Re-revealing dark-field scanning imaging via modal decomposition of bright-field light in the OAM domain

An alternative dark-field (DF) scanning imaging is re-revealed via modal decomposition of the bright-field (BF) light in the orbital angular momentum (OAM) domain. A Gaussian beam containing only the zeroth-order OAM mode illuminates the surface. The BF light scattered from the surface carries the phase shift induced by the surface feature and then diffracts into a distorted light field, which is a coherent sum of the different OAM modes. The amplitude weight and inter-modal phase of each mode are obtained by modal decomposition. As the surface is scanned transversely, the DF image contrast results from either the square amplitude or the inter-modal phase at the nonzero orders of OAM modes. The results verified that the first-order OAM mode produces a DF image with the highest contrast for the high-spatial-frequency surface features. The inter-modal phase DF image is less sensitive to the rough surface than the square amplitude DF image, thus producing a cleaner DF background. The square amplitude DF image is more advantageous in observing high-spatial-frequency surface features. The proposed DF scanning imaging in the OAM domain provides an additional degree of freedom for surface characterization.