## 4 Applications
### Photoelectron Spectroscopy
The SFPH technique differs from LIED since it is based on the acquisition of an interference pattern, which is due to the superposition of a scattered and an unscattered photoelectron wave packet, both emitted from the same site by laser-induced tunnel ionization. Proof of principle of photoelectron holography was provided in 2011 by [PERSON] et al. [102] with the study of metastable xenon atoms with a hundred-fs Free Electron Laser (FEL) beam at 7 \(\upmu\)m. Subsequently, a Ti:Sa femtosecond laser system, delivering 40-fs pulses at 800 nm, was used for implementing SFPH in \(N_{2}\)[103]. Up to now, the possibility to extend SFPH to the few-cycle mid-IR regime is still under investigation [104]. Moreover, the few-cycle regime would allow one to confine photoelectrons, steering motion into a short temporal window, thus preventing multiple rescattering events and simplifying phase-retrieval procedures. Thus, few-cycle mid-IR parametric sources can currently be addressed as the most promising table-top alternative to FELs in SFPH experiments. Strong-field processes are intrinsically connected to the control of electron currents at an atomic scale with sub-optical cycle temporal resolution. The perspective for future technological applications in photonic devices is promoting research on strong-field phenomena at the condensed-matter level. The possibility to access this regime of interaction is however limited by the necessity to use an exciting radiation with an intensity below the material damage threshold. As mentioned in the HHG section, the use of mid-IR is beneficial, hindering multiphoton absorption and, thus, optical damage. Moreover, plasmonic nanostructures have been identified as ideal candidate for supporting strong-field physics in condensed-matter systems, as field confinement can lead to a strong field regime even with a lower intensity driving radiation. Recently, experimental results have been published demonstrating strong-field photoemission from plasmonic nanostructures in a non-destructive regime by means of few-cycle mid-IR pulses. [PERSON] and coworkers [105] observed sub-cycle photoelectron emission from isolated metal nano-tips over a broad spectral window, extending up to hundreds of eV. A femtosecond source tunable in the near-IR and in the mid-IR was used for exciting photoemission and for triggering space-charge effects in the nanostructures. The sub-wavelength field confinement at the tip was here identified as responsible for a huge enhancement of the kinetic energy of photoelectrons escaping from the metal. The experimental and simulated kinetic energy distributions of photoelectrons acquired at a number of IR field intensities for a driving wavelength of 3.8 \(\upmu\)m are shown in Figure 12a. Figure 12: **(a)** Experimental (circles) and simulated (solid lines) kinetic energy distributions of photoelectrons emitted from the nano-tips for increasing intensities at a wavelength of 3.8 \(\upmu\)m; **(b)** Electron energy distribution as a function of the wavelength. Curves represent the number of electrons with energies above the analyzer potential. Adapted with permission from [105], Copyright NPG, 2012. In addition, measurements at various wavelengths were performed in order to investigate the scaling dynamics of photoelectron kinetic energy (see Figure 12b). In this sense, an anomalous behavior was observed for increasing wavelength. Indeed, for wavelengths longer than 3.8 \(\upmu\)m, a new sub-cycle regime was identified, in which the photoelectrons' spectrum becomes substantially wavelength independent. The transition to this wavelength independent regime was attributed to the local field distribution, decaying over distances shorter than the photoelectron quiver amplitude. Apart from plasmon-assisted strong-field experiments, this regime is proven to be widely inaccessible because of the diffraction limit affecting the beam dimension.