The development of real-time and sensitive humidity sensors is in great demand from smart home automation and modern public health. We hereby proposed an ultrafast and full-color colorimetric humidity sensor that consists of chitosan hydrogel sandwiched by a disordered metal nanoparticle layer and reflecting substrate. This hydrogel-based resonator changes its resonant frequency to external humidity conditions because the chitosan hydrogels are swollen under wet state and contracted under dry state. The response time of the sensor is ~10 4 faster than that of the conventional Fabry-Pérot design. The origins of fast gas permeation are membrane pores created by gaps between the metal nanoparticles. Such instantaneous and tunable response of a new hydrogel resonator is then exploited for colorimetric sensors, anti-counterfeiting applications, and high-resolution displays.

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As the quest for active photonic devices continues, materials with exotic and exploitable properties have become paramount to enable new advancements. In recent years, phase‐change materials (PCMs) have emerged as an exciting option that open new gateways for both fundamental physics and material science research, while also being applicable to actively tunable photonic devices. One PCM in particular, vanadium dioxide (VO2), has gained a lot of attention due to its interesting reversible insulator‐to‐metal transition that comes along with a drastic change in its optical properties. In this review, starting with a brief description and explanation of the origin of the exciting optical characteristics of VO2, we then present examples of the latest research trends of VO2 in photonic applications. This includes manipulation of the reflection, transmission, and absorption of light to produce color filters and near‐infrared perfect absorbers, as well as applications of telecommunications modulators, and we round up with applications in the energy field, in smart windows and adaptive radiative cooling. We conclude with a discussion about our views on the future perspectives for VO2 platforms in active photonic devices.

The application of hydrogels in nanophotonics has been restricted due to their low fabrication feasibility and refractive index. Nevertheless, their elasticity and strength are attractive properties for use in flexible, wearable-devices, and their swelling characteristics in response to the relative humidity highlight their potential for use in tunable nanophotonics. We investigate the use of nanostructured polyvinyl alcohol (PVA) using a one-step nanoimprinting technique for tunable and erasable optical security metasurfaces with multiplexed structural coloration and metaholography. The resolution of the PVA nanoimprinting reaches sub-100 nm, with aspect ratios approaching 10. In response to changes in the relative humidity, the PVA nanostructures swell by up to ~35.5%, providing precise wavefront manipulation of visible light. Here, we demonstrate various highly-secure multiplexed optical encryption metasurfaces to display, hide, or destroy encrypted information based on the relative humidity both irreversibly and reversibly.