Broadband terahertz wave remote sensing using coherent manipulation of fluorescence from asymmetrically ionized gases Jingle Liu 1 , Jianming Dai 1 , See Leang Chin 2 and X.-C. Zhang 1 * Terahertz wave sensing and imaging have received a great deal of attention because of their significant scientific and techno- logical potential in multidisciplinary fields 1–3 . However, owing to the challenge of dealing with high ambient moisture absorp- tion, the development of remote open-air broadband terahertz spectroscopy is lagging behind the urgent need for the technol- ogy that exists in homeland security and the fields of astron- omy and environmental monitoring 3,4 . The requirement for on-site bias or forward collection of the optical signal in con- ventional terahertz detection techniques has inevitably prohib- ited their use in remote sensing 5–7 . We introduce an ‘all-optical’ technique of broadband terahertz wave detection by coherently manipulating the fluorescence emission from asymmetrically ionized gas plasma interacting with terahertz waves. Owing to the high atmospheric transparency and omnidirectional emission pattern of the fluorescence, this technique can be used to measure terahertz pulses at standoff distances with minimal water vapour absorption and unlimited directionality for optical signal collection. We demonstrate coherent tera- hertz wave detection at a distance of 10 m. Photoconductive antennas 5 , electro-optic (EO) sampling 6 and terahertz air detection 7 have been widely used in recent decades for the detection of broadband terahertz radiation in an increasing variety of applications including biomedical imaging, non-destruc- tive inspection and material characterization 1–3 . In attempts to meet the emerging needs of homeland security and environmental science, a large amount of research effort has been directed at devel- oping broadband remote terahertz spectroscopy. Focusing two-colour optical beams remotely provides a solution for remote terahertz wave generation 8 . However, the realization of broadband terahertz remote sensing is even more challenging because of the strong absorption of ambient water vapour in the terahertz band and the difficulties inherent to remote optical signal collection. Using a biased photoconductive antenna 5 or EO crystal 6 for tera- hertz-wave remote sensing is not practical. In terahertz wave detec- tion using a gas sensor 7 , the second-harmonic beam, generated from a four-wave-mixing process involving the terahertz beam and the fundamental laser beam, has to be measured in the forward direc- tion, so collecting it is difficult at standoff distances due to weak scattering. Here, we report on an ‘all-optical’ technique for standoff (10-m) broadband coherent terahertz wave detection by probing the tera- hertz pulse with a fully controllable two-colour laser-induced gas plasma and analysing the interaction by detecting the omnidirec- tional fluorescence emission. The high transparency of UV fluor- escence in the atmosphere can circumvent the sensing distance limitation that arises due to strong water vapour absorption in the terahertz region. Instead of being used for terahertz wave generation as demonstrated in ref. 9, the two-colour laser field functions as a remote ‘optical modulator’ for the terahertz radiation enhanced emission of fluorescence (THz-REEF) signal through coherent manipulation of the ionized electron drift velocity and subsequent collision-induced fluorescence emission. We will further reveal the complex physical picture of the light–plasma interaction by investi- gating the relation between the fluorescence and the electron momentum distribution. THz-REEF from gas plasma excited by single-colour, multicycle laser pulses has been studied and demon- strated for terahertz wave detection 10 . However, this method only detects terahertz wave intensity, and not phase information, which makes it non-ideal for remote sensing due to the requirement for an on-site external electric bias to provide a local oscillator. Unlike the inherently incoherent scheme in ref. 10, this technique using symmetry-broken laser fields to control electron momentum is inherently coherent and directly measures the terahertz field E THz (t ) instead of the vector potential A THz (t ). The performance of this technique regarding terahertz wave detection is one to two orders better than that using bias as in ref. 10 due to the larger modulation of the electron momentum and elimination of noise induced by the derivative relation E THz (t ) ¼ dA THz (t )/dt. Furthermore, this technique circumvents the limitations of the on-site bias requirement, water vapour attenuation and signal-col- lection direction at standoff distances. By applying this technique we have realized the detection of broadband terahertz radiation from a distance of 10 m. Figure 1 presents a schematic of experiments on terahertz wave remote sensing, using coherent manipulation of terahertz wave enhanced fluorescence from asymmetrically ionized gas. The two- colour laser beam with parallel polarization was focused into air to generate plasma, with the relative phase being controlled by an in-line phase compensator 9 . A single-cycle terahertz pulse with a peak field of 100 kV cm 21 was focused collinearly with the optical beam onto the plasma. The shaded area in Fig. 1 shows the fluor- escence detection system, which has translational mobility on a horizontal plane. The fluorescence emitted from the two-colour laser-induced plasma was collected by a rotatable UV-enhanced concave mirror (M1) with a diameter of 200 mm and focal length of 500 mm, and was then guided by another UV plane mirror (M2) with a diameter of 75 mm through a monochromator into a photomultiplier tube (PMT). Terahertz wave sensing was performed as the distance between the plasma and fluorescence detection system was varied. In the laser-induced ionization processes, electrons newly released from molecules or atoms acquire a constant drift velocity after passage of the laser pulse 11 . The drift velocity is determined 1 Center for Terahertz Research, Rensselaer Polytechnic Institute, Troy, New York 12180, USA, 2 Department of Physics, Center for Optics, Photonics and Laser, Laval University, Quebec City, Quebec, G1V 0A6, Canada. *e-mail: zhangxc@rpi.edu LETTERS PUBLISHED ONLINE: 11 JULY 2010 | DOI: 10.1038/NPHOTON.2010.165 NATURE PHOTONICS | VOL 4 | SEPTEMBER 2010 | www.nature.com/naturephotonics 627 © 2010 Macmillan Publishers Limited. All rights reserved.