Although it's already possible to create colored images on metal surfaces using lasers, different types of lasers have to be used for different effects. Now, however, Russian scientists have developed a method of producing multiple effects using one commercially-available laser.
Led by Galina Odintsova, researchers at Saint Petersburg's ITMO University were able to change the "processing parameters" of the laser, allowing them to control how much it heats a metal surface over a wide temperature range. This, in turn, makes it possible to produce color images in three ways. None of the techniques are particularly time-consuming, and all can be used to create large or small images.
First of all, in a process known as laser oxidation, the laser is used to produce a thin oxide film over a wide area of the metal surface. That film subsequently appears as a given color, due to light interference occurring within it.
In the second process, the laser produces subwavelength nanoparticles on the metal. When these particles are hit by ambient light, an optical phenomenon called surface plasmon resonance causes the treated area to take on a certain color – that color is determined by selecting nanoparticles of different shapes or sizes. It is suggested that this process would be particularly well-suited to precious metals such as those used in jewellery (see photo below), as it requires no pretreatment of the material.
Finally, the third process utilizes the laser to partially melt a surface layer of the metal, creating an array of fine parallel grooves. Known as a periodic grating, this pattern scatters light in such a way that the surface takes on different colors depending on the angle from which it's being viewed. A possible application for this process is anti-counterfeiting tags, that could be printed directly onto metal products.
According to the scientists, the laser oxidation and periodic grating processes are already practical and reliable enough to be used both in large-scale production, and for individual projects. They are continuing to develop the nanoparticle process, in order to make it more stable for everyday use.
A paper on the research was recently published in the journal Optical Materials Express.
Source: The Optical Society
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