Polarization-controlled photoswitching resolves dipole directions with subwavelength resolution.

We present that modulation of fluorescence emission by linearly polarized excitation light can allow us to resolve spatially two fluorescent molecules within a diffraction limit and to determine simultaneously their precise dipole directions. Using polarization-dependent photoswitching, we imaged the 2D geometry of the DNA Holliday junction in a 10-nm length scale by measuring both the distance and the in-plane dipole angle between Cy3 emitters stacked onto the ends of two adjacent branches of the Holliday junction. The proposed polarization-modulated imaging technique provides a simple and nonstochastic imaging process to visualize the nanostructure, including directional information, of biomolecules beyond the diffraction limit.