# Ultraviolet Transient Absorption Spectrometer with Sub-20-fs Time Resolution
## UV Pulse Generation and Characterization
### Pulse Characterization  
The application of UV pulses in TA spectroscopy requires the precise management of dispersion to keep the pulse duration as short as possible, thus maximizing the time resolution of the experiment. The measurement of the temporal duration requires the knowledge of the spectral amplitude and phase [54]. The characterization of the spectral phase is however not straightforward in the UV range since, due to the lack of transparent materials and of suitable phase-matching configurations, no traditional techniques based on SHG or SFG can be applied. For this reason, autocorrelation techniques in the UV exploit multi-photon absorption as nonlinear process. A first approach consists in utilizing the two-photon conductivity of a substrate and measuring the generated charge by using a pair of electrodes [55; 56], with the main drawback of a complex manufacture of the electrodes. In gas phase, two-photon induced fluorescence of excimers [57] or gas ionization [58] have been exploited. Another group of techniques uses two-photon absorption (TPA) in liquids and bulk materials [59; 60; 61; 62]. For this purpose, the beam is divided in two replicas: when they temporally overlap, a contribution due to TPA arises, thus creating an autocorrelation signal. Finally, [PERSON] et al. proposed a sharper-assisted cross-correlation setup by combining a pulse modulator and a solar-blind photomultiplier tube [63]. Such techniques, however, do not allow the full reconstruction of the light pulse in amplitude and phase. Among the approaches for complete pulse reconstruction, the so-called spectrographic techniques aim at evaluating the relative arrival time of spectral slices of the pulse. A measurement generates thus a 2D map as a function of time and frequency--known as spectrogram--from which it is possible to extract the spectral phase. The most well-known example of these techniques is the frequency-resolved optical gating (FROG) [64]. In FROG the pulse is split in two replicas which are superimposed in a nonlinear crystal. The delay of one of the replicas is varied and the SHG signal is registered in a spectrometer as a function of the delay yielding the spectrogram. With an iterative algorithm it is then possible to retrieve the spectral phase of the pulse. Unfortunately, this approach cannot be directly applied in the UV due to the lack of suitable SHG crystals in this range. Another scheme, the polarization gating (PG) FROG, which exploits a \(\chi^{(3)}\) polarization-gate arrangement, was successfully extended by [PERSON] et al. to the UV using quartz as the nonlinear medium [65]. Other variants of FROG have been also demonstrated for the measurement of ultrashort pulses in the UV. One of the most common approaches is the cross-correlation FROG (XFROG) [66], which exploits a fully-characterized reference pulse beam for difference frequency generation (DFG) with the unknown UV pulse. The amplitude and phase of the test pulse are therefore retrieved from the corresponding spectrogram and from the fully characterized reference. Self-diffraction (SD) FROG has also been applied in the UV range, although it requires comparatively high energies in the uJ range. SD-FROG retrieves the temporal information on the pulse from the diffracted beam scattered by the crossing of two replicas in a material. [PERSON] et al. measured 16-fs pulses at a central wavelength of 266 nm by using this method [32]. [PERSON] et al. combined XFROG with a technique of tomographic ultrafast retrieval of transverse light E-fields (TURTLE) [67]. In XTURTLE two orthogonal linear projections at 0\({}^{\circ}\) and 90\({}^{\circ}\) and an additional projection, for example at 45\({}^{\circ}\), of the vector-shaped test fields in the UV are down-converted with a fully characterized reference pulse. The first two maps (at 0\({}^{\circ}\) and 90\({}^{\circ}\)) are inserted in the XFROG algorithm to retrieve the phase. In a second step, a TURTLE algorithm combines those measurements to model the third component at 45\({}^{\circ}\).