Abstract
Broadband laser sources based on supercontinuum generation in femtosecond laser filamentation have enabled applications from stand-off sensing and spectroscopy to the generation and self-compression of high-energy few-cycle pulses. Filamentation relies on the dynamic balance between self-focusing and plasma defocusing – mediated by the Kerr nonlinearity and multiphoton or tunnel ionization, respectively. The filament properties, including the supercontinuum generation, are therefore highly sensitive to the properties of both the laser source and the propagation medium. Here, we report the anomalous spectral broadening of the supercontinuum for filamentation in molecular gases, which is observed for specific elliptical polarization states of the input laser pulse. The resulting spectrum is accompanied by a modification of the supercontinuum polarization state and a lengthening of the filament plasma column. Our experimental results and accompanying simulations suggest that rotational dynamics of diatomic molecules play an essential role in filamentation-induced supercontinuum generation, which can be controlled with polarization ellipticity.
Highlights
Broadband laser sources based on supercontinuum generation in femtosecond laser filamentation have enabled applications from stand-off sensing and spectroscopy to the generation and selfcompression of high-energy few-cycle pulses
It has proven critical to the understanding of laser-matter interactions underlying filamentation, the generation and compression of few-cycle pulses[3,4,5], and the generation of secondary sources for sensing (LIDAR)[6] and countermeasure applications, for instance
There remains a need for more complete understanding of the mechanisms underlying filament-induced Supercontinuum generation (SCG)
Summary
Polarization of the beam after filamentation was determined by fitting the polarization ellipse obtained by measuring the transmitted energy (attenuated using a neutral density filter) through a rotating polarizing cube, for each QWP angle. For both the spectral and polarization measurements, the detectors were placed approximately 2.2 meters after the geometric focus of the lens. The nonlinear polarization density, PNL = Pelec + Prot + P free + Pion, includes the instantaneous (Kerr) response, the delayed molecular rotational response, the free electron response, and ionization energy losses respectively[22,30]. Free electron density,UI is the ionizaAdditional details about the response model can be found in ref. 22
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