Controlling Two-Center Interference in Molecular High Harmonic Generation

We experimentally investigate the process of intramolecular quantum interference in high-order harmonic generation in impulsively aligned ${\mathrm{CO}}_2$ molecules. The recombination interference effect is clearly seen through the order dependence of the harmonic yield in an aligned sample. The experimental results can be well modeled assuming that the effective de Broglie wavelength of the returning electron wave is not significantly altered by the Coulomb field of the molecular ion. We demonstrate that such interference effects can be effectively controlled by changing the ellipticity of the driving laser field.

Figure 1

Modulation of the harmonic yield in impulsively aligned ${\mathrm{CO}}_2$ molecules due to half-revival of rotational wave packet (a) for the 33rd and (b) 49th harmonic order. Pump and probe pulses with parallel polarization. Pump pulse: energy $200\mu \mathrm{J}$, duration 60 fs. Probe pulse: intensity $2\times {10}^{14}\mathrm{W}/{\mathrm{cm}}^2$, duration 30 fs. Dashed curve in (a) and (b) displays the calculated temporal evolution of the corresponding $⟨{cos}^2\theta ⟩$. (c) Polar plot of the $\theta$ distribution for different pump-probe delays: $\tau =20.7$, 21.1, and 21.3 ps.

Figure 2

(a) Measured harmonic spectra generated by 30 fs pulses with (solid curve) and without (dashed curve) pump beam (pump and probe pulses with parallel polarization). Pump-probe delay $\tau =21.1\mathrm{ps}$. (b) Ratio $\Phi /{\Phi }_0$, between the harmonic intensities measured with ($\Phi$) and without (${\Phi }_0$) the pump pulse, measured at three pump-probe delays: $\tau =20.7\mathrm{ps}$ (circles), 21.1 ps (squares), and 21.3 ps (triangles). The lines show the modulation in harmonic yield predicted by Eq. (1) (with a vertical offset term added) for $\theta =30°$ [solid (red online) curve] and $\theta =40°$ [dashed (blue online) curve].

Figure 3

Ellipticity dependence of the harmonic yield for three different harmonic orders measured in aligned (solid curve) and not aligned (dashed curve) normalized to the zero ellipticity case. Pump-probe delay $\tau =21.1\mathrm{ps}$.