This paper presents a new concept to tackle this challenge using the combination of femtosecond direct laser writing (FsDLW) in polymer and external diffusion of a gaseous monomer. FsDLW with two-photon absorption was used to initiate cross-linking following a programmed trajectory to form the waveguide core. A thermal treatment was then needed to complete cross-linking. Afterwards, a low-index monomer from a gas atmosphere was diffused into the uncross-linked cladding. Since this diffusion hardly occurred in the already cross-linked pattern, the subsequent UV flood exposure only cross-linked the diffused monomer with host oligomer in the cladding. This low-index monomer decreased the refractive index of the cladding and, therefore, created enough refractive index contrast for total internal reflection. Finally, the whole structure was hard-baked for polymerization and stabilization.
The peak refractive index change of 0.012 was revealed using refractive near field method. Measured near-field intensity at the end facet of waveguides showed single-mode Gaussian profiles. We further demonstrated how feature sizes can be linearly adjusted in the range of 5-12 μm by varying scanning speed and laser intensity. Moreover, changing the voxel shape by a field aperture in front of the objective was investigated. Our fabrication method requires only one layer of a single material without masks, contact or wet processing. Free-form waveguides with high index contrast have high potential to improve the density and flexibility of optical interconnects at board level. Some applications of this concept are three-dimensional arrays of optical waveguide network routers, optofans, pitch converters or splitters.