Laser Ablation and Nanoimprint Lithography for the Fabrication of Embedded Light Redirecting Micromirrors

Light redirecting devices usually increase the daylight illuminance level far from the window but also affect the visual comfort to some extent. Some designs achieve high redirection rate but are not transparent, others offer a partial view through but with reduced performance. Miniaturizing the light redirecting mirrors and encapsulating them has the potential to increase both view and performance. The shape of such encapsulated micro mirrors was optimised in a Monte Carlo ray-tracing model. In simulations, the redirected proportion of light could be increased with minimal influence on the transparency of the device. Maximal transparency was conserved at near to normal incidence with a strong redirection of light beams providing both daylight and glare protection. In this study, the fabrication process of micro mirrors embedded in a transparent medium is described. The later process consists of a succession of four steps: mould fabrication, replication in an ultraviolet (UV) curable resin, partial coating with a reflective material at an imposed deposition angle and embedding using the same UV curable resin. The mould was fabricated by laser ablation at EMPA and replicated into polydimethylsiloxane (PDMS) to enable correct unmoulding of the resin. This negative mould was used to replicate the original structure into a transparent low-shrinkage hyperbranched acrylate polymer (HBP). It was identified that the direction in which the UV curable resin is polymerised is of crucial importance. Embedded micro mirrors provide transparency at normal incidence and the redirected proportion of light impinging at 60° was measured to be greater than 80%.