Molecular harmonic generation in bowtie-shaped nanostructure

Molecular high-order harmonic generation (MHHG) from H-2(+) in the bowtie-shaped nanostructure has been investigated. The results show that (i) due to the surface plasmon polaritons in the bowtie-shaped nanostructure, the laser intensity can be enhanced and present the nonhomogeneous effect in space. As a result, the extension of the harmonic cutoff can be achieved when the position of H-2(+) is injected away from the gap center of the nanostructure. However, due to the limited gap size of the nanostructure, the inherent threshold value of the harmonic cutoff can be found. (ii) Due to the asymmetric enhancement of the laser intensity in space, the extended higher harmonics come from E(t) > 0 or E(t) < 0 for the cases of the positive or the negative injected positions of H-2(+), respectively. (iii) In the few-cycle pulse, the extended higher harmonics mainly comes from the multi-photon resonance ionization (MPRI) around R = 4.0 and R = 4.5 but from only one harmonic emission event (HEE), thus leading to the intensity difference of the extended higher harmonics from the positive and the negative positions of H-2(+). In the multi-cycle pulse, the extended higher harmonics are also attributed to the MPRI but from R = 3.5 to R = 6.0 and there are three HEEs contributed to the extended higher harmonics, thus leading to the similar intensity of the extended higher harmonics from the positive and the negative positions of H-2(+). (iv) Due to the slower nuclear motion of the heavy nuclei, the intensities of the harmonics from T-2(+) are lower than those from H-2(+). (v) The contributions of the MHHG from the two-H nuclei show that when E(t) > 0 or E(t) < 0, the left-H or the right-H plays the dominating role in the MHHG. Moreover, as H-2(+) moves away from the gap center, the intensity difference of the higher harmonics from the two-H nuclei can be enhanced. But, it can be decreased as the pulse duration increases.