Polarimetry is a crucial tool in both fundamental and applied physics, ranging from the measurement of parity nonconservation (PNC) in atoms, to the determination of biomolecule structure, and the probing of interfaces. These measurements tend to be extremely challenging as the change of the polarization of light is usually extremely small; typical differences in polarization states are of the order of 10^-5 to 10^-8. Current experimental techniques often require acquisition times of the order of seconds or, in the case of PNC, even many days, limiting the possibilities of time-resolved measurements. Here, I propose to develop optical-cavity-based techniques which will enhance measurements of the polarization sensitivity and/or the time-resolution by 3-6 orders of magnitude. Preliminary data from prototypes and feasibility studies are presented. I propose to demonstrate how these breakthroughs will revolutionize polarimetry, by addressing some of the most important multidisciplinary problems in fundamental physics, biophysics, and material science: a) Testing the limits of the Standard Model with atomic PNC measurements. Current PNC experiments, and more importantly theory, for cesium atoms are limited to precision of about 0.5%. The novel and robust experimental technique I am proposing here affords 4 orders-of-magnitude higher sensitivity, thus giving access to lighter atoms, where the theory can be better than 0.1%, for the most stringent test of the Standard Model, while seeking new physics. b) The measurement of protein folding dynamics. Highly sensitive time-resolved spectroscopic ellipsometry, providing novel dynamical information on protein folding: nanosecond resolved, position measurements of functional groups of surface proteins, which map out the time-dependent protein structure. c) Determination of thin film thickness and surface density with nanosecond resolution, for the study of processes such as laser ablation and polymer growth.