A new three-zone amplitude-only filter for increasing the focal depth of near-field solid immersion lens systems

The near-field solid immersion lens (SIL) system with high numerical aperture exhibits high-quality resolution but its focal depth is at the nanoscale. This drawback limits its applications in high-density optical storage and photolithography. A new three-zone amplitude-only filter is proposed, to increase the focal depth and further improve the resolution of SIL systems. The procedure for designing a rotationally symmetric pupil-plane filter is presented, to control the three-dimensional intensity distribution of a SIL system near focus. The spot size, focal depth, Strehl ratio, and sidelobe intensity are analysed in detail. Numerical simulations show that, using the proposed filter, focal depth and resolution can be increased and sidelobe intensity can be suppressed.