By rotating the four-section π-shifted phase plate in the transverse plane relatively to the axes of the elliptical beam of 800-nm, 1.1-mJ, 35-fs pulse propagating in air, we switch between the regime of four parallel plasma channels and the regime of spatial symmetry breakup followed by on-axis plasma channel formation identified on the burnt paper images of the beam. Relaxation of the π-phase shift for 45° phase plate rotation is demonstrated explicitly in 3D+time carrier wave resolved numerical simulations yielding the initial step-like phase distribution degradation along the plasma region. This degradation becomes negligible as the angle between the ellipse major axis and the π-phase break line decreases to 15°.

Nonuniform azimuthal distribution of N 2 + fluorescence emitted from the femtosecond laser filament in air was discovered. The fluorescence is stronger when the detector is placed perpendicular or parallel to the laser polarization. The experimental results have been confirmed by the theoretical calculation that the azimuthal distribution of fluorescence is reproduced by the convolution of the transition of the dipole and the molecular alignment in the strong laser field. The results would provide new insight into laser−molecule interactions during filamentation, which are important in practice for remote sensing using filamentation.

The regulation of multiple filaments during an intense femtosecond laser propagation in a transparent optical medium is still challenging. In this study, by tracing the phase variation of the laser pulse during the propagation, the effect of the beam ellipticity on the generation of multiple filaments was investigated using the efficient π-phase plate. The spatial distribution of the generated multiple filaments did not follow the predesigned pattern of the phase plate when the laser beam was elliptic owing to the asymmetric change of the laser phase by the linear diffraction and nonlinear Kerr effect.

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