  Imaging of the optical properties of individual nanosystems beyond
fluorescence can provide a wealth of information. However, the minute signals
for absorption and dispersion are challenging to observe, and only specialized
techniques requiring sophisticated noise rejection are available. Here we use
signal enhancement in a scanning optical microcavity to demonstrate
ultra-sensitive imaging. Harnessing multiple interactions of probe light with a
sample within an optical resonator, we achieve a 1700-fold signal enhancement
compared to diffraction-limited microscopy. We demonstrate quantitative imaging
of the extinction cross section of gold nanoparticles with a sensitivity below
1 nm2, we show a method to improve spatial resolution potentially below the
diffraction limit by using higher order cavity modes, and we present
measurements of the birefringence and extinction contrast of gold nanorods. The
demonstrated simultaneous enhancement of absorptive and dispersive signals
promises intriguing potential for optical studies of nanomaterials, molecules,
and biological nanosystems.
