Signature of the transversal coherence length in high-order harmonic generation

It is well known that high-order harmonic generation (HHG) from extended targets differs substantially from that of isolated atoms. Phase matching during propagation plays a major role in defining the extent of the spatial regions in the target that contribute constructively to the final yield. Typically, the understanding of this process is simplified by considering phase-matching effects along the field propagation axis, defining a longitudinal coherence length. In this paper we explore the role of phase matching in the transverse direction. The definition of a transversal coherence length appears fundamental to understanding propagation of harmonics generated by focalized laser beams. We present experimental results—-supported by theory—in which transversal phase matching plays the leading role in the macroscopic HHG.We acknowledge technical support from Oscar Varela, Juan Hernández Toro, and Cruz Méndez. We acknowledge support by a Marie Curie International Outgoing Fellowship within the EU Seventh Framework Programme for Research and Technological Development, under REA Grant Agreement No. 328334. We acknowledge support from Junta de Castilla y León (Consejería de Educación and Fondo Social Europeo and Projects No. SA002B08 and No. SA116U13), Spanish MINECO (Grant No. FIS2009-09522, Consolider Program SAUUL CSD2007-00013, and Ramón y Cajal Program), and Centro de Láseres Pulsados, CLP