2010, Brée, Carsten, Demircan, Ayhan, Steinmeyer, Günter

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.

2011, Brée, Carsten, Demircan, Ayhan, Bethge, Jens, Nibbering, Erik T.J., Skupin, Stefan, Bergé, Luc, Steinmeyer, Günter

Self-compression of multi-millijoule laser pulses during filamentary propagation is usually explained by the interplay of self-focusing and defocusing effects, causing a substantial concentration of energy on the axis of the propagating optical pulse. Recently, it has been argued that cell windows may play a decisive role in the self-compression mechanism. As such windows have to be used for media other than air their presence is often unavoidable, yet they present a sudden non-adiabatic change in dispersion and nonlinearity that should lead to a destruction of the temporal and spatial integrity of the light bullets generated in the self-compression mechanism. We now experimentally prove that there is in fact a self-healing mechanism that helps to overcome the potentially destructive consequences of the cell windows. We show in two carefully conducted experiments that the cell window position decisively influences activation or inhibition of the self-healing mechanism. A comparison with a windowless cell shows that presence of this mechanism is an important prerequisite for the exploitation of self-compression effects in windowed cells filled with inert gases.

2013, Borchers, Bastian, Brée, Carsten, Birkholz, Simon, Demircan, Ayhan

We discuss the influence of the higher-order Kerr effect (HOKE) in wide band gap solids at extreme intensities below the onset of optically induced damage. Using different theoretical models, we employ multiphoton absorption rates to compute the nonlinear refractive index by a Kramers-Kronig transform. Within this theoretical framework we provide an estimate for the appearance of significant deviations from the standard optical Kerr effect predicting a linear index change with intensity. We discuss the role of the observed saturation behavior in practically relevant situations, including Kerr lens mode-locking and supercontinuum generation in photonic crystal fibers. Furthermore we present experimental data from a multi-wave mixing experiment in BaF2 which can be explained by the appearance of the HOKE.

2010, Brée, Carsten, Demircan, Ayhan, Steinmeyer, Günter

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.

2011, Brée, Carsten, Demircan, Ayhan, Steinmeyer, Günter

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 $n_2$ 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 $n_2$ 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.