Lensless focusing with subwavelength resolution by direct synthesis of the angular spectrum

We consider the coherent superposition of unfocused wave fronts for lensless focusing of electromagnetic waves with subwavelength resolution. Near the focal point, intensity distributions generated using the approach approximate those generated using lenses. Far from the focal point, discretization of spatial frequencies results in a trade-off between the number of wave fronts and the accuracy of the approximation. We experimentally demonstrate the feasibility of the approach by generating an approximation of an azimuthally polarized Bessel beam with a focal spot diameter (full width at half maximum intensity) of 0.37 lambda.