Surface profilometry using vortex beams generated with a spatial light modulator

We present a common path interference setup for detection of surface depth differences of the order of 200 nm, using a vortex and a Gaussian beam generated with a spatial light modulator. The phase of a vortex with a topological charge + 1 and the phase of a lens are encoded next to each other onto the spatial light modulator. Both beams are incident onto a surface with a step feature; the Gaussian beam is incident onto a flat mirrored surface, while the vortex beam scans through the step. Upon interaction with the testing surface, both beams are superposed and their interference pattern is captured, collecting a set of images as a result of the scanning process. A phase detection algorithm is applied to these images and by tracking the vertical positions of the phase singularity along the scanning, the depth variations are estimated. The proposed set up is able to detect a 179 nm step feature, working with light at a wavelength of 830 nm and using a vortex with a diameter of 177 mu m.