Pulse-shape control of two-color interference in high-order-harmonic generation

Abstract

We report on calculations of harmonic generation by neon in a mixed (800-nm + time-delayed 400-nm) laser pulse scheme. In contrast with previous studies we employ a short (few-cycle) 400-nm pulse, finding that this a↵ords control of the interference between electron trajectories contributing to the cuto↵ harmonics. The inclusion of the 400-nm pulse enhances the yield and cuto↵ energy, both of which exhibit a strong dependence on the time delay between the two pulses. Using a combination of time-dependent R-matrix theory and a classical trajectory model, we assess the mechanisms leading to these e↵ects.

The imaginary part of the high-harmonic cutoff

“…In one dimension, this caustic can be precisely localized to a point, known as the 'fold' (more technically, the 'bifurcation set'), which marks the transition between the two regimes, and our harmonic-cutoff times are the precise embodiment of that understanding of the caustic. This view of the harmonic cutoff as a caustic has been used for some time [36][37][38], but recent years have seen a marked increase in interest in that perspective on the dynamics [39][40][41][42][43][44], as laser sources achieve better control over polychromatic combinations with high relative intensities (which is slated to continue to increase [80]). This has opened the door to the observation of higher-order catastrophes in strong-field observables, involving higherdimensional bifurcation sets; however, the theoretical understanding remains somewhat behind, and a quantitative understanding that reflects those structures has not yet followed.…”

“…In this sense, the complex harmonic-cutoff energy Ω hc is the bifurcation set for the catastrophe -suitably generalized to the complex variables involved -and it marks the location of the caustic. The formal study of caustics has seen increasing interest within attoscience [36][37][38][39][40][41][42][43][44], and our approach provides a useful tool for exploring and describing the higher-dimensional bifurcation sets at the heart of the higherorder catastrophes that become available as our control over short pulses of light becomes more finely tuned.…”

Ellipticity dependence of excitation and ionization of argon atoms by short-pulse infrared radiation

“…Time propagation of the wave function is achieved by the Krylov subspace method of Arnoldi and Lanczos [24]. RMT has been successfully applied to a wide range of strong-field problems, from High Harmonic Generation (HHG) in two-color [25] and near-IR fields [26], XUV-initiated HHG [27,28], and strong-field rescattering [29].…”

Cooper minimum in singly ionized and neutral argon

“…Moreover, the RMT code has been adapted to include a two-electron finite difference outer region to allow the modelling of double-ionization [31][32][33]. Much recent success in the application of the RMT method has been to compliment and even preempt cutting edge experimental techniques such as attosecond transient absorption spectroscopy [34], high-harmonic spectral caustics [35] and extreme-ultraviolet-initiated high harmonic generation [16,36].…”