Creation of dual foci structures with tunable depth of focus and spot size of high-order radially polarized beams

Here, we present a numerical method that exceeds conventional optical limit of high-order radially polarized beams by tuning the spot size and depth of focus (DoF) and reveals their dual and quad foci behavior, under tight focusing conditions. Our analysis, grounded on the vector diffraction theory, demonstrates that the phase mask containing only two regions of opposite phases, effectively suppresses peripheral energy, resulting in a notable reduction in spot size and simultaneous increase in DoF, particularly at lower numerical apertures (NA). The advantage of the proposed method is that it performs all three tasks, such as reduction in the spot size, enhancement in the DoF and creation of dual foci and quad foci patterns in a controlled manner. This study demonstrates that the method can be used to enhance the efficiency of high-order radially polarized beams, which could lead to breakthroughs in optical data storage, multifocal imaging, and other advanced optical applications.