Scaling strong-field interactions towards the classical limit

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 mu m. The development of intense, ultrafast laser sources in the mid-infrared ( 1 mu m < lambda < 5 mu m) region enables exploration of the wavelength scaling of the Keldysh picture while enabling new opportunities in strong-field physics, control of electronic motion and attosecond science. Here we report a systematic experimental investigation of the wavelength scaling in this region by concurrently analysing the production of energetic electrons and photons emitted by argon atoms interacting with few-cycle, mid-infrared fields. The results support the implicit predictions contained in Keldysh's work, and pave the way to the realization of brighter and shorter attosecond pulsed light sources using longer-wavelength driving fields.