Abstract

Dynamics of the ionization process attracts significant attention over many years. Recently, this interest intensified because novel techniques based on attosecond pulse metrology [1] allowed direct insight into this process. Ionization dynamics is very different for different optical intensities. For relatively low intensities (multiphoton ionization regime) ionization process takes place over many optical cycles, leading to slow growth of free electron number [Fig. 1(b)]. For higher intensities, this dynamics takes place on the sub-femtosecond time scale [tunnel ionization, Fig. 1(a)]. In contrast to the pre-created plasma, ionization process creates strong nonlinearity, thus resulting in harmonic generation called Brunel harmonics [2]. Brunel harmonics include also low frequency components in terahertz (THz) range [3-5], if the pump field has asymmetric temporal shape, which can be achieved by, for instance, two-color pumping.

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