Lenses for Terahertz Applications: Development of New Materials and Production Processes

Up to now, polymer-based terahertz lenses have been made of pure polymers, such as high-density polyethylene, polytetrafluoroethylene or polypropylene. They are fabricated by injection molding, sintering or mechanical machining. Yet, the refractive indices of the abovementioned materials lie in a range between 1.40 to 1.55. Materials with a higher refractive index would allow for the fabrication of thinner lenses with a smaller curvature, shorter focal lengths and thus improved imaging properties. In this work innovative THz-lenses based on highly filled polymeric compounds, possessing a refractive index significantly higher than that of pure polymers, were developed. Furthermore, a simple and versatile approach for the fabrication of THz-lenses by means of extrusion and subsequent compression molding is presented. It was shown, that the new lenses exhibit refractive indices around 2.0 and simultaneously a low absorption. Other relevant material properties were satisfying as well. The new lenses show a very good surface quality. Compared to conventional lenses made of pure polymers, the new lenses possess an improved imaging quality and functionality. In addition, substrate lenses filled with highly refracting titanium dioxide could be used as an adequate replacement for their expensive, silicon made counterparts. Furthermore, compression molding proved to be a suitable method to fabricate lenses in a fast, efficient and precise way.