Ultrashort laser pulse propagation in materials with strong-coupling carriers

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The propagation of ultrashort, intense femtosecond laser pulses in optical materials is determined by light-matter-interactions. It results in a spatial and temporal development of energy density, phase and polarisation and in technologically relevant effects, such as self-phase modulation and self-focussing.Only recently, it has been shown that additional, non-instantaneous contributions to the nonlinear polarisation appear in materials with strong-coupling carriers, that rely on a carrier-induced lattice deformation. Characteristically, the formation dynamics is found in the sub-500-fs time regime, i.e. typical for modern, commercial ultrashort laser pulse systems. Thereby, an interaction with propagating femtosecond laser pulses becomes possible, that result in self-contained nonlinear optical effects and pulse coupling phenomena.From the viewpoint of solid state physics, the non-instantaneous, nonlinear polarisation relies on the self-trapping of carriers by strong coupling to the crystal lattice as self-trapped excitons or small, strong-coupling polarons and the resulting change of the electronic dipole moment. For the understanding of propagating ultrashort pulses, a comprehensive knowledge on the nonlinear interaction of coupled charge carriers with light is necessary, that is missing in literature, so far. Established, ultrafast measurement techniques rely on fixed pulse durations, that hinder the selective resolution of pulse propagation influence due to quasi-instantaneous and non-instantaneous contributions to the nonlinear polarisation. Multiscale modelling of nonlinear optical effects dependent on the pulse duration are missing; a complete control of pulse propagation in materials with strong-coupling carriers is not possible.The project aims to fill this gap and a systematic experimental and numeric study of pulse propagation and coupling using the pulse duration as central parameter will be performed.By combination of spectroscopic and nonlinear optical methods, a selective assignment of contributions of the non-instantaneous nonlinear polarisation to self-trapped excitons and small, strong-coupling polarons and their separation to the quasi-instantaneous polarization becomes possible. Due to pronounced resonant interactions of ultrashort laser pulses with these quasiparticles, it is necessary to precisely characterize the complex spectral signature over a broad spectral range. We expect that our results, with lithium niobate as a model system, will bring important information about pulse propagation in the applicationary field of ultrafast photonics, such as e.g. for optical parametric amplifiers, but also may trigger novel applications.