13 Feb 2008
A femtosecond laser process can create a variety of colours on a metal surface, and could lead to control of material's optical properties from UV to terahertz.
A team at the University of Rochester has discovered that irradiating aluminium with femtosecond pulses produces surface structuring at nano-, micro- and submillimeter scales that affect the reflectance properties of the metal, and so change the colour. "I had wanted to see what would happen to a metal under different radiation conditions from high-intensity ultrashort pulses," Chunlei Guo told optics.org. "As the research progressed, I realized that we could actually enhance the absorbance properties permanently." (Appl Phys Lett 92 041914.)
Guo's team used a femtosecond Ti:sapphire laser that generated 65 fs pulses at 800 nm. Irradiating polished Al at an intensity of 0.16 J/cm2 and a repetition rate of 100 Hz resulted in the metal becoming golden in colour. "The reflectance of the Al after treatment drops over the entire measured wavelength range, but is more pronounced at shorter wavelengths," explained Guo. "The greater absorption at blue and green wavelengths leads to a golden colour in the aluminium, which is visible at various viewing angles." Since the process is a surface alteration and not a coating, the new colour should not fade or peel away.
Modifying the laser parameters produced other colourful results. "We produced black aluminium, where the reflectance approaches zero over the entire wavelength range, and an Al sample having two different shades of grey," said Guo. Applying the same principles to other metals resulted in coloured gold, platinum and titanium as well.
"This technique should be quite general in terms of the types of metals we can colourize," Guo commented. "The intrinsic electronic, optical and transport properties of metals can affect the process, but I believe we should be able to tailor our experimental parameters to compensate for these differences."
Developing the idea further, Guo also produced a sample of aluminium exhibiting different colours at different viewing angles. In this case the laser produced a unique type of laser-induced periodic surface structure (LIPSS), so-called nanostructure-covered LIPSS or NC LIPSS, with a period of about 540 nm. This value is considerably shorter than the laser wavelength, which the researchers believe is caused by an increase in the refractive index at the air-metal interface. According to Guo, the complex reflective properties of the NC LIPSS produces the variation in colours.
"Producing an NC LIPSS is a very versatile way of modifying optical properties, since the period of the NC LIPSS can be controlled by changing the laser wavelength, incidence angle or the real part of the refractive index."
Another more direct way to modify a surface structure is by scanning a laser beam across it to produce grooves. Using this approach, Guo produced grating structures with periods of 5-1000 µm. "These allowed us to modify optical properties in infrared, terahertz and even longer wavelengths," he said.
Guo believes such altered metals could be used in a variety of applications. "Some examples include laser marking, military camouflage and stray light suppression in optical instruments."