Controlling and Enhancing Atmospheric Optical/Plasma Filaments

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As intense laser pulses propagate in atmosphere, they experience dramatic self-focusing, spectral broadening and phase modulation, and they ionize atmospheric molecules. The self-focusing and ionization-induced defocusing are competing effects that keep parts of the beam, called filaments, at high intensity over many Rayleigh lengths. Optical filaments and the plasma filaments that follow them are useful tools for remote sensing and ionization, atmospheric monitoring, terahertz generation, guiding of electrical discharges and optical pulse compression even to the few-cycle regime. Some of these applications may only be realized when the filamentation process is stabilized and plasma density is enhanced.

Our experiments have shown that the rotational response of atmospheric nitrogen and oxygen is large enough and fast enough to dominate Kerr-induced self-focusing for optical pulses propagating with FWHM time duration > 40 fs. Moreover, our measurements have pointed to a way to greatly enhance the filament electron density by controlling the alignment of ambient N2 and O2 molecules and thereby controlling the optical nonlinearity or air. In addition, our group pointed out for the first time that quantum effects could dominate the propagation of intense femtosecond pulses in the atmosphere.

This effect was demonstrated in our experiment that showed the quantum beats from laser-excited rotational wavepackets were able to steer, enhance or destroy laser filaments, depending on laser pulse timing. Our more recent work demonstrates that these quantum effects can increase the length of the plasma filament by a factor of three and can also promote soliton-like behavior of the pulse, cleaning and compressing it temporally. We performed direct measurements of the plasma density left behind by the filamenting optical pulses to confirm enhancement and extension of the electron density and laser intensity. Compression was measured with SPIDER, a technique for measuring the complex envelope and phase of optical pulses with sub-5 fs features.