Manipulation of doughnut focal spot by image inverting interferometry.

Based on image inverting interference combined with phase modulation, we theoretically demonstrate that the doughnut focal spot can readily be manipulated, and either shrinkage or expansion of size of the central dark spot is possible in a large scale (peak-to-peak value: 0.555λ-0.830λ, or 93.3%-140.8% compared with the standard one). As the interference and phase modulation can both be achieved by a double Porro prism, it is feasible to introduce this approach into optical tweezers to improve their performance. As much as 33.9% intensity of stimulated emission depletion (STED) beam can be reduced if the further optimized configuration is utilized in STED microscopy.