A novel micro-optical electric field sensor exploits morphology-dependent shifts of the optical modes of dielectric cavities to measure temporally- and spatially-resolved of electric field with extremely high sensitivity. The measurement principle is based on the electrostriction effect on the optical modes of dielectric micro-resonators (or micro-cavities) and exploits recent developments in optical fiber and switching technologies. The optical modes are commonly referred to as “whispering gallery modes” (WGM) or “morphology dependent resonances” (MDR). By monitoring the WGM shifts, the electric field causing the electrostriction effect can be determined. Different sensitivities and measurement ranges (maximum measured electric field) can be obtained by using different cavity geometries (for example solid or hollow spheres), polymeric materials (PMMA, PDMS, etc) as well as poling the dielectric material.