Spotlight on “Freeform Three-Dimensional Embedded Polymer Waveguides Enabled by External-Diffusion Assisted Two-photon Lithography”

High-index contrast waveguides now go 3D on a photonic chip. One of the main limits of planar lightwave circuits is that they are… planar, indeed. Giving optical waveguides a third dimension would offer more flexibility to overcome the density and complexity limits of conventional photonic integrated architectures, with large benefits in optical interconnects applications. Direct laser writing is a well established technique to fabricate free-form 3D optical waveguides, but until now it was supposed to be not suitable to realize high-index-contrast single-mode waveguides. Untrue, actually, because H. H. D. Nguyen and coworkers have just got it. They exploited 3D femtosecond-laser curing of a newly developed photopolymer, followed by diffusion of a lower-index gaseous monomer. The diffusion of the guest monomer is the key of their method. It leads to a strong reduction of the refractive index of the uncured host polymer, as large as 0.013; in contrast, the refractive index of the laser-cured polymer, that is the waveguide core, is almost not affected by the monomer diffusion process. This technique maintains all the main advantages of conventional 3D laser writing schemes, such as the possibility to write multi layers of waveguides without any stacking or alignment effort, the need for neither wet chemical nor etching process, the use of only one layer of a single material. And waveguide propagation loss (0.37 dB/cm at 850 nm) is nicely low. Monomer diffusion time, now in the order of several tens of hours, is a bit long, but H. H. D. Nguyen and coworkers are confident that it can be reduced to better meet commercial requirements.