Fabrication of concave spherical microlenses on silicon by femtosecond laser irradiation and mixed acid etching.

All-silicon plano-concave microlens arrays with spherical profiles and good image performance were obtained using femtosecond laser direct irradiation and mixed acid etching. A femtosecond laser was employed to produce microhole arrays on silicon, and the microholes were expanded and smoothed by the mixed acid to form concave microlenses. The effects of the etching time, laser power, and pulse number on the microlens morphology were investigated. This method has potential applications in the fabrication of all-silicon plano-concave microlenses for use in infrared devices.