  We propose the use of nanostructured photonic nanocavities made of
second-order nonlinear materials as prospective passive devices to generate
strongly sub-Poissonian light via single-photon blockade of an input coherent
field. The simplest scheme is based on the requirement that the nanocavity be
doubly resonant, i.e. possess cavity modes with good spatial overlap at both
the fundamental and second-harmonic frequencies. We discuss feasibility of this
scheme with state-of-the art nanofabrication technology, and the possibility to
use it as a passive single-photon source on-demand.
