| Abstract [eng] |
In recent years, a growing interest in ultrafast mid-infrared (MIR) laser sources, widely used in spectroscopy, high-harmonic generation, and as pump sources for MIR optical parametric amplifiers was observed. Potential applications of MIR radiation in nonlinear optics have driven an increasing demand for new nonlinear materials suitable for this spectral range. Particularly, MIR optical parametric amplifiers have gained significant popularity, requiring a supercontinuum, generated in an MIR-transparent medium, as a seed source. In this work, supercontinuum generation, pumped by 180 fs duration, 2,3 µm central wavelength pulses from optical parametric amplifier, was studied in thirteen different nonlinear optical crystals: sapphire, YAG, GGG, yttrium oxide, diamond, potassium gadolinium tungstate, calcium tungstate, thallium bromo-iodide, two samples of polycrystalline ZnS and one monocrystalline, and two samples of polycrystalline ZnSe. A relation between the achievable supercontinuum red-shift extent and the material's properties: band gap, group velocity dispersion (GVD), and effective dispersion length was investigated. The tested materials have a wide range of properties: tested media ranged from wide bandgap dielectrics to narrow bandgap semiconductors. Some of the crystals were in the normal GVD regime for pump wavelength; others fell into the anomalous GVD regime. For used pump radiation, no relation was observed between achieved spectral broadening to the red side and band gap or dispersion properties of the material. However, a well-known supercontinuum blue-shift dependence on the band gap was observed during our experiments. Supercontinuum generation in polycrystalline ZnSe and ZnS crystals was analyzed. Results show, that spectral extent of the supercontinuum depends on collection of crystallites in the pulse path. Spectral extent, harmonic generation efficiency and optimal driving pulse energy of the supercontinuum, generated in polycrystalline media, is highly dependent on the polycrystalline sample. The influence of focusing conditions on supercontinuum spectral broadening was investigated. Results show, that maximum spectral broadening to the red-side in normal GVD materials is achieved when light filament forms inside the crystal. However, in the anomalous GVD regime, best red-shifted spectral broadening is produced when a filament forms on the back surface of the crystal. The results of numerical simulations suggests, that in the anomalous GVD regime, red wavelengths, trailing behind the most intense part of the pulse, are absorbed by the plasma, unless filament forms on the back surface of the medium. Supercontinuum generation in sapphire and YAG samples was successfully achieved with driving pulse energy below self-focusing threshold. Such result can be explained the fact that the materials with anomalous GVD for the driving pulse, pulse self-compressed before the supercontinuum generation starts, which results in the rise of a peak power of the pulse. Thus, supercontinuum can be generated in the anomalous GVD regime even though peak power of the driving pulse is below critical self-focusing value. |
