Terahertz detection at a distance

Marie Freebody, marie.freebody@photonics.com

When it comes to safe screening and detection, terahertz imaging offers considerable advantages but one major drawback. Terahertz waves occupy a large segment of the electromagnetic spectrum between the infrared and microwave bands and can provide imaging and sensing capabilities not available through conventional technologies such as x-ray and microwave. But remote sensing using broadband terahertz waves remains a great challenge for scientists due to the high absorption of terahertz waves by water vapor in the atmosphere.

Now, thanks to researchers at Rensselaer Polytechnic Institute (RPI), broadband detection of ultrashort terahertz pulses at a distance of 10 m and remote terahertz generation at up to 30 m have been demonstrated. The work was funded by the US Departments of Defense and Homeland Security.

Pictured from left to right are Jingle Liu, professor Xi-Cheng Zhang and Jianming Dai at Rensselaer Polytechnic Institute. The team has demonstrated broadband detection of ultrashort terahertz pulses at a distance of 10 m and remote terahertz generation at up to 30 m. Image courtesy of Rensselaer Polytechnic Institute.

“Major applications are homeland security and environmental sensing. For example, remote terahertz detection can be employed on the battlefield and at road checkpoints to detect hidden explosives, weapons, biochemical agents or illegal drugs,” said Dr. Xi-Cheng Zhang, who headed up the RPI study. “Mounting the system on tanks, mobile vehicles or aircraft can further enhance the mobility and accessibility of the system.”

Terahertz radiation can penetrate almost any material that is not metal or a polar liquid. Effectively, the waves can “see” through certain materials that might be used to conceal explosives or other dangerous materials, such as packaging, corrugated cardboard, clothing, shoes and backpacks, making it extremely attractive for homeland security and military purposes. What’s more, unlike x-rays, terahertz radiation poses little or no health threat.

The trouble is that ambient moisture attenuation can be as high as 1000 dB/km at selected terahertz frequencies. This means that some terahertz signals will be greatly attenuated after propagating just a few meters in the atmosphere, which limits the effective use of terahertz spectroscopy in the air.

To overcome this limitation, the researchers took advantage of laser-induced fluorescence and used the subsequent plasma fluorescence to carry information through the air. Zhang and his team demonstrated terahertz generation and detection separately in two individual experiments, which are described in a letter published online in Nature Photonics on July 11, 2010.

“Our system works a little differently compared with the terahertz scanners currently employed in some airports. Our system could access the broadband fingerprint information of materials via terahertz time-domain spectroscopy at remote distances. Since fluorescence is used as the media to carry the terahertz signal, we can avoid strong water vapor attenuation of terahertz waves,” Zhang said. “On the other hand, it is very difficult for conventional terahertz scanners to detect a target that is located far away.”

In the setup, two laser beams with center wavelengths at 800 and 400 nm, respectively, are focused in the air to create a plasma near the target material and terahertz source. The pulse duration of the two beams ranges from around 60 to 80 fs. An in-line phase compensator finely controls the relative phase between these two laser pulses to manipulate the electron motion within the plasma.

Once the laser-induced plasma overlaps with the terahertz pulse both spatially and temporally, the terahertz pulse will enhance the fluorescence emission from the plasma. The fluorescence from the plasma can then be measured and used to determine the amplitude and phase of the terahertz wave, which contains spectral fingerprint information of the materials in the target.

By using a UV-coated optical telescope and spectrometer, the team can detect the intensity of the fluorescence emission at remote distances. Typically, the strongest nitrogen fluorescence line is at 357 nm, and it was this wavelength that was chosen for signal detection.

Zhang and his team are optimistic about the commercial potential of the remote terahertz system and are hoping to bring it to market within a couple of years.

“We have not yet demonstrated remote terahertz spectroscopy but are working toward this by combining the remote generation and detection techniques we developed,” Zhang said. “We aim to extend the maximum sensing distance by using a fluorescence detector of higher sensitivity and collecting optics of a larger size.”