Photoelectron spectroscopy of 3s3p doubly excited helium dressed with strong near-infrared laser fields
We report time-resolved photoelectron spectroscopy of the $3s3p$ doubly excited states of helium dressed by an intense near-infrared (NIR) laser field. Using synchronized XUV free-electron-laser and 800-nm NIR laser pulses, we observe a pronounced delay-dependent shift of resonance-related spectral minima together with the emergence of additional structures around the NIR sideband energy. \textit{Ab initio} theoretical calculations support these observations and identify the features as signatures of NIR-induced coupling of the bright ($3s3p {}^{1}P^{o}$) autoionizing state to nearby dark ($^{1}D^{e}$ and $^{1}S^{e}$) resonances below the $N = 3$ threshold. A multichannel Fano resonance analysis of the measured spectra yields delay-dependent line-shape parameters and resonance energies, establishing a quantitative route to characterize and control correlated two-electron resonances in strong laser fields.