Vapor condensation plays a crucial role in solar water-purification technologies. Conventional condensers in solar water-purification systems do not provide sufficient cooling power for vapor condensation, limiting the water production rate to 0.4 L m<sup>-2</sup> hour<sup>-1</sup>. On the other hand, radiative dew condensation, a technique used by existing radiative dew condensers, only works at nighttime and is incompatible with solar water-purification technologies. Here, we develop daytime radiative condensers that reflect almost all solar radiation, and can thus create dew water even in direct sunlight. Compared to stateof- art condensers, our daytime radiative condenser doubles the production of purified water over a 24-hour period.
Efficient theoretical modeling of metasurface is highly desired for designing metasurfaces. However, most of current modeling of metasurfaces relies on full-wave numerical simulation methods that solve the Maxwell’s equations. As a metasurface typically consists of many meta-units, solving Maxwell’s equations is computationally expensive and thus inefficient for designing metasurface. Here, we develop a general theoretical framework for modeling metasurface based on the coupled mode theory (CMT), which fully describes the interaction between the meta-units and light by a simple set of coupled-mode equations. Consequently, the CMT formulism is far less computationally demanding than the Maxwell’s equations. We show that our CMT approach allows us to quickly and efficiently optimize the design of a beam-steering metagrating. The optimal design obtained from our CMT model is further validated by numerical simulation. The proposed CMT model provides an efficient tool to model and design optical devices based on multiple optical resonators.
Sensing the direction of sounds provides animals clear evolutionary advantage. For large animals in which the distance between the ears is larger or comparable to the audible sound wavelength, directional hearing is simply accomplished by recognizing the intensity and time differences of the wave impinging on the two ears. In small (subwavelength) animals, angle sensing seems instead to rely on coherent coupling of soundwaves from the two ears. Inspired by this natural design, here we present a subwavelength photodetection pixel that can measure both the intensity and the incident angle of light. It consists of two silicon nanowire optical resonators spaced at subwavelength distance that are electrically isolated but optically coupled. We exploit this effect to fabricate a subwavelength angle-sensitive pixels.