## 3 High-Energy Few-Optical Cycle Parametric Sources in the IR Spectral Region
It involves spectral broadening of the pumping pulses by means of a combination of three-photons processes as Four Wave Mixing (FWM) and Self-Phase Modulation (SPM) [17]. By filtering the low-frequency components, a seed is obtained extending up to the near-IR. In this process, the CEP value of the driving pulses is preserved [18; 19; 20] so that a CEP-stable seed is produced provided that CEP-stable pump pulses are used. On the other hand, if the pump and seed have a well-defined phase relationship between them, a CEP-stable idler beam is obtained under parametric amplifications by phase differencing between the two contributing pulses. Finally, DFG occurs in second-order non-linear crystals excited by highly intense ultra-broadband pump pulses. It can be described as a two-photon process in which frequency differencing takes place, so that new components are generated in the long wavelength domain. As opposed to PSF and WLG, DFG potentially has the advantage of generating CEP-locked pulses in the near-IR and mid-IR, starting from non-CEP-stabilized driving pulses [21; 22]. Indeed, intrapulse DFG is associated with a phase differencing operation between the two mixing frequencies, thus providing insensitivity to pump CEP fluctuations [23]. In the following sections, a number of OPA implementations will be described based on different seed generation approaches, depending on the IR region of interest for amplification and on the CEP stabilization strategy. As a technological issue, the development of the high-energy ultrafast parametric source in the IR is strongly affected by the availability of suitable Non-Linear Optical (NLO) crystals. Indeed, NLO crystals have to be properly selected according to a number of requirements. In particular, crystals exhibiting a large second-order nonlinear coefficient and a broadband birefringent phase-matching over the near-IR and mid-IR spectral windows are required for an efficient parametric amplification. Moreover, in order to support high pumping power, crystals with high thermal conductivity and low absorption losses at the pump-pulse wavelengths have to be used. Importantly, transparency in the signal and idler spectral domain has to be fulfilled. Most OPA and OPCPA systems in the near-IR rely on \(\beta-BaB_{2}O_{4}\) (BBO) as a nonlinear crystal. A number of other materials, belonging to the oxide-family may be cited as ideal candidates for near-IR parametric amplification, as \(KH_{2}PO_{4}\) (KDP), \(KTiOAsO_{4}\) (KTA), \(KTiOPO_{4}\) (KTP), \(LiB_{3}O_{5}\) (LBO) and \(LiNbO_{3}\). However, so far, their exploitation was fundamentally limited by the difficulties associated with the fabrication process, preventing the growth of large-size crystals with a good optical quality. By using the above-mentioned NLO media, tunability can be achieved up to wavelengths on the order of a few \(\upmu\)m (shorter than \(\sim\)3 \(\upmu\)m) by the onset of IR absorption. Parametric amplification in the IR domain is at the cutting edge of parametric source technology. When the DFG process generates a seed pulse in the terahertz (THz) domain, the process is called optical rectification, and it is widely benefiting from a new generation of non-oxide materials capable of providing optical transparency up to tens of \(\upmu\)m, among which \(AgGaSe_{2}\) (AGSe), \(ZnGeP_{2}\) (ZGP), GaSe, GaP and ZnTe. A full list of NLO materials is provided in [24] for applications in the near-IR and in [25; 26; 27] for applications in the mid-IR. An overview on the main technological solutions applied in the currently available high-energy few-cycle parametric amplifiers can be inferred from the following paragraphs, where a detailed description of the different experimental setups is reported.