Abstract-Highly enhanced terahertz conversion by two-color laser filamentation at low gas pressures

Yung-Jun Yoo, Dogeun Jang, and Ki-Yong Kim

Fig. 1 Experimental setup for THz generation from two-color laser filamentation inside a long gas tube. The emitted THz radiation is refocused by an off-axis parabolic mirror onto a pyroelectric detector for energy measurement and an uncooled microbolometer focal plane array for imaging. Synchronized probe pulses at variable delays are used to measure THz waveforms via electro-optic sampling with a thin GaP crystal. The inset shows a long plasma filament in argon captured by optical side imaging.

https://www.osapublishing.org/oe/abstract.cfm?uri=oe-27-16-22663

We present an experimental study on pressure-dependent terahertz generation from two-color femtosecond laser filamentation in various gases. Contrary to short-focusing geometry, we find that long filamentation yields higher terahertz energy at lower gas pressures in most gases. This counter-intuitive phenomenon occurs due to multiple peculiar properties associated with filamentation. In practice, filamentation in low-pressure argon provides a maximum laser-to-terahertz conversion efficiency of 0.1%, about 10 times higher than in atmospheric air. In addition, our pressure-dependent study reveals an anticorrelation between terahertz output energy and local plasma fluorescence brightness. This determines the absolute phase difference between two-color laser fields for maximal terahertz generation, as well as verifies the microscopic mechanism of terahertz generation in two-color laser mixing.

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