## 4 Results and Discussion  
### Experimental Characterization of Self-Phase Modulation Broadened UV Pulses  
With the insights gained by the simulations, we experimentally tested the broadening of UV pulses by focusing them into a CaF\({}_{2}\) crystal and subsequently compensate the phase of the broadened spectra. Figure 3: Setup for the generation, compression and characterization of UV pulses spectrally broadened by SPM. PC: prism compressor; SFM (SHG): sum frequency (second harmonic) generation; FS-W: Fused silica wedges; Ap: aperture; L1, L2 lenses. First we tuned the chirp of the input pulse and investigated the influence of the peak intensity on the SPM process. Subsequently, we varied the type and thickness of the broadening crystal. When the UV beam is tightly focused into a crystal we observe changes in the spectral intensity distribution (see Figure 4d-f). Experimental conditions can be found that allow the generation of new frequency components due to SPM. As commonly known for SPM of ultrashort laser pulses, we find that for a negatively chirped input pulse spectral narrowing occurs. For positively chirped pulses--the red components arrive before the blue ones--the output spectra are broadened [28]. Furthermore, for a specific positive chirp (measured to be about 260 fs\({}^{2}\)) spectra with a square like shape which are symmetric about the central frequency can be generated. Pulses with such spectra correspond to the maximal broadening observed in the experiment while the beam profile still has a homogeneous spatial intensity distribution. These characteristics are advantageous for applications like pump-probe spectroscopy. A simple way to vary and optimize the beam waist for a given crystal length and type and thus precisely adjust the peak intensity while using the full pulse energy is to move the crystal with respect to the focus of the UV beam. For this experiment we chose a 1 mm CaF\({}_{2}\) crystal which we move through the focus of a positively chirped pulse centered at 312 nm with a pulse energy of 1.96 \(\upmu\)J. The pulse length of the input pulse was measured by the UV-AC and found to be 63 fs. To determine the peak intensity at the entrance of the crystal we measured the size of the beam profile at the respective crystal positions with a CCD camera. When moving the crystal towards the focus, the output energy remains constant and is equal to the input energy minus the losses due to reflection. A drop in the output energy occurs for crystal positions  
Figure 4: Correlation between the peak intensity and the measured output energy (**a**), output Fourier limit (**b**) and the calculated self-focal length (**c**) obtained by moving a 1 mm CaF\({}_{2}\) crystal relative to the beam focus of a positively chirped pulse centered at 312 nm. Measured spectra and calculated Fourier limits of the input pulse (**d**) and output pulses for selected crystal positions (**e**, **f**). within \(\pm\) 1 cm of the focus. This resembles the profile of a _z-scan_ and is therefore attributed to multi-photon absorption (MPA) (Figure 4a, purple line). For every position of the crystal we measure the spectrum and calculated the Fourier limit of the output pulse (Figure 4b). The Fourier limit steadily decreases from 20.6 fs (input spectrum) down to 8 fs when the crystal is placed near the focus. In addition to the above measurements the self-focal length was determined for all respective crystal positions (see Figure 4c). The calculations were done by taking into account the convergence or divergence caused by the focusing of the beam according to ref. [29]. For crystal positions within \(\pm\) 1 cm of the focus we obtain a self-focal length smaller than the crystal thickness. When comparing the three plots in Figure 4a-c, it becomes clear, that the MPA induced drop in output power accompanied by increased shot-to-shot fluctuations occurs when the self-focus lies within the crystal.