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LAMPTA

Duration: 12 months, Start: February 2015

Low-cost additive manufacturing of polymeric components for THz applications: LAMPTA

Academic project leader: Thomas Feurer, Institution: IAP Laser Physics, University of Bern. Industrial project partner: AME GmbH - Philippe Raisin, Florian Reinhard, Adrian Ryser, Leandro von Werra

Over the last decades, additive manufacturing - commonly known as 3D printing - has emerged as a promising fabrication technology, offering opportunities from research to engineering. The different additive manufacturing technologies offer a wide range of printable polymers. In general, these polymers happen to be transparent for radiation in the terahertz frequency regime. Furthermore, the manufacturing resolution of the machines fulfills surface roughness requirements for these frequencies. Hence an opportunity to print optical elements for THz setups presents itself. The goal of AME GmbH is to explore these possibilities in collaboration with the University of Bern. They strive to apply the unique qualities of additive manufacturing to develop optical elements such as waveguides, lenses, mirrors as well as more complex integrated optical systems. In addition, different coatings and after-treatments will be employed to match desired properties.

LAMPTA aims at utilizing lower-cost desktop 3D-printers that have emerged over the last three years. With these machines, a variety of optical parts will be developed. Furthermore, our ambition is to assign not only form but also physical properties to objects created with additive manufacturing technology. The development of advanced optical parts benefits from this ability. Above cited papers have shown that insertion of nanoparticles such as graphene, carbon nanotubes or metallic powders into the printing material leads to significant changes in physical properties such as conductivity. These characteristics, combined with the freeform nature of additive manufacturing, opens the door to fabricate unique optical parts.

Results
In the course of the project, various techniques have been employed to manufacture low cost terahertz polymer components. Most applications favor components with high refractive indices and low absorption coefficients. The base material of choice for low cost components are polymers, however, most polymers exhibit a low refractive index. This problem can be tackled by mixing the polymer with high refractive material powders (Al₂O₃, ZnS, TiO₂). Throughout the project, this option to mix materials was explored.

In the first phase, two additive manufacturing technologies have been tested: Fused deposition modelling (FDM) as well as stereo lithography (SLA), each with desktop-sized printers. In general, both machines can print features upwards of 300µm, meaning that these printers have turned out not to be capable of printing Terahertz optics with sub-wavelength features, which is often of interest. In order to compare the results of the 3D printer study to conventional manufacturing methods, several lenses were produced with the 3D printers and a desktop sized CNC mill and compared. The CNC mill proved to be more reliable and yielded smoother surface properties. This shows that standard manufacturing processes are not easily replaced by 3D printers. In a second phase, aspheric THz lenses were compressed from polyethylene (PE) micro-powders on a laboratory-scale hydraulic press. With minimal effort one can mix metal powders into the micro-powders, which enables production of polymer/metal compound optical components. Furthermore, PE features very low absorption in the THz range. Therefore, pressure molding is a useful technique for the production of low cost prototypes or small volume production. The press molds were fabricated on a CNC milling machine.

After performing satisfyingly well, a series of lenses with focal lengths of [200, 100, 50, 33]mm was externally manufactured and is currently being sold to interested parties. Save for Fresnel losses, no absorption in the polymer has been measured. So in total, each lens (regardless of focal length) has a total transmission of about 90%. In addition, alignment of lenses during testing was very straightforward, with only minimal adjustment needed to optimize the THz signal. This ease of handling is in stark contrast with the tedious alignment needed in the case of off-axis parabolic mirrors. Given the aspheric design, effective focal lengths down to 33mm were realized, which resulted in measured spot sizes on the order of 300µm at 1THz. In addition, several multi-lens arrays with a lens pitch down to 1.1mm and radius of curvature of 0.75mm were fabricated with the micro-powder compression method for use in a Terahertz Hartman Shack sensor to reconstruct the phase front of Terahertz radiation emitted from a non-linear crystal.


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