Terahertz technology used in unique remote sensing system

Citation

(2011), "Terahertz technology used in unique remote sensing system", Sensor Review, Vol. 31 No. 1. https://doi.org/10.1108/sr.2011.08731aaf.001

Terahertz technology used in unique remote sensing system

Article Type: Mini features From: Sensor Review, Volume 31, Issue 1

A research team from the American Rensselaer Polytechnic Institute has demonstrated a unique optical system that can determine the chemical composition of remote targets at a range of up to 10 m. Led by Professor Zhang, Director of the Centre for THz Research at Rensselaer, the team (Figure 1) recently announced its findings in Nature Photonics (Liu et al., 2010).

Figure 1 Members of the Rensselaer team

Terahertz sensing and imaging technologies have attracted much recent attention due to their potential in fields such as healthcare, criminology, material testing and security. However, developments in open-air terahertz spectroscopy have been inhibited by the high absorption of ambient moisture. The Rensselaer group has now overcome this limitation by developing an “all optical” technique dubbed terahertz radiation enhanced emission of fluorescence (THz-REEF). This uses laser-induced fluorescence, essentially by focusing two laser beams together into the air to create a plasma that interacts with a generated terahertz wave. The plasma fluorescence carries information from a target to a detector where it is compared with material spectra in a terahertz library, making it possible to identify the target’s composition.

Figure 2 The plasma, visible as the central bright spot

Broadband terahertz wave detection has been achieved by coherently manipulating the fluorescencent emission from asymmetrically ionised gas plasma interacting with terahertz waves. Owing to the high-atmospheric transparency and omnidirectional emission pattern of the fluorescence, the technique can be used to measure terahertz pulses at stand-off distances with minimal water vapour absorption effects and unlimited directionality. In the experimental system, a two-colour laser beam (400 and 800 nm) with parallel polarisation was focused into air to generate a plasma (Figure 2), with the relative phase being controlled by an in-line phase compensator. A single-cycle terahertz pulse with a peak field of 100 kV cm⁻¹ was focused co-linearly with the optical beam onto the plasma. The UV fluorescence (at 357 nm) emitted from the laser-induced plasma was collected by a rotatable UV-enhanced concave mirror with a diameter of 200 mm and focal length of 500 mm and was then guided by another UV plane mirror with a diameter of 75 m through a monochromator into a photomultiplier tube. The principle is shown schematically in Figure 3. In anticipation of potential applications in homeland security, the absorption spectrum of a pellet of the explosive 4A-DNT (4-amino-2,6-dinitrotoluene) in polyethylene has been measured with the technique (Figure 3). This compared favourably with data obtained by free-space electro-optic sampling, an alternative means of characterising freely propagating terahertz beams.

Figure 3 REEF system schematic

Although most of the research has been conducted in the laboratory, the technology is portable and could eventually be used to examine luggage in airports for explosives or illegal drugs and locate concealed explosives and IEDs on the battlefield. Professor Zhang sees strong prospects for the technique and stated “I think I can predict that, within a few years, the THz science and technology will become more available and ready for industrial and defense-related use”. The work has been funded by the US National Science Foundation, the Defense Threat Reduction Agency and the Department of Homeland Security.