  A dielectric vertical cavity is used to study the spin dynamics of
molecularly self-assembled colloidal CdSe quantum dots (QDs). Using this
structure, a nearly 30-fold enhancement of Faraday rotation is observed, which
scales with the quality factor of the cavity. In this classical nonperturbative
regime, the amplified rotation is attributed to optically excited spins
interacting with multiple passes of the probe photons in the cavity. By
applying this general planar cavity motif to Faraday rotation, dynamical
measurements are accessible at extremely low powers on relatively small numbers
of quantum confined spins. In CdSe QDs, low power measurements reveal that
contributions from exciton and electron spin precession are largely dependent
upon the power of excitation. We demonstrate that this scheme is amenable to
both soft and hard systems as a means to increase detection sensitivity.
