Bringing Dark States to Light after Frustrated Tunneling Ionization

High-harmonic generation (HHG) in gases is typically regarded as a sub-laser-cycle process, well described by the semiclassical three-step model. In this work, we report the extreme-ultraviolet (XUV) emission observed in an HHG experiment arising from a longer temporal process associated with dark states populated through frustrated tunneling ionization. The emission exhibits a larger spatial divergence than typical high-order harmonics, resembling the polarization decay of optically bright states. Supported by the time-frequency analysis of the theoretical calculations, we show that the emission originates from the nonradiative capture of the tunneling electrons into excited dark states of the neutral atom, followed by Floquet-like laser-dressing effects induced by the extended tail of the driving pulse. Our results reveal a largely unexplored pathway for XUV generation through laser-dressed dark states, thereby enabling the enhancement of coherent XUV emission at targeted wavelengths with high repetition rates.