Boaz Ilan

Multiple filamentation induced by input-beam ellipticity

We provide what is to our knowledge the first experimental evidence that multiple filamentation (MF) of ultra-short pulses can be induced by input beam ellipticity. Unlike noise-induced MF, which results in complete beam breakup, the MF pattern induced by small input beam ellipticity appears as a result of nucleation of annular rings surrounding the central filament. Moreover, our experiments show that input beam ellipticity can dominate the effect of noise (transverse modulational instability), giving rise to predictable and highly reproducible MF patterns. The results are explained with a theoretical model and simulations.

Self-Focusing with Fourth-Order Dispersion

We analyze self-focusing and singularity formation in the nonlinear Schrodinger equation (NLS) with high-order dispersion

Vectorial and random effects in self-focusing and in multiple filamentation

i \psi_t \pm \Delta^q \psi + |\psi|^{2 \sigma} \psi = 0,

Control of multiple filamentation in air

in the isotropic mixed-dispersion NLS

Self-focusing of elliptic beams: an example of the failure of the aberrationless approximation

i \psi_t + \Delta \psi +\epsilon \Delta^2 \psi + |\psi|^{2 \sigma} \psi = 0

Self-focusing Distance of Very High Power Laser Pulses.

, and in nonisotropic mixed-dispersion NLS equations which model propagation in fiber arrays.

Deterministic vectorial effects lead to multiple filamentation

The standard explanation for multiple filamentation of laser beams is that breakup of cylindrical symmetry is initiated by noise in the input beam. In this study we propose an alternative deterministic explanation based on vectorial effects. We derive a scalar equation from the vector Helmholtz equation that describes self-focusing in the presence of vectorial and nonparaxial effects. Numerical simulations of the scalar equation show that when the input beam is sufficiently powerful, vectorial effects lead to multiple filamentation. We compare multiple filamentation due to vectorial effects with the one due to noise, and suggest how to decide which of the two leads to multiple filamentation in experiments. We also show that vectorial effects and nonparaxiality have the same effect on self-focusing of a single filament, leading to the arrest of catastrophic collapse, followed by focusing–defocusing oscillations. The magnitude of vectorial effects is, however, significantly larger than that of nonparaxiality.

Theory of magnetodynamics induced by spin torque in perpendicularly magnetized thin films.

In this Letter we provide what is believed to be the first experimental evidence of suppression of the number of filaments for high-intensity laser pulses propagating in air by beam astigmatism. We also show that the number, pattern, and spatial stability of the filaments can be controlled by varying the angle that a focusing lens makes with the axial direction of propagation. This new methodology can be useful for applications involving atmospheric propagation, such as remote sensing.

Monte-Carlo simulations of light propagation in luminescent solar concentrators based on semiconductor nanoparticles

We show that the increase in critical power for elliptic input beams is only 40% of what had been previously estimated based on the aberrationless approximation. We also find a theoretical upper bound for the critical power, above which elliptic beams always collapse. If the power of an elliptic beam is above critical, the beam self-focuses and undergoes partial beam blowup, during which the collapsing part of the beam approaches a circular Townesian profile. As a result, during further propagation additional small mechanisms, which are neglected in the derivation of the nonlinear Schrodinger equation (NLS) from Maxwell’s equations, can have large effects, which are the same as in the case of circular beams. Our simulations show that most predictions for elliptic beams based on the aberrationless approximation are either quantitatively inaccurate or simply wrong. This failure of the aberrationless approximation is related to its inability to capture neither the partial beam collapse nor the subsequent delicate balance between the Kerr nonlinearity and diffraction. We present an alternative two-stage approach and use it to analyze the effect of nonlinear saturation, nonparaxiality, and time dispersion on the propagation of elliptic beams. The results of the two-stage approach are found to be in good agreement with NLS simulations.

On the Galerkin/Finite-Element Method for the Serre Equations

We show numerically for continuous-wave beams and experimentally for femtosecond pulses propagating in air, that the collapse distance of intense laser beams in a bulk Kerr medium scales as 1/P;1/2 for input powers P that are moderately above the critical power for self focusing, but that at higher powers the collapse distance scales as 1/P.