Experimental high numerical aperture focusing with high contrast gratings

We demonstrate high aperture (up to NA similar to 0.64) three-dimensional focusing in free space based on wavefront-engineered diffraction gratings. The grating lens' optical response is tailored by spatially varying the grating ridge and groove width in two dimensions to achieve focal lengths of order 100 mu m that are crucial for micro-optical applications. The phase profile of the lens includes multiple 2 pi phase jumps and was obtained by applying an algorithm for finding the optimal path for both phase and amplitude. Experimental measurements reveal a lateral spot size of 5 mu m that is close to the size of a corresponding Airy disk. (C) 2013 Optical Society of America