Dual-Refractive-Index Photonic Crystal Waveguides Fabricated by Combining Two-Photon Polymerization 3D Nanoprinting with Dose-Modified One-Photon Polymerization

Abstract

A difference in refractive indices between polymers produced by two-photon polymerization (2PP) and one-photon polymerization (1PP) opens up a possibility for fabricating multi-refractive-index nanostructures. It significantly expands the design capabilities of 3D nanoprinting technologies based on 2PP-enabled direct laser writing (DLW). One key application is waveguide design, where the refractive index contrast between the core and the cladding materials determines light propagation properties. This study demonstrates the combination of 2PP and 1PP to fabricate photonic crystal fiber (PCF) segments, creating structures with a complex 3D refractive index distribution. Refractive index properties of commercially available IP-Dip and IP-S photoresins, commonly used in 2PP nanoprinting, are analyzed based on supplier data and previous research. Using these findings, PCF structures are designed to facilitate light propagation through either index-guiding (IG) or photonic bandgap (PBG) guiding mechanisms. The fabrication process is carried out using 2PP, exploiting the refractive index contrast between polymerized and unpolymerized resin regions. Subsequently, controlled UV exposure induces refractive index modifications in previously unpolymerized regions, enabling transitions between IG-to-PBG, PBG-to-IG, and IG-to-no guiding. This approach facilitates the fabrication of waveguides with tailored propagation properties, and by adjusting the PCF's transverse geometry and refractive index contrast, specific mode distributions can be achieved.