Spatial position scaling on harmonic generation from He atom in bowtie-shaped nanostructure

Spatial position scaling on the harmonic generation from He in the bowtie-shaped nanostructure has been theoretically investigated. It shows that (i) due to the surface plasmon polaritons in the nanostructure, the laser intensity can be enhanced and presents the nonhomogeneous effect in space. As a result, the extension of the harmonic cutoff can be achieved when He is away from the gap center of the nanostructure. However, due to the limit of the gap size, there is a maximum harmonic cutoff extension for a given nanostructure. (ii) Due to the asymmetric enhancement of the laser intensity in space, the extended harmonics are mainly from E(t) >0 a.u. and E(t) <0 a.u. when He is injected into the positive and the negative positions, respectively. Moreover, the intensities and the cutoffs of the extended harmonics can be controlled by changing the pulse duration or by adding the second controlling pulse. Finally, by properly superposing the harmonics from the two-color field, four single attosecond pulses with the durations of 30 as can be produced.