## 1 Introduction  
For nonlinear optics and time resolved spectroscopic studies of atoms, molecules and biological processes there is a rising need for ultrashort light pulses in the UV and deep UV spectral domain. Straightforward frequency upconversion of short visible pulses in non-linear crystals already allows for the generation of sub-20 fs UV pulses [1] and 30 fs pulses down to 189 nm [2]. The phase-matching bandwidth can be increased by advanced angular dispersion methods leading to sub-10 fs pulses [3,4]. Other important techniques capable of generating UV pulses are four wave mixing in a gas filled hollow wave guide providing sub-10 fs pulses at 270 nm [5]. Third harmonic generation of the fundamental pulses at around 800 nm provide sub-30 fs pulses in air [6] or even 3-fs pulses in a Neon filled gas cell [7]. These techniques are characterized by a low conversion efficiency and require an elaborate experimental implementation. Pulse shortening can also be achieved through spectral broadening of longer UV pulses typically generated by frequency up conversion. Spectral broadening can be achieved by nonlinear phase modulation of the UV pulse in an optically transparent medium. In cross phase modulation the nonlinearity is induced by an auxiliary high intensity laser pulse. To increase the interaction length, gas filled hollow waveguides are often used. With this approach cross phase modulation between the fundamental and the third harmonic of a Ti:sapphire laser amplifier allows to compress the 266 nm pulses by a factor of 2 down to 23 fs [8]. For a 35 fs Ti:sapphire laser system this process helps to further broaden the intense 266 nm pulses originating from four-wave-mixing of blue and red pulses down to a length of 8-fs [9]. Efficient spectral pulse broadening in gas-filled hollow fibers or gas cells has also been achieved by self-phase modulation (SPM). With input pulse lengths of around 100 fs, deep UV pulses with an pulse energy of several hundred uJ as short as 20 fs centered at 268 nm [10] and 25 fs centered at 248 nm [11] were obtained. This technique however needs high input UV pulse energies up to several hundred uJ due to the low nonlinearity of the gas. Such high energy pulses are typically not available in spectroscopic setups, particularly at high repetition rates, and therefore the practical use of spectral broadening in gases is limited. Bulk materials and fibers in contrast offer medium to high nonlinearities and are suitable candidates for nonlinear phase modulation. In single-mode fibers the pulse power is typically limited to tens of nJ. Fibers are therefore best suited for high repetition laser systems [12]. Bulk materials can be operated at any input energy as the thickness of the material can be suitably adjusted so that self-focusing in the crystal is controlled and any serious beam distortion or material destruction can be avoided. Spectral broadening by SPM in bulk material has been first observed in the visible by [PERSON] _et al._[13] and later in the UV by [PERSON] _et al._[14]. Theoretically, it was shown that in bulk material cross phase modulation should be capable of generating chirp free nearly-single cycle pulses [15]. The shortening of IR and VIS pulses by SPM in bulk materials has been realized successfully in the 1980s [16,17] and it was envisioned that this approach can be extended into the UV spectral domain [17]. In this paper we demonstrate that SPM can indeed be used to shorten UV pulses below their input Fourier limit. Taking into account the nonlinear processes of self-focusing and SPM we use simulations to find the optimal conditions for UV spectral broadening in common crystals. We then describe our experimental results on spectral broadening of UV pulses in a CaF\({}_{2}\) plate and describe theway to achieve shorter pulses by incorporating a few supplemental parts into the setup. Compression of the pulses to nearly their Fourier limit is demonstrated by implementing an acousto-optical pulse shaper not necessary if the residual higher order chirp is of no concern.