Superresolution at the singularities of phase images

Measurements performed with an Airyscan laser phase microscope showed the possibility of localisation of structural elements of a surface with a large gradient of the profile height and linear dimensions as small as 25 – 40 nm, which is 10 – 15 times lower than the Rayleigh criterion. The width of the structural elements was estimated from the extent of a region of the enhanced intensity of phase fluctuations. A metallised CD substrate with the known surface structure was used as a test object. Fluctuations of the optical path difference, whose intensity increased at steep profile slopes, were measured by periodically scanning the surface profile. It is shown that the extent of the region of enhanced intensity of phase fluctuations is close to the measured width of the steep profile slope. The intensity of fluctuations increased proportionally to the square of the height gradient, while their spatial extent decreased noticeably with increasing the objective aperture. These measurements explain a high sensitivity of the dynamic phase microscopy to fluctuations of the optical path difference in the regions of high gradient of the refractive index. The increase in the intensity at the slopes of the phase height profile and the spatial – temporal correlation of fluctuations were observed in mitochondria and other biological objects.