Light can be used to modify and control properties of media, as in the case of electromagnetically induced transparency or, more recently, for the generation of slow light or bright coherent XUV and X-ray radiation. Particularly unusual states of matter can be created by light fields with strengths comparable to the Coulomb field that binds valence electrons in atoms, leading to nearly-free electrons oscillating in the laser field and yet still loosely bound to the core [1,2]. These are known as Kramers-Henneberger states [3], a specific example of laser-dressed states [2]. Here, we demonstrate that these states arise not only in isolated atoms [4,5], but also in rare gases, at and above atmospheric pressure, where they can act as a gain medium during laser filamentation. Using shaped laser pulses, gain in these states is achieved within just a few cycles of the guided field. The corresponding lasing emission is a signature of population inversion in these states and of their stability against ionization. Our work demonstrates that these unusual states of neutral atoms can be exploited to create a general ultrafast gain mechanism during laser filamentation.

We present the experimental realization of a method to generate predetermined, arbitrary pulse shapes after transmission through an optical fiber in the nonlinear regime. The method is based on simulating the reverse propagation of the desired pulse shape in the fiber. First, linear and nonlinear parameters of a single-mode step-index fiber required for the simulation are determined. The calculated pulse shapes are then generated in a pulse shaper.

We investigate the evolution of ultrashort pulses with an antisymmetric spectral phase during propagation through an optical fiber in presence of nonlinear effects. The shaped pulses are then applied for selective excitation of nonresonant two-photon transitions. Both numerical simulations and measurements confirm that a certain class of antisymmetric phase, a π-step, remains approximately antisymmetric-and is therefore suitable for the selective excitation-even though the pulse spectrum is significantly modified by self-phase modulation. Second-harmonic generation is used as a model two-photon transition. Furthermore, the capability of generating two perpendicularly polarized subpulses with independently shaped phase is demonstrated.

We show that nonlinear effects like self-phase modulation in a sample have to be considered for phase control of three-photon excitations. Furthermore, we demonstrate the control of three-photon excitation of L-Tryptophan in water using a pulse-shaping setup. Simulations of the propagation of the laser pulses in the cuvette exhibit good agreement with the experimental fluorescence scans at different laser intensities and show large discrepancies when neglecting nonlinear effects prior to the three-photon process. This can lead to improvements in selective excitation of amino acids by a near-infrared femtosecond laser source.

We demonstrate selective excitation of dyes with overlapping absorption spectra in solution with pulses transmitted through a hollow-core fiber. Thereto we show how dispersive effects occurring in the fiber can be compensated and what the limiting pulse energies are. Furthermore, an overview over various phase parametrizations is given and we examine which are best used when optimizing a two-photon fluorescence contrast of two dyes in a sample. This could be relevant for future endoscopic applications as well as state of the art two-photon microscopy.