Resonantly-initiated quantum trajectories and their role in the generation of near-threshold harmonics
We present a theoretical study of the role that resonant enhancement plays in the temporal and spectral properties of near-threshold harmonics in argon, driven by a moderately intense, near-infrared laser pulse. By studying how the temporal profile of the eleventh harmonic (H11) changes with peak intensity and pulse duration, we show that H11 is predominantly emitted when the instantaneous intensity of the laser pulse is such that a high-lying excited state is Stark-shifted into multiphoton resonance with the ground state. We demonstrate that if this resonant intensity is lower than the peak intensity, the harmonic pulse will in general exhibit two peaks in time, on the rising and falling edges of the laser pulse. The resonantly enhanced harmonic radiation exhibits strong characteristics of semi-classical long and short quantum paths, and in general both paths are enhanced by the resonance. We find that the resonant enhancement leads to the harmonic radiation being emitted between .5 and 1.1 optical cycles after the time of multiphoton resonance, indicating that the resonance introduces a delay as compared to non-resonant emission. We further demonstrate that the resonantly enhanced long-trajectory contribution to the harmonic radiation manifests in the spectral domain as red- and blueshifted features near the central harmonic frequency. Finally, we compare the single argon atom response to the macroscopic response of an argon gas and show that spectral and temporal effects of the resonance are still recognizable after propagation.