# Attosecond Pulse Shaping by Multilayer Mirrors
## 4 Achieved Results and Examples of Multilayer Mirrors for Attosecond Pulses
### Above the Silicon L-Edge in the 100-150 eV Photon Range
#### 4.3.2 Cr/Sc Multilayer Mirror for Attosecond Pulses at 145 eV
The Cr/Sc material system combines all the advantages required for attosecond pulse applications: sufficient photon throughput due to the optimized reflectivity [43], suppression of out-of-band radiation (in the case of lanthanum-based systems, a thicker filter could increase the suppression in the 120 eV range, accompanied by a lower overall throughput), and a nearly Gaussian pulse profile close to the Fourier-limit, both in the temporal and spectral domain. The experimental realization of the Cr/Sc multilayer mirror was performed by a dual ion-beam sputtering technique [67] together with a tailored interface polishing process [43] for a higher mirror reflectivity. We also analyzed the mirror by two independent measurement techniques, EUV/soft X-ray reflectometry and hard X-ray reflectometry, to support the later characterization by attosecond electron streaking. These analyses are not shown here. The well-established EUV-pump/NIR-probe streaking technique [61] was used to characterize the attosecond pulses upon reflection from the (Cr/Sc) multilayer mirror. Here, both an attosecond (soft X-ray) pulse and a near-infrared laser pulse are focused by a double mirror into a neon (Ne) gas jet. The soft X-ray pulse photoionizes the Ne atoms (this frees photoelectrons from the 2\(p\) shell), which are then momentum-streaked by the co-propagating (temporally) synchronized and the phase-stabilized electric field of the NIR laser. The double mirror's inner core can be moved with respect to the outer ring. This introduces a temporal delay between the attosecond (soft X-ray) pulse (reflected from the mirror core) and the laser pulse (reflected from the outer ring). A typical streaking spectrogram is recorded by measuring the electron energy as a function of the delay between both pulses (Figure 7a). Figure 7: Measured attosecond electron streaking at a photon energy of 145 eV and the corresponding retrievals. This technique was used for attosecond pulse characterization in a neon gas jet utilizing the Cr/Sc multilayer mirror. (**a**) Measured electron streaking trace; (**b**) the retrieved trace performed by FROG/CRAB (frequency-resolved optical gating/complete reconstruction of attosecond burst) analyses; (**c**) the retrieved attosecond (soft X-ray) pulse (solid blue) and the phase (dotted red) in the spectral domain and for the temporal domain in (**d**). FROG/CRAB [63] analysis enables a complete reconstruction of both the intensity and the phase of the attosecond (soft X-ray) pulse, as well as the retrieval of the vector potential of the streaking laser field from a recorded spectrogram (Figure 7a). The result of the corresponding FROG/CRAB retrieval is shown in Figure 7b. Its theory is described in [65; 74]. The retrieved intensity and phase of the attosecond (soft X-ray) pulse are depicted in Figure 7, once in the spectral (c) and once in the temporal (d) domain. To get the photon energy, the electron energy is shifted by the binding energy of the Ne-2\(p\) electrons (\(E_{\mathrm{bin}}=21.6\) eV). The attosecond (soft X-ray) pulse shows a central energy of 145.6 eV, which is in good agreement with the target design and the previously described results. With the retrieved spectral bandwidth and phase being the most prominent sources of uncertainty, the temporal retrieval error can be approximated to about 20 as. The retrieved amplitude and phase give a pulse duration of about 580 as in the temporal domain. This is also in excellent agreement with the previous estimations (which was based solely on the reflectivity of the mirror and the assumption of a flat mirror phase). To compare this retrieved value with the duration limit, consider that a perfect Fourier-limited Gaussian pulse with a bandwidth of 2.9 eV (FWHM) has a duration of 629 as. To summarize this work, a Cr/Sc multilayer mirror was developed and optimized for reflecting isolated attosecond pulses at a central energy of 145 eV. The successful application was proven by attosecond electron streaking, which revealed a pulse duration of 580 as.