Hydrogels for active photonics

Conventional photonic devices exhibit static optical properties that are design-dependent,including the material’s refractive index and geometrical parameters.However,they still possess attractive optical responses for applications and are already exploited in devices across various fields.Hydrogel photonics has emerged as a promising solution in the field of active photonics by providing primarily deformable geometric parameters in response to external stimuli.Over the past few years,various studies have been undertaken to attain stimuli-responsive photonic devices with tunable optical properties.Herein,we focus on the recent advancements in hydrogel-based photonics and micro/nanofabrication techniques for hydrogels.In particular,fabrication techniques for hydrogel photonic devices are categorized into film growth,photolithography(PL),electron-beam lithography(EBL),and nanoimprint lithography(NIL).Furthermore,we provide insights into future directions and prospects for deformable hydrogel photonics,along with their potential practical applications.

Nanoimprint lithography for high-throughput fabrication of metasurfaces

Metasurfaces are composed of periodic subwavelength nanostructures and exhibit optical properties that are not found in nature.They have been widely investigated for optical applications such as holograms,wavefront shaping,and structural color printing,however,electron-beam lithography is not suitable to produce large-area metasurfaces because of the high fabrication cost and low productivity.Although alternative optical technologies,such as holographic lithography and plasmonic lithography,can overcome these drawbacks,such methods are still constrained by the optical diffraction limit.To break through this fundamental problem,mechanical nanopatteming processes have been actively studied in many fields,with nanoimprint lithography(NIL)coming to the forefront.Since NIL replicates the nanopattem of the mold regardless of the diffraction limit,NIL can achieve sufficiently high productivity and patterning resolution,giving rise to an explosive development in the fabrication of metasurfaces.In this review,we focus on various NIL technologies for the manufacturing of metasurfaces.First,we briefly describe conventional NIL and then present various NIL methods for the scalable fabrication of metasurfaces.We also discuss recent applications of NIL in the realization of metasurfaces.Finally,we conclude with an outlook on each method and suggest perspectives for future research on the high-throughput fabrication of active metasurfaces.