Abstract
We measure the instantaneous electronic nonlinear refractive index of N(2), O(2) and Ar at room temperature for a 90 fs and 800 nm laser pulse. Measurements are calibrated by post-pulse molecular alignment through a polarization technique. At low intensity, quadratic coefficients n(2) are determined. At higher intensities, a strong negative contribution with a higher nonlinearity appears, which leads to an overall negative nonlinear Kerr refractive index in air above 26 TW/cm(2).
Highlights
The propagation of intense short laser pulses in the atmosphere is an important domain of investigation regarding applications in remote sensing
Optical filamentation is usually defined as the balancing between self-focusing (Kerr effect) and plasma defocusing
We show that the nonlinear Kerr index exhibits a large variation and becomes negative above few tens of TW/cm2
Summary
The propagation of intense short laser pulses in the atmosphere is an important domain of investigation regarding applications in remote sensing. This propagation is accompanied by a number of phenomena among which one finds self-focusing and self-phase modulation, ionization and plasma defocusing, filamentation, third harmonic generation, continuum generation, and terahertz emission [1]. It has been recently proposed that the nonlinear Kerr refractive index could undergo a saturation with the intensity through a negative contribution n4I2. This term was considered in order to stabilize the filament. Drastic effects on pulse propagation are awaited from this strong nonlinear behaviour
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