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Remote sensing with pulsed THz wave


IGERT faculty Prof. X.-C. Zhang (Physics and Electrical Engineering) and his team including IGERT trainee Ben Clough (Electrical Engineering) and fellow graduate student Jingle Liu (Physics) at the THz Center, Rensselaer Polytechnic Institute, Troy, NY are the first to develop remote sensing with pulsed THz waves. This development will be useful in many applications. One example is remote sensing of explosives to save lives.

The significant scientific and technological potential of terahertz (THz) wave sensing and imaging has been the focus of considerable attention within many fields of research. However, the development of remote, spectroscopic sensing technologies for THz waves is lagging behind compelling real-world needs. This is due to the challenge posed by high attenuation by tremendous absorption by ambient moisture in the THz range. As a result, remote sensing with pulsed THz waves was considered impossible.

The Rensselaer team developed new methods to provide unique solutions for remote sensing. A focused optical pulse (mJ pulse energy and femtosecond pulse duration) in air creates a plasma. It is commonly known that pulsed, laser-induced plasma emits UV and visible fluorescence, as well as sound waves (photo-acoustics). It is less known that the fluorescence brightness and acoustic amplitude increase when a THz pulse is applied on the plasma. It is generally unknown that a broadband THz signal can be coherently detected with the use of dual-color laser beam excitation. Jingle Liu and Ben Clough developed THz-enhanced acoustic (TEA) techniques and THz-enhanced fluorescence (TEF) with standoff sensing capability. By “seeing” THz radiation-enhanced-emission-of-fluorescence (REEF), or “hearing” THz-enhanced acoustics (TEA), coherent detection of THz waves at standoff distance is feasible. If successful, the research would be transformative by enabling remote, pulsed THz wave sensing with fluorescence and acoustic techniques. The demonstrated sensing distance is 10 meters. They wish to extend the distances to greater than 100 meters.

Fig. 1 shows the schematics of TEF and its remote sensing capability, with the measured THz waveforms by electro-optic sampling and REEF at different distances from plasma, respectively. Waveforms are normalized and shifted for clarity.

Address Goals

A similar remote sensing concept could be extended from the fluorescence to the photo-acoustic wave. For example, THz waves, incident on a femtosecond laser-induced plasma, enhance translational motion of electrons and molecules, thereby locally heating the plasma and magnifying the acoustic pressure wave that results after relaxation of the photoacoustic shock wave. At a single cycle THz pulse with its peak field of 100 kV/cm, a pressure enhancement of 10% is observed throughout the acoustic spectrum up to 140 kHz and the THz-enhanced acoustics (TEA) signal is found to increase linearly with THz wave intensity. Using dual-color laser excitation to manipulate free electron drift, allows modulation of the enhanced acoustic signal and recovery of the coherent THz time-domain waveform. (Phys. Rev. Letts., submitted 2010).