Isolation of phase edges using off-axis q-plate filters

Edge enhancement plays an important role in optical biological imaging that highlights the contours of an object. Edge-enhanced microscopes are widely used to enhance the edges of phase-amplitude objects due to their capability for all-directional edge enhancement, while differential interference-contrast microscopy enhances edges in only one-direction. However, edge-enhanced microscopes cannot distinguish the edges of phase and amplitude objects, as both edges are equally enhanced. This study introduces a novel method for isolating the edge of a phase object from an amplitude object using an off-axis q-plate filter in a $4f$ system. Herein, we combined off-axis q-plates with four different displacements to isolate the phase object edge from the amplitude object. To verify the effectiveness of the proposed method, we conducted experiments using two distinct samples. The first sample comprised a phase test target surrounded by an aperture, and the second sample involved an overlap between the phase test target and a white hair (with nonzero transmittance). In the samples, the isolated phase object edge met the requirements of theoretical expectations, and the amplitude object edge was reduced by approximately 93\%. In summary, the proposed method is a novel and effective approach for isolating the edge of a phase object from an amplitude object and can be useful in various biological imaging applications.