Two decades of two-photon lithography: Materials science perspective for additive manufacturing of 2D/3D nano-microstructures

Two-photon lithography (TPL) is a versatile technology for additive manufacturing of 2D and 3D micro/nanostructures with sub-wavelength resolved features. Recent advancement in laser technology has enabled the application of TPL fabricated structures in several fields such as microelectronics, photonics, optoelectronics, microfluidics, and plasmonic devices. However, the lack of two-photon polymerizable resins (TPPRs) induces bottleneck to the growth of TPL to its true potential, and hence continuous research efforts are focused on developing efficient TPPRs. In this article, we review the recent advancements in PI and TPPR formulation and the impact of process parameters on fabrication of 2D and 3D structures for specific applications. The fundamentals of TPL are described, followed by techniques used for achieving improved resolution and functional micro/nanostructures. Finally, a critical outlook and future prospects of TPPR formulation for specific applications are presented.