Three photonic laser techniques to transfer colour images with high resolution to metal surfaces: oxidation of metal and interferences, formation of nanoparticles and plasmon resonance, structuring of a periodic circuit and diffraction of light.
Scientists at the ITMO University of St. Petersburg have developed a method to produce various colour effects on a metal surface with a single laser. It is a question of controlling the processing parameters in order to control the correct amount of heat delivered and, consequently, the temperature obtained. The study consisted of three processes based on different physical effects to produce images in colour.
Colouring by oxidation
The laser is used to produce a thin layer of oxide, line by line, on a large part of the metal surface by significantly increasing the temperature. Because of the light interferences occurring in this layer, the colour appears. For example, a Ti surface can be coloured via the light interferences on a layer of TiO2 obtained either at 1875°C or at 1575°C depending on the desired effect.
Colouring by creating nanoparticles
With this working method, when the high intensity laser radiation causes the surface temperature to rise above the evaporation threshold, a metal vapour is created. Due to the rapid temperature drop caused by convection, condensation occurs in the form of ball-shaped nanoparticles. These particles oxidise and partly fall down on the surface.
When particles smaller than the length of the light wave are exposed to the radiation, the phenomenon of surface plasmon resonance causes the colouring, which is determined by the shape, size and dispersion of the particles created. This method is ideally suited for decorating precious metals because it does not require any pretreatment of the material.
For example, a silver surface can be coloured line by line by creating silver nanoparticles and inducing plasmon resonance in those particles.
Colouring by creating nanoparticles
With this working method, when the high intensity laser radiation causes the surface temperature to rise above the evaporation threshold, a metal vapour is created. Due to the rapid temperature drop caused by convection, condensation occurs in the form of ball-shaped nanoparticles. These particles oxidise and partly fall down on the surface. When particles smaller than the length of the light wave are exposed to the radiation, the phenomenon of surface plasmon resonance causes the colouring, which is determined by the shape, size and dispersion of the particles created. This method is ideally suited for decorating precious metals because it does not require any pretreatment of the material. For example, a silver surface can be coloured line by line by creating silver nanoparticles and inducing plasmon resonance in those particles.
Conclusion
The three effects described above provide high-resolution images. Non-contact, relatively environmentally friendly (no solvent...) and fast (1.4 cm²/minute).
The laser oxidation and forming process for a temporary circuit can be transferred to industrial applications. With regard to the colouring through the production of nanoparticles, the researchers are continuing their developments because the stability of the colours leaves much to be desired.
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