Molecular structural effects in below- and near-threshold harmonics in XUV-comb generation

The below- and near-threshold harmonics generated by ${\mathrm{O}}_2$ molecules and Xe atoms have been investigated experimentally by using an extreme-ultraviolet (XUV)-comb setup that is driven by an intense 1040-nm laser field. It is found that the yields of the below- and near-threshold harmonics generated by ${\mathrm{O}}_2$ molecules show a significant suppression when compared to Xe, although the two species have nearly identical ionization potentials. Our theoretical analysis shows that the harmonic yield suppression is related to the two-center interference effect of ${\mathrm{O}}_2$ and the tightly bounded orbital of the O atom. Our study provides useful information for the XUV-comb technique when extending the working media from atoms to complex systems.

Figure 1

Schematic view of our experimental setup. PZT, piezo transducer; IC, input coupler; PD, photodiode; NaS plate, sodium salicylate plate; MA–MF, mirror A to mirror F.

Figure 2

Image of the experimentally observed high-order harmonics fluorescing on a sodium salicylate plate of (a) Xe and (b) ${\mathrm{O}}_2$, respectively. The intensity of the laser field is about $3.3\times {10}^{13}$ W/${\mathrm{cm}}^2$. (c) The spectra of the out coupled high-order harmonic radiation of Xe and ${\mathrm{O}}_2$ obtained from the images of the fluorescent plate. The order of each harmonic is also marked.

Figure 3

Harmonic yields as a function of harmonic order for Xe and ${\mathrm{O}}_2$, respectively, calculated with the modified semiclassical method. The laser wavelength is 1040 nm and the peak intensity is $3.3\times {10}^{13}$ W/${\mathrm{cm}}^2$. For comparison with the experimental result, the alignment average of the molecular axis is considered in our simulation.

Figure 4

Absolute value of $exp\left(i{S}_q^j\right)$ and $a_j$ of each trajectory for ${\mathrm{O}}_2$ and Xe, respectively. Panels (a) and (b) are for $exp\left(i{S}_q^j\right)$; panels (c) and (d) are for $a_j$. The left column is for the first trajectory; the right column is for the second trajectory.

Figure 5

The value of $|a_j|$ as a function of the harmonic order for ${\mathrm{O}}_2$ with (blue diamond) and without (red dot) considering the two-center interference terms for (a) the first trajectory and (b) the second trajectory. The values of $|a_j|$ but without the two-center interference terms as a function of the harmonic order for the xenon atom with the orbital $5p_z$ and the oxygen atom with the orbital $2p_x$ for (c) the first trajectory and (d) the second trajectory.