## 6 Summary and Perspectives  
In this work we have shown that SPM in bulk materials can be readily used to broaden the spectrum of \(\upmu\)J femtosecond UV pulses and to reduce the pulse length. The broadening stage is most conveniently incorporated into an optical setup by using two lenses or spherical mirrors with a slight off-axis geometry and a crystal plate and thus relies only on components present in most ultrafast spectroscopy laboratories. There is no need for a specially designed fiber or meter long gas cells.  
By focusing the UV beam into the crystal the high pulse intensity leads to self-phase modulation and self-focusing. In contrast to techniques using gas filled cells [10] where self-focusing induced filamentation is a prerequisite for broadening, we find that the self-focus should be kept just outside the crystal, about 25% of the crystal thickness. This leads to broadening without serious beam distortions and to no beam break-up. Under this condition the pulse energy is nearly conserved as multi-photon absorption is minimized. It allows a maximum achievable broadening of up to a factor of three. Thin crystals with high nonlinearity in conjunction with reasonably tight focusing are found optimal as this keeps the negative influence of the strong group delay dispersion in the UV and the  
Figure 8: SPM broadened UV pulses in collinear two-dimensional spectroscopy as pump and probe pulses. The absorptive signal resolves the pump-probe frequency correlation of the ground-state bleach of Pyrene in methanol for a population delay of 100 ps (**a**). The spectral intensity of the pulses (blue hashed areas) and the absorption spectrum of Pyrene (black line) are shown in the right (**b**) and lower side panel (**c**).  
associated pulse lengthening to a minimum. A slight up chirp of the input pulse helps to enhance the broadening. Experimentally we find that a few millimeter thick CaF\({}_{2}\) crystal delivers the best results in terms of broadening and transmission efficiency. Once the pulse energy and length, and the crystal thickness is known, the beam size at the plate can be determined from the well known formula for the self-focal length.  
Temporal compression of the broadened pulses is done with a simple prism compressor for the regime down to about 50 fs. A compression by a factor of two to three is demonstrated. For even shorter pulses an AOPDF (DAZZLER) is added for the compensation of higher order chirp components. For pulses at 312 nm and 273 nm this allowed for a near Fourier limited compression down to 15 fs and 20 fs. If the pulses do not have to be perfectly compressed, even larger spectral broadening can be achieved.  
The beam profile is quite acceptable for spectroscopic applications. The use of a circular aperture to select the central part of beam is often helpful. Still many tens of percent of the pulse energy can be utilized with the resulting new-Gaussian transversal profile.  
We expect that SPM in bulk material can be employed for a large range of energies as the focusing conditions can be adapted for pulses less than a \(\upmu\)J to tens of \(\upmu\)J. Although here we only investigated the broadening of 120 and 25 fs input pulses, we envision that pulses at various pulse length can be broadened and compressed. The method likely will be best suited to situations where just an extra factor around two is needed for a successful experiment. This is, to our own experience, quite often the case.  
We thank [PERSON] for valuable discussions and suggestions. This work was supported by the Austrian Science Fund within the framework of the Special Research Program F16 (Advanced Light Sources), by the SFB749 and by the DFG-Cluster of Excellence: Munich-Centre for Advanced Photonics.