A process for manufacturing an optical system includes forming a first hydrophobic surface at a semiconductor substrate, providing a first drop of transparent material having a first shape on the first hydrophobic surface, and allowing the first drop to harden to form a first optical element having the first shape. The optical system may be a particle detector, and the process may optionally further include forming a light source at the semiconductor substrate configured to generate a light beam that passes through the first optical element and a cavity to a photodetector.