By Patricia Daukantas
Two University of Rochester (New York, U.S.A.) scientists who figured out how to blacken metals with femtosecond lasers have found that these treated metals absorb radiation all the way into the terahertz region of the electromagnetic spectrum. The finding could lead to new kinds of medical detectors.
Associate professor of optics Chunlei Guo, research assistant Anatoliy Vorobyev and three colleagues at V.N. Karazin Kharkov National University (Ukraine) recently published an article on the enhanced absorption of metals after femtosecond laser microstructuring technique.
Obviously, any object that we call “black” has that color because it absorbs light at visible wavelengths. The Rochester-Karazin team used a laser-calorimetry method to measure the absorptance of blackened tungsten samples, which were patterned with grooves with periods of 66, 120 and 430 µm. They measured the materials’ absorption of terahertz radiation with wavelengths of 70.7 and 118.8 µm (4.24 and 2.53 THz, respectively).
The highest absorptance they recorded was 51 percent of the 70.7-µm radiation by the 120-µm-grooved surface. By contrast, the authors write, metals are virtually perfect reflectors of far-infrared and terahertz radiation. Further spectral studies showed that the blackened tungsten absorbs radiation at wavelengths into the terahertz region.
Some experts say that the terahertz-absorbing materials could be used in medical equipment that could detect pathogens at the cellular level.
OPN first wrote about the metal-blackening work of Guo and Vorobyev in the Scatterings column of the February 2007 issue. I described some additional work of theirs – making metals look colored, not just black, with the laser technique – in a February 2008 blog post that also mentioned a popular article in the New York Times. Finally, the pair’s work was summarized in the Optics in 2007 and Optics in 2008 issues of OPN.