The recent progress on radiative cooling reveals its potential for applications in highly efficient passive cooling. This approach utilizes the maximized emission of infrared thermal radiation through the atmospheric window for releasing heat and minimized absorption of incoming atmospheric radiation. These simultaneous processes can lead to a device temperature substantially below the ambient temperature. Although the application of radiative cooling for nighttime cooling was demonstrated a few decades ago, significant cooling under direct sunlight has been achieved only recently, indicating its potential as a practical passive cooler during the day. In this article, the basic principles of radiative cooling and its performance characteristics for nonradiative contributions, solar radiation, and atmospheric conditions are discussed. The recent advancements over the traditional approaches and their material and structural characteristics are outlined. The key characteristics of the thermal radiators and solar reflectors of the current state‐of‐the‐art radiative coolers are evaluated and their benchmarks are remarked for the peak cooling ability. The scopes for further improvements on radiative cooling efficiency for optimized device characteristics are also theoretically estimated.

COMMUNICATIONTo demonstrate effi cient radiative coolers, selective IR emitters have been extensively studied. [1][2][3][4][5][6][7][8][9][10][11] In particular, composite materials, [ 2,9 ] white pigmented paints, [ 5,8 ] SiO fi lms, [ 6,7,10 ] and polymeric materials [ 3,11 ] are demonstrated to possess IR emission within the atmospheric transparency window. However, almost all of these materials either lack near-unity emission or broadband emission within the entire 8-13 μm window. [ 1,2,[5][6][7][8][9][10] In addition, signifi cant IR absorption outside the transparency window, where the atmosphere is highly emissive, also restricts the materials to cool down well below the ambient temperature. [2][3][4][5]11 ] On the other hand, artifi cial metallic nanostructures, such as, plasmonic nanostructures, [12][13][14][15][16][17][18] and metallic photonic crystals [19][20][21][22] possess highly selective IR optical absorptions. However, in most cases, their absorption spectra are diffi cult to optimize for wide-band absorption. Metamaterials, on the contrary, can provide both selective and broadband IR absorption. [23][24][25][26][27] Recently, multilayer metal-dielectric anisotropic metamaterials have been demonstrated to possess intriguing optical properties. [ 25,[28][29][30] By employing dispersive properties and anisotropy, ultra-broadband, spectrally selective and polarization sensitive absorption was achieved in the visible to microwave frequencies. Here, for the fi rst time, we propose the use of anisotropic metamaterials toward the application of highly effi cient radiative cooling. To achieve an ideal thermal emitter, we design and demonstrate a microstructure consisted of an array of symmetrically shaped conical metamaterial (CMM) pillars leading to a near unity absorption of unpolarized light. By selectively matching the thermal emission (absorption) to the entire 8-13 μm atmospheric transparency window, the CMM structure can possess a practical radiative cooling power of 116.6 W m −2 .Our design concept of an elementary metal-dielectric CMM pillar consists of alternating layers of aluminum and germanium as depicted in Figure 1 a. Each of the metal and dielectric layers maintains the circular symmetry along the vertical axis and the diameters of the layers decrease gradually from bottom to top which gives the structure a conical shape. The thickness of the aluminum layer is 30 nm and the thickness of the germanium layer is 110 nm. The top and bottom diameters of the CMM pillars are defi ned t and b . The substrate of the CMM structure is set to 150 nm thick aluminum which is optically thick enough to diminish any IR transmission through the substrate. The dispersive permittivity of aluminum is defi ned from reference [ 31 ] and the permittivity of germanium is 16. The periodicity of the CMM pillars, p is set to 1.3 times b , providing a suffi cient gap between the adjacent CMM pillars to avoid any proximity effects. [ 29 ] The aspect ratio, s , of t and b is optimized to 0.6 and seven periods of metal...

Tailoring emissivity and absorptivity of structured material surfaces to match atmospheric transmission spectral windows can lead to radiative cooling that consumes no external energy. Recent advances in nanofabrication technology have facilitated progress in the realization of structured metasurfaces. In particular, subwavelength dielectric resonator metasurface supporting various resonance modes can be efficient absorbers. Here, such metasurfaces are proposed and experimentally demonstrated enhanced by metal loading to obtain strong broadband thermal emission over a wide angle at mid‐infrared frequencies. This concept results in passive cooling devices that can lower temperature by 10 °C below ambient temperature. Importantly, the utilization of standard constituent materials and processes lead to scalable fabrication compatible with silicon photonics integration, which will enable effective and energy‐efficient applications in passive cooling and thermodynamic control.

Finding new ways to control and slow down the group velocity of light in media remains a major challenge in the field of optics. For the design of plasmonic slow light structures, graphene represents an attractive alternative to metals due to its strong field confinement, comparably low ohmic loss and versatile tunability. Here we propose a novel nanostructure consisting of a monolayer graphene on a silicon based graded grating structure. An external gate voltage is applied to graphene and silicon, which are separated by a spacer layer of silica. Theoretical and numerical results demonstrate that the structure exhibits an ultra-high slowdown factor above 450 for the propagation of surface plasmon polaritons (SPPs) excited in graphene, which also enables the spatially resolved trapping of light. Slowdown and trapping occur in the mid-infrared wavelength region within a bandwidth of ~2.1 μm and on a length scale less than 1/6 of the operating wavelength. The slowdown factor can be precisely tuned simply by adjusting the external gate voltage, offering a dynamic pathway for the release of trapped SPPs at room temperature. The presented results will enable the development of highly tunable optoelectronic devices such as plasmonic switches and buffers.

Periodic metallic nano/microstructures have received a great a deal of attention in the photonics research community over the last few decades due to their intriguing optical properties. Three‐dimensional metallic nano/microstructures such as metallic photonic crystals, metamaterials, and plasmonic devices possess unique characteristics of tailored thermal radiation, negative refraction and deep subwavelength confinement of light. In this article, the recent progress on the experimental methods for the realisation of three‐dimensional periodic metallic and thin metal film coated dielectric nano/microstructures operating from optical to mid‐infrared frequencies has been reviewed. Advancement of the state‐of‐the‐art nanofabrication methods over the last few decades have led to the development of metallic nano/microstructures of diverse geometries, high resolution features and large scale production. The recent progress in the novel fabrication methods have inspired the development of functional and exciting photonic devices based on periodic metallic nano/microstructures with various applications in photonics including communications, photovoltaics, and biophotonics.

Graphene has been identified as an emerging horizon for a nanoscale photonic platform because the Fermi level of intrinsic graphene can be engineered to support surface plasmons (SPs). The current solid back electrical gating and chemical doping methods cannot facilitate the demonstration of graphene SPs at the near-infrared (NIR) window because of the limited shift of the Fermi level. Here, we present the evidence for the existence of graphene SPs on a tapered graphene-silicon waveguide tip at a NIR wavelength, employing a surface carrier transfer method with molybdenum trioxides. The coupling between the graphene surface plasmons and the guiding mode in silicon waveguides allows for the observation of the concentrated field of the SPs in the tip by near-field scanning optical microscopy. Thus the hot spot from the concentrated SPs in the graphene layer can be used as a key experimental signature of graphene SPs. The NIR graphene SPs opens a new perspective for optical communications, optical sensing and imaging, and optical data storage with extreme spatial confinement, broad bandwidth and high tunability.

Localized plasmon resonances are proposed in a new concept of 3D photonic crystals stacked by hybrid rods made of dielectric-cores and metallic-nanoshells. The resonant plasmon coupling of inner and outer surfaces of the metallic-nanoshells forms the localized plasmon resonances which can be flexibly tuned by mediating the dielectric cores. At the resonance wavelengths, the strong electromagnetic wave-plasmon interaction leads to the enhancement in the structural absorption by more than 20 times. The tunability of the enhanced absorption is demonstrated in experiments.

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