Scaling strong-field interactions towards the classical limit

A systematic experimental study of the ionization of argon by mid-infrared light confirms half-a-century-old predictions and paves the way to the development of brighter, shorter attosecond pulse sources. In 1964 Keldysh 1 helped lay the foundations of strong-field physics by introducing a theoretical framework that characterized atomic ionization as a process that evolves with the intensity and wavelength of the fundamental field. Within this context, experiments 2 have examined the intensity-dependent ionization but, except for a few cases, technological limitations have confined the majority to wavelengths below 1 μm. The development of intense, ultrafast laser sources in the mid-infrared (1 μm< λ