Scaling strong-field interactions towards the classical limit

Abstract

In 1964 Keldysh1 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 midinfrared (1 µm < l < 5 µm) 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. Keldysh 1 described the two main effects an intense lowfrequency laser field has on an atom as (1) a bending of the Coulomb potential by the field, forming a sufficiently narrow barrier for the electron to tunnel into the continuum, and (2) an oscillating motion of the free electron induced by the field of strength E and frequency ω. The cycle-averaged kinetic energy of the oscillating electron (ponderomotive energy) is given in atomic units as U p = E 2 /4ω 2 . The limit of validity of the Keldysh approach is defined by the condition that the adiabaticity parameter γ = √

Theory of Nonlinear Propagation of High Harmonics Generated in a Gaseous Medium

“…The Cooper minimum (CM) in the HHG spectra of Ar has been studied intensively by using the traditional 800-nm lasers [127][128][129], or longer-wavelength lasers [130,131]. This minimum always occurs in the single-atom HHG spectrum of Ar based on the QRS theory, however, it is not necessary to appear in the macroscopic spectrum.…”

“…Macroscopic propagation effect has to be included in order to compare with experimental HHG spectra. A few investigations on the wavelength scaling of HHG experimentally [32,130,136] have been reported. However, theoretical analysis is still rather scarce.…”

“…3.8(b) in which the gas jet is placed at z=3 mm. Since the laser intensity is fixed at the center of the thin gas jet, we calculate that the intensities at the laser focus are Experimentally, Colosimo et al [130] reported that the HHG yields between 35-50 eV for 2000 nm lasers are about 1000 times smaller than that for 800 nm lasers, for experimental conditions that were kept "as fixed as possible". This would give a λ −9 dependence in this narrow energy region which is not too far off from our scaling of λ −10.5 .…”

“…Colosimo et al [130] it was reported that the HHG yields between 35-50 eV generated by using 2000 nm lasers can be as high as 50% of that from 800 nm lasers if the experimental conditions were optimized independently. Furthermore, Chen et al [139] demonstrated that it was possible to use much higher pressure to generate HHG for long-wavelength lasers, thus achieving usable photon yields even in the water-window region.…”

Intensity enhancement in the molecular ionization and dissociation dynamics in the presence of noise

“…In this area, many interesting phenomena have been revealed, including the bond softening and hardening [5,6], the electron localization in dissociation channels [7,8], the above threshold Coulomb explosion (CE) [9,10], the high-order harmonic generation (HHG) [11,12], the abovethreshold ionization (ATI) and dissociation (ATD) [13,14] etc. Usually, the ionization probabilities (IPs) and/or the dissociation probabilities (DPs) are critical to gain high signals or high yields for these photochemical and photophysical processes.…”