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The rapid development of additive manufacturing has fueled a revolution in various research fields and industrial applications. Among the myriad of advanced 3D printing techniques, two-photon polymerization lithography (TPL) uniquely offers a significant advantage in nanoscale print resolution, and has been widely employed in diverse fields, for example, life sciences, materials sciences, mechanics, and microfluidics. More recently, by virtue of the optical transparency of most of the resins used, TPL is finding new applications in optics and photonics, with nanometer to millimeter feature dimensions. It enables the minimization of optical elements and systems, and exploration of light-matter interactions with new degrees of freedom, never possible before. To review the recent progress in the TPL related optical research, it starts with the fundamentals of TPL and material formulation, then discusses novel fabrication methods, and a wide range of optical applications. These applications notably include diffractive, topological, quantum, and color optics. With a panoramic view of the development, it is concluded with insights and perspectives of the future development of TPL and related potential optical applications.

Architectural windows that smartly regulate indoor solar radiation by changing their optical transmittance in response to thermostimuli are developed as a promising solution toward reducing the energy consumption of buildings. Recently, energy‐efficient smart window technology has attracted increasing scientific interest, with the exploration of energy‐efficient novel materials as well as integration with practical techniques to generate various desired multifunctionalities. This review systematically summarizes emerging thermoresponsive materials for smart window applications, including hydrogels, ionic liquids, perovskites, metamaterials, and liquid crystals. These are compared with vanadium dioxide (VO2), a conventional and extensively studied material for thermochromic smart window applications. In addition, recent progress on cutting‐edge integrated techniques for smart windows is covered, including electrothermal techniques, self‐cleaning, wettability, and also integration with solar cells for bifunctional energy conservation and generation. Finally, opportunities and challenges relating to thermochromic smart windows and prospects for future development are discussed. NRF (Natl Research Foundation, S’pore) MOE (Min. of Education, S’pore) Accepted version

A printable hybrid hydrogel is fabricated by embedding poly(N‐isopropylacrylamide) (PNIPAm) microparticles within a water‐rich silica‐alumina(Si/Al)‐based gel matrix. The hybrid gel holds water content of up to 70 wt%, due to its unique Si/Al matrix. The hybrid hydrogel can respond to both heat and electrical stimuli, and can be directly printed layer‐by‐layer using a commercial 3‐dimensional printer, without requiring any curing. The hybrid ink is printed onto a transparent, flexible conductive electrode composed of silver nanoparticles and sustains bending angles of up to 180°, which enables patterning of various flexible devices such as smart windows and a 3D optical waveguide valve. NRF (Natl Research Foundation, S’pore) Accepted version