Two-photon lithography for three-dimensional fabrication in micro/ nanoscale regime: A comprehensive review

With the advent of femtosecond lasers in the early 1990s, ultrafast laser processing has proven to be an imperative tool for micro/nano machining. Two-photon lithography (TPL) is one such unique microfabrication technique exploiting the nonlinear dependency of the polymerization rate on the irradiating light intensity to produce true three-dimensional structures with feature sizes beyond the diffraction limit. This characteristic has revolutionized laser material processing for the fabrication of micro and nanostructures. In this paper, an overview of TPL including its working principle, experimental setup, and materials is presented. Then, the effect of resolution with a focus on techniques adopted to improve the final resolution of the structures is covered. Insights to improve throughput and speed of fabrication to pave a way for industrialization of this technique are provided. Finally, TPL for microfabrication of structures with the emphasis on metamaterials is thoroughly reviewed and presented.