C4F8 plasma treatment for the modification of the focal length of liquid-based plano-convex lenses on different substrates

Abstract

In this contribution, we present a plasma-based approach for modifying the focal length of plano-convex lenses realised by the deposition of droplets made of optical cement. For this purpose, PMMA and soda-lime glass substrates were treated with a direct dielectric barrier discharge plasma at low pressure. To obtain plasma-induced changes in surface energy of the substrates by the formation of functional groups, octafluorocyclobutane (C4F8) was used as process gas. Such treatment lead to a notable decrease in surface energy and especially its polar fraction, which is attributed to the generation of fluorocarbon compounds such as polytetrafluoroethylene (PTFE) as also confirmed by FTIR spectroscopic measurements. It is shown that this effect has a considerable impact on the contact angle and radius of curvature, respectively, of the deposited cement droplets where differences by a factor of about two were obtained. After curing of the cement, front focal lengths in the range from approx. 5 to 13 mm were realised depending on the duration of the plasma pre-treatment. The cement lenses finally feature dimensions in the range of some millimetres.