Theoretical exploration of asymmetric molecular harmonic emission and attosecond pulse generation in the presence of spatially inhomogeneous plasmon-enhanced field

Asymmetric molecular harmonic generation from HeH²⁺ ion in the presence of the spatially inhomogeneous plasmon-enhanced field has been theoretically investigated by solving the two-dimensional non-Born–Oppenheimer time-dependent Schrödinger equation (both the electron and nuclear motions are considered only along the laser polarisation axis). It shows that (1) for the normal homogeneous field, the harmonic cut-off from the asymmetric molecule is beyond the classical harmonic cut-off energy from the atom (I_p + 3.17 U_p). Further analyses show that the multi-channels ionisation–recombination process is responsible for the harmonic extension. (2) For the spatially inhomogeneous plasmon-enhanced field, the harmonic cut-off can be further enhanced, and an optimal 3.0 dB enhanced field with a 482 eV supercontinuum has been obtained. Quantum time-frequency harmonic analyses have been shown to explain the harmonic characteristics and the harmonic extension process. (3) Initial vibrational state and isotopic effects show that intense harmonics can be generated from higher initial vibrational state and the heavier molecule. Finally, by properly superposing the harmonics, a series of sub-30 as attosecond pulses can be obtained.