59 research outputs found
Adiabatic Floquet model for the optical response in femtosecond filaments
The standard model of femtosecond filamentation is based on phenomenological
assumptions which suggest that the ionization-induced carriers can be treated
as free according to the Drude model, while the nonlinear response of the bound
carriers follows the all-optical Kerr effect. Here, we demonstrate that the
additional plasma generated at a multiphoton resonance dominates the saturation
of the nonlinear refractive index. Since resonances are not captured by the
standard model, we propose a modification of the latter in which ionization
enhancements can be accounted for by an ionization rate obtained from
non-Hermitian Floquet theory. In the adiabatic regime of long pulse envelopes,
this augmented standard model is in excellent agreement with direct quantum
mechanical simulations. Since our proposal maintains the structure of the
standard model, it can be easily incorporated into existing codes of filament
simulation.Comment: 14 pages, 6 figures, submitted to New Journal of Physic
Femtosecond filamentation by intensity clamping at a Freeman resonance
We demonstrate that Freeman resonances have a strong impact on the nonlinear
optical response in femtosecond filaments. These resonances decrease the
transient refractive index within a narrow intensity window and strongly affect
the filamentation dynamics. In particular, we demonstrate that the peak
intensity of the filament can be clamped at these resonances, hinting at the
existence of new regimes of filamentation with electron densities considerably
lower than predicted by the standard model. This sheds a new light on the
phenomenon of filamentary intensity clamping and the plasmaless filaments
predicted by the controversial higher-order Kerr model
Saturation of the all-optical Kerr effect
Saturation of the intensity dependence of the refractive index is directly computed from ionization rates via a Kramers-Kronig transform. The linear intensity dependence and its dispersion are found in excellent agreement with complete quantum mechanical orbital computations. Higher-order terms concur with solutions of the time-dependent Schrödinger equation. Expanding the formalism to all orders up to the ionization potential of the atom, we derive a model for saturation of the Kerr effect. This model widely confirms recently published and controversially discussed experimental data and corroborates the importance of higher-order Kerr terms for filamentation
Kramers--Kronig relations and high order nonlinear susceptibilities
As previous theoretical results recently revealed, a Kramers-Kronig transform of multiphoton absorption rates allows for a precise prediction on the dispersion of the nonlinear refractive index in the near IR. It was shown that this method allows to reproduce recent experimental results on the importance of the higher-order Kerr effect. Extending these results, the current manuscript provides the dispersion of for all noble gases in excellent agreement with reference data. It is furthermore established that the saturation and inversion of the nonlinear refractive index is highly dispersive with wavelength, which indicates the existence of different filamentation regimes. While shorter laser wavelengths favor the well-established plasma clamping regime, the influence of the higher-order Kerr effect dominates in the long wavelength regime
Modulation instability in filamentary self-compression
We numerically analyze filamentary propagation for various medium- and input pulse parameters and show that temporal self-compression can greatly benefit from refocusing events. Analyzing the dynamical behavior in the second focal spot, it turns out that a dispersive temporal break-up may appear due to the emission of a hyperbolic shock-wave from the self-steepened trailing edge of the pulse. This break-up event enhances the self-compression capabilities of laser filaments, enabling up to 12-fold temporal compression. Only slightly perturbing the input pulse parameters, we further identify a regime in which refocusing events give rise to extended subdiffractive propagation in a weakly ionized channel
Spatio-temporal pulse propagation in nonlinear dispersive optical media
We discuss state-of-art approaches to modeling of propagation of ultrashort optical pulses in one and three spatial dimensions.We operate with the analytic signal formulation for the electric field rather than using the slowly varying envelope approximation, because the latter becomes questionable for few-cycle pulses. Suitable propagation models are naturally derived in terms of unidirectional approximation
Cancellation of Raman self-frequency shift for compression of optical pulses
We study to which extent a fiber soliton can be manipulated by a specially chosen continuous pump wave. A group velocity matched pump scatters at the soliton, which is compressed due to the energy/momentum transfer. As the pump scattering is very sensitive to the velocity matching condition, soliton compression is quickly destroyed by the soliton self-frequency shift (SSFS). This is especially true for ultrashort pulses: SSFS inevitably impairs the degree of compression. We demonstrate numerically that soliton enhancement can be restored to some extent and the compressed soliton can be stabilized, provided that SSFS is canceled by a second pump wave. Still the available compression degree is considerably smaller than that in the Raman-free nonlinear fibers
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