Extreme-ultraviolet spectral compression by four-wave mixing

Selecting two-photon sequential ionization pathways in H2 through harmonic filtering

“…Moreover, the characteristics of APTs make them suitable for specific applications such as the RABBITT (reconstruction of attosecond beating by interference of two-photon transitions) technique that exploits the high temporal resolution provided by each attosecond pulse within the train and the spectral definition given by the total duration of the train [7][8][9] . The increasing number of applications using APTs makes worth current investments to accomplish XUV conversion with high efficiency [10][11][12][13] and to design In the present manuscript, we investigate the use of a frequency filter as a mean to unravel the coupled electron-nuclear dynamics in an excited (ionized) molecule by suppressing/favoring specific multiphoton absorption paths. We first examine the outcome of realistic XUV APT-pump/ IR-probe schemes employed in previous experimental studies [25][26][27][28] in the hydrogen molecule.…”

Phase-Dependence of Resonant and Antiresonant Two-Photon Excitations

“…Multi-photon absorption [1] is a fundamental nonlinear response of materials to intense laser radiation, and a key aspect of a wide range of applications of lasers that include four-wave mixing [2,3] and highharmonic generation (HHG) [4,5], as well as non-linear microscopy techniques such as Two-Photon-Excited Fluorescence (TPEF) microscopy [6]. According to perturbation theory, the efficiency of multi-photon absorption processes is significantly enhanced by the occurrence of intermediate state resonances.…”

“…An experimental setup previously described in detail [3,21,22] was used to measure RABBITT signals from (near-)resonant two-photon ionization of helium.…”

“…As expected in a RABBITT experiment(see Eqs. (3,4)), the intensities of the harmonic and SB photoelectron signals oscillate with XUV-NIR time delay, with one full oscillation occurring in half an NIR optical period (1.3 fs). When H15 is resonant with a Rydberg state, a resonance enhancement of SB16 can be observed, which can then become the dominant contribution to the PES (see Figure 1(b), ω NIR = 784 nm).…”

“…In the experiment described here, however, the cc transition only contributes to the H17-NIR pathway, thus providing direct experimental access to the magnitude of this phase. The afore-mentioned hydrogenic model predicts that the Wigner and cc transition phase combined add a phase of ≈ 5 4 π to the SB16 RABBITT phase, where the contribution of the cc phase alone increases from ≈ 3 4 π (λ NIR = 765 nm) to ≈ π (λ NIR = 806 nm). This has recently been investigated in more detail by Kheifets and Bray [37].…”