Focal-point shaping of microlenses by amplitude or phase masks

We present results of numerical investigations into the possibility to shape the focal field distribution by means of amplitude or phase masks on top of a microlens that works in the transition regime between a diffractive and a refractive element. For different applications, such as photolithography or optical tweezers, a specialized intensity distribution in the focal region is desirable. For the trapping of small dielectric particle e.g. a large gradient in the field distribution becomes favourable to compensate for the scattering force, which pushes the particle towards the optical axis. We apply scalar as well as rigorous diffraction theory to calculate the interaction of light with the microlens and analyse the influence of different types of masks (ringaperture, central stop). As one application we analyse in the depth optical tweezers and calculate the forces in a two-dimensional geometry on particles with different sizes. The field in the focal region of a lens with an aperture suffers from a squeezing in lateral as well as transversal direction causing higher gradients of the field, which allows to trap relative large particles.