465 research outputs found
Resonantly enhanced filamentation in gases
In this Letter, a low-loss Kerr-driven optical filament in Krypton gas is
experimentally reported in the ultraviolet. The experimental findings are
supported by ab initio quantum calculations describing the atomic optical
response. Higher-order Kerr effect induced by three-photon resonant transitions
is identified as the underlying physical mechanism responsible for the
intensity stabilization during the filamentation process, while ionization
plays only a minor role. This result goes beyond the commonly-admitted paradigm
of filamentation, in which ionization is a necessary condition of the filament
intensity clamping. At resonance, it is also experimentally demonstrated that
the filament length is greatly extended because of a strong decrease of the
optical losses
Field-free two-direction alignment alternation of linear molecules by elliptic laser pulses
We show that a linear molecule subjected to a short specific elliptically
polarized laser field yields postpulse revivals exhibiting alignment
alternatively located along the orthogonal axis and the major axis of the
ellipse. The effect is experimentally demonstrated by measuring the optical
Kerr effect along two different axes. The conditions ensuring an optimal
field-free alternation of high alignments along both directions are derived.Comment: 5 pages, 4 color figure
Orientation and Alignment Echoes
We present what is probably the simplest classical system featuring the echo
phenomenon - a collection of randomly oriented free rotors with dispersed
rotational velocities. Following excitation by a pair of time-delayed impulsive
kicks, the mean orientation/alignment of the ensemble exhibits multiple echoes
and fractional echoes. We elucidate the mechanism of the echo formation by
kick-induced filamentation of phase space, and provide the first experimental
demonstration of classical alignment echoes in a thermal gas of CO_2 molecules
excited by a pair of femtosecond laser pulses
Higher-order Kerr terms allow ionization-free filamentation in gases
We show that higher-order nonlinear indices (, , , )
provide the main defocusing contribution to self-channeling of ultrashort laser
pulses in air and Argon at 800 nm, in contrast with the previously accepted
mechanism of filamentation where plasma was considered as the dominant
defocusing process. Their consideration allows to reproduce experimentally
observed intensities and plasma densities in self-guided filaments.Comment: 11 pages, 6 figures (11 panels
Spectral dependence of purely-Kerr driven filamentation in air and argon
Based on numerical simulations, we show that higher-order nonlinear indices
(up to and , respectively) of air and argon have a dominant
contribution to both focusing and defocusing in the self-guiding of ultrashort
laser pulses over most of the spectrum. Plasma generation and filamentation are
therefore decoupled. As a consequence, ultraviolet wavelength may not be the
optimal wavelengths for applications requiring to maximize ionization.Comment: 14 pages, 4 figures (14 panels
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