Although many time-domain terahertz-wave detection techniques have been developed, high ambient-moisture absorption rules out "remote" terahertz sensing, shutting out a host of applications in homeland security, astronomy, and environmental monitoring. But finally that picture is changing, thanks to a new all-optical technique from researchers at Rensselaer Polytechnic Institute (RPI; Troy, NY) and Laval University (Quebec City, QC, Canada).1
The key is the exploitation of omnidirectional fluorescence emission that interacts directly with a terahertz wave. The signal-detection method is sensitive enough to temporally resolve terahertz pulses at standoff distances up to 10 m with minimal water-vapor absorption and unlimited directionality.
An ultraviolet (UV)-coated telescope and a spectrometer collect and measure laser-induced nitrogen fluorescence from a plasma at remote distances. Irradiation of nitrogen atoms by intense laser pulses causes a portion of excited electrons to be trapped in high-lying Rydberg states of atoms and molecules. A single-cycle terahertz pulse causes the atoms in those trapped states to be more easily ionized, causing terahertz radiation-enhanced emission of fluorescence (THz-REEF).
By using a two-color (fundamental and second-harmonic) laser pulse to asymmetrically ionize the gas, the researchers control the electron-drifting velocity in the laser-induced plasma and thus coherently manipulate the fluorescence emission from plasma that interacts with terahertz waves. The relative optical phase between the fundamental laser pulse and second-harmonic laser pulse, which is controllable with attosecond accuracy, is set to create asymmetrical electron drifting. The time-resolved THz-REEF waveforms (obtained by measuring fluorescence emission as the time delay between the terahertz pulse and optical pulses is continuously changed) are measured when the electron-drift velocity is parallel and antiparallel to the terahertz polarization direction, respectively. The difference between these two waveforms is linearly proportional to the time-dependent terahertz field (terahertz time-domain waveform). Amplitude and phase information of the terahertz pulse can be extracted from the fluorescence signal simultaneously.
Using the THz-REEF technique, high-resolution broadband spectra of water vapor and 4-Amino-2, 6-dinitrotoluene (4A-DNT) were obtained from 0 to 7 THz and 0 to 1.5 THz ranges, respectively. The 4A-DNT spectra compared favorably with electro-optic sampling, wherein an electro-optic crystal (300 µm thick <110> gallium phosphide) is used to measure the time-domain terahertz waveform via the Pockels effect. Due to the absence of intrinsic phonon absorption and Fabry-Perot effect, this technique is able to provide broadband and high-resolution spectroscopy signatures.
"With sufficient laser power, the combination of this terahertz-wave remote-sensing technique and previously demonstrated terahertz-wave generation at long distances using a two-color laser beam with stable control of the relative phase, would realize remote terahertz spectroscopy for the identification of chemical biological agents," says professor X.-C. Zhang, J. Erik Jonsson '22 Professor of Science and the director of the Center for Terahertz Research at RPI. "Moreover, this offers a promising way to characterize electron behavior in strong light-matter interactions by revealing the detailed interplay process of strong-field ionization, plasma dynamics, and terahertz-wave-induced electron heating."
Zhang adds, "The great challenge of terahertz-wave remote sensing in ambient conditions is the strong water-vapor attenuation at terahertz frequencies. High atmospheric transparency of the UV fluorescence enables remote sensing; we are excited to achieve this without worrying about the humidity anymore."
1. J. Liu et al., 2010 CLEO Postdeadline paper CPDB8, San Jose, CA (May 2010).