Controlling the XUV Transparency of Helium Using Two-Pathway Quantum Interference

Atoms irradiated with combined femtosecond laser and extreme ultraviolet (XUV) fields ionize through multiphoton processes, even when the energy of the XUV photon is below the ionization potential. However, in the presence of two different XUV photons and an intense laser field, it is possible to induce full electromagnetic transparency. Taking helium as an example, the laser field modifies its electronic structure, while the presence of two different XUV photons and the laser field leads to two distinct ionization pathways that can interfere destructively. This work demonstrates a new approach for coherent control in a regime of highly excited states and strong optical fields.

Figure 1

(a) Tuning of the harmonics in the frequency domain into the IR-created $2p$ double slit by varying the phase-matching conditions in a xenon-filled waveguide. The dashed vertical lines represent the calculated sidebands of the dressed Floquet $2p$ state of He. The two interfering quantum pathways are $11\mathrm{th}+5\omega$ and $13\mathrm{th}+3\omega$. (b) Calculated He excitation cross section, with and without the presence of an IR field of intensity $4\times {10}^{12}\mathrm{W}/{\mathrm{cm}}^2$. The relative amplitudes of the harmonics are shown on the right axis. By changing the absorption cross section of He using the IR laser field, the amplitudes of the two interfering quantum pathways are controlled.

Figure 2

(a) ${\mathrm{He}}^{+}$ yields for XUV and IR illumination (laser intensity of $4\times {10}^{12}\mathrm{W}/{\mathrm{cm}}^2$), in the case of the three different two-color XUV spectra of Fig. 1a. The inset shows the theoretical and experimental photoelectron momenta at 200 fs IR delay, along the negative and positive $p_z$ axis, respectively. (b) Theoretical calculation of the ${\mathrm{He}}^{+}$ yield under the same conditions as (a). The inset shows calculated photoelectron energies corresponding to the $750/11\mathrm{nm}$ and $750/13\mathrm{nm}$ XUV beams. The $13\mathrm{th}+2\omega$ (2 IR photon) channel appears at $\pm 25\mathrm{fs}$ IR delays.

Figure 3

(a)–(c) Calculated modulation of the He ionization probability for different ratios of the $11\mathrm{th}:13\mathrm{th}$ harmonic fields, and for two-color $765/(11,13)\mathrm{nm}$, $785/(11,13)\mathrm{nm}$, and $750/(11,13)\mathrm{nm}$ XUV beams, respectively. (d) Experimental and theoretical modulation of the ${\mathrm{He}}^{+}$ yield for a laser intensity of $4\times {10}^{12}\mathrm{W}/{\mathrm{cm}}^2$, and a $785/(11,13)\mathrm{nm}$ XUV field. The red curve of (a) demonstrates that a ratio of $256:1$ for the 11th vs the 13th harmonic can result in nearly complete cancellation of XUV absorption.