  We describe a high-resolution, high-bandwidth technique for determining the
local viscoelasticity of soft materials such as polymer gels. Loss and storage
shear moduli are determined from the power spectra of thermal fluctuations of
embedded micron-sized probe particles, observed with an interferometric
microscope. This provides a passive, small-amplitude measurement of rheological
properties over a much broader frequency range than previously accessible to
microrheology. We study both F-actin biopolymer solutions and polyacrylamide
(PAAm) gels, as model semiflexible and flexible systems, respectively. We
observe high-frequency omega^(3/4) scaling of the shear modulus in F-actin
solutions, in contrast to omega^(1/2) scaling for PAAm.
