| Abstract [eng] |
High intensity and short duration laser pulses can experience self-focusing phenomena while traveling a nonlinear optical medium and focus down to a light filament. During this process supercontinuum occurs and the optical spectrum of the pulse suddenly broadens, covering the whole visible spectrum. As the laser pulse is focusing down to very small dimensions, the energy density inside the pulse can reach the damage threshold of the material, causing it to degrade or eventually break down. The breakdown of the material is unwanted since it reduces the effectiveness of supercontinuum generation or eventually stops it. Therefore, using and finding new methods to avoid laser-induced material damage is of great interest. In this work a numerical simulation was written which can model the propagation of a laser pulse and various nonlinear phenomena which are key for supercontinuum generation. The pulse is modelled in a medium with a modified spatial dispersion – photonic crystal. Photonic crystals have various uses due to their optical bandgaps, but the effect of photonic crystals on beam propagation was more important for this work. An optimized photonic crystal can oppose the self-focusing of the pulse, disallowing it from reaching energy thresholds responsible for laser-induced damage. The modified spatial dispersion curve was chosen such that the top of it would be flat, which allows the beam to travel without experiencing the effect of diffraction. The width of the flat top influences how the beam is affected by diffraction and other spatial phase altering effects. Supercontinuum generation was modelled in a regular material after writing the numerical simulation. A 50 "fs" pulse of 1030 "nm" central wavelength and 200 "μm" diameter was used. This pulse propagated through a fused silica crystal of 4 "cm" length. Initial pulse energy was varied from 0.6 "μJ" to 1.3 "μJ" while measuring the output spectra, minimal pulse diameter, intensity, plasma density, supercontinuum signal, spectrum width. Later an initial pulse of 0.95 "μJ" was chosen to model how photonic crystals of different modified spatial dispersion curve top width would affect the same parameters as for the regular material. During this work was shown, that a photonic crystal with a modified spatial dispersion curve which was sufficiently flat enough could stop the supercontinuum generation entirely. Additionally, photonic crystals can reduce the plasma density and the peak intensity of the pulse during supercontinuum generation. |
