Maximizing the solar heat gain through windows in winter and minimizing the solar radiation entering the room in summer are of great significance for the energy saving of buildings. Here, we present a new idea for transparent metasurfaces, based on asymmetric metal/insulator/metal (MIM) nanostructures, which can be switched back and forth between absorbing and reflecting solar radiation by reversing the sample orientation. Owing to the fundamental mode of a low-quality-factor resonance, a selective near-infrared absorption is obtained with an absorption peak value of 90% upon front illumination. The average solar absorption (45%) is about 10% higher than that (35%) of reported transparent absorbers. The near-infrared light is also strongly and selectively reflected upon back illumination and a reflection peak value above 70% is observed. Meanwhile, the average visible transmission of the metasurface is above 60%, which is about 1.6 times that (36%) of previous transparent metasurface absorbers. In addition, Cu material can replace the noble metals in this work, which will greatly reduce the manufacturing cost. Owing to the attractive properties of directional and selective absorption, passive operation mode, and low cost of the materials, the metasurfaces have promising prospects in building energy saving or other solar applications where surface transparency is desirable.

A self-reference plasmonic sensor is numerically demonstrated based on surface plasmon polaritons (SPP) and cavity resonances. The sensitivity of the SPP mode is up to 1220 nm/RIU for dectecting refractive index variations. The cavity mode is insensitive to refractive index variations, which is used to monitor the environment variations. Owing to the low radiative loss of SPP mode propagating on smooth silver surface, a ultranarrow linewidth (0.9 nm) of SPP mode is obtained. Benefiting from high sensitivity and ultranarrow linewidth, the figure of merit (1356 RIU-1) is much larger than that (31 RIU-1) of reported self-reference plasmonic sensors.

The direction-controllable and fast color transition is achieved simultaneously by utilizing a needle to direct the flow of water and gas pressure to drive water. The transition time is minimal for the metal materials so far.