University of Rochester
optics professor Chunlei Guo made headlines in the past couple of years when
he changed the color of everyday metals by scouring their surfaces with precise,
high-intensity laser bursts.
Suddenly it was possible to make sheets of golden tungsten, or black aluminum.
A recent discovery in Guo's lab has shown that, beyond the aesthetic opportunities
in his find lie some very powerful potential uses, like diagnosing some diseases
with unprecedented ease and precision.
Along with his research assistant, Anatoliy Vorobyev, Guo has discovered that
the altered metals can detect electromagnetic radiation with frequencies in
the terahertz range (also known as T-rays), which have been challenging, if
not impossible, to detect prior to his discovery.
"When we turned metals black, we knew that they became highly absorptive
in the visible wavelength range because the altered metals appear pitch black
to the eye. Here, we experimentally demonstrated that the enhanced absorption
extends well into the far infrared and terahertz frequencies," Guo said.
With wavelengths shorter than microwaves, but longer than infrared rays, T-rays
occupy a place in the electromagnetic spectrum that is capable of exciting rotational
and vibrational states of organic compounds, like pathogens. This quality could
allow doctors and biomedical researchers to get previously impossible glimpses
of diseases on the molecular level.
In addition, unlike X-rays, T-rays are non-ionizing, which means that people
who are exposed to them don't risk the possible tissue damage that can result
University of California, Berkeley, bioengineering Professor Thomas Budinger
says terahertz radiation is like much-higher-frequency radar, except that it
theoretically can allow its users to see intricate details of tissue architecture,
on the scale of one-thousandth of a millimeter and smaller, instead of large
objects like airplanes and boats.
"Terahertz electromagnetic radiation has the capability to interrogate
tissues at the cellular level. If applied within microns of the subject of interest,
this form of imaging has the theoretical capability to detect properties of
molecular assemblages that could be attributes of disease states," Budinger
What made terahertz radiation so difficult to detect in the past was that typical
materials do not readily absorb that frequency. However, after undergoing Guo's
femtosecond structuring technique, metals become over 30 times more absorptive.
The key to creating the black metal in terahertz is a beam of ultra-brief,
ultra-intense laser pulses called femtosecond laser pulses. The laser burst
lasts less than a quadrillionth of a second. To get a grasp of that kind of
speed, consider that a femtosecond is to a second what a second is to about
32 million years. During its brief burst, Guo's laser unleashes as much power
as the entire grid of North America onto a spot the size of a needle point.
That intense blast forces the surface of the metal to undergo some dramatic
changes and makes them extremely efficient in absorbing terahertz radiation.