  Soft matter consists of meso-scale (nm~{\mu}m) structures that are formed by
weak interactions and reorganized under thermal fluctuations. The resulting
complex relaxation phenomena may be probed with microrheology, by observing the
movement of embedded probe particles. Because of the softness of the material,
however, perturbations to the probe that are inevitably added during
microrheology experiments prevent direct translation of those movements to
rheological properties. In this study, we conducted optical-trap-based
microrheology with significantly reduced mechanical perturbations; dual
feedback technology allowed us to apply well-determined optical-trapping forces
to a fluctuating embedded probe and precisely measure its response and
fluctuations with high spatiotemporal resolution. We demonstrate the improved
performance of this technique by studying an reconstituted network of actin
cytoskeletal filaments, observing their slow dynamics, homogeneous thermal
fluctuations as well as activated hopping between mesoscale microenvironments.
