Nanofiller based spin-on materials for negligible reflection of silicon photonic external coupling

Optical out coupling is one of the key research items of silicon photonic packaging. It is necessary to realize robust, high-bandwidth, low-power optical data communication system in an economical way. Because material interfaces are always involved for the coupling, the reflection loss at the interface often occupies a major part of loss. Here we report on an ideal anti-reflection coating method for use with silicon nanophotonics devices. Because the refractive index of silicon is large at about 3.5, the optical reflection at the silicon waveguide boundary is large. We describe a set of nanoparticle-based spin-on materials having indices of 1.8 and 1.2. The former material is suitable for a single layer anti-reflection coating on a silicon surface and realizes a first order thin reflection suppression which allows large bandwidth and large angle tolerance. The latter material is for coating of the fiber or polymer interface. We studied the feasibility of uniform coating over microlens structures. High index material is also photo patternable, which is desirable for silicon chip integration.