Terahertz radiation (THz), says Yujie Ding, offers critical advantages over the radiation of other frequencies.
Unlike x-rays, THz is non-ionizing and not harmful to tissue. It penetrates clothing and plastics. Its wavelength is shorter than microwave and millimeter wave radiation, giving it superior spatial resolution for imaging. And THz shows promise in identifying polar materials by revealing their spectral “fingerprints.”
Two challenges – generating and detecting THz rays – have impeded the development of THz systems, says Ding, a professor of electrical and computer engineering.
In 10 years of research funded by the Air Force Office of Scientific Research, Ding has made breakthroughs in generating THz radiation by mixing the frequencies from two narrow-linewidth lasers.
When Ding began his research, scientists seeking to generate THz rays typically focused a single ultrafast laser beam on a nonlinear medium that acted as a “rectifier.”
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“Ultrafast lasers have a broad frequency bandwidth, which requires many pairs of frequencies to be efficiently rectified by a medium,” says Ding. “We have mixed the radiation from an Nd:YAG laser and a Master Oscillator/Power Oscillator emitting narrow-linewidth frequencies in a gallium-selenide crystal. We have generated the highest peak power yet for THz – about 390 watts.”
Ding and his group produced the highest average output power to date – 260 microwatts – by frequency-mixing two CO2 lasers. They reported their results last year in Applied Physics Letters.
“This is the first report of high-power THz generation based on frequency-mixing two CO2 lasers in a bulk nonlinear crystal,” Ding said. “Usually when people generate THz radiation, they end up with a broad frequency bandwidth. We have been able to generate a very narrow linewidth with much greater spectral density.”
The narrow linewidth, says Ding, is crucial for chemical sensing.
“Because of its frequencies, THz is a perfect match to the transitions among rotational energy states of molecules. But the THz linewidth has to be at least as narrow as what you’re probing. Furthermore, since the THz frequencies within a relatively narrow band can be tailored to be within one of the atmospheric windows, they are perfect for remote chemical sensing through air.
“Fingerprinting used to examine just one wavelength, or one peak. But this is not accurate. Our goal is to select, for instance, 10 peaks, analyze them as a set and fingerprint an unknown molecule or a mixture of gases without making a mistake.”
Ding plans to continue studying the absorption of THz waves by various gases and to work on the imaging of remote objects. He is also fingerprinting a nerve-gas simulant in its liquid and vapor phases. “We’re currently using Fourier transform infrared spectroscopy to analyze all the transitions within the THz region. We have identified 12 signature peaks so far.”
