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Freeform optics can reduce the cost, weight, and size of advanced imaging systems, but it is challenging to manufacture the complex rotationally asymmetric surfaces to optical tolerances. To address the need for disruptive, high-precision sub-aperture forming and finishing techniques for freeform optics, we investigate an alternative, non-contact polishing methodology using femtosecond lasers, combining modeling, experiments, and demonstrations. Femtosecondlaser- based polishing of germanium was investigated using an experimentally-validated twotemperature model of laser/germanium interaction to guide the understanding and selection of laser parameters to achieve near-nonthermal ablation for polishing and figuring. For the first time to our knowledge, model-guided femtosecond laser polishing of germanium was successfully demonstrated, achieving precision material removal while maintaining single-digit nanometer optical surface quality. The demonstrated femtosecond-laser-based polishing technique lays the foundation for semiconductor optics polishing/fabrication using femtosecond lasers and opens a viable path for high-precision, complex sub-aperture optical polishing tasks on various materials. © 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement 

Sensitivity of femtosecond laser ablation to laser parameters was investigated to evaluate controllability of germanium material removal. A processing metric was determined, enabling a method to control laser polishing with flexible combinations of laser parameters.