“hearing” terahertz electromagnetic waves

Benjamin Clough, Jingle Liu, and X.-C. ZhangCenter for Terahertz Research, Rensselaer Polytechnic Institute, Troy, NY 12180

We develop a unique method to coherentlyinterrogate terahertz (THz) pulses using theacoustic emission from a bichromatic laser-induced air plasma that interacts with THzelectromagnetic transients as depicted in FIG.1 (a). The near instantaneous expansion ofthe gas during ionization releases an acousticpulse that is broad in frequency and can travelgreat distances in the atmosphere as seen inFIG. 1 (b). The electric field of the THz waveinfluences the plasma dynamics byaccelerating or decelerating free electronsand inducing electron-molecule energytransfer. FIG 2 shows the consequent plasmatemperature increase results in uniformenhancement of acoustic emission, and thatthe pressure enhancement is a linear functionof the THz intensity incident the plasma. Thismakes TEA useful as a THz detector.Furthermore, the coherent time-resolved THztransient can be recovered by subtracting theTEA curves for the two electron driftasymmetries produced by changing the phasebetween the optical pulses as seen in FIG 3.

By using bichromatic laser fields tomanipulate the free electron drift velocityinside a plasma, and measuring the acousticsignals under opposite electron driftsymmetry, we demonstrate that informationconcerning polarity and magnitude of the THzwave can be obtained in all directions withouta need for direct line of sight to the detector.Coherent detection of ultrashort THz pulsesthrough photoacoustic encoding has beenrealized and will minimize strong ambientwater vapor absorption of THz wavepropagating through the atmosphere whenextended to larger distances. Theseachievements open new ways to uncover thepotential of THz wave technology inhomeland security for use in remoteexplosives detection and environmentalmonitoring.

This material is based upon work supported by the U.S. Department of Homeland Security under Award Number 2008-ST-061-ED0001. The views and conclusions contained in this document are those of the authors and should not be interpreted as necessarily representing the official policies, either expressed or implied of the U.S. Department of Homeland Security.

The photoacoustic THz-encoding technique for sensingelectromagnetic transients, with femtosecond timeresolution and unlimited signal collection directionality, hasprovided a method to circumvent 100 dB/km attenuationof THz in the atmosphere. This has opened new avenues toapply the powerful characteristics of THz waves, whileovercoming fundamental limitations.

Currently, detection and generation of THz waves from aremote distance have been performed separately. We aimto close this loop by demonstrating a fully remote systemthat incorporates the techniques discussed here incombination with existing remote generation techniquesutilizing air photonics (laser-plasma formed throughionization of the air).

1. B. Clough, J. Liu, and X.-C. Zhang, Proc. SPIE 7938,

793804 (2011).

2. B. Clough, J. Liu, and X.-C. Zhang, Opt. Lett. 35, 2544

(2010).

3. J. Liu, B. Clough, and X.-C. Zhang, Phys. Rev. E 82,

066602 (2010)

“Hearing” Terahertz Electromagnetic Waves

Technical ApproachAbstract

Relevance Accomplishments Through Current Year

Future Work

Publications Acknowledging DHS SupportOpportunities for Transition toCustomer

Patent Submissions

Terahertz remote sensing was once thoughtimpossible due to several fundamentallimitations such as high atmosphericabsorption, a need for forward signalcollection, and a need for direct line of sightto the detector. This photoacoustic THzdetection method shows promise forextending THz capabilities for non-invasivematerial spectral “fingerprinting,” from a safedistance.

Clough, B. , Liu, J. & Zhang, X. C. “Method for terahertz wave detection utilizing terahertz-enhanced acoustics.” (Submitted April 2010, Invention disclosure No. 1357)

FIG. 1 (a) Experimental schematic for the THz-enhanced acoustics using bichromatic (ω+2ω)femtosecond laser excitation. (b) Normalizedacoustic pulses and corresponding spectra at 0.5and 11 meters from the plasma acoustic source.

FIG. 3 (a) THz emission as ‘tR’delay between ω and 2ω isscanned. The THz emissionreveals the electron driftasymmetry and is indicated bythe green electrons andarrows. (b) Time resolved THz-enhanced-acoustics (TEA) forvarious electron driftingconditions. Red curve: ‘TR’ isset to π/2 and the delaybetween THz and opticalpulses ‘TD’ is scanned. Bluedashed curve: ‘TR’ is set to -π/2and the delay between THzand optical pulses ‘TD’ isscanned. (c) Coherent THzwaveform obtained throughacoustics compared with a THzwaveform acquired throughconventional methods.

a b

FIG. 2 (a) Single photoacousticwaveforms and spectra with andwithout THz interaction. (b)Acoustic emission vs THz intensityincident on the plasma.

Microphone

Lemelson-MIT Rensselaer Student Prize

b

aa

b

c

Benjamin Clough, a student in the

Department of Electrical, Computer,

and Systems Engineering at Rensselaer,

is one of three finalists for the 2011

$30,000 Lemelson-MIT Rensselaer

Student Prize. His project is titled

“Terahertz Enhanced Acoustics.” For

more information on the ceremony

visit: http://www.eng.rpi.edu/lemelson