Lehigh researchers hone radiation source for THz devices

Potential applications in medicine, remote sensing, imaging and satellite communications

A world that consumes information in gigabytes may one day find terahertz-sized solutions for some of its most pressing problems.

While one gigabyte is equal to one billion (109) bytes of information, a terahertz (THz) is a unit of electromagnetic-wave frequency equivalent to one trillion (1012) hertz, with one hertz equaling one cycle per second.

Terahertz (THz) frequencies, ranging from 0.1 to 10 THz, have potential applications in medicine, remote sensing, imaging and satellite communications, but are nonetheless one of the most under-utilized frequency ranges. That is because the THz range lies between the microwave frequency range and the near-infrared and optical frequency ranges, in which conventional semiconductor devices are usually operated.

Yujie J. Ding, professor of electrical and computer engineering and a member of Lehigh University’s Center for Optical Technologies, is working to solve challenges that must be overcome for THz devices to become readily accessible and cost-effective.

“We need a source to generate coherent THz waves and we need detectors,” says Ding, a specialist in optoelectronics, nonlinear optics and quantum electronics.

“This is very challenging because the concepts that govern infrared light and visible light don’t work with THz.”

Ding hopes to develop a compact THz radiation source with wide tunability in the wavelength range of 30 to 3,000 microns (a micron is equal one one-millionth of a meter). Several methods have been advanced by other researchers, but most have shortcomings. Free-electron lasers are bulky and costly. Ultrafast lasers generate very weak THz beams with low output powers and pulse energies.

Ding and his research group, which includes four Ph.D. candidates, one M.S. candidate and two post-doctoral researchers, have developed a method of focusing two high-frequency lasers to generate tunable and coherent THz waves in the range of 58 to 3540 microns.

In the Aug. 4 issue of Applied Physics Letters, Ding described his work in an article titled “Continuously tunable and coherent terahertz radiation by means of phase-matched difference-frequency generation (DFG) in zinc germanium phosphide ZnGeP2.”

In the article, Ding reported a highest-output peak power seven orders of magnitude higher than any output power previously reported for a THz source. He also reported a tuning range of output wavelengths that was about five times wider than a range reported previously by researchers generating THz waves in ZnGeP2 using two carbon-dioxide laser lines.

Last year, Ding reported successful THz radiation using gallium-selenide crystals in an article titled “Efficient, tunable and coherent 0.18-5.27-THz source based on GaSe crystal,” which was published Aug. 15, 2002, in the journal Optics Letters.

A properly tuned source emitting THz frequencies, says Ding, would be ideally suited for imaging, spectroscopy and medical diagnostics, including cancer detection and, potentially, gene therapy.

Because vibrations of DNA and RNA chains resonate in THz, Ding says, “with a proper THz radiation source, you can tune across the resonances and sense very slight changes of the atomic chain arrangement.”

Cancer cells, especially melanoma tissues, also vibrate in THz, says Ding, and lend themselves to early detection by doctors equipped with THz devices.

THz devices are also promising for homeland security tasks such as detecting the presence of toxic and semitoxic gases, says Ding. When subjected to THz waves, he says, gaseous materials reveal a limited number of sharp peaks that form a distinct pattern like a fingerprint. When the same material is subjected to the much shorter visible or mid-infrared light waves, the peaks that are revealed are too congested to show an observable pattern. Ding has already performed experiments on water vapor using THz waves at Lehigh.

Ding’s next challenge is to scale down his THz radiation device, which now approaches a large shoe box in size. His ultimate goal is to fit 10 arrays, each equipped with an emitter, a detector and photonic bandgap crystals, and each measuring millimeters in size, onto one computer chip wafer of standard dimensions.

To miniaturize his THz device, Ding is using nanostructure quantum dots and including photonic bandgap crystals that act as a special waveguide by tightly focusing the THz beam on a particular detector. The result is a more sensitive detecting tool that detects the presence of a specific toxic chemical when that chemical blocks part of the THz beam. “Without the photonic bandgap crystals, the beam will diverge,” says Ding.