Strong resonance enhancement of a single harmonic generated in the extreme ultraviolet range.

We report the demonstration of strong resonance enhancement of a single high-order harmonic in the extreme ultraviolet (XUV) region generated from the interaction of a femtosecond pulse with low-ionized In ablation. A strong 13th harmonic (61.2 nm) of Ti:sapphire laser radiation with output intensity almost two orders of magnitude higher than neighboring harmonics was observed in these studies. The high conversion efficiency of the 13th harmonic (8 x 10(-5)) is attributed to multiple collisions of electron trajectories with the origin due to multiphoton resonance with the In ion.