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Generation of Elliptically Polarized Terahertz Waves from Laser-Induced Plasma with Double Helix Electrodes


The NSF sponsored IGERT trainee Xiaofei Lu and her collaborators at the Center for Terahertz Research, Rensselaer Polytechnic Institute, Troy, NY have observed the generation of elliptically polarized far-field terahertz wave pattern and clarified the mechanisms responsible for polarization control of air-plasma-based terahertz sources. The velocity mismatch between optical excitation and propagation of terahertz waves was verified to be the underlying mechanism.

The air plasma, produced by focusing an intense laser beam to ionize atoms or molecules, was demonstrated recently to be a promising source of broadband terahertz waves. However, simultaneous broadband and coherent detection of such broadband terahertz waves is still challenging. Electro-optical sampling and photoconductive antennas are the typical approaches for terahertz wave detection. The bandwidth of these detection methods is limited by the phonon resonance or carrier’s lifetime. Unlike solid-state detectors, gaseous sensors have several unique features, such as no phonon resonance, less dispersion, no Fabry-Perot effect, and a continuous renewable nature.

Xiaofei and her colleagues developed a broadband terahertz time-domain spectrometer having both a gaseous emitter and sensor that features high efficiency, perceptive sensitivity, broad bandwidth, adequate signal-to-noise ratio, sufficient dynamic range, and controllable polarization. Figure 1A shows the basic experimental setup of double helix electrodes and corresponding electric field applied on plasma region with left- and right- handedness. The laser pulses were focused to ionize air where a pair of double helix electrodes was positioned. The trajectory of electrons (arrows) follows the external electric field direction, resulting in an elliptically polarized THz wave in the far field. The laboratory coordinate and definition of handedness are shown in the figure. The origin is defined at the center of the plasma.

The calculated far-field terahertz waves in x and y direction at various position of electrodes from right-handed and left-handed helical and linear electrodes are in good agreement with the experimental observation. See Figure 1B for a comparison between experimental (EXP) and calculations (SIM).

This finding provides new perception of plasma-based terahertz emitters, opens interesting perspectives to coherently control polarization properties of terahertz emission in a broader spectral range, and offers a useful diagnostics tool for biomolecular and spintronics study in the terahertz frequency.

For further details, please see published paper “Generation of Elliptically Polarized TerahertzWaves from Laser-Induced Plasma with Double Helix Electrodes” by Xiaofei Lu et al., Physical Review Letters 108, 123903 (2012), DOI: 10.1103/PhysRevLett.108.123903

Address Goals

In the process of elliptically polarized terahertz waves discovery the IGERT trainee and collaborators learned to work together and developed science and engineering skills as well as disseminated findings to society through outreach activities and professional conferences.