title: Experimental high numerical aperture focusing with high contrast gratings.
abstract: We demonstrate high aperture (up to NA~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 μm that are crucial for micro-optical applications. The phase profile of the lens includes multiple 2π 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 μm that is close to the size of a corresponding Airy disk.
authors.affiliation: Department of Physics, University of York, Heslington, York, UK. annett.klemm@york.ac.uk
authors.forename: Annett B
authors.initials: AB
authors.lastname: Klemm
authors.forename: Daan
authors.initials: D
authors.lastname: Stellinga
authors.forename: Emiliano R
authors.initials: ER
authors.lastname: Martins
authors.forename: Liam
authors.initials: L
authors.lastname: Lewis
authors.forename: Guillaume
authors.initials: G
authors.lastname: Huyet
authors.forename: Liam
authors.initials: L
authors.lastname: O'Faolain
authors.forename: Thomas F
authors.initials: TF
authors.lastname: Krauss