Broad Overview
Strong-field physics, as the general research field of laser-matter interaction, aims to induce and control ultrafast processes. The matter here refers to atoms, molecules, solids, etc.; whereas the laser field is described here by its electric component (as its magnetic component is by two orders of magnitude smaller). Possible ultrafast processes that can occur include single excitation, double excitation followed by autoionization, partial fragmentation including or not excitation of the residual ion, complete fragmentation, and all rescattering events including high-order harmonic generation, nonsequential double ionization, nonsequential single ionization, etc.
In general, the notion of strong field is relative to the target as the same applied electromagnetic field (having a carrier frequency ω and an intensity I) on different targets can lead to different processes. However, depending upon the relative strength of the laser field compared to the internal properties of the target, it is common in strong-field physics to distinguish two different fundamental regimes depending on the value taken by the Keldysh parameter γ=(Ip/2Up)1/2, where Ip is the binding energy of the active electron and Up=I/4ω2 (in a.u.) is its ponderomotive energy in the laser field. Whereas the tunneling regime is defined by γ≤1, the multiphoton regime is defined by γ>1. It thus follows that the tunneling regime corresponds to either to a superintense laser field or to a low-frequency (i.e., long-wavelength) laser field.
A goal of strong-field physics is to understand the mechanism underlying any of these ultrafast processes. Among all these processes, one of these called high-order harmonic generation (HHG) has been a key focus of strong-field physics for more than two decades owing to both its intrinsic interest and its many important applications, such as, e.g., for producing attosecond pulses, for producing high-energy harmonics in the important water-window region, and for imaging atomic, molecular, and other target systems. In the important low-frequency tunneling regime, the famous three-step scenario (i.e., ionization by tunneling, laser-driven electron motion away from and back to the target ion, and recombination of the electron with harmonic emission) has proved to be an invaluable guide for understanding HHG.
Some Open Questions
In contrast, in the multiphoton regime the three-step scenario is no longer applicable, and the role of multielectron correlations on HHG in this regime was still an open question until the pioneering works by Ngoko Djiokap and Starace. Our focus is on investigating the role played by electron correlations on HHG in the multiphoton regime from two-active-electron systems. First, this study is a part of the very active research field of increasing intensity of attosecond pulses by means of HHG, as electron correlations can be used to improve the efficiency of production of harmonics in the gas phase; specifically to enhance particular harmonics, and to produce HHG plateau. Second, this study shows that HHG processes can be viewed as a powerful tool to reveal target orbital structures. Finally, controlling the HHG processes in the multiphoton regime by the polarization and the chirp of the driving laser pulse or by the time delay and relative phase of a pair of laser drivers are of great interest.
Strong-field physics, as the general research field of laser-matter interaction, aims to induce and control ultrafast processes. The matter here refers to atoms, molecules, solids, etc.; whereas the laser field is described here by its electric component (as its magnetic component is by two orders of magnitude smaller). Possible ultrafast processes that can occur include single excitation, double excitation followed by autoionization, partial fragmentation including or not excitation of the residual ion, complete fragmentation, and all rescattering events including high-order harmonic generation, nonsequential double ionization, nonsequential single ionization, etc.
In general, the notion of strong field is relative to the target as the same applied electromagnetic field (having a carrier frequency ω and an intensity I) on different targets can lead to different processes. However, depending upon the relative strength of the laser field compared to the internal properties of the target, it is common in strong-field physics to distinguish two different fundamental regimes depending on the value taken by the Keldysh parameter γ=(Ip/2Up)1/2, where Ip is the binding energy of the active electron and Up=I/4ω2 (in a.u.) is its ponderomotive energy in the laser field. Whereas the tunneling regime is defined by γ≤1, the multiphoton regime is defined by γ>1. It thus follows that the tunneling regime corresponds to either to a superintense laser field or to a low-frequency (i.e., long-wavelength) laser field.
A goal of strong-field physics is to understand the mechanism underlying any of these ultrafast processes. Among all these processes, one of these called high-order harmonic generation (HHG) has been a key focus of strong-field physics for more than two decades owing to both its intrinsic interest and its many important applications, such as, e.g., for producing attosecond pulses, for producing high-energy harmonics in the important water-window region, and for imaging atomic, molecular, and other target systems. In the important low-frequency tunneling regime, the famous three-step scenario (i.e., ionization by tunneling, laser-driven electron motion away from and back to the target ion, and recombination of the electron with harmonic emission) has proved to be an invaluable guide for understanding HHG.
Some Open Questions
In contrast, in the multiphoton regime the three-step scenario is no longer applicable, and the role of multielectron correlations on HHG in this regime was still an open question until the pioneering works by Ngoko Djiokap and Starace. Our focus is on investigating the role played by electron correlations on HHG in the multiphoton regime from two-active-electron systems. First, this study is a part of the very active research field of increasing intensity of attosecond pulses by means of HHG, as electron correlations can be used to improve the efficiency of production of harmonics in the gas phase; specifically to enhance particular harmonics, and to produce HHG plateau. Second, this study shows that HHG processes can be viewed as a powerful tool to reveal target orbital structures. Finally, controlling the HHG processes in the multiphoton regime by the polarization and the chirp of the driving laser pulse or by the time delay and relative phase of a pair of laser drivers are of great interest.