Liquid deposition photolithography for submicrometer resolution three-dimensional index structuring with large throughput

Abstract

Photonic devices increasingly require three-dimensional control of refractive index, but existing fabrication methods such as femtosecond micromachining, multilayer lithography and bulk diffusion can only address a select scale range, are often limited in complexity or thickness and have low throughput. We introduce a new fabrication method and polymeric material that can efficiently create mm3 optical devices with programmable, gradient index of refraction with arbitrary feature size. Index contrast of 0.1 is demonstrated, which is 100 times larger than femtosecond micromachining, and 20 times larger than commercial holographic photopolymers. This is achieved by repetitive microfluidic layering of a self-developing photopolymer structured by projection lithography. The process has the unusual property that total fabrication time for a fixed thickness decreases with the number of layers, enabling fabrication 105 faster than femtosecond micromachining. We demonstrate the process by sequentially writing 100 layers to fabricate a mm thick waveguide array. A photolithography scheme for making polymer-based photonic circuitry can be used to create complex 3D devices with large refractive index contrasts. Adam Urness and co-workers from the University of the USA say that their ‘liquid deposition photolithography’ approach can rapidly create millimetre-scale structures with submicrometre resolution and refractive index contrasts of up to 0.1. The scheme works by projecting a pattern of ultraviolet (365 nm) light onto a layer of a liquid photosensitive polymer that solidifies into a gel upon exposure. Regions that receive greater light intensity have a higher density and refractive index. The process can be repeated with many layers to create large, complex structures. For example, the team has used their approach to fabricate miniature logos, waveguide arrays and photonic crystal fibres.