A tabletop femtosecond laser has been used to change the surface properties of metals to reflect a specific color or combination of colors. Silver, platinum, gold, and other metals have been turned colors such as blue, gray, black, and purple.Today Photonics.com and Physorg.com reported Unversity of Rochester's Professor Guo's recent research achievement.
The intense blast forces the surface of the metal to form nanostructures -- pits, globules and strands that response incoming light in different ways depending on the way the laser pulse sculpted the structures. Since the structures are smaller than the wavelength of light, the way they reflect light is highly dependent upon their specific size and shape, Guo said. Varying the laser intensity, pulse length, and number of pulses, allows Guo to control the configuration of the nanostructures, and hence control what color the metal reflects.
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The Darkest Material Ever Made
Consists of a carpet of vertically oriented carbon nanotubes. The darkness or lightness of any object depends on the fraction of light falling on the object that gets reflected back. The reflectivity of the nanotube array developed by physicists at the Rensselaer Polytechnic Institute (RPI) is only 0.045%, three times smaller than the best previous dark object (see figure at http://www.aip.org/png/2008/296.htm). Shawn Lin and his colleagues grow the nanotubes on iron nanodots atop a silicon wafer. The resulting mat is thin (10-800 microns) and lightweight (.01-.02 g/cm^3).
Possible applications include the revision of darkness standards, such as are used by photographers. The lowest dark scale defined by NIST right now is for reflectances of about 1.5%. The material might also be useful in astronomical detectors (where you want to soak up stray radiation) or in photovoltaic cells which turn sunlight into electricity. Lin (sylin@rpi.edu) says that an additional feature of this new material is that it represents a controllably porous substance with an index of refraction (1.02) not very different from that of air. (Yang et al., NanoLetters, 9 January 2008).
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