Interplay between resonant enhancement and quantum path dynamics in harmonic generation in helium

Abstract

We present a theoretical study of the influence of resonant enhancement on quantum path dynamics in the generation of harmonics above and below the ionization threshold in helium. By varying the wavelength and intensity of the driving field from 425 to 500 nm and from 30 to 140 TW/cm 2 , respectively, we identify enhancements of harmonics 7, 9, and 11 that correspond to multiphoton resonances between the ground state and the Stark-shifted 1s2p, 1s3p, and 1s4p excited states. A time-frequency analysis of the emission shows that both the short and the long quantum path contributions to the harmonic yield are enhanced through these bound-state resonances. We analyze the subcycle time structure of the ninth harmonic yield in the vicinity of the resonances and find that on resonance the long trajectory contribution is phase shifted by approximately π/4. Finally, we compare the single atom and the macroscopic response of a helium gas and find that while the subcycle time profiles are slightly distorted by propagation effects, the phase shift of the long-trajectory contribution is still recognizable.

Role of Excited States In High-order Harmonic Generation

“…This means that the e-HHG harmonics can be identified through their spectral shift. Last, we note that the long lasting narrowband xFID emission from the excited states is visible between 9 and 14 eV at later times, in agreement with previous observations [13].…”

“…What is the origin of these two components? The narrow structures below the ionization threshold are clearly associated to bound-bound emission from Rydberg states, as recently predicted [13]. Because the exci- tation mechanism is coherent, all atoms in the medium decay in phase to the ground state by spontaneous emission, resulting in coherent emission referred to here as xFID.…”

“…They also demonstrated that the emission showed the same ellipticity dependence as the above-threshold harmonics, and suggested that polarization gating techniques [12] could thus be employed for temporal confinement. On the other hand, a recent theoretical study demonstrated that the narrowband emission emerged for long-lived dipoles which coherently emit radiation for times much longer than the pulse duration [13]. This process can be seen as XUV Free Induction Decay (xFID) [14][15][16][17].…”

“…The coherence of the xFID process explains the collimated nature of the emission. Since the lifetime of the Rydberg states is much longer than the IR pulse duration, the xFID mostly takes place after the laser pulse is over, at photon energies corresponding to the field-free resonance energies [13].…”

Angular momentum distribution in Rydberg states excited by a strong laser pulse

“…The important role of Rydberg states in various strong-field ionization processes and harmonic generation has been discussed. For example, resonant enhancement of below-threshold harmonics [8,12,33], emission from excited states via free induction decay [12,23], high harmonic emission through ionization from excited states and recombination to the ground state [23,34] have been predicted and observed.…”