Fabrication of microlens arrays in polycarbonate with nanojoule energy femtosecond laser pulses.

We report fabrication of smooth microlens arrays by focusing a nanojoule energy, high repetition rate femtosecond laser oscillator inside a polycarbonate sample. Heat accumulation at the laser focal point and subsequent material expansion leads to localized swelling at the sample surface that behaves as a microlens. By changing the depth of the laser focus in the sample, the focal length of the microlens can be controlled and varies from 40 to 80 μm while maintaining a high numerical aperture of ~0.6. This fabrication technique is a single step, controllable, and economical process that can produce arrays of optically smooth microlenses.