The Fraunhofer Institute for Silicate Research ISC has accelerated a fabrication process that uses two-photon polymerisation (2PP) in the production of arbitrarily shaped micro-structures suited for innovative optical applications.
According to the institute, the fabrication of larger structures or larger batches of optical elements via 2PP is currently far too time consuming to be used in industrial scale production: 3D structures have to be processed point-by-point and only a single tiny focal volume of focused femtosecond laser pulses is used to solidify the material. To overcome this throughput bottleneck, the institute is currently investigating new approaches, methods and materials.
One such approach features the application of nanoimprint techniques in order to replicate 2D or 2.5D structures for volume manufacturing. Two-photon polymerisation is used to create a ‘master’ structure from which a silicone cast is moulded. This mould is then used to replicate the master structure using either the same polymer as the original, or instead by using other materials.
The approach was tested by the Fraunhofer ISC by fabricating a complex array of 10,000 individual 60µm micro-prisms using a galvo scanner and dip-in lithography. The copying of the master pattern took less than an hour, while the entire prism pattern took several days to complete.
In-house synthesised inorganic-organic hybrid polymers were used for the test, of which the properties can be customised via chemical modification or formulation. They can be especially designed for 2PP applications and also maintain their key properties, such as refractive index and low absorption, under harsh conditions. In addition, they offer superior optical properties, high stability and biocompatibility.
In combining 2PP with suitable methods of replication, the Fraunhofer ISC can provide both true freeform 3D micro-structures and a rapid and reliable fabrication technology. The technique also means different forms like prisms, lenses, or pyramids can be combined in a single substrate.
The institute looks to further optimise its materials and methods for the industrial upscaling and replication of micro optical structures in the future, particularly for large area diffractive optical elements, which continue to prove a challenge.
