Atmospheric Window Research Articles

Strong Nonreciprocal Thermal Radiation Within Atmospheric Window With Lithography-Free Structure

Strong Nonreciprocal Thermal Radiation Within Atmospheric Window With Lithography-Free Structure

Atmospheric reaction of CH2=CHCH2OCF2CHF2 with OH radicals and Cl atoms, UV and IR absorption cross sections, and global warming potential.

In this work, the rate coefficients for OH radical, k1(T), and Cl atom, k2(T), reaction with allyl 1,1,2,2-tetrafluoroethyl ether, CH2=CHCH2OCF2CHF2, were studied as a function of temperature and pressure in a collaborative effort made between UCLM, Spain, and LAPKIN, Greece. OH rate coefficients were determined in UCLM, between 263 and 353K and 50-600Torr, using the absolute rate method of pulsed laser photolysis-laser-induced fluorescence technique, while Cl kinetics were studied in temperature (260-363K) and pressure (34-721Torr) ranges, using the relative rate method of the thermostated photochemical reactor equipped with Fourier transform infrared spectroscopy as the detection technique. In both OH and Cl reactions, a negative temperature dependence of the measured rate coefficients was observed, which is consistent with complex association reactions. The temperature dependence of OH rate coefficients was found to be well represented by the following expression: k1(T) = (2.30 ± 0.35) × 10-12 exp[(544 ± 46) K/T] cm3 molecule-1s-1. In the case of the Cl-initiated reaction, a slight curvature was observed in the Arrhenius plot for k2(T), and the kinetic data were fitted to a modified Arrhenius expression: k2(T) = (4.42 ± 0.32) × 10-16 T2 exp[(610 ± 22) K/T] cm3 molecule-1s-1. No pressure dependence was observed in either case. These results are consistent with a complex reaction mechanism that is not uncommon in radical association reactions to the unsaturated bond. As part of this work, UV (200-400nm) and infrared absorption spectra (500-3200cm-1) were also measured to further evaluate CH2=CHCH2OCF2CHF2 atmospheric impact. Atmospheric lifetimes with respect to OH radical and Cl atom reactions were estimated to be 19.8h and 38days, respectively, showing that OH radicals dominate atmospheric oxidation. CH2=CHCH2OCF2CHF2 is a very weak absorber in the solar actinic region, while its relatively low radiative efficiency in the atmospheric IR window, 0.0034 W m-2 ppbv-1, and the short lifetime led to a very low GWP value relative to CO2, 1.2 × 10-2 and 3.3 × 10-3, at time horizons of 20 and 100years, respectively.

Inverse Design of Aberration-Corrected Hybrid Metalenses for Large Field of View Thermal Imaging Across the Entire Longwave Infrared Atmospheric Window.

Long wave infrared (LWIR) cameras play a pivotal role across diverse applications due to their distinctive features. The growing demand for high-performance thermal imaging optics, characterized by a broad working bandwidth, large field of view (FOV), aberration-free design, lightweight construction, compactness, and cost-effectiveness, poses significant challenges for LWIR lens design. Here, we propose an inverse design method for LWIR hybrid metalenses, specifically aiming to achieve aberration-corrected thermal imaging with both a large FOV and a broad working bandwidth. Our approach involves optimizing phase profiles of metalens' unit cells guided by a loss function that compares the hybrid lens design to diffraction-limited results for various incident angles and wavelengths. As a result, we demonstrate an aberration-corrected thermal camera with a 30° FOV and an achromatic working bandwidth spanning the entire LWIR atmospheric window (8 to 14 μm). Significantly, the total optical path length, the entrance pupil to the sensor plane of the charge-coupled device (CCD), is a mere 13.6 mm. Our work merits advantages in the FOV, working bandwidth, and compactness, which surpass state-of-the-art LWIR hybrid metalens designs and find numerous imaging and sensing applications.

Hot phonon effect in mid-infrared HgTe/CdHgTe quantum wells evaluated by quasi-steady-state photoluminescence

Room-temperature photoluminescence (PL) spectra of intensely pumped HgTe/CdHgTe quantum well (QW) heterostructures emitting at around 5 μm wavelength have been investigated. Based on the model description of the PL spectra using a free-electron recombination band approach, effective electronic temperatures were determined depending on the excitation density. Within the quasi-steady-state approximation, we establish the balance between pump-induced heating of the electron gas in the QWs and phonon-mediated dissipation of this excess energy and deduce hot-phonon lifetime of ∼0.47 ps. Maximum operating temperatures for optically pumped HgTe/CdHgTe QW laser heterostructures emitting at around 5 μm are estimated depending on the excitation wavelength, and lasing at Peltier temperatures appears feasible for the pump wavelength of about 3 μm. Thus, the entire 3∼5 μm atmospheric transparency window can be potentially covered by thermoelectrically cooled HgCdTe-based laser sources.

Wide-band dual-mode adaptive angle anisotropic thermal emitter

The phenomenon of controlling far-field thermal radiation with large-angle emission has gradually attracted attention, but it was previously only present in the TM mode. The ability to make the TE mode have a similar emission trend to the TM mode is important. By introducing a grating-like phase change material:vanadium dioxide (VO2), the emitter has achieved wide-band controllable differences in TE emission rates between the two modes in the atmospheric window range (8–13 um). The range of variation in the emission rate is close to 0.9. Furthermore, temperature-controlled color displays at different angles have been added to the emitter, which makes it easier to judge the working state of the emitter visually. This adjustable broadband large-angle directional emitter makes angle-dependent thermal radiation control possible. It has potential application value in the fields of thermal camouflage, directional radiative cooling, solar cooling waste and heat recovery.

Advances in Lanthanide-Based NIR-IIb Probes for In Vivo Biomedical Imaging.

The past decades have witnessed the significant development and practical interest of in vivo biomedical imaging technologies and optical materials in the second-near infrared (NIR-II, 1000-1700 nm) window. Imaging with the extended emission wavelength toward the long-wavelength end (NIR-IIb, 1500-1700 nm) further offers micrometer imaging resolution and centimeter tissue penetration depth by taking advantage of the much-reduced photon scattering and near-zero tissue autofluorescence background, which have become a very hot research area. This review focuses on the recent advances in the development of lanthanide-based NIR-IIb probes for in vivo biomedical applications. The progress including ratiometric imaging, multiplexed imaging for wide-field and microscopy, lifetime multiplexing and sensing, persistent luminescence, and multimodal imaging is summarized. Challenges and future directions concerning the investigation of the photophysical and photochemical properties of NIR-IIb probes, the selection of near-infrared cameras as well as the potential extension of the NIR-IIb imaging sub-window are pointed out. This review will inspire readers who have a strong interest in developing optical imaging technology and long-wavelength fluorescence probes for high-contrast in vivo biomedical applications.

Daytime radiative cooling using pattern free metal dielectric coating

Radiative coolers, which dumps the excess heat to the cold outer space by releasing the thermal radiation through the atmospheric window, have offered an alternate and feasible solution to the conventional coolers that are fuelled on the electricity produced by either using renewal or non-renewal sources like fossil fuels. Particularly, daytime passive radiative coolers have paved the way for many energy free technology that are used in reducing the temperature of building tops, power plants, and water harvesting. In this, we propose a pattern free and large area scalable bi-layered thin film based daytime radiative cooler. The proposed design illustrates an average reflectivity of 98.5% for the solar spectrum, while showing average emittance of 91% within the atmospheric window (8-13μm). The design consists of thin films of transparent dielectrics such as ZnS or BaF2 placed on top of a thick glass substrate, that is backed by a thin film of silver. We theoretically obtained a cooling power of 119 W m−2 with a temperature reduction of 9 °C below the ambient.

Powder‐Sintered Hierarchically Porous PMMA with Optimal Pore Parameters for Passive Daytime Radiative Cooling

AbstractPorous radiative cooling polymers have drawn significant attention for passive daytime cooling due to their desirable cooling performance and easy scalability. Since optimal pore parameters (i.e., pore diameter and porosity) will enhance light scattering, polymethyl methacrylate (PMMA) powders are sintered to fabricate samples with different pore parameters by the one‐step full‐dry powder sintering method. These samples have a hierarchically porous structure with pore diameters <5.0 µm. The most porous sample has 43% porosity, 0.964 reflectance in the 0.3–2.5 µm wavelength range, and 0.967 emittance in 8–13 µm wavelength range (which is the atmospheric window). The measured cooling temperature difference decreases when the porosity is low for hierarchically porous PMMA with similar pore size distribution. The high sub‐ambient cooling temperature reduction provided by hierarchically porous PMMA with optimal pore parameters is due to high solar reflectance when illuminated by the sun. Similar cooling trends are evident when sunlight is simulated by a Xenon lamp and an optical filter. The powder‐sintered hierarchically porous PMMA with optimal pore parameters can be an excellent radiative cooler for passive daytime cooling applications.

Durable and multifunctional coating design with superhydrophobicity, high transparency, radiative cooling for photovoltaic application

The market-dominant silicon-based solar cells are facing great challenges in further improving their benchmark efficiency. However, due to dust deposition and temperature rise, the actual operating efficiency is still far from the benchmark efficiency. The goal of this study is to develop a durable and multifunctional coating with superhydrophobicity, high light transmittance and strong infrared radiation, which is applied to the surface of photovoltaic glass to reduce dust deposition and lower the module temperature. Based on a silicon wafer template and die casting process, epoxy resin microcavities are prepared on the glass surface, and SiO2 nanoparticles are sprayed into the microcavities to complete the preparation of the multifunctional coating. The experimental test results show that the coating has a contact angle of about 160°, a visible transmittance over 91 %, and an infrared emissivity of 94.5 % among the atmospheric window, demonstrating the potential of self-cleaning and radiative cooling functions. The coating also shows good durability through sandpaper wear, scraper wear, tape peeling, and water jet tests. The multifunctional coating developed in this study is expected to be applied to different types of photovoltaic cells to improve their photoelectric conversion efficiency in outdoor environments.

A two-dimensional thin film structure with spectral selective emission capability suitable for high-temperature environments

Spectral selective emission materials, characterized by their high emission efficiency within specific wavelength ranges, play a crucial role in various fields such as infrared stealth and radiative cooling. In this study, leveraging the tunneling effect of ultrathin metal layers and the impedance matching principle, we designed a one-dimensional multilayer thin film structure composed of Mo/Ge/Al. This design successfully achieved spectral selective emission within the 3-14 μm infrared spectrum range, showcasing superior high-temperature infrared stealth capabilities. Research findings indicate that the infrared atmospheric window (ε3-5 μm=0.40, ε8-14 μm=0.30) maintains a low emissivity within the temperature range from room temperature to 400°C, while high emissivity in non-infrared atmospheric windows (ε5-8 μm=0.81) enables radiative cooling. The use of high emissivity in the non-infrared atmospheric window reduced the surface temperature of this structure by 15.3°C compared to untreated Si substrate under similar heating conditions. The temperature reduction was 34.1°C when compared to a Si substrate coated with a 200 nm Al film. This straightforward and easily scalable structure not only presents novel solutions for applications in infrared stealth and radiative cooling but also holds vast potential for diverse fields including military, aerospace, and industrial manufacturing, injecting fresh impetus and possibilities into technological advancement.

Performance investigations of the easily manufactured composite all-day radiative cooling materials based on PDMS

The passive radiative cooling technology can be employed to obtain the cooling capacity without any energy consumption, which has been garnering significant attention nowadays. This work proposes and experimentally studies the all-day radiative cooling material based on layered composite material made from Polydimethylsiloxane (PDMS). This work adds SiO2 nano-particles, which act as visible light scatters, and the proposed film can realize an excellent all-day cooling capacity when the thicknesses of the PDMS and Al films are respectively chosen as 200 μm and 4 μm, and the concentration of the SiO2 particles in the film is 40 %. The theoretical explorations indicated that the presented composite film can exhibit the high reflectivity (90.36 %) and emissivity (90.19 %) in the visible spectrum and atmospheric transparency window (ATW) spectrum. Under direct sunlight of a highest solar irradiance of 688.89 W/m2, there is an average temperature difference between cooling film and white paper of 6.3 °C. Furthermore, in clear weather conditions at night with an average humidity level of 58.15 %, it can generate a temperature difference of 4.8 °C. This work proposes an easily manufactured and eco-friendly all-day radiative cooling material, which has the potential applications in future refrigeration technology.

Absolute Reference for Microwave Polarization Experiments. The COSMOCal Project and its Proof of Concept

Context. The cosmic microwave background (CMB), a remnant of the Big Bang, provides unparalleled insights into the primordial universe, its energy content, and the origin of cosmic structures. The success of forthcoming terrestrial and space experiments hinges on meticulously calibrated data. Specifically, the ability to achieve an absolute calibration of the polarization angles with a precision of <0.°1 is crucial to identify the signatures of primordial gravitational waves and cosmic birefringence within the CMB polarization. Aims. We introduce the COSmological Microwave Observations Calibrator project, designed to deploy a polarized source in space for calibrating microwave frequency observations. The project aims to integrate microwave polarization observations from small and large telescopes, ground-based and in space, into a unified scale, enhancing the effectiveness of each observatory and allowing robust combination of data. Methods. To demonstrate the feasibility and confirm the observational approach of our project, we developed a prototype instrument that operates in the atmospheric window centered at 260 GHz, specifically tailored for use with the NIKA2 camera at the IRAM 30 m telescope. Results. We present the instrument components and their laboratory characterization. The results of tests performed with the fully assembled prototype using a Kinetic Inductance Detectors-based instrument, similar concept of NIKA2, are also reported. Conclusions. This study paves the way for an observing campaign using the IRAM 30 m telescope and contributes to the development of a space-based instrument.

Self-adaptive radiation recuperator with double phase transition temperature switches based on VO2 intelligent films

Developing renewable energy for heating and cooling to achieve carbon neutrality and environmental protection is of great significance. The combination of photothermal (PT) and radiative cooling (RC) technology to continuously obtain energy throughout the day is gradually attracting interest, in which vanadium dioxide (VO2) intelligent phase transition film serves as a bridge between the daytime PT mode and the nighttime RC mode. The high phase transition temperature of VO2 poses a challenge to environmental adaptability and dynamic switching in practical applications. In this work, a novel self-adaptive radiation recuperator with double phase transition temperature (37 °C and 68 °C) switches is proposed to exchange energy from the sun and outer space by radiation. Compared with VO2 and W-doped VO2 single-layer structure, this method has a higher self-adaptability to easily achieve the phase transition temperature of mode switching. The absorptivity of the radiation recuperator is 0.75 in the solar band (0.3–2.5 μm), and the emissivity switching capability is present from 0.3 to 0.86 in the atmospheric window band (8–14 μm). The transition mode generated by the presence of double phase transition temperature improves temperature self-adaptability and improves energy collection efficiency in dynamic environments.

Short-Wave Infrared Upconverting Nanoparticles.

Optical technologies enable real-time, noninvasive analysis of complex systems but are limited to discrete regions of the optical spectrum. While wavelengths in the short-wave infrared (SWIR) window (typically, 1700-3000 nm) should enable deep subsurface penetration and reduced photodamage, there are few luminescent probes that can be excited in this region. Here, we report the discovery of lanthanide-based upconverting nanoparticles (UCNPs) that efficiently convert 1740 or 1950 nm excitation to wavelengths compatible with conventional silicon detectors. Screening of Ln3+ ion combinations by differential rate equation modeling identifies Ho3+/Tm3+ or Tm3+ dopants with strong visible or NIR-I emission following SWIR excitation. Experimental upconverted photoluminescence excitation (U-PLE) spectra find that 10% Tm3+-doped NaYF4 core/shell UCNPs have the strongest 800 nm emission from SWIR wavelengths, while UCNPs with an added 2% or 10% Ho3+ show the strongest red emission when excited at 1740 or 1950 nm. Mechanistic modeling shows that addition of a low percentage of Ho3+ to Tm3+-doped UCNPs shifts their emission from 800 to 652 nm by acting as a hub of efficient SWIR energy acceptance and redistribution up to visible emission manifolds. Parallel experimental and computational analysis shows rate equation models are able to predict compositions for specific wavelengths of both excitation and emission. These SWIR-responsive probes open a new IR bioimaging window, and are responsive at wavelengths important for vision technologies.

Three-layered films enable efficient passive radiation cooling of buildings

Abstract To address the excessive energy consumption of building cooling, the coverage of passive radiation cooling materials on the surface of buildings can effectively save the global refrigeration power resources and reduce the greenhouse gas emissions generated by refrigeration equipment. In this work, passive radiation hydrophobic fabric cooling materials with three functional layers (i.e., top polydimethylsiloxane [PDMS] film layer for solar emissivity, middle polymethylmethacrylate [PMMA] film layer for solar reflectivity, and bottom cotton fabric layer for support) were prepared. This passive radiation cooling material with optimized thickness of PDMS (1.5 mm) and PMMA (3.5 mm) have a rich uneven filament structure and ideal internal bonding structure, which enabled 94% of solar reflectivity and 93.4% of atmospheric window emissivity (8–14 μm). Top layer of the composite film was hydrophobic (a contact angle of 117°) and allowed the rolling of water droplets to remove most of the surface dust. Moreover, these composites presented an excellent cooling of 7.7–15.0°C in the outdoor real cooling test. For medium-sized houses, the roof covered with composite was expected to reduce the emission CO2 by 17% every year. The findings of this work indicated that the prepared three-layered radiation cooling materials have great potentials in thermal energy storage buildings.

Pb2TeV2O10: A Lead Vanadate Tellurate with Wide Mid-IR Transparency and Large Birefringence Induced by Multiple Birefringence-Active Groups.

Birefringent materials as the key materials in laser science and technology have attracted continuous attention due to their ability to modulate polarized light. Herein, a new lead vanadate tellurate, Pb2TeV2O10, has been synthesized through the rational integration of different kinds of birefringence-active functional units. Pb2TeV2O10 features a unique two-dimensional (2D) [TeV2O10]∞ layered structure consisting of [VO6]7- and [TeO6]6- octahedra, and Pb2+ cations reside between the [TeV2O10]∞ layers. In addition, the rare edge-sharing mode of [VO6]7- and [TeO6]6- octahedra was found in this structure. Attributed to the high polarizability and appropriate arrangement of PbO8, VO6, and TeO6 units, Pb2TeV2O10 possesses a great theoretical birefringence of 0.275 at 532 nm, which is the largest among the vanadate tellurate family. The spectral tests also prove that Pb2TeV2O10 showcases a broad transparency window (439 nm-10 μm), covering an important mid-infrared (IR) atmospheric window (3-5 μm). In addition, in order to improve the transparency, alkali and alkaline earth metal cations were introduced by the substitution strategy, and then the compound K2Sr2Te2O9 was synthesized. It owns a shorter ultraviolet (UV) cutoff edge of 234 nm and a wider transparency window (234 nm-13.8 μm). The findings of Pb2TeV2O10 and K2Sr2Te2O9 enrich the structure chemistry of the tellurate family and provide new insights for designing new compounds with large optical anisotropy and wide spectral transparency.

All-day freshwater harvesting enabled by designing benzoxazine molecule with intrinsic photothermal conversion and radiation cooling ability

The importance of clean water resources for human health and economic development cannot be overlooked. Providing a way for fully passive all-day freshwater harvesting presents a significant challenge. Herein, we propose a strategy for all-day freshwater harvesting passively through a freestanding Janus film which combining solar-driven interfacial desalination during day time and dew water generation driven by passive radiative cooling at night. Specifically, the core of the strategy was a new bio-based benzoxazine which containing high-infrared emittance in the atmospheric window and the resultant cured black freestanding film owns a steady high light absorbance in the whole range of the solar irradiation wavelength. The home-made device made by the Janus film achieved a drinkable freshwater collection efficiency of 90.9 % when desalinating seawater under 1 sun illumination and achieved a dew water collection rate of 68.36 g m−2 day−1 at night. Moreover, the film has an excellent antibacterial effect, which ensures the quality of water collection and enhances the service life. Combining the advantages of bio-based raw materials, a simple fabrication process and scalability of the film, the all-day freshwater collection system makes a promising solution to alleviate freshwater shortage in many arid regions, islands and even boats sailing on the sea.

The Radiative Efficiency and Global Warming Potential of HCFC-132b.

Hydro-chloro-fluoro-carbons (HCFCs) are potent greenhouse gases which strongly absorb the infrared (IR) radiation within the 8-12 μm atmospheric windows. Despite international policies schedule their phasing out by 2020 for developed countries and 2030 globally, HCFC-132b (CH2ClCClF2) has been recently detected with significant atmospheric concentration. In this scenario, detailed climate metrics are of paramount importance for understanding the capacity of anthropogenic pollutants to contribute to global warming. In this work, the radiative efficiency (RE) of HCFC-132b is experimentally measured for the first time and used to determine its global warming potential (GWP) over 20-, 100- and 500-year time horizon. Vibrational- and rotational-spectroscopic properties of this molecule are first characterized by exploiting a synergism between Fourier-transform IR (FTIR) spectroscopy experiments and quantum chemical calculations. Equilibrium geometry, rotational parameters and vibrational properties predicted theoretically beyond the double-harmonic approximation are employed to assist the vibrational assignment of the experimental trace. Finally, FTIR spectra measured over a range of pressures are used to determine the HCFC-132b absorption cross section spectrum from 150 to 3000 cm-1, from which istantaneous and effective REs are derived and, in turn, used for GWP evaluation.

Macro-Nanoporous Film with Cauliflower-Shaped Fibers for Highly Efficient Passive Daytime Radiative Cooling.

Passive daytime radiative cooling (PDRC) is a simple and effective cooling approach that does not consume any extra energy just by highly reflecting shortwave sunlight and highly radiating infrared heat through the atmospheric windows. In recent years, the application of photonic coolers and metamaterials for PDRC has been studied. However, they usually have complex processes and high precision requirements, which seriously limit large-scale fabrication. In this paper, a high-performance polyvinylidene difluoride-hexafluoropropylene (PVDF-HFP) PDRC fiber film was prepared by a simple and efficient electrospinning method combined with phase separation, and the obtained PVDF-HFP fibers have a cauliflower-shaped macro-nanoporous structure. The cauliflower-shaped structure provides more scattering sites of the fibers, and the fiber film with the macro-nanoporous morphology has a high scattering ability in the solar region, resulting in a high solar reflectivity of 99.65%. The PVDF-HFP porous film possesses an emissivity of 90.44% in the atmospheric window, and it can reach a maximum cooling temperature of 10.2 °C during the daytime. In addition, the excellent mechanical strength provides a guarantee for its large-scale practical application. This study offers an effective improvement strategy for the spectral performance of polymer fiber films, which is meaningful for green cooling management and reduction of carbon emission.

Experimental and theoretical study on building integrated radiative cooling with a limited sky-view factor

Building integrated radiative cooling (BIRC) technology leverages the coldness of outer space through the 8–13 μm atmospheric window for cooling. While BIRC's thermal performance has been widely studied across various climates, urban areas with obstructed sky views pose unique challenges often overlooked. To address this gap, the sky-view factor (SVF) was introduced as a metric to gauge BIRC effectiveness in urban settings. The novelty of this study lies in the experimental validation of SVF's impact on radiative cooling. A novel theoretical model was developed and validated, which was then applied to calculate BIRC's hourly, monthly, and year-round cooling power under Shenzhen's varying SVF conditions. The results revealed significant temperature drops in the RC module, ranging from 4.57 °C to 0.11 °C as SVF decreased from 0.7 to 0.1. Additionally, a numerical model was created based on experimental data, further validating the findings. This comprehensive analysis of SVF's effects on BIRC in Shenzhen's climate demonstrated that RC capacity averaged 57.27 W/m2 for SVF 0.9 but dropped below 22.55 W/m2 when SVF decreased to 0.3 or lower throughout the year. These insights enhance our understanding of BIRC's potential and limitations in densely populated urban environments.