Many-electron dynamics in high-order harmonic generation of niobium: A time-dependent density-functional-theory study

The strong-field many-electron dynamics is studied via the resonance-enhanced high-order harmonic generation (HHG) of niobium. In intense IR lights excited states of Nb with the configuration $4d^{3}5snp$ play a vital role in the enhanced HHG below the ionization potential, even though the electron initially occupying the $5s$ orbital is the most active. The energies of these states increase linearly with the laser intensity, which causes the effective ionization potential to increase as well. Above the ionization potential, many-electron effects may cause an electronic state of Nb to be in resonance with that of ${\mathrm{Nb}}^{+}$, which leads to enhanced HHG and which influences the tunneling ionization as well.

Figure 1

Orbital contributions to the induced dipole moment of a Nb atom in a linearly polarized 1500-nm laser field. The laser intensity is ${10}^{13}$ W/${\mathrm{cm}}^2$.

Figure 2

The HHG power spectra of a Nb atom in a linearly polarized 1500 nm (black solid) and 1353 nm (green) calculated with the TDDFT method. The laser intensity is ${10}^{13}$ W/${\mathrm{cm}}^2$. The $I_p$ is represented by the vertical dotted red line.

Figure 3

The HHG of a Nb atom in a linearly polarized 1500-nm pulsed laser calculated with the TDDFT method. The laser intensity is 1 (black solid), 2 (red), and 3 (green) $\times {10}^{13}$ W/${\mathrm{cm}}^2$, respectively. The $I_p$ is represented by the vertical solid blue line.