Direct laser writing of superimposed phase gratings for visible color-selective transmission

Direct laser writing using two-photon polymerization (TPP) is a powerful tool for fabrication of nano/micro-structures. However, the fabrication of high aspect ratio structures remains challenging due to mechanical instability and distortion induced by capillary forces during development. In this work, the zeroth order transmission of two types of diffraction phase gratings is investigated, with the goal of developing an optical filter exhibiting high spectral purity across the visible range. Fabrication limitations are addressed by introducing a reinforced grating geometry formed through the superposition of phase square wave gratings, enabling wider, shorter, and mechanically robust features. This superimposed grating enhances structural stability while simultaneously improving the optical response of the resulting bandpass filter and observed color purity. The designs of both conventional single line and superimposed gratings are determined using particle swarm optimization. The numerical and experimental results demonstrate that the superimposed gratings overcome the limitations of standard square wave structures, achieving improved spectral selectivity and offering enhanced mechanical resilience. This approach provides a promising pathway toward robust, high performance micro structured optical filters fabricated via TPP. The optimized IP-L structures can be directly transferred to imprint lithography, enabling straightforward replication and cost-effective large-scale production of color-selective filters.