Sub-diffraction-limit imaging based on the topographic contrast of differential confocal microscopy

Using the nanometer depth sensitivity of differential confocal microscopy, we detect surface features of lateral dimensions smaller than the diffraction limit without fluorescence labeling. The lateral resolution of the topographic images is further enhanced by a maximum-likelihood estimation algorithm. Based on the comparison of signal and noise at high spatial frequency, we estimate the best lateral resolution of the enhanced images to be 0.15 lambda. In addition, on composite samples this technique can simultaneously display sub-diffraction-limit topographic features and reflectivity heterogeneity.