Substrate Reshaping for Optically Tuned Liquid-Printed Microlenses Beyond Their Wetting Properties

Abstract

Several strategies exist capable of fabricating microlenses for applications such as cameras and solar cells. Among them, techniques based on printing curable liquid prepolymers including inkjet and electrohydrodynamic-jet printing, or laser-induced forward-transfer (LIFT) offer unique advantages in terms of ease of integration, cost, and compatibility with flexible substrates. However, the optical properties of the so-fabricated microlenses depend on the wettability of the liquid prepolymer, preventing the broad implementation of printing technologies for micro-optics. Herein, how printing microdroplets on top of reconfigurable substrates allows overcoming this issue is reported. The strategy, called print-n-release, is based on depositing prepolymer microdroplets on top of mechanically stretched elastomeric substrates. Once the stress applied to the substrate is released, and provided pinning of the contact line, the microdroplet's base diameter decreases producing an increase in contact angle and reduction in radius of curvature. Following a curing step, the microdroplets are converted into microlenses whose shape no longer depends exclusively on their wetting properties, but also on the substrate elongation. It is demonstrated that, by combining LIFT with substrates elongated up to 80%, microlenses can be fabricated with a 400% increase in contact angle and a 90% reduction in focal length, in good agreement with theoretical predictions.