# Ultraviolet Transient Absorption Spectrometer with Sub-20-fs Time Resolution
## 3 Experimental Setup
### UV Pulse Characterization  
In TA spectroscopy with WLC probe and broadband detection, the time resolution is determined essentially by the duration of the pump pulse [11]; for this reason, it is crucial to precisely characterize its temporal profile. For this purpose, we adopt a modified scheme of two-dimensional spectral shearing interferometry (2 DSI), recently developed by some of the authors [75], which is suitable for the full characterization of UV pulses. The 2 DSI method [76; 77] relies on spectral shearing interferometry with zero delay; with respect to the SPIDER technique, which encodes phase as a sensitively calibrated fringe in the spectral domain, 2 DSI robustly encodes phase along a second dimension. In traditional 2 DSI, the pulse under test is nonlinearly mixed with two collinear fields, named ancillae, sheared by a frequency \(\Omega\), resulting in two co-propagating replicas \(A(\omega)\) and \(A(\omega-\Omega)\), which give rise to a spectral interference pattern. If one of the ancillae, at angular frequency \(\omega_{\text{CW}}\), is delayed by \(\tau_{\text{CW}}\), the corresponding interferogram can be written as:  
\[\begin{array}{rcl}I(\omega,\tau_{\text{CW}})&=&\left|A(\omega)\right|^{2}+ \left|A(\omega-\Omega)\right|^{2}\\ &&\qquad+2\left|A(\omega)\right||A(\omega-\Omega)|cos[\omega_{\text{CW}}\tau_ {\text{CW}}+\phi(\omega)-\phi(\omega-\Omega)]\end{array} \tag{1}\]  
where \(\phi(\omega)\) is the spectral phase of the pulse under test. By recording the spectra as a function of \(\tau_{\text{CW}}\), one obtains a 2D map which displays, for each value of \(\omega\), interference fringes along the \(\tau_{\text{CW}}\) axis with phase: \(\theta(\omega)\ =\ \phi(\omega)-\phi(\omega-\Omega)\), from which one obtains by integration the spectral phase as:  
\[\phi(\omega)\cong\frac{1}{\Omega}\int\theta(\omega)d\omega \tag{2}\]  
Equation (2) shows that a precise knowledge of the spectral shear \(\Omega\) is critical for an accurate retrieval of the spectral phase. In traditional 2 DSI the two sheared ancillae are generated by passing a  
Figure 1: Experimental setup for the generation of few-optical-cycle UV pulses, based on: (a) SHG of the NOPA pulse and (b) SFG between the NOPA pulse and a fraction of the FW laser source. DL: delay line; DM: dichroic mirror; C: combiner.  
strongly chirped pulse (which may be a replica of the pulse under test or an auxiliary pulse) through an unbalanced Michelson interferometer which creates two delayed replicas, and which is also used to scan the delay \(\tau_{\text{CW}}\). This configuration requires an indirect determination of the spectral shear based on the chirp of the pulse and the delay of the replicas. Our 2 DSI scheme, on the other hand, produces the sheared ancillae by spectral filtering, which allows the direct measurement of \(\Omega\) with a spectrometer. The experimental setup is shown in Figure 2: a fraction of the FW is split off and sent to a 4f zero-dispersion pulse shaper [78], using a 2000 lines/mm diffraction grating as dispersive element. The two nearly monochromatic ancillae, with adjustable bandwidth and spectral shear, are produced by a double-slit mask placed close to the Fourier plane of the pulse shaper. Finally, the folding mirror of the shaper is replaced by two independent mirrors, one of which is mounted on a precision translation stage which allows the scanning of \(\tau_{\text{CW}}\). The two collinear ancillae are then synchronized with the pulse under test, and both beams are non-collinearly focused onto a nonlinear crystal for DFG (10-\(\upmu\)m-thick BBO cut for Type I phase matching with \(\theta=29^{\circ}\)). The nonlinear signal is then coupled to a spectrometer, and its spectrum is recorded as a function of \(\tau_{\text{CW}}\), giving the 2 DSI map, from which the spectral phase of the pulse \(\phi(\omega)\) is extracted. The correctness of the calibration of the shear is typically checked by characterizing the dispersion introduced by materials of known thickness and comparing it to tabulated values.