A method for extending depth of focus in STED nanolithography

Abstract

We present a theoretical approach to extend depth of focus (DOF), which is based on the integrated utilization of the cylindrical polarization effect, circular symmetrical phase mask and interference effect. Simulation results show that this method can generate over 4λ and 2λ DOFs when objective lenses with NA = 0.9 and 1.4 are introduced respectively, while maintaining the horizontal size beyond the diffraction limit. The DOF is over two times that by conventional approaches under the same conditions. Such a focal spot with extended DOF, although stemming from the STED microscope concept, is applicable in nanolithography.