Usually, materials with perfect structures possess excellent properties, but it is not always the case. Here, a new approach is reported to construct structural colored hydrogel films with excellent ultraviolet (UV) blocking performance for contact lenses. The theoretical simulation predicts that with perfect periodic structures, the hydrogel films can strongly reflect incident light in a narrow visible wavelength range and thus exhibit extraordinarily brilliant colors. However, such hydrogel films cannot effectively block UV light. By slightly breaking the structural periodicity (quasi-periodic structure), strong diffuse scattering or pseudoabsorption of light can occur for all of the wavelengths shorter than a structural characteristic length, leading to perfect UV blocking. According to the theoretical prediction, a structural colored hydrogel film with nearly periodic polystyrene sphere arrays in poly­(hydroxyethyl methacrylate) hydrogel matrix is fabricated; this hydrogel film possesses brilliant colors and perfect UV blocking, and the core particle composition and size have been investigated in detail for the optimized properties of contact lenses. Meanwhile, the cell proliferation assay proves the cytocompatibility of the hydrogel for real application. Regarding its unique optical characteristics, the as-prepared structural colored hydrogel shows great promise in the fields of UV-protective equipment, medical device, soft robot, sensor, and so on.

We demonstrate that when a charged particle moves on top of a metal-slot metasurface consisting of metallic slot resonators, strong Smith-Purcell electromagnetic (EM) radiation can be produced at resonant frequency. By adjusting the period of the metasurface, the resonant (or working) frequency can be tuned from gigahertz to terahertz and infrared regions. Since the EM field is localized in the slots rather than at the metal surface, the metasurfaces are found to exhibit a very low absorption loss ratio (<1%) in low working frequencies (<1  THz). Although it becomes larger in high frequencies (>1  THz), the loss ratio remains relatively low (<12%). In addition, a nonlinear relationship is also uncovered between the resonant frequency and the reciprocal of the period. Our results could benefit the construction of efficient, compact terahertz, and infrared free-electron light sources.

When a charged particle moves parallel and close to the surface of a metasurface, intense Smith-Purcell radiation can be observed at resonant frequencies. Here, we present a systematic investigation on the Smith-Purcell radiation and evanescent-to-propagating wave conversion in metal-groove metasurfaces. Based on a coupled mode theory, analytic formulas are derived for the resonant frequency, Q-factor, and wave conversion efficiency at resonant frequency. The accuracy of the formulas is verified by numerical simulations. It is found that the resonant frequency and Q-factor depend on the depth and filling ratio of the grooves, respectively. A high Q-factor can be obtained by decreasing the filling ratio of the grooves. As the Q-factor increases, the wave conversion efficiency at resonant frequency increase but exhibits an upper limit. Such an upper bound of efficiency (Cr,max = 4) can be approached at a moderate Q-factor (Q = 16) or an optimal filling ratio of the grooves (fs = 0.05). Our results may benefit the construction of compact high-power free-electron light sources.

The signal-to-noise ratio of infrared photodetectors can be improved by using resonant cavities, whereas the enhancement effect usually occurs in a narrow wavelength range. Here, we propose a dual-mode plasmonic resonant cavity which can enhance the performance of infrared photodetectors in a wide range of wavelengths from 3.5 μm to 5.5 μm. The optical cavity consists of an Au grating, an ultrathin (310 nm) detective layer of mercury cadmium telluride, and an Au film, which can exhibit nearly perfect absorption at resonant wavelengths with using optimal parameters. For the target wavelength range, the wavelength-averaged absorption in the detective layer can also be 62%, about 12 times of that without the resonant cavity. Such a high enhancement of absorption can occur for incident light in a broad range of angle (θ < 45⁰) and with different polarizations.

The signal-to-noise ratio of infrared photodetectors can be improved by using resonant cavities, whereas the enhancement effect usually occurs in a narrow wavelength range. Here, we propose a dual-mode plasmonic resonant cavity which can enhance the performance of infrared photodetectors in a wide range of wavelengths from 3.5 µm to 5.5 µm. The optical cavity consists of an Au grating, an ultrathin (310 nm) detective layer of mercury cadmium telluride, and an Au film, which can exhibit nearly perfect absorption at resonant wavelengths with using optimal parameters. For the target wavelength range, the wavelength-averaged absorption in the detective layer can also be 62%, about 12 times of that without the resonant cavity. Such a high enhancement of absorption can occur for incident light in a broad range of angle (θ < 45 0 ) and with different polarizations.