min), the processing time can be significantly reduced (to less than 20s per hologram) by introducing a laser beam galvo-scanning system into the laser marking setup. Further work should be focused on the generation of the surface features with a square contour and a flat bottom.
”Our simulations] show that the holograms with “square pixels” can produce diffractive images with a high image contrast and a diffraction efficiency of more than 25%. Moreover, such holograms can produce the diffraction images without the presence of the 0th order beam and even the “twin” image in the case of the multi-level holographic structures. In order to generate the surface deformations with a square contour and a flat bottom, we plan to test laser mask projection techniques.”