Electron-Nuclear Dynamics in Molecular Harmonic Generation Driven by a Plasmonic Nonhomogeneous Field

Electron (z)-nuclear (R) dynamics in the molecular high-order harmonic generation (MHHG) from H2+ driven by the plasmonic nonhomogeneous field, generated by the surface plasmon polaritons in the bowtie-shaped nanostructure, have been theoretically investigated through solving the two dimensional time-dependent Schrodinger equation with the Non-Bohn-Oppenheimer approximation. It is found that (i) due to the plasmonic enhancement of the laser intensity, the harmonic cutoff can be extended when the spatial position of H2+ is away from the gap center of the nanostructure. However, due to the limit of the gap size, the threshold value of the harmonic cutoff can be obtained at a given position of H2+. (ii) Due to the asymmetric enhancement of the laser intensity in space, the extended higher harmonics are respectively from E(t) > 0 a.u. or E(t) < 0 a.u. for the cases of the positive and the negative spatial position of H2+. As a result, the intensities of the extended higher harmonics are different and can be controlled by changing the carrier-envelope phase and the pulse duration of the laser field. (iii) In the few-cycle pulse duration, the MHHG mainly comes from the multi-photon resonance ionization (MPRI), while as the pulse duration increases, the MPRI, the charge-resonance enhanced ionization (CREI) and even the dissociative ionization (DI) are contributed to the MHHG. Moreover, as the spatial position of H2+ moves, the contributions of the MHHG from the MPRI, the CERI and the DI can be controlled. (iv) The contributions of the MHHG from the two-H nuclei have been investigated and found that when E(t) > 0 a.u., the intensities of the harmonics from the negative-H is higher than those from the positive-H; while when E(t) < 0 a.u., the intensities of the harmonics from the positive-H plays the main role in the MHHG. Moreover, the multi-minima, caused by the two-center interference can also be found. (v) Finally, by superposing a properly selected harmonics, a single isolated attosecond pulse (SIAP) with the full width at half maximum (FWHM) of 34 as can be obtained.