T-Ray Surprise: Rice Lab Makes Unexpected Discovery

Frequently, the unexpected results in science are the most exciting. That’s the case with the latest findings from the lab of Rice electrical engineer Daniel Mittleman, who was trying to find new ways to use terahertz energy, or T-rays, for chemical sensing when he noticed a strange tendency of the signals to travel more slowly if they were sent down smaller wires.

The odd phenomenon arises from the unique way T-rays interact with the sea of electrons flowing across the surface of a wire, says Mittleman, associate professor in electrical and computer engineering. “A similar variation in wave velocity is well-documented for higher frequency radiation in the visible portion of the spectrum, but this was a real puzzle because no one had predicted it for such low frequencies.”

Mittleman and graduate student Kanglin Wang discovered the phenomenon during follow-up experiments to last year’s groundbreaking development of the first T-ray wire waveguides. Their discovery that T-rays propagate along bare wires has allowed them to make T-ray endoscopes that can carry T-rays around corners and into tight places—like pipes and metal containers—where it isn’t feasible to place a T-ray generator. Mittleman hopes to use the technique to design a new class of chemical sensors that port security officers can use to quickly determine whether explosives are hidden inside shipping containers.

That kind of sensing is possible because of the unique properties of T-rays, which fall between microwaves and infrared light in the least-explored region of the electromagnetic spectrum. Metals and other electrical conductors are opaque to T-rays, but like X-rays, T-rays can penetrate plastic, vinyl, paper, dry timber, and glass. Unlike X-rays, however, T-rays pose no health hazards.

The reason bare wires can be used as T-ray waveguides is due to the way light from the terahertz frequency interacts with the sea of electrons flowing over the surface of the wire. When a wave of light strikes the wire, it creates a corresponding wave, called a plasmon, in the electrons flowing over the wire’s surface.

A new field of optics dedicated to the study of plasmons has sprung up within the past decade, and Rice boasts several leading plasmonics labs, most of which are dedicated to the design, testing, and use of metallic nanoparticles that are tailored to interact in particular ways with specific wavelengths of light.

Plasmonics is the key to understanding the movement of T-rays down wires, Mittleman says. When T-rays strike the wire, they create plasmons, and it is via these electron waves that the T-ray energy propagates down the wire. As the diameter of the wire gets smaller, the curvature becomes more pronounced, and this changes the plasmonic properties of the wire. It is this reduction in curvature, coupled with properties of the metal, that slows the movement.

“This is but one example of the interesting new physics coming out of T-ray labs across the country,” Mittleman says. “With more researchers taking an interest in T-rays, we’re well on our way to answering some of the fundamental questions that must be addressed for the field to progress.”

The research is funded by Advanced Micro Devices Inc., the Welch Foundation, and the National Science Foundation and was reported in the April 21 issue of Physical Review Letters.

—Jade Boyd