# Ultraviolet Transient Absorption Spectrometer with Sub-20-fs Time Resolution
## UV Pulse Generation and Characterization  
### Pulse Generation  
Few-optical-cycle light pulses are nowadays routinely generated in the visible and NIR ranges using different approaches: directly from a laser oscillator [24], by spectral broadening in a nonlinear medium (solid or hollow-core optical fiber) [25; 26] or by a non-collinear optical parametric amplifier (NOPA) [27; 28]. The extension of these techniques to the UV range is challenging for a variety of reasons. First of all, there are no broadband laser gain media in the UV, with the exception of excimers of noble gas halides [29; 30]. Nonlinear gain media based on OPAs are also difficult to implement in the UV, because they require pumping with high energy photons, which undergo strong two-photon absorption in all nonlinear media [31]. For these reasons, broadband UV pulses are typically generated by nonlinear frequency up-conversion of few-optical-cycle pulses in the visible or NIR ranges. Few-optical-cycle UV pulses have been generated by nonlinear optical processes in noble gases in a guiding geometry, using either a hollow-core optical fiber or a filament. The exploited nonlinear processes were third or higher order harmonic generation or four-wave-mixing (FWM) between the fundamental wavelength (FW) and the second harmonic (SH) of the driving pulse, typically generated by a Ti:sapphire laser. [PERSON] et al. produced 16-fs, 1-uJ pulses at 266 nm by third-harmonic generation (THG) in air of the 20-fs pulses produced by an amplified Ti:sapphire laser [32]. [PERSON] et al. obtained considerably shorter sub-4-fs pulses at 270 nm with 1-uJ energy by THG in a Ne cell of the 6-fs output of Ti:sapphire spectrally broadened in a hollow-core fiber [33]; the pulses were further shortened to 2.8-fs duration by [PERSON] et al. using a sub-4-fs driver pulse [34]. [PERSON] et al. produced 11-fs, 4-nJ vacuum UV (VUV) pulses at 166 nm from fifth harmonic generation of Ti:sapphire in an Ar-filled cell [35]. In a series of pioneering studies, [PERSON] et al. generated 8-fs pulses at 266 nm with uJ energy by FWM between the SH and the FW of a 20-fs Ti:sapphire laser [36; 37]. [PERSON] et al. reported on sub-20-fs VUV pulses at 160 nm with energies exceeding 400 nJ generated by FWM between the third harmonic of Ti:sapphire and the FW broadened by filamentation [38]. [PERSON] et al. employed broadband four-wave mixing for the generation of sub 10-fs deep-UV [39; 40; 41], which were further exploited as pump pulses in a TA spectrometer [42; 43; 44]. Even shorter wavelengths were produced by [PERSON] et al., who generated 5-nJ pulses at 133 nm by cascaded FWM between third, fourth and fifth harmonic of Ti:sapphire in a filament [45]. Nonlinear interactions in gases typically require high-energy short-duration (<50 fs) driving pulses and, due to the high order of the involved nonlinear process, they generate pulses whose shot-to-shot stability is not optimum for TA applications. For this reason, in many TA setups the UV pulses are generated by second-order nonlinear frequency conversion in crystals. Frequency up-conversion processes generally need to compromise between efficiency, which scales as the square of the thickness L of the nonlinear crystal, and phase matching bandwidth, which is inversely proportional to L. For the case of SHG, the phase-matching bandwidth is given by [46]\(\Delta v_{SH}~{}=~{}0.886/|\delta_{SH}|L\) where \(\delta_{SH}~{}=~{}\frac{1}{v_{gSH}}-\frac{1}{v_{gFW}}\) is the group velocity mismatch (GVM) between the FW and the SH. For the case of \(\beta\)-barium borate (BBO) and Type I (oeoe) phase matching, in the case of a FW visible pulse at \(\lambda\) = 600 nm wavelength, one has \(\delta_{\text{SH}}\) = 410 fs/mm, so that a crystal thickness of L = 8 um would be required to obtain a sub-5-fs UV pulse. With a thicker 50-um crystal UV pulses with \(\approx\)20-fs duration and 5-10 uJ energy were generated by frequency doubling a high-energy visible NOPA [47].