REPORT◥

OPTICAL METAMATERIALS

Scalable-manufactured randomizedglass-polymer hybrid metamaterialfor daytime radiative coolingYao Zhai,1* Yaoguang Ma,1* Sabrina N. David,2 Dongliang Zhao,1 Runnan Lou,3

Gang Tan,4 Ronggui Yang,1† Xiaobo Yin1,2†

Passive radiative cooling draws heat from surfaces and radiates it into space asinfrared radiation to which the atmosphere is transparent. However, the energydensity mismatch between solar irradiance and the low infrared radiation flux from anear-ambient-temperature surface requires materials that strongly emit thermalenergy and barely absorb sunlight. We embedded resonant polar dielectric microspheresrandomly in a polymeric matrix, resulting in a metamaterial that is fully transparentto the solar spectrum while having an infrared emissivity greater than 0.93 across theatmospheric window. When backed with a silver coating, the metamaterial shows anoontime radiative cooling power of 93 watts per square meter under direct sunshine.More critically, we demonstrated high-throughput, economical roll-to-roll manufacturingof the metamaterial, which is vital for promoting radiative cooling as a viableenergy technology.

Radiative cooling—deposition of blackbodyradiation from a hot object through theinfrared transparency window of the atmo-sphere to the cold sink of outer space—isan appealing concept for the 21st century,

when most daily necessities, from power gener-ation to data centers, generate excess heat. Incontrast to most of the currently used coolingmethods that require energy and resources tocarry heat away, radiative cooling is a passiveenhancement of Earth’s natural method of cool-ing itself. Efficient nighttime radiative coolingsystems have been extensively investigated in thepast, with promising infrared emissivity in bothorganic and inorganic materials, including pig-mented paints (1–5). Daytime radiative cooling,however, presents a different challenge becausesolar absorbance of just a few percent exceedsthe cooling power and effectively heats the sur-face. Recently proposed nanophotonic devicescan effectively reject solar irradiance but emitstrongly in infrared (6, 7), which is promising fordaytime radiative cooling. However, the nano-photonic approach requires stringent, nanometer-precision fabrication, which is difficult to scale upcost-effectively in order to meet the large area

requirements of the residential and commercialapplications that can benefit most from radia-tive cooling.Polymeric photonics is a growing field and at-

tractive for economy and scalability (8–11). Hy-bridization of random optical metamaterialswith polymer photonics can be a promising ap-proach for efficient daytime radiative cooling; todate, harnessing randomness in photonic sys-tems has yielded amplified spontaneous emis-sion (12, 13), extremely localized electromagnetichot spots (14–16), improved light-trapping effi-ciency of photovoltaic cells (17, 18), and nega-tive permeability and switching devices withmultistability (19, 20). When electromagneticresonators in a random metamaterial are col-lectively excited, the extinction and the opticalpath length in the material are both enhanced,resulting in nearly perfect absorption at the res-onance (21, 22). This implies great potential forusing metamaterials with randomly distributedoptical resonators for effective radiative cooling,if perfect absorption (emissivity) across the entireatmospheric transmission window can be achieved.Here, we demonstrate efficient day- and night-

time radiative cooling with a randomized, glass-polymer hybrid metamaterial. The metamaterialconsists of a visibly transparent polymer encap-sulating randomly distributed silicon dioxide (SiO2)microspheres. The spectroscopic response spanstwo orders of magnitude in wavelength (0.3 to25 mm). Our hybrid metamaterial is extremelyemissive across the entire atmospheric transmis-sion window (8 to 13 mm) because of the phonon-enhanced Fröhlich resonances of the microspheres.A 50-mm-thick metamaterial film containing 6% of

microspheres by volume has an averaged infraredemissivity of >0.93 and reflects ~96% of solarirradiance when backed with a 200-nm-thick sil-ver coating. We experimentally demonstrate anaverage noontime (11 a.m. to 2 p.m.) radiativecooling power of 93 W/m2 under direct sun-shine during a 3-day field test and an averagecooling power of >110 W/m2 over the continuous72-hour day-and-night test. The metamaterialwas fabricated in 300-mm-wide sheets at a rateof 5m/min, so that in the course of the experiment,we produced hundreds of square meters of thematerial.The proposed structure of the randomized,

glass-polymer hybrid metamaterial containsmicrometer-sized SiO2 spheres randomly dis-tributed in a matrix material of polymethyl-pentene (TPX) (Fig. 1A). We used TPX becauseof its excellent solar transmittance. Other vis-ibly transparent polymers such as poly(methylmethacrylate) and polyethylene can be used butwould slightly increase solar absorption. Becauseboth the polymer matrix material and the en-capsulated SiO2 microspheres are lossless inthe solar spectrum, absorption is nearly absent,and direct solar irradiance does not heat themetamaterial.At infrared wavelengths, the encapsulated SiO2

microspheres have optical properties drasticallydifferent from that of the surrounding matrixmaterial, owing to the existence of strong phonon-polariton resonances at 9.7 mm (23). We calculatedthe normalized absorbance (sabs/a

2), scattering(ssca/a

2), and extinction (sext/a2) cross sections

of an individual microsphere encapsulated inTPX as a function of its size parameter, k0a,for an incident wavelength of 10 mm (Fig. 1B).Here, k0 is the wave vector in free space, and a isthe radius of the microsphere. The extinctionpeaks at a size parameter of ~2.5, correspond-ing to a microsphere radius of ~4 mm. The sizeparameter of the microsphere plays a key rolein designing the hybrid metamaterial for radia-tive cooling. At the small particle (quasi-static)limit, the resonance is purely electric-dipolarin character (Fig. 1B, inset). At the extinctionpeak, high-order Fröhlich resonances, includ-ing both electric and magnetic modes, are alsostrongly excited, which is evidenced by the strongforward scattering shown in Fig. 1C, the three-dimensional power scattering function (far-fieldscattering pattern) (24).The intrinsically narrow linewidth of phonon

polaritons—often a superior advantage in appli-cations such as infrared sensing (25, 26)—canhere limit the bandwidth of the highly emissiveinfrared region. We obtained broadband emis-sivity across the entire atmospheric window byaccessing the high-order Fröhlich resonances ofthe polar dielectric microspheres (27). The realand the imaginary parts of the extracted effec-tive index of refraction, nþ ik ¼ ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi

eeff ⋅ meffp

, arefunctions of wavelength and microsphere sizes,as illustrated in Fig. 2 for 1- and 8-mm-diametermicrospheres. Given the low concentration (6%by volume) and assuming that the microspheresare uniform in size and distribution, we retrieved

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1Department of Mechanical Engineering, University ofColorado, Boulder, CO 80309, USA. 2Materials Science andEngineering Program, University of Colorado, Boulder, CO80309, USA. 3Department of Aerospace EngineeringSciences, University of Colorado, Boulder, CO 80309, USA.4Department of Civil and Architectural Engineering,University of Wyoming, Laramie, WY 82071, USA.*These authors contributed equally to this work. †Correspondingauthor. Email: xiaobo.yin@colorado.edu (X.Y); ronggui.yang@colorado.edu (R.Y.)

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the effective permittivity and permeability of thehybrid metamaterial from eeff = ep · [1 + ig(S0 +S1)] and meff = 1 + ig(S0 + S1), respectively (28),where S0 and S1 are the forward- and backward-scattering coefficients of an individual microspherein the encapsulating medium and the factor gincorporates the volume fraction, f, and the size

parameter, k0a g ¼ 3f2ðk0aÞ3

h i. In the case of large

microspheres, modal interference between higher-order modes makes the hybrid metamaterialstrongly infrared-absorbing and nearly disper-sionless in the infrared. The dispersion of boththe real and imaginary part of the effective indexof refraction is less than 9 × 10−5/nm across theentire infrared wavelength range (Fig. 2), instark contrast to the strong dispersion of polar,dielectric bulk SiO2, which is ~5 × 10−3/nm inthis same range. The low dispersion provides ex-cellent broadband impedance matching of themetamaterial to free space, resulting in extremelylow reflectance for both solar and infrared radi-ation. A hybrid metamaterial as thin as 50 mmcan provide uniform and sufficiently strong ab-sorbance across the entire atmospheric window,resulting in perfect broadband infrared emissionfor radiative cooling (Fig. 2C). In contrast, whenthe microspheres are small (k0a << 1), a sharpresonance occurs (Fig. 2B), which limits the highinfrared emissivity to the polariton resonancewavelength only. Moreover, the resonance intro-duces strong reflectance, further reducing theoverall emissivity.The hybrid metamaterial strongly reflects solar

irradiation when backed with a 200-nm-thicksilver thin film (Fig. 3A) prepared by means ofelectron beam evaporation. We characterizedthe spectroscopic performance of the metama-terial thin film in both the solar (0.3 to 2.5 mm)and infrared (2.5 to 25 mm) regions using anultraviolet-visible–near-infrared (UV-Vis-NIR) spec-trophotometer and Fourier transform infraredspectrometer (FTIR), respectively (Fig. 3, C andD). We used integrating spheres to account forthe scattered light from the full solid angle inboth spectral regions. The measured spectral ab-sorptivity (emissivity) of the sample (Fig. 3D) in-dicates that the 50-mm-thick film reflects ~96%solar irradiation while possessing a nearly satu-rated emissivity of >0.93 between 8 and 13 mm—yielding greater than 100 W/m2 radiative coolingpower under direct sunlight at room temperature.The experimental results agree well with theory,in which the spectroscopic discrepancies near 3-and 16-mm wavelengths are primarily due to theabsorbance of water and air during the FTIRmeasurement in ambient conditions. We mustuse different theoretical approaches for calculat-ing the emissivity in the solar and infrared wave-length ranges. We used the generalized, incoherenttransfer-matrix method in the infrared region(29). In the solar region, we instead used rig-orous coupled wave analysis (RCWA) becausethe extracted effective parameters of the meta-material are inaccurate when the size of themicrosphere is greater than the relevant wave-lengths (30). The high emissivity in the second

Zhai et al., Science 355, 1062–1066 (2017) 10 March 2017 2 of 4

x

y

z

Size parameter ( )

Fig. 1. Glass-polymer hybridmetamaterial. (A) A schematic of the polymer-based hybrid metamaterialwith randomly distributed SiO2 microsphere inclusions for large-scale radiative cooling. The polarizablemicrospheres interact strongly with infrared light, making the metamaterial extremely emissive acrossthe full atmospheric transmission window while remaining transparent to the solar spectrum. (B) Nor-malized absorption (blue), scattering (red), and extinction (black) cross sections of individual microspheresas functions of size parameter (k0a). The extinction—the sum of the scattering and absorption—peaks at asize parameter of 2.5, which corresponds to a microsphere radius of 4 mm. (Inset) The electric field dis-tributions of two microspheres with 1- and 8-mm diameters, illuminated at a 10-mmwavelength. Scale bar,4 mm.The smaller microsphere resonates at the electric dipolar resonance, whereas higher-order electricand magnetic modes are excited in the larger microsphere. (C) Angular diagram for the scattering far-fieldirradiance of an 8-mm-diameter microsphere with 10-mm wavelength illumination. The incident field ispolarized along the y direction and propagating along the z direction.

Fig. 2. Fröhlich resonance andbroadband infrared absorbance ofthe hybrid metamaterial. (A and B)The real (A) and imaginary (B) part ofthe effective index of refraction for theglass-polymer hybrid metamaterials.The metamaterial with 1-mm-diameterSiO2 microspheres (black curves)shows a strong Fröhlich resonance atits phonon-polariton frequency of 9.7 mm,whereas the metamaterial with 8-mm-diameter microspheres (red curves)shows significantly more broadbandabsorption across infrared wavelengths.The strong Fröhlich resonance not onlylimits the bandwidth of strong emissivitybut also introduces strong reflectanceof incident infrared radiation. In bothcases, the metamaterial contains6% SiO2 by volume. (C) The attenuationlengths of the two hybrid metamaterials,with the 8-mm-diameter SiO2 microsphere case showing an average attenuation length of ~50 mm froml = 7 to 13 mm.

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AM1.5

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Room Temperature Surface Radiation

Fig. 3. Spectroscopic response of the hybrid metamaterial. (A) Schematicof the hybrid metamaterial backed with a thin silver film. The silver filmdiffusively reflects most of the incident solar irradiance, whereas the hybridmaterial absorbs all incident infrared irradiance and is highly infrared emis-sive. (B) Three-dimensional confocal microscope image of the hybrid meta-material. The microspheres are visible because of the autofluorescence ofSiO2. (C) Power density of spectral solar irradiance [air mass (AM) 1.5] andthermal radiation of a blackbody at room temperature. The sharply varyingfeatures of both spectra are due to the absorbance of the atmosphere.

The radiative cooling process relies on strong emission between 8 and13 mm, the atmospheric transmission window. (D) The measured emissivity/absorptivity (black curve) of the 50-mm-thick hybrid metamaterial from300 nm to 25 mm. Integrating spheres are used for the measurement ofboth solar (300 nm to 2.5 mm) and infrared (2.5 to 25 mm) spectra. Theo-retical results for the same hybrid metamaterial structure (red curves) areplotted for comparison. Two different numerical techniques, RCWA and in-coherent transfer matrix methods, are used for the solar and infrared spectralranges, respectively.

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Fig. 4. Performance of scalable hybrid meta-material for effective radiative cooling. (A) Aphoto showing the 300-mm-wide hybrid meta-material thin film that was produced in a roll-to-roll manner, at a speed of 5 m/min. The film is50 mm in thickness and not yet coated with silver.(B) A 72-hour continuous measurement of the am-bient temperature (black) and the surface temper-ature (red) of an 8-in-diameter hybrid metamaterialunder direct thermal testing. A feedback-controlledelectric heater keeps the difference between ambi-ent and metamaterial surface temperatures lessthan 0.2°C over the consecutive 3 days.The heatingpower generated by the electric heater offsets theradiative cooling power from the hybrid metamate-rial. When the metamaterial has the same temper-ature as the ambient air, the electric heating powerprecisely measures the radiative cooling power ofthe metamaterial. The shaded regions representnighttime hours. (C) The continuousmeasurementof radiative cooling power over 3 days shows anaverage cooling power of >110W/m2 and a noontimecooling power of 93 W/m2 between 11 a.m. and2 p.m. The average nighttime cooling power ishigher than that of the daytime, and the coolingpower peaks after sunrise and before sunset. Themeasurement error of the radiative cooling poweris well within 10 W/m2 (32).

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atmospheric window between 16 and 25 mmmight be harnessed for additional radiativecooling (31).Using a polymer as the matrix material for

radiative cooling has the advantages of beinglightweight and easy to laminate on curved sur-faces. It can accommodate small variations inmicrosphere size and shape, with negligible im-pact on the overall performance. TPX has excel-lent mechanical and chemical resistance, offeringpotentially long lifetimes for outdoor use. How-ever, one of the most compelling advantages ofdeveloping a glass-polymer hybrid metamateriallies in the possibility of cost-effective scalablefabrication. We produced a roll of 300-mm-wideand 50-mm-thick hybrid metamaterial film ata rate of 5 m/min (Fig. 4A). We controlled thevolume concentration of the SiO2 microspheresby using gravimetric feeders. The resultant filmhas a homogeneous distribution of microspheres,with fluctuations in concentration of less than0.4% (fig. S1) (32). The hybrid metamaterialfilms are translucent because of the scatteringof visible light from the microsphere inclusions(fig. S2) (32). Additionally, when backed with a200-nm-thick reflective silver coating, the hybridmetamaterial has a balanced white color (fig. S2)(32). The strongly scattering and nonspecularoptical response of the metamaterial will avoidback-reflected glare, which can have detrimen-tal visual effects for humans and interfere withaircraft operations (33).We demonstrated real-time, continuous radi-

ative cooling by conducting thermal measure-ments using an 8-in-diameter, scalably fabricatedhybrid metamaterial film over a series of clearautumn days in Cave Creek, Arizona (33°49′32′′N,112°1′44′′W, 585 m altitude) (Fig. 4, B and C).The metamaterial was placed in a foam con-tainer that prevents heat loss from below. Thetop surface of the metamaterial faced the skyand was directly exposed to the air (fig. S3) (32).We kept the surface temperature of the meta-material the same as the measured ambienttemperature using a feedback-controlled electricheater placed in thermal contact with the meta-material so as to minimize the impacts of con-ductive and convective heat losses. The total

radiative cooling power is therefore the sameas the heating power generated by the electricheater if there is no temperature difference be-tween the surface and the ambient air. With thefeedback control, the surface temperature followsthe measured ambient temperature within ±0.2°Caccuracy during the day and less than ±0.1°Cat night (fig. S4) (32). We continuously measuredradiative cooling power, which gives an averageradiative cooling power of >110 W/m2 over a con-tinuous 72-hour day- and nighttime measurement(Fig. 4C). The average cooling power around noonreaches 93 W/m2, with normal-incidence solarirradiance greater than 900 W/m2. We observedhigher average nighttime radiative cooling thanduring the day. However, the cooling power peaksafter sunrise and before sunset, when the am-bient temperature is changing rapidly and solarirradiance is incident at large oblique angles. Tofurther demonstrate the effectiveness of radia-tive cooling, we also used water as a cold storagemedium and show cold water production withthe scalably fabricated hybrid metamaterial (fig.S6) (32). Although we did not determine the re-liability and lifetime of the glass-polymer hybridmetamaterials for outdoor applications, applyingchemical additives and high-quality barrier coat-ings may enhance their outdoor performance.Many polymeric thin films are currently avail-able and designed with extended outdoor life-times (34).

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ACKNOWLEDGMENTS

This research was supported by the U.S. Department of Energy’sAdvanced Research Projects Agency–Energy (ARPA-E) grantDE-AR0000580. X.Y. also thanks the Defense Advanced ResearchProjects Agency (DARPA) for the support in Young FacultyAward grant D15AP00106, which enabled the modeling ofrandomized optical materials, and a seed grant from the SoftMaterials Research Center under NSF Materials Research Scienceand Engineering Center grant DMR-1420736. We have filed aprovisional patent application (U.S. Full Patent Application15/056680) on the work described here. Data are available inthe manuscript and supplementary materials.

SUPPLEMENTARY MATERIALS

www.sciencemag.org/content/355/6329/1062/suppl/DC1Materials and MethodsSupplementary TextFigs. S1 to S6

11 August 2016; resubmitted 17 November 2016Accepted 27 January 2017Published online 9 February 201710.1126/science.aai7899

Zhai et al., Science 355, 1062–1066 (2017) 10 March 2017 4 of 4

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coolingScalable-manufactured randomized glass-polymer hybrid metamaterial for daytime radiative

Yao Zhai, Yaoguang Ma, Sabrina N. David, Dongliang Zhao, Runnan Lou, Gang Tan, Ronggui Yang and Xiaobo Yin

originally published online February 9, 2017DOI: 10.1126/science.aai7899 (6329), 1062-1066.355Science 

, this issue p. 1062; see also p. 1023Sciencequantities for a variety of energy technology applications.material near the theoretical limit for daytime radiative cooling. The translucent and flexible film can be made in largemicrospheres, backed with a thin layer of silver (see the Perspective by Zhang). The result is an easy-to-manufacture

solve this riddle by constructing a metamaterial composed of a polymer layer embedded withet al.through. Zhai Passive radiative cooling requires a material that radiates heat away while allowing solar radiation to pass

The lazy way to keep cool in the sun

ARTICLE TOOLS http://science.sciencemag.org/content/355/6329/1062

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