Quantum-classical correspondence in circularly polarized high harmonic generation

Abstract

Using numerical simulations, we show that atomic high order harmonic generation, HHG, with a circularly polarized laser field offers an ideal framework for quantum-classical correspondence in strong field physics. With an appropriate initialization of the system, corresponding to a superposition of ground and excited state(s), simulated HHG spectra display a narrow strip of strong harmonic radiation preceded by a gap of missing harmonics in the lower part of the spectrum. In specific regions of the spectra, HHG tends to lock to circularly polarized harmonic emission. All these properties are shown to be closely related to a set of key classical periodic orbits that organize the recollision dynamics in an intense, circularly polarized field. The interaction of a strong, short laser pulse with atoms/molecules is of great interest to the strong field and attosecond science communities because of the insights it provides in probing matter at the atomic scale [1]. These systems have been investigated from various angles ranging from experimental to numerical and analytical approaches using quantum, semi-classical and classical models (see and references therein). Among these, (semi)-classical models offer an insightful trajectory interpretation of the electronic dynamics which often compensates for the loss of purely quantum mechanical effects. A famous example is the recollision mechanism [4-6] which, for linearly polarized fields, explains many events such as nonsequential double (or multiple) ionization [1], high order harmonic generation (HHG) [7,8] or laser induced electron diffraction [9][10][11]. The quantum nature of the system at hand raises the question of the applicability of such a classically-based interpretation. In this Letter we investigate the quantumclassical correspondence in the framework of HHG. Using quantum mechanical simulations, we demonstrate the existence of atomic HHG with an intense circularly polarized laser field and show that some of the harmonics are circularly polarized (see Fig. 1). The properties of the HHG spectrum are later explained through specific classical electronic trajectories. The perspective offered by nonlinear dynamics allows one to fully interpret the observed HHG spectra and to devise quantitative predictions which are not accessible by standard interpretation [12,13]. The close connection we reveal between HHG and classical trajectories with circular polarization opens a way for controlling the highly nonlinear radiation spectrum properties through, e.g., nonlinear dynamical tools applied to orbit stability control [14]. The robustness of the process with the initial preparation of the system, the atomic species and the laser parameters, along with the strong intensity of the radiation, hints at the universality of the mechanism described in this Letter and its accessibility to experimental observations with currently available intense laser technology.Classical simulations have shown that not all two electron configurations lead to recollision in a strong, circula...

Bicircular High-Harmonic Spectroscopy Reveals Dynamical Symmetries of Atoms and Molecules

“…The sensitivity of the polarization of the emitted harmonic radiation to the angular momentum of the highest-occupied orbital of the generating medium provides a convenient pathway for isolating XUV radiation with a specific helicity. Potential applications of this aspect include the generation of isolated elliptically polarized attosecond pulses [28,59]. Extending BHHS to molecules, we demonstrated its extreme sensitivity to both electronic structure (shape resonance in N 2 ) and symmetry breaking at the ensemble level.…”

Molecular high-order-harmonic generation due to the recollision mechanism by a circularly polarized laser pulse

“…It is worth noting that in [37], where an excited initial condition is considered, the cutoff follows 3.17U p + I p from the TDSE calculation. And both the upper and lower cutoffs can be well predicted by the calculation based on the recolliding periodic orbits (RPO) analysis [36].…”

“…In our work, the cutoff is judged where the spectrum begins to drop. And in [37], the upper cutoff is judged where the spectrum has already vanished. Because of this, two "different" cutoff laws are found.…”

“…Therefore, the recollision process under a circularly polarized laser pulse (CPLP) and the recollision induced HHG from (pre-excited) atomic systems are also investigated [35][36][37][38]. These works show that the recollision under CPLP does occur and has a significant effect on the recollision-driven processes (such as nonsequential double ionization and HHG).…”

“…It is found that the higher efficiency is mainly contributed in the recombination step and is at least partly due to the higher recollision probability for molecules. Since this HHG process involves the unique recollision mechanism under a circularly polarized laser field, it will stimulate new potential applications, such as offering an ideal framework for study of strong-field dynamics [37] and (one-shot) imaging or structure detection of molecules with all optical methods [39][40][41][42].…”