Two-photon lithography for microlenses: principles and applications

The rapid miniaturization of modern photonic systems has created an urgent demand for high-performance, integrated micro-optics. This review presents a comprehensive overview of two-photon lithography (TPL) as a transformative technique for fabricating microlenses. Unlike conventional planar manufacturing, TPL leverages non-linear laser absorption to achieve true three-dimensional (3D) direct writing with sub-wavelength resolution. We systematically categorize advancements in microlens architectures, ranging from refractive freeform surfaces to diffractive elements and metalenses. The discussion extends to emerging design methodologies, such as topology optimisation and inverse design, alongside innovations in fabrication speed and functional material diversity. Furthermore, we highlight critical applications in endoscopic biomedical imaging, optical fibre integration, and advanced 3D display technologies. By bridging the gap between sophisticated optical design and manufacturing capabilities, TPL is positioned to revolutionize the field of integrated photonics, offering a versatile platform for the next generation of smart, multifunctional micro-optical systems.