Decoding Nanoscale Asymmetries with Engineered Electromagnetic Nulls

Conventional far-field optical microscopy fails to resolve nanoscale perturbations in symmetric nanostructures due to the diffraction limit. Here, we report a symmetry-matched Fourier phase modulation microscopy (SM-FPM²), which enables the direct visualization of nanometric perturbations in symmetric nanostructures. SM-FPM² utilizes specifically designed binary π-phase Fourier modulation patterns to match the geometric symmetry of the target nanostructures. By converting in-phase scattering into out-of-phase interference, SM-FPM² creates engineered electromagnetic nulls, allowing for the high-contrast separation of subwavelength features. We experimentally demonstrate the visualization of 40 nm nanostructures spaced by 35 nm under 421 nm illumination, as well as the identification of cutting, extrusion, and particle-type defects in nanostructure assemblies. SM-FPM² operates in the far-field, requires no labeling or nonlinear effects, and integrates directly into standard bright-field microscopes. These attributes make it promising for wafer inspection, materials characterization, and biosensing, particularly for detecting structural irregularities in symmetric nanoscale assemblies.