PhysRevApplied.4.014023

Plasmonic Metasurface for Directional and Frequency-Selective Thermal Emission

D.Costantini,1,*A.Lefebvre,2A.-L.Coutrot,1I.Moldovan-Doyen,1J.-P.Hugonin,1

S.Boutami,2F.Marquier,1H.Benisty,1and J.-J.Greffet1,?

1Laboratoire Charles Fabry,Institut d’Optique,CNRS,UniversitéParis-Sud,

2avenue Augustin Fresnel,91127Palaiseau cedex,France

2UniversitéGrenoble Alpes F-38000Grenoble,France CEA,LETI,

MINATEC Campus,F-38054Grenoble,France

(Received5November2014;revised manuscript received20March2015;published30July2015)

Incandescent filaments and membranes are often used as infrared sources despite their low efficiency,

broad angular emission,and lack of spectral selectivity.Here,we introduce a metasurface to control

simultaneously the spectrum and the directivity of blackbody radiation.The plasmonic metasurface

operates reliably at600°C with an emissivity higher than0.85in a narrow frequency band and in a narrow

solid angle.This emitter paves the way for the development of compact,efficient,and cheap IR sources and

gas detection systems.

DOI:10.1103/PhysRevApplied.4.014023

I.INTRODUCTION Electromagnetic radiation in the midinfrared(mid IR) spectral range provides useful information for matter analysis.The infrared absorption spectrum contains “fingerprints”of the most common molecular bonds,key

to sample composition analysis.As a result,there is a strong demand for efficient and cheap light sources operating at room temperature(RT).Simple semiconductor structures such as light-emitting diodes(LED)based on interband transitions have a very low efficiency in the mid IR(namely,λ>3μm).Since the radiative decay time of the electron is proportional toλ3and becomes longer than nonradiative decay times,nonradiative processes become predominant whenλincreases and the LED radiative efficiency decreases accordingly.On the other hand,more sophisticated semiconductor devices,such as quantum cascade lasers(QCLs)based on intersubband transitions, are powerful sources that operate at RT for wavelengths 3μm<λ<25μm[1].However,QCLs are fabricated with an epitaxial growth technique that is very expensive. In this landscape,incandescent sources are still the best candidate,surely in terms of cost,but also in regards to the absolute power whenever broad spectral features are considered(say,Δλ=λ>1%).

The simplest available incandescent sources are either globar or hot membranes.They are cheap but suffer from a number of limitations that have hampered their deploy-ment.Their emitted radiation is quasi-isotropic and broad-band.Hence,most of the emitted radiation is useless.From the point of view of efficiency,most of the heating energy goes into convection or conduction losses and unwanted

radiation,leading to wall-plug efficiencies typically on the

order of a few percent for lighting applications but as low as 10?4for spectroscopic applications where the useful bandwidth is on the order of100nm and the operating

temperature on the order of1000K.Low efficiencies are

due to heat leakage through conduction,convection,and

radiation into unwanted frequencies and directions.Since

convection can be suppressed by operating under vacuum

and conduction can be suppressed by a proper design as

recently proposed[2],it appears that the ultimate efficiency

limit for incandescent sources is due to emission into

unwanted frequencies and directions.The upper bound of

the efficiency is given by the radiative efficiency which we

define as the ratio of the power emitted in the useful

spectral and directional window divided by the total power

emitted by the source.In other words,there is no funda-

mental limit preventing incandescent sources from reaching

100%efficiency of incandescent sources.It is,thus,of

critical importance to develop novel sources allowing the

simultaneous control of the emission spectrum and the

angular emission pattern so as to suppress unwanted

radiation.In this paper,we introduce a metasurface that

has a peak emissivity of95%in the normal direction at

4.25μm corresponding to a CO2absorption line.It emits in

a limited solid angle(0.84sr)and in a frequency interval

with a full width at half maximum(FWHM)of200nm.Its

radiative efficiency is3.1%.This is to be compared with the

theoretical radiative efficiency of a blackbody membrane,

which is0.5%,as implemented in state-of-the-art sources

[3,4].Furthermore,it can be operated at650°C in ambient

conditions reliably.

The emitted power at a given frequencyω,direction u, and temperature T is characterized by the emissivity εeω;uTdefined by Leω;u;TT?εeω;uTL BBeω;TTwhere

*Present address:Saint Gobain Recherche,39quai L.Lefranc,

93303Aubervilliers Cedex,France.

?jean?jacques.greffet@institutoptique.fr

PHYSICAL REVIEW APPLIED4,014023(2015)

L eω;u ;T Tis the spectral radiance of the structure [5],and L BB eω;T Tis the spectral radiance of the blackbody.The emissivity depends on the emission direction as well as on the frequency and acts as a filter of the blackbody spectral radiance to shape the emitted radiation from the thermal source.The polarization dependence is also included in the emissivity.In what follows,we consider only the averaged emissivity over the two polarizations ε?eεs tεp T=2.According to Kirchhoff ’s law,the emissivity εis equal to the absorption α[5],reducing the problem of the εcalculation to a study of the reflectivity R for an opaque medium.Indeed,energy conservation for an incident beam yields α?1?R .For a periodic metasurface of period p ,the reflectivity is the sum of the power reflected in all propagating orders in both polarizations s and p :R ?P n;m eR s nm tR p nm T,namely,the orders n;m such that the z wave vector component in vacuum γnm ?fe4π2=λ2T??K x tn e2π=P T 2??K y tm e2π=P T 2g 1=2is a real number with K x and K y the in-plane projections of the vector eω=c Tu ,c being the speed of light.

Many different approaches have been reported to control the emission spectrum of surfaces [6–21].A review can be found in Ref.[22].These incandescent sources are quasi-isotropic sources (spatially incoherent)as opposed to anten-nas or lasers which produce directional beams (spatially coherent).Yet,highly directional incandescent sources have been demonstrated in the last ten years [23–27].Despite a large number of contributions regarding either spectrally selective or directional thermal sources,to the best of our knowledge,there is only one report of a thermal source that combines both properties.By inserting 63quantum wells in an otherwise transparent semiconductor,a narrow spectrum absorber has been realized [28].In addition,by patterning the material with a photonic-crystal structure,it is possible to control the angular pattern of emission.This realization is,however,nearly as demanding technologically as QCLs.From the data published in Ref.[28],we estimate the radiative efficiency (i.e.,the power emitted in the spectral window and the solid angle normalized by the electrical power used to heat the source)to be 0.45%.

II.DIRECTIONAL AND SPECTRALLY SELECTIVE PLASMONIC METASURFACE In this work,we use a plasmonic metasurface for the design and fabrication of an incandescent source which is both spectrally selective,directional,and fairly simple.Metasurfaces have been introduced in the last ten years for a number of applications [29–35].Here,we use a 2D periodic array of metal-insulator-metal (MIM)cavities.Each MIM supports a gap surface plasmon (GSP)mode responsible for the resonant absorption or emission.Because of the subwavelength size of the cavity,the GSP resonance of a single antenna is obviously isotropic,which means that no directivity can be obtained.We show

that while the individual MIM geometry allows the control of the resonance frequency,the directivity of the emitted radiation is,nonetheless,driven by the periodicity of the MIM array.We introduce a design procedure that allows us to vary the solid angle and the operating frequency by changing the metasurface parameters with great flexibility.The central result of this paper is the experimental measurement of the metasurface emissivity shown in Fig.1.It takes significant values in a narrow frequency interval centered at ω?2353cm ?1(λ?4.25μm,relative FWHM of approximately 16%)and for angles θ<30°,in a solid angle 0.84sr.The choice of materials (W =SiN =Pt)borrowed from Refs.[3,4]is MEMS compatible and high-temperature compatible.

III.METASURFACE DESIGN

AND FABRICATION

The metasurface is composed of a metal backplane,a continuous silicon nitride (SiN)insulator layer of height h SiN ,and a 2D periodic square array (period p )of metal squares of height h metal ?100nm and side L .Since the backplane is much thicker (h backplane ?200nm)than the mid IR metal skin depth,it is opaque.The actual substrate (i.e.,silicon)is not playing any role in the device emissivity.We choose a geometry in which the GSP mode is confined along surface directions thanks only to the discontinuity induced by the metal patch edges (the insulator film is continuous [34])and well depicted by the associated GSP lateral reflection factor.In order to explain the metasurface properties we will,from now on,show the numerical and experimental results obtained with a structure made

of

(b)

(a)Angle (deg)F r e q u e n c y (c m -1)

0.0

0.20.4

0.6

0.8

1.0

4500

3500250015004000

300020001000

50

25

050

25Emissivity

Angle (deg)

FIG.1(a)Direct measurement of the emissivity at 600°C as a function of the frequency and the angle of the metasurface reported in this paper (W =SiN =Pt).The emissivity peak is located at ω?2353cm ?1(λ?4.25μm)and between 0°and 26°.The complete characterization of the structure is shown in Fig.5.(b)Schematics of the ideal source in the frequency angle plane.The emissivity should be 1in the useful window and 0elsewhere.

D.COSTANTINI et al.

PHYS.REV .APPLIED 4,014023(2015)

Au =SiN =Au (backplane,spacer,and patches,respectively).Au is chosen as a model material to demonstrate the electromagnetic proof of concept but cannot be used to operate reliably at high temperature and is not comple-mentary metal-oxide-semiconductor (CMOS)compatible.In the last part of the paper,we report the results obtained with a structure made of W =SiN =Pt.These materials can be used up to 900K.Note that the SiN layer prevents the oxidation of the tungsten backplane.

Once the materials are chosen,the frequency and the intensity of the GSP resonance merely depend on the geometrical parameters h SiN and L .We first engineer the MIM cavity to tune the resonance with the CO 2absorption line in the range 2353?60cm ?1using a refractive index of 1.77for the SiN and Palik ’s data [36]for Au.We perform a numerical calculation with the rigorous coupled-waves analysis (RCWA)method in order to calculate and optimize the absorption at ω?2353cm ?1,with a fixed period (p ?3μm),as a function of the spacer height h SiN and the patch side L .The result in Fig.2shows an absorptivity α>99%for the operating point h SiN ?120nm and L ?0.9μm e?0.3p T.We also observe that the absorptivity remains close to unity for the height range h SiN ?120?20nm,a large tolerance interval exceeding that of common SiN deposition techniques.The inset of Fig.2shows the section of the MIM cavity along the Oxz symmetry plane.The plotted absolute value of the total electric field for the chosen optimal case of h SiN ?120nm and L ?0.9μm exhibits a strong confinement in the SiN.After defining the MIM size and height,we perform an absorptivity calculation of the structure as a function of the frequency and the parallel component of the wave vector

(K ?K x ).The calculation result shown in Fig.3(a)clearly displays a MIM resonance at ω?2353cm ?1,visible for all the K range and standing out against the low back-ground (equivalent to no absorption).The grating perio-dicity introduces obviously diffracted orders.As a result,on Fig.3(a),an oblique line ω?2πc=p ?cK cuts the MIM horizontal resonance line in two parts.On the left part (smaller values of K ),only one propagating order exists,and the absorption is maximum.For larger K ,a ?1diffracted order appears,and this new channel for scattering is the reason why the absorptivity decreases as discussed below.In other words,by controlling the period p ,we can

z

y x

Absorption at λ= 4.25 μm

MIM side (μm)

S i N t h i c k n e s s (μm )

0.00.20.40.6

0.8

1.00.05

0.10.150.20.250.30.5

0.6

0.70.80.9

1

FIG.2Calculated absorption at normal incidence obtained with RCWA of a 2D MIM square grating (period of 3μm)as a function of the SiN thickness and of the MIM side.The calculation is performed on the Au =SiN =Au structure (see inset with the electric-field absolute value map at a cross section;minimum is blue,maximum is red)under normal incidence at λ?4.25μm and provides,at this specific wavelength,the geometrical parameters ensuring maximum MIM absorption (critical coupling conditions).The 100%absorption takes place for L ?900nm and h SiN ?120nm (black

dot).

FIG.3(a)Calculated absorptivity as a function of the frequency and the incident K .The absorptivity is averaged over the two polarizations.The parameters of the simulated structure are L ?900nm,h SiN ?120nm,and p ?3μm.The MIM reso-nance displays an absorption peak at ω?2353cm ?1(λ?4.25μm)for all incident wave vectors (angles).The oblique line ω?2πc=p ?cK (c being the speed of light)cuts the horizontal resonance line in two parts,with large absorption on the side of smaller K .(b)Schematics to explain the reduction of the absorption for large K :(i)for small K values,there is a single diffracted order,whereas (ii)for larger K values,there are two diffracted orders.The existence of the ?1order introduces an additional radiative decay channel so that the critical coupling condition is no longer fulfilled.(c)Calculated absorptivity at λ?4.25μm as a function of the normalized k x and k y .It is clearly seen that the presence of the grating allows controlling of the directivity of the absorption and emission.

PLASMONIC METASURFACE FOR DIRECTIONAL AND …PHYS.REV .APPLIED 4,014023(2015)

design the angular width of the emitter.More precisely, the position of this first-order cutoff is given by sin?1?eλ?pT=p .Here,with p?3μm andλ?4.25μm, we findθ?24.6°.

IV.PHYSICAL MECHANISM

FOR DIRECTIONALITY

The physical mechanism responsible for the absorptivity control can be understood as follows:The total absorption is due to a critical coupling to the GSP resonator[15]at normal incidence in a regime having a single(specular) diffracted order;Fig.3(b)panel(i).When a second non-specular order is allowed[Fig.3(b)panel(ii)],radiative losses are modified(basically,they are increased)so that the critical coupling condition is no longer fulfilled.It follows that the absorption is reduced.We explore the absorption for different azimuthal angles by solving the full 3D problem.We plot in Fig.3(c)the polarization-averaged absorptivityeαstαpT=2as a function of the emission direction atω?2353cm?1.We observe a large absorption in a square-bounded(pyramidlike)solid angle rather than a cone of cylindrical symmetry,with a minimum apex semiangle of26°along the x axis.The subtended square-shaped solid angle is larger by a factor of only approximately4=πthan that of the inscribed cone,not a crucial change of directivity for most instrumental optics. In summary,tuning the parameters of a periodic array of gap-plasmon patch resonators provides enough room for designing a metasurface with an emissivity close to unity in a controlled spectral bandwidth and in a controlled solid angle.

V.FABRICATION AND ROOM-TEMPERATURE

CHARACTERIZATION

The device simulated above based on Au cavities with h SiN?120nm,L?920nm,and p?3μm is fabricated and experimentally measured;see Fig.4(a).The Au backplane of thickness200nm is evaporated on a Si wafer after deposition of a10-nm-thick Ti adhesion layer.SiN is deposited by plasma-enhanced chemical-vapor deposition. The square patches are defined by electron-beam lithog-raphy with a lift-off process using the evaporation of a100-nm-thick gold layer again onto a3-nm-thick Ti adhesion layer.Unlike the first gold adhesion layer,this thin Ti layer has to be as thin as possible in order to avoid broadening the resonance.A micrograph of the sample is presented in Fig.4(a)with typical values of the patch side and period. We measure first the room-temperature specular reflec-tance of this device.Measurements are performed with a Vertex70Bruker Fourier-transform infrared spectrometer (FTIR)using a reflectivity module.We use the internal MIR light source,the KBr beam splitter,and a liquid-nitrogen-cooled mercury-cadmium-telluride detector.The result of the measurement is shown in Fig.4(b)by mapping1?R0,where R0is the measured(specular)reflectance in the

zeroth order.When only the specular(zeroth)order exists,

in the central part of the Brillouin zone(smaller K),the

quantity1?R0corresponds to the absorption,and Fig.4(b)

can be directly compared to Fig.3(a).In this region,R0is

close to0so that the absorption is maximum,as predicted in

the calculations.When K becomes larger,the diffracted

order?1becomes propagating,and the absorption becomes 1?R0?R?1so that1?R0is no longer the absorptivity. To better appreciate the degree of control on the source

directivity,we report in Fig.4(c)two reflection factor

measurements for two different periods differing by10%, p?3μm,p?2.7μm,and for p polarization only.It can be observed that,as predicted,the period does not affect the frequency of the MIM resonance but unambiguously shifts the crossing point indicated by dashed lines.

VI.HIGH-TEMPERATURE EMITTER

USING TUNGSTEN

To perform emission measurements at high temperature, we fabricate a W=SiN=Pt sample with a tungsten(W) backplane,a SiN insulator layer,and sturdy platinum

(Pt) FIG.4(a)SEM image of the MIM grating showing the size of the square side and of the period.(b)Measured reflectivity R0 normalized by a gold surface,plotted as the map of1?R0.The experimental measurement spans from13°to90°and allows observation of the anticrossing point due to strong coupling between the MIM resonance and the plasmon.On the left side (smaller K)of this crossing line,the quantity1?R0equals the absorption,and the figure can be compared to Fig.3(a).

(c)Enlarged anticrossing zone in the measured1?R0map in p polarization for samples with L?900nm and different periods,(i)p?2.7μm,(ii)p?3μm.By decreasing the period, the anticrossing point shifts towards higher K(vertical dashed line).

D.COSTANTINI et al.PHYS.REV.APPLIED4,014023(2015)

square https://www.wendangku.net/doc/0118113835.html,ing the Palik [34]refractive indices for W and Pt,we numerically optimize the MIM cavity parameters obtaining h SiN ?210nm and L ?855nm,the largest change being the larger height.The fabrication process is the same as described above except that W and Pt are deposited by sputtering.The emissivity measurement is performed by heating the sample at 600°C (873K)on a holder,which is fixed on a motorized rotating goniometer.The emitted thermal radiation is collected with a mirror system and sent into the Bruker FTIR spectrometer [24].By replacing the sample with a calibrated blackbody (SR 200ECI system),we get an adequate reference in order to fully determine the emissivity.This setup provides us with a direct measurement of the emissivity εeω;u Twith an angular resolution of 0.5°.

The sample characterization through its reflectivity is shown in Fig.5(a).It is quite similar to the previous sample,with only a slightly broader absorption band,as expected.The experimental emissivity map is shown in Fig.5(b)and is in very good agreement with the map predicted by the numerical calculation,Fig.5(c).The 13°limit is indicated for comparison to Fig.5(a).The decrease of emissivity for large K ,which cannot be inferred from specular reflection measurement on Fig.5(a),appears clearly now.The resonance peak culminates at 95%at ω?2390cm ?1and has a FWHM of 370cm ?1,whereas the numerical calculation gives a peak of 100%at 2350cm ?1with a FWHM of 250cm ?1.We attribute the peak broadening at high temperature to a thermal evolution of the refractive indices.The rightmost panel of Fig.5shows the theoretical radiance of a blackbody at 600°C highlighting the useful frequency interval for CO 2detection and its proximity to the Wien wavelength in that case.

VII.FIGURE OF MERIT FOR CO 2SPECTROSCOPY AND CONCLUSION To quantify the gain with respect to the blackbody,we define a useful spectral window (ω?2353?60cm ?1),which corresponds to the CO 2absorption band,while for

the solid angle,we take into account a cone with a semiangle of 25°.Using the data of Fig.5(b)at normal incidence,we find that the device emits 85%of the power of a blackbody in the useful spectral window .Hence,the device is almost nearly as good as a blackbody for the desired radiation.We now examine the radiative efficiency that we defined as the total power emitted in the useful spectral and spatial window divided by the total power emitted by the source.This figure of merit is 0.5%for a blackbody membrane given the small cone and reaches experimentally a 6times higher value of 3.1%for the engineered metasurface.

In summary,we experimentally demonstrate a metasur-face for directional and frequency-selective thermal emis-sion designed for operation at 600°C.We suggest a practical application as a source for CO 2detection showing radiative performances equivalent to a blackbody but with a radiative efficiency enhanced by a factor of 6.2.The emission of the metasurface can be easily tuned with geometrical parameters well within process and litho-graphic capabilities.The concept can be adapted to multichannel emission using various MIM cavities of different sizes.All the technology is CMOS compatible allowing integration in cheap wafer-scale industrially processed sources.This type of design can also be implemented for thermophotovoltaic applications.

ACKNOWLEDGMENTS

This work is supported by the French National Research Agency (Project No.ANR-12-NANO-0005“IDEE ”).We thank P.Barritault for useful discussions.We thank M.Besbes for the help in finite element simulations and D.Daineka for the support in fabrication.The device fabri-cation has been performed in the Thales and IOGS

facilities.

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FIG.5Characterization of the W =SiN =Pt metasurface,with height h SiN ?210nm and patch side L ?855nm,as a function of the frequency and wave vector K .(a)Measured map of 1?R 0normalized by a gold surface (from 13°to 90°).(b)Measured emis-sivity at 600°C obtained by normaliz-ing with a blackbody reference (from 0°to 70°angles,with the 13°line as a guide to the eye).(c)Calculated ab-sorption for a direct comparison with the emissivity.(d)The calculated black-body radiance at 600°C with the fre-quency interval of the CO 2absorption (from 4.16to 4.36μm).

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