## 4 Applications
### Attosecond Pulse Generation  
The HHG process is at the base of attosecond pulse generation. As discussed in the previous section, this process allows for the emission of a broad spectrum of radiation as a result of an upconversion mechanism for which high-order harmonics of the driving pulse frequency are generated. Such an extended spectrum supports pulse duration on the attosecond time scale. As we have already discussed, the HHG spectrum depends on the wavelength of the driving radiation, so that the longer is the wavelength, the broader is the spectrum. In particular, when near-IR and mid-IR pulses are used, a harmonics signal can be obtained, extending up to the soft X-ray spectral region. Thus, long-wavelength driving pulses provide the advantage of supporting the generation of attosecond pulses shorter than those achievable using common Ti:Sa 800-nm driving pulses. Moreover, the soft X-ray spectral window is attained, which is particularly attractive for spectroscopy. X-ray-based attosecond pulses can be exploited for studying electron dynamics involving deep electronic states in atomic, molecular and condensed-matter systems. An additional benefit, connected to the use of pulses in the long-wavelength range, is related to the temporal structure of the high-order harmonics radiation that is affected by a non-linear temporal dispersion, known as attochirp. Attochirp arises from the temporal distribution of the ponderomotive energies acquired by the electrons under the effect of the driving field. This chirp depends on the wavelength of the driving radiation according to an inverse scaling law. As a consequence, long-wavelength pulses allow for a reduction of the attochirp, thus helping in the generation of transform-limited attosecond pulses [79]. In HHG, the emission of XUV and X-ray radiation is distributed all along the driving pulse duration. However, in order to obtain a single attosecond pulse for each driving pulse (namely an Isolated Attosecond Pulse (IAP)), emission from a single half-cycle of the driving field must be selected. Selection can be achieved either by confining radiation in a sub-cycle temporal window or by filtering a well-defined portion of the HHG spectrum. In the first case, a control on the HHG process is required, which can be achieved by properly tailoring the driving pulse waveform. For this purpose, a number of techniques have been developed, exploiting the dependence of HHG on pulse polarization [80; 81] and field amplitude profile [82]. In the second case, a high pass filter is applied to the harmonics spectrum [83]. Indeed, if few-cycle driving pulses are used, the high-energy part of the harmonics emission can be attributed to a single half-cycle, the one in which the amplitude gets its maximum value. All of these techniques are implemented by means of few-cycle CEP-stable driving pulses. Indeed, few cycle pulses provide a higher peak-to-peak contrast among successive optical cycles, thus favoring single half-cycle selection. Moreover, for optimizing such a contrast, the CEP has to be locked to a proper value. So far, IAPs at 800 nm have been successfully generated by means of the above-mentioned schemes. The extension to near-IR and mid-IR window is still under investigation, the main limiting factor being the low HHG efficiency at a longer wavelength. In this framework, the development of powerful parametric sources operating in these spectral regions has recently driven research. In the last few years, a number of results have been obtained demonstrating IAPs' emission in the long-wavelength range. The first experimental characterization of isolated attosecond pulses driven by 1.7 \(\upmu\)m was recently published by [PERSON] and coworkers [84]. Before [PERSON]'s works, fingerprints of IAPs have been already observed in HHG experiments performed with IR pulses. Millijoule-level few-cycle CEP-stable pulses at 1.5 \(\upmu\)m were used in combination with 800-nm pulses for performing HHG in noble gas [85; 86].