Microfabrication of SU-8 Fresnel lenses for THz imaging

Free space focusing of terahertz light is normally achieved through the use of bulky parabolic mirrors. Alternatively, for focusing onto a substrate or sample, polished high resistivity silicon lenses are commonly used. This paper presents the design, fabrication and testing of an alternative approach, based on Fresnel microlenses which have been optimised for use in the terahertz region. The microlenses are fabricated using layers of SU-8 photoresist and conventional UV photolithography. The lens design approach presented here provides a low cost, mass production ready alternative to silicon lenses. Fresnel lenses can have a large numerical aperture and a short focal length and are well suited for use in terahertz imaging systems. The focal point of the demonstrated Fresnel microlens has been calculated to be approximately 5 mm at 1 THz using a commercial FDTD solver, Lumerical. Characterization of the microlenses by VNA (Vector Network Analyzer) operating in the frequency range of 750 GHz to 1.1 THz is presented and discussed. The measured focal length using the VNA approach corresponds well to the values calculated using the FDTD solver and demonstrates effective focusing from highly compact lenses.