  Mesoscopic self-collimation in mesoscopic photonic crystals with high
reflectivity is exploited to realize a novel high-Q factor cavity by means of
mesoscopic PhC planar mirrors. These mirrors efficiently confine a mode inside
a planar Fabry-Perot-like cavity, due to a beam focusing effect that stabilises
the cavity even for small beam sizes, resembling the focusing behaviour of
curved mirrors. Moreover, they show an improved reflectivity with respect to
their standard distributed Bragg reflector counterparts that allows higher
compactness. A Q factor higher than 10^4 has been achieved for an optimized
5-period-long mirror cavity. The optimization of the Q factor and the
performances in terms of energy storage, field enhancement and confinement are
detailed.
