  We present a surface passivation method that reduces surface-related losses
by almost two orders of magnitude in a highly miniaturized GaAs open
microcavity. The microcavity consists of a curved dielectric distributed Bragg
reflector (DBR) with radius $\sim 10$ $\mu$m paired with a GaAs-based
heterostructure. The heterostructure consists of a semiconductor DBR followed
by an n-i-p diode with a layer of quantum dots in the intrinsic region.
Free-carrier absorption in the highly doped n- and p-layers is minimized by
positioning them close to a node of the vacuum electromagnetic-field. The
surface, however, resides at an anti-node of the vacuum field and results in
significant loss. These losses are much reduced by surface passivation. The
strong dependence on wavelength implies that the main effect of the surface
passivation is to eliminate the surface electric field, thereby quenching
below-bandgap absorption via a Franz-Keldysh-like effect. An additional benefit
is that the surface passivation reduces scattering at the GaAs surface. These
results are important in other nano-photonic devices which rely on a
GaAs-vacuum interface to confine the electromagnetic field.
