## 3 High-Energy Few-Optical Cycle Parametric Sources in the IR Spectral Region  
In order to achieve high energy pulses in the IR as a result of a parametric amplification, high-energy pumping pulses are required. Indeed, the efficiency of the pump-to-signal energy conversion fundamentally depends on the system design and usually stands around a few tens of percent [8]. For this reason, mJ-level OPAs are mainly pumped by amplified femtosecond Ti:sapphire (Ti:Sa) lasers systems providing pulses at 800 nm with terawatt peak powers [12]. Ti:Sa has been established as the leading technology in OPA systems, both for the peak-power achievable and for the unsurpassed broad bandwidth they exhibit, enabling the generation of high-energy pump pulses below 20 fs. Pump pulse duration is a critical issue in parametric systems. For attaining an efficient energy transfer in a few-cycle parametric regime, a duration-matching is beneficial, with the pump pulse duration being on the same scale of the signal pulse duration. Femtosecond Ti:Sa-based OPAs have currently been sided by high-energy narrow-band sources, based on Nd- and Yb-doped active media, in a new class of parametric amplifiers, namely Optical Parametric Chirped Pulses Amplifiers (OPCPAs). In OPCPAs, restrictions related to the use of femtosecond pump sources are overcome by means of a technique inspired by the Chirped Pulse Amplification (CPA). Specifically, picosecond pump lasers (at 800-1100 nm) are used for amplifying temporally pre-dispersed IR signal pulses [13]. Sections 3.1 and 3.2 are devoted to the description of femtosecond-pumped OPAs and picosecond-pumped OPCPA high energy few-cycle IR sources, respectively. The amplified signal inherits the temporal and spectral properties of the seed pulse. Thus, the possibility to produce IR signal pulses supporting the few-cycle regime relies on the exploitation of the broadband seed generation techniques. Another crucial characteristic of the seed pulse in the few-cycle regime is the Carrier-Envelope Phase (CEP), which is defined as the phase offset between the carrier and the envelope of the pulse. The stabilization of the CEP enables the generation of optical waveforms with a reproducible electric field profile. In the few-cycle regime, a change in the CEP produces a strong variation in the waveform, affecting strongly non-linear optical phenomena, such as multiphoton absorption, above-threshold ionization and high-harmonic generation. CEP control is a crucial pre-requisite for the generation of isolated attosecond pulses. The main techniques adopted for generating a broadband seed in the IR spectral region are Parametric SuperFluorescence (PSF), White Light Generation (WLG) and Difference Frequency Generation (DFG). PSF consists of a spontaneous emission process, such that the seed is generated in a second-order non-linear crystal from vacuum fluctuations or quantum noise, under the effect of a pump pulse radiation with a relatively low intensity [14]. A broadband seed tunable from the near- to the mid-IR range depending on the non-linear media is generated, with the IR emission being subject to phase-matching conditions with the pump beam [15]. Since the process is initiated by noise, large random pulse-to-pulse energy fluctuations occur. However, this is generally a minor issue in PSF-based OPAs; indeed, those fluctuations undergo attenuation as a result of the subsequent parametric amplification. Moreover, due to the stochastic nature of emission, a timing jitter between pump and seed exists, making the synchronization of pulses in the OPA a fundamental issue. As a major drawback of PSF, the CEP relation between the pump and the PSF pulses is completely lost [16], and seed pulses exhibit a completely random CEP. Higher pump pulse intensity is required for WLG, but significant advantages can be obtained in terms of shot-to-shot stability and CEP-control. WLG occurs in dielectric media with a third-order non-linearity.