  We analyze the formation of the low-energy structure (LES) in above-threshold
ionization spectra first observed by Quan et al.\ \cite{quan09} and Blaga et
al.\ \cite{blaga09} using both quasi-classical and quantum approaches. We show
this structure to be largely classical in origin resulting from a
two-dimensional focusing in the energy-angular momentum plane of the
strong-field dynamics in the presence of the atomic potential. The peak at low
energy is strongly correlated with high angular momenta of the photoelectrons.
Quantum simulations confirm this scenario. Resulting parameter dependences
agree with experimental findings \cite{quan09,blaga09} and, in part, with other
simulations \cite{liu10,yan10,kast11}.
