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Radiative cooling is a well-researched area. For many years, surfaces relying on radiative cooling failed to exhibit a sub-ambient surface temperature under the sun because of the limited reflectance in the solar spectrum and the reduced absorptivity in the atmospheric window. The recent impressive developments in photonic nanoscience permitted to produce photonic structures exhibiting surface temperatures much below the ambient temperature. This paper aims to present and analyze the main recent achievements concerning daytime radiative cooling technologies. While the conventional radiative systems are briefly presented, the emphasis is given on the various photonic radiative structures and mainly the planar thin film radiators, metamaterials, 2 and 3D photonic structures, polymeric photonic technologies, and passive radiators under the form of a paint. The composition of each structure, as well as its experimental or simulated thermal performance, is reported in detail. The main limitations and constraints of the photonic radiative systems, the proposed technological solutions, and the prospects are presented and discussed.

In recent years, an increasing number of passive radiative cooling materials are proposed in the literature, with several examples relying on the use of silica (SiO2) due to its unique stability, non‐toxicity, and availability. Nonetheless, due to its bulk phonon‐polariton band, SiO2 presents a marked reflection peak within the atmospheric transparency window (8‐13 µm), leading to an emissivity decrease that poses a challenge to fulfilling the criteria for sub‐ambient passive radiative cooling. Thus, the latest developments in this field are devoted to the design of engineered SiO2 photonic structures, to increase the cooling potential of bulk SiO2 radiative coolers. This review seeks to identify the most effective photonic design and fabrication strategies for SiO2 radiative emitters by evaluating their cooling efficacy, as well as their scalability, providing an in‐depth analysis of the fundamental principles, structural models, and results (both numerical and experimental) of various types of SiO2 radiative coolers. Among several materials emerged for passive radiative cooling applications, SiO2 stands out due to its stability, abundance, recyclability, and overall safety. Patterning strategies of the SiO2‐air interface are reviewed, which can enhance its emissivity within the atmospheric transparency window (8–13 µm) by engineering its photon‐polariton band, opening new prospects in many application fields where SiO2 cover layers are routinely used.

Photonic crystals are artificial structures with a spatial periodicity of dielectric permittivity on the wavelength scale. This feature results in a spectral region over which no light can propagate within such a material, known as the photonic band gap (PBG). It leads to a unique interaction between light and matter. A photonic crystal can redirect, concentrate, or even trap incident light. Different materials (dielectrics, semiconductors, metals, polymers, etc.) and 1D, 2D, and 3D architectures (layers, inverse opal, woodpile, etc.) of photonic crystals enable great flexibility in designing the optical response of the material. This opens an extensive range of applications, including photovoltaics. Photonic crystals can be used as anti-reflective and light-trapping surfaces, back reflectors, spectrum splitters, absorption enhancers, radiation coolers, or electron transport layers. This paper presents an overview of the developments and trends in designing photonic structures for different photovoltaic applications.

Controlling the emissivity of a thermal emitter has attracted growing interest, with a view toward a new generation of thermal emission devices. To date, all demonstrations have involved using sustained external electric or thermal consumption to maintain a desired emissivity. In the present study, we demonstrated control over the emissivity of a thermal emitter consisting of a film of phase-changing material Ge 2 Sb 2 Te 5 (GST) on top of a metal film. This thermal emitter achieves broad wavelength-selective spectral emissivity in the mid-infrared. The peak emissivity approaches the ideal blackbody maximum, and a maximum extinction ratio of >10 dB is attainable by switching the GST between the crystalline and amorphous phases. By controlling the intermediate phases, the emissivity can be continuously tuned. This switchable, tunable, wavelength-selective and thermally stable thermal emitter will pave the way toward the ultimate control of thermal emissivity in the field of fundamental science as well as for energy harvesting and thermal control applications, including thermophotovoltaics, light sources, infrared imaging and radiative coolers. Phase-change materials: control over emissivity The use of phase-change materials can provide tunable control over the emissivity of a thermal emitter. This discovery, made by Kaikai Du and co-workers at Zhejiang University in China, could be useful for applications in thermophotovoltaics, infrared imaging and radiative cooling. The team coated gold substrates with thin layers of the phase-change material Ge 2 Sb 2 Te 5 (GST). They found that the emissivity of these samples changed with the thickness of the GST layer and the sample temperature. Specifically, the wavelength of the emissivity peak shifted from around 9 to 13 micrometres as the thickness of the GST layer increased from 360 to 540 nanometres. Furthermore, gradually changing the temperature of the GST layer to switch it between its amorphous and crystalline states provided continuous control over the emissivity.

Passive radiative cooling, which cools an item without any electrical input, has drawn much attention in recent years. In many radiative coolers, silica is widely used due to its high emissivity in the mid-infrared region. However, the performance of a bare silica film is poor due to the occurrence of an emitting dip (about 30% emissivity) in the atmospheric transparent window (8–13 μm). In this work, we demonstrate that the emissivity of silica film can be improved by sculpturing structures on its surface. According to our simulation, over 90% emissivity can be achieved at 8–13 μm when periodical silica deep grating is applied on a plane silica film. With the high emissivity at the atmospheric transparent window and the extremely low absorption in the solar spectrum, the structure has excellent cooling performance (about 100 W/m2). The enhancement is because of the coupling between the incident light with the surface modes. Compared with most present radiative coolers, the proposed cooler is much easier to be fabricated. However, 1-D gratings are sensitive to incident polarization, which leads to a degradation in cooling performance. To solve this problem, we further propose another radiative cooler based on a silica cylinder array. The new cooler’s insensitivity to polarization angle and its average emissivity in the atmospheric transparent window is about 98%. Near-unit emissivity and their simple structures enable the two coolers to be applied in real cooling systems.

In this paper, the potential of passive radiative cooling for Indian cities of various climatic conditions covering maximum climate diversity is evaluated for different (ideal, broadband, and selective) emissive radiative cooler surfaces. The effects of various environmental parameters on the cooler performance are studied. The cooling energy saving potentials of various photonic radiative coolers as roof envelope integrated with the conventional air conditioning system are evaluated as well. Results reveal that the selective radiative cooler performs better in low humidity locations, namely, Jaisalmer and Delhi, with the maximum temperature drops of 20.6°C and 17.3°C, respectively. However, it is less likely to be recommended for monsoon season and high humid cities. The cooling load reduction of 36% was observed for low humidity locations with an integrated system. (12 Figures, 3 Tables, 36 References)

In this paper, the photonic radiative cooler performance is analysed for year 2019 at various Indian locations considering the diversity of climate and forecasted for the year 2030. The effects of three types of climate change are considered: geographical, seasonal, and year-wise. Some photonic coolers with different emissive profiles are also compared. As the radiative cooling depends upon air temperature, humidity, wind speed, and solar flux intensity, the effects of influencing climatic or weather parameters during summer months are studied extensively and major performance influencing factors are identified. Photonic radiative cooler performance in energy saving as rooftop envelope assisted to the conventional air conditioning system is assessed and cooling energy saving of 25-32 kWhth/month for selected locations was observed. The study reveals that windshield is the necessary condition to get net cooling flux through the rooftop. The reduction in cooling load on active systems of 34% is observed for low humidity locations with the integration of radiative cooler as a roof envelope.

In an optical communication system, providing a stable and constant temperature environment for the optical chip is the key for a less loss communicating performance. In this paper, a new design of temperature controller for photonics is set up based on the combination of the Thermoelectric Coolers (TEC) and Arduino. Through the process of the whole design, as we rely on Arduino to collect the temperature data and make the corresponding response according to the varying temperature. Thus, the design of the coding also plays a significant role in this project. One great merit of using the coding to play a role as a controller and data collector, the characteristic and the function of one specific code do not change when the environment change. It can offer a stable constant temperature environment within ±0.05℃. The temperature controller for photonics offers a reliable temperature stabilizing approach for the optical chip. At last, it also offers a simple as well as useful structure of temperature controller which can be a useful reference to other fields besides the optical communication area.

Aperiodic photonic crystals can open up novel routes for more efficient photon management due to increased degrees of freedom in their design along with the unique properties brought about by the long-range aperiodic order as compared to their periodic counterparts. In this work we first describe the fundamental notions underlying the idea of thermal emission/absorption control on the basis of the systematic use of aperiodic multilayer designs in photonic quasicrystals. Then, we illustrate the potential applications of this approach in order to enhance the performance of daytime radiative coolers and solar thermoelectric energy generators.

Entangled photon pairs are essential for a multitude of photonic quantum applications. To date, the best performing solid-state quantum emitters of entangled photons are semiconductor quantum dots operated around liquid-helium temperatures. To favor the widespread deployment of these sources, it is important to explore and understand their behavior at temperatures accessible with compact Stirling coolers. Here we study the polarization entanglement among photon pairs from the biexciton-exciton cascade in GaAs quantum dots at temperatures up to 65 K. We observe entanglement degradation accompanied by changes in decay dynamics, which we ascribe to thermal population and depopulation of hot and dark states in addition to the four levels relevant for photon pair generation. Detailed calculations considering the presence and characteristics of the additional states and phonon-assisted transitions support the interpretation. We expect these results to guide the optimization of quantum dots as sources of highly entangled photons at elevated temperatures.