Coherent extreme-ultraviolet emission generated through frustrated tunnelling ionization

Coherent extreme-ultraviolet emission can be obtained through high-harmonic generation and multiphoton excitation from atoms exposed to a strong laser field. We report the generation of a new kind of coherent extreme-ultraviolet emission from He atoms excited by intense few-cycle laser pulses. An atom can be excited after tunnelling in a strong laser field, in the process known as frustrated tunnelling ionization (FTI). We find that excitation through FTI leads to coherent extreme-ultraviolet emission, and its intensity strongly depends on the ellipticity and carrier-envelope phase of the laser pulses. Additionally, the propagation direction of the emission can be coherently controlled by employing the attosecond lighthouse technique. This coherent control of tunnelling and recombination dynamics, which has provided the fundamental basis of attosecond physics, promises the utilization of FTI emission as a coherent light source and offers new opportunities in ultrafast spectroscopy.