Black metal detects medically important terahertz radiation

Hank Hogan, hank.hogan@photonics.com

Black metal, so called because of its pitch-dark appearance, could be important in medical applications because the material is highly absorptive. According to researchers from the University of Rochester, the enhanced absorption provided by the material extends into the terahertz region.

As a result, according to optics professor Chunlei Guo, black metal could be used in terahertz detectors. “This has a lot of biomedical applications because terahertz radiation is coupled to the vibrational and rotational spectrum of many biomedical compounds.”

In addition to being able to highlight important molecules, another advantage is that terahertz radiation is not ionizing, as are x-rays. Thus, terahertz-based techniques could be used without fear of harming patients or research subjects.

Black metal is made using femtosecond laser pulses in a technique originally discovered and developed several years ago by Guo and research assistant Anatoliy Vorobyev. The approach doesn’t need any additional materials and can be done in a vacuum or in a standard atmosphere.

The femtosecond pulses reshape the metal surface, leading to a series of nano- and microscale structures. One result is increased absorption efficiency because the structured surface becomes extremely effective in trapping radiation and then absorbing it, Guo explained.

In the terahertz black metal research, which was reported in a recent issue of Applied Physics Letters, the researchers used a heavily modified commercial Ti:sapphire laser, generating 65-fs pulses at a wavelength of 800 nm. With these pulses, they irradiated polished titanium samples that measured 25 mm on a side and 1 mm thick. They used a motorized stage, creating parallel grooves of surface structures in the metal that were spaced at 66, 120 and 430 µm.

A titanium chip (left) appears black because femtosecond laser pulses reshaped the surface (right), making it highly absorptive from the visible to the terahertz ranges. Courtesy of Chunlei Guo, University of Rochester.

They then exposed the samples to terahertz radiation at 70.7 and 118.8 µm, measuring the absorption by looking at the temperature rise in the metal. The untreated metal had almost no absorption at those wavelengths, while the processed metal showed a maximum of 51 percent absorption. The enhancement in absorption due to the laser treatment, the researchers reported, was more than 30-fold.

Even better performance might be possible through the adjustment of pulse energy, pulse duration, groove pitch and other parameters. Other fine tuning could be done to make processing of the metal faster and more efficient, something that Guo believes might be needed for commercial use of black metal in a terahertz application.

He noted that higher absorption already makes the material a good candidate for a detector, and that it could also prove useful in a terahertz source. “When you have higher absorption, you also have higher emission as well, typically,” he said.