Dependence upon the molecular and atomic ground state of higher-order harmonic generation in the few-optical-cycle regime

High-order harmonic generation from molecules (${\mathrm{O}}_2$, ${\mathrm{N}}_2$, ${\mathrm{H}}_2$, and $\mathrm{C}{\mathrm{O}}_2$) and atoms (Xe, Ar, and Kr) has been studied in the few optical cycle domain. Two laser peak intensities; $2\times {10}^{14}$ and $6\times {10}^{14}\mathrm{W}{\mathrm{cm}}^{-2}$, were compared. At the lower intensity spectra were approximately the same for molecules and atoms with the same ionization potential, at higher laser intensity the cutoff of ${\mathrm{O}}_2$ and $\mathrm{C}{\mathrm{O}}_2$ extends far beyond the cutoff of Xe and Kr, respectively, in contrast with ${\mathrm{N}}_2$ and ${\mathrm{H}}_2$ which exhibit cutoffs very close to that of Ar. This behavior is well explained by adopting an atomlike approximation for the molecule response in the high-field regime and employing the Lewenstein’s model, properly modified in order to account for the nonlinear dipole moment of a randomly oriented molecule ensemble.

Figure 1

Measured high-order harmonic spectra from ${\mathrm{O}}_2$ (solid line) and Xe (dashed line) obtained with a Ti:sapphire laser ($760\mathrm{nm}$, $6\mathrm{fs}$) at (a) $2\times {10}^{14}$ and (b) $6\times {10}^{14}\mathrm{W}{\mathrm{cm}}^{-2}$.

Figure 2

Measured-high order harmonic spectra at $2\times {10}^{14}\mathrm{W}{\mathrm{cm}}^{-2}$ (thick solid line) and at $6\times {10}^{14}\mathrm{W}{\mathrm{cm}}^{-2}$ (thin solid line) from (a),(d) Ar, (b),(e) ${\mathrm{N}}_2$, and (c),(f) ${\mathrm{H}}_2$.

Figure 3

Measured high order harmonic spectra from $\mathrm{C}{\mathrm{O}}_2$ (solid line) and Kr (dashed line) at (a) $2\times {10}^{14}$ and (b) $6\times {10}^{14}\mathrm{W}{\mathrm{cm}}^{-2}$. The $\mathrm{C}{\mathrm{O}}_2$ curve in (b) has been divided by a factor of 10 for the sake of clarity.

Figure 4

Calculated single-atom spectra at $6\times {10}^{14}\mathrm{W}{\mathrm{cm}}^{-2}$ for (a) Xe, (b) ${\mathrm{O}}_2$, (c) Ar, and (d) ${\mathrm{N}}_2$ for a laser pulse having $6\text{‐}\mathrm{fs}$ pulse duration (FWHM of the intensity), $760\text{‐}\mathrm{nm}$ carrier wavelength, and ${\mathrm{sech}}^2$ intensity temporal envelope. Simulations result from an average over the carrier-to-envelope phase.