Controllable long focal length microlens based on thermal expansion

A novel method to fabricate a microlens array with a long focal length has been developed in this paper. It is based on the fabricating and heating of a microlens consisting of two materials with a great difference in coefficient of thermal expansion. A thermal expansion process leads to considerable deformation of the microlens surface and significant increase in focal length, which could be controlled by altering the processing temperature. Cylindrical polymeric microlens arrays with different focal lengths were successfully fabricated. By measuring the focal length and temperature dependence of the cylindrical microlens geometry, the formation mechanism was analyzed and validated. While the temperature is ranged from 20°C to 50°C, the focal length of the cylindrical microlens has been extended by 38.2% and the longest focal length was obtained up to 6.6 mm for the microlens with a linewidth of 240 μm.