Valley in the efficiency of the high-order harmonic yield at ultra-high laser intensities Pérez-Hernández, J. A. ; Roso, L. ; Zaïr, A. ; Plaja, L. Abstract Publication: Optics Express Pub Date: September 2011 DOI: 10.1364/OE.19.019430 Bibcode: 2011OExpr..1919430P