hv photonics

Article Incandescent Light Bulbs Based on a Refractory † Metasurface

Hirofumi Toyoda 1, Kazunari Kimino 1, Akihiro Kawano 1 and Junichi Takahara 1,2,*

1 Graduate School of Engineering, Osaka University, Osaka 565-0871, Japan; [email protected] (H.T.); [email protected] (K.K.); [email protected] (A.K.) 2 Photonics Center, Graduate School of Engineering, Osaka University, Osaka 565-0871, Japan * Correspondence: [email protected]; Tel.: +81-6-6879-8503 Invited paper. †


Received: 5 September 2019; Accepted: 9 October 2019; Published: 12 October 2019


Abstract: A thermal radiation light source, such as an incandescent light bulb, is considered a legacy light source with low luminous efficacy. However, it is an ideal energy source converting light with high efficiency from electric power to radiative power. In this work, we evaluate a thermal radiation light source and propose a new type of filament using a refractory metasurface to fabricate an efficient light bulb. We demonstrate visible-light spectral control using a refractory metasurface made of tantalum with an optical microcavity inserted into an incandescent light bulb. We use a nanoimprint method to fabricate the filament that is suitable for mass production. A 1.8 times enhancement of thermal radiation intensity is observed from the microcavity filament compared to the flat filament. Then, we demonstrate the thermal radiation control of the metasurface using a refractory plasmonic cavity made of hafnium nitride. A single narrow resonant peak is observed at the designed wavelength as well as the suppression of thermal radiation in wide mid-IR range under the condition of constant surface temperature.

Keywords: incandescent light bulb; thermal radiation; refractory metal; microcavity; metamaterial; metasurface; surface plasmon; infrared emitter; nanoimprint

1. Introduction An incandescent filament shining in a transparent glass bulb is the origin of the beauty of lighting. The presence of incandescent light can stimulate a brilliant human mind and exert a calming effect. In addition, incandescent light displays a continuous spectrum of thermal radiation that is attractive from the point of view of architecture, lighting design, and print color-matching. However, a thermal radiation light source like an incandescent light bulb is a legacy light source, and in recent years, its applications have decreased. This is because the luminous efficacy of an incandescent light bulb is only 15 lm W 1, which is much lower than the emerging solid-state light sources, viz., the light-emitting · − diode (LED) light bulbs. Most of the radiative power emitted from an incandescent light bulb is in infrared (IR) light. Hence the luminous efficacy of an incandescent light bulb is low compared to an LED light bulb because the luminous efficacy is limited to the narrow absorption band of the human eye. Additionally, the lifetime of LED light bulbs is 40,000–50,000 h which is much longer than that of incandescent light bulbs (1000–2000 h). In contradiction to the trend favoring the use of LEDs over incandescent light bulbs, we propose that a thermal radiation light source is an ideal, high-efficiency converter of electric input power to a radiative output power source. The energy conversion efficiency of incandescent light bulbs is higher than 90%, which is a result of the Joule heating of a filament [1,2]. Hence thermal radiation light

Photonics 2019, 6, 105; doi:10.3390/photonics6040105 www.mdpi.com/journal/photonics Photonics 2019, 6, 105 2 of 20

Photonics 2019, 6, x FOR PEER REVIEW 2 of 20 sources have great potential for use as efficient light sources. Although the thermal radiation spectrum spectrumobeys Planck’s obeys law Planck’s that depends law that on depends physical constantson physical and constants temperature, and itstemperature, luminous e ffiitscacy luminous can be efficacyimproved can beyond be improved that suggested beyond by that Planck’s suggested law, ifby the Planck’s thermal law, radiation if the spectrathermal from radiation a filament spectra can frombe controlled a filament artificially can be controlled and optimized artificially to the and absorption optimized band to the of humanabsorption eyes. band of human eyes. TheThe basic basic concept concept for for improving improving the the efficiency efficiency of an an incandescent incandescent light light bulb bulb by by thermal thermal radiation radiation controlcontrol of of a a nanostructured nanostructured filament filament was was proposed proposed by by Waymouth Waymouth inin 1989 1989 [3,4]. [3,4 He]. He reasoned reasoned that that IR radiationIR radiation (longer (longer wavelengths) wavelengths) can be can suppressed be suppressed by the by cut-off the cut-o effectff ofeff aect microcavity of a microcavity while visible while lightvisible (shorter light (shorter wavelengths) wavelengths) is radiated is radiated into free-space, into free-space, which which is analogous is analogous to the to theoperation operation of ofa microwavea microwave waveguide. waveguide. This This idea idea is known is known as asthe the Waymouth Waymouth hypothesis, hypothesis, and and the the lamp lamp is iscalled called a “microcavitya “microcavity lamp.” lamp.” Figure Figure 1 shows1 shows the the concept concept of the of microcavity the microcavity lamp, lamp, where where a cuboidal a cuboidal hole array hole isarray formed is formed on the onsurface the surface of a tungsten of a tungsten (W) filament (W) filament set into seta bulb. into Following a bulb. Following the viewpoint the viewpoint of modern of photonics,modern photonics, such an suchartificial an artificial surface surfacestructure structure is called is a called “metasurface,” a “metasurface,” i.e., a two-dimensional i.e., a two-dimensional (2D) metamaterial.(2D) metamaterial.

FigureFigure 1. TheThe concept concept of of a a microcavity lamp: lamp: an an incand incandescentescent light light bulb bulb with with a a microcavity microcavity array array filamentfilament acting as a refractory metasurface. Independent of the Waymouth hypothesis, the modification of a thermal radiation spectrum Independent of the Waymouth hypothesis, the modification of a thermal radiation spectrum (deviation from Planck’s law) using a microstructured surface was demonstrated first in 1986 by a deep (deviation from Planck’s law) using a microstructured surface was demonstrated first in 1986 by a silicon grating [5]. This was studied further using various types of micro and nanostructures such deep silicon grating [5]. This was studied further using various types of micro and nanostructures as an open-end metallic microcavity [6,7], photonic crystal [8,9], plasmonic cavity [10,11], and Mie such as an open-end metallic microcavity [6,7], photonic crystal [8,9], plasmonic cavity [10,11], and resonators [12]. Additionally, narrow-band thermal radiation has been reported using surface waves Mie resonators [12]. Additionally, narrow-band thermal radiation has been reported using surface such as surface phonon polaritons [13], surface plasmon polaritons [14,15], spoof surface plasmons [16], waves such as surface phonon polaritons [13], surface plasmon polaritons [14,15], spoof surface and Tamm plasmons [17]. In 2008, perfect absorbers based on metamaterials were proposed [18,19]. plasmons [16], and Tamm plasmons [17]. In 2008, perfect absorbers based on metamaterials were In addition, thermal radiation emitters based on metasurfaces using metal-dielectric-metal (MDM) proposed [18,19]. In addition, thermal radiation emitters based on metasurfaces using metal- structures were demonstrated in the infrared (IR) range [20,21]. In the last decade, much research dielectric-metal (MDM) structures were demonstrated in the infrared (IR) range [20,21]. In the last was done on perfect absorbers based on MDM metasurfaces, and their applications such as thermal decade, much research was done on perfect absorbers based on MDM metasurfaces, and their radiation emitters or gas sensing in the IR range [22–29]. There are many papers about thermal applications such as thermal radiation emitters or gas sensing in the IR range [22–29]. There are many radiation control based on metasurfaces in the IR range. A comprehensive list of these efforts can be papers about thermal radiation control based on metasurfaces in the IR range. A comprehensive list found in the literature [30,31]. of these efforts can be found in the literature [30,31]. In contrast to the IR range, there are a few studies about thermal radiation control in the visible In contrast to the IR range, there are a few studies about thermal radiation control in the visible spectrum. This occurs because the melting points of typical plasmonic materials such as gold (Au) spectrum. This occurs because the melting points of typical plasmonic materials such as gold (Au) and silver (Ag) are below 1500 K. Additionally, damage during the nanofabrication process induces and silver (Ag) are below 1500 K. Additionally, damage during the nanofabrication process induces a decrease in the melting points of refractory metals due to defects. This reduces the durability of a decrease in the melting points of refractory metals due to defects. This reduces the durability of nanostructures compared with bulk materials. It was reported that Tungsten microcavity structures nanostructures compared with bulk materials. It was reported that Tungsten microcavity structures degrade the melting point to just above 1500 K, which was then held at 1400 K, less than one-half its degrade the melting point to just above 1500 K, which was then held at 1400 K, less than one-half its melting point [32]. In 2015, we fabricated a microcavity lamp utilizing nanoimprint technology and melting point [32]. In 2015, we fabricated a microcavity lamp utilizing nanoimprint technology and demonstrated the enhancement of visible light using a microcavity filament [33,34]. In 2016, Ilic et al. reported an efficient thermal light source that radiated only visible light (cut IR light emission from

Photonics 2019, 6, 105 3 of 20 demonstrated the enhancement of visible light using a microcavity filament [33,34]. In 2016, Ilic et al. Photonics 2019, 6, x FOR PEER REVIEW 3 of 20 reported an efficient thermal light source that radiated only visible light (cut IR light emission from the filament)the filament) by directlyby directly sandwiching sandwiching a dielectric a dielectric multilayer multilayer filter filter [35 ].[35]. However, However, it is it still is still challenging challenging to constructto construct an an effi efficientcient incandescent incandescent light light bulb bulb with with a filamenta filament controlled controlled by by a a refractory refractory metasurface. metasurface. In thisthis paper,paper, we we review review our our recent recent work work on thermalon thermal radiation radiation control control for incandescent for incandescent light bulbslight based-onbulbs based-on refractory refractory metasurfaces. metasurfaces. First, we First, demonstrate we demonstrate the spectral the spectral control ofcontrol visible of light visible using light an opticalusing an microcavity optical microcavity array fabricated array on fabricated a filament insertedon a filament in a light inserted bulb. Here, in a nanoimprintlight bulb. Here, method a isnanoimprint used to mass-produce method is theused filaments. to mass-produce Then, to overcomethe filaments. the drawback Then, to ofovercome a microcavity, the drawback we introduce of a amicrocavity, refractory metasurface we introduce based a refractory on a plasmonic metasurface cavity based made on froma plasmonic hafnium cavity nitride made (HfN), from which hafnium is a newnitride kind (HfN), of refractory which is plasmonica new kind material. of refractory We demonstrate plasmonic mate spectralrial. controlWe demonstrate in the mid spectral IR range control using thisin the new mid refractory IR range metasurface.using this new refractory metasurface.

2. The Efficiency of Thermal Radiation Light Sources 2. The Efficiency of Thermal Radiation Light Sources Figure2 2 shows shows thethe powerpower flowflow for for typical typical commercial commercial light light sources: sources: anan incandescentincandescent lightlight bulbbulb with andand withoutwithout inert inert gases gases (inert (inert gases gases were were not no usedt used in old in incandescentold incandescent lamps), lamps), a fluorescent a fluorescent lamp, andlamp, an and LED an light LED bulb light [ 1bulb,2]. In[1,2]. Figure In Figure2, we show 2, we the show ratio the (percentage) ratio (percentage) of output of output power power to input to electricinput electric power. power. Here, allHere, power all power flow data flow are data taken are fromtaken [ 36from–40 ].[36–40].

Figure 2. The power flowflow ratio (percentage)(percentage) of typicaltypical commercialcommercial lightlight sources:sources: (a) anan incandescentincandescent light bulbbulb withwith inert inert gases gases (100 (100 W) W) [36 [36,37],,37], (b )(b an) an incandescent incandescent light light bulb bulb without without inert inert gases gases (10W) (10W) [38], ([38],c) a fluorescent(c) a fluorescent lamp (40lamp W) (40 [39 W)], and [39], (d and) a light-emitting (d) a light-emitting diode (LED)diode light(LED) bulb light (blue bulb LED (blue+ yellowLED + phosphor)yellow phosphor) [40]. [40].

For an LED light bulb, the conversion ratio from input power to visible light is 30–50% while it is only 10% for an incandescent light bulb with inert gas. The LED light bulb is more efficient than the incandescent light bulb. The energy loss of an LED light bulb is caused by various physical processes such as wavelength-conversion losses, inner absorption, or non-radiative phonon

Photonics 2019, 6, 105 4 of 20

For an LED light bulb, the conversion ratio from input power to visible light is 30–50% while it is only 10% for an incandescent light bulb with inert gas. The LED light bulb is more efficient than the incandescent light bulb. The energy loss of an LED light bulb is caused by various physical processes such as wavelength-conversion losses, inner absorption, or non-radiative phonon excitation, resulting in the dissipation of energy to the environment around the bulb. However, considering the conversion ratio from input power to total electromagnetic radiation, it is higher than 80% for the incandescent light bulb. Additionally, it exceeds 90% (~94%) for an incandescent light bulb without inert gas. This latter result is obtained because the loss of heat conduction from the filament to inert gas is negligible. Thus, we can conclude that a thermal radiation light source is an ideal high-efficiency energy converter from input electric power to output radiative power. If we suppress the IR light and convert it to visible light, incandescent light bulbs can be recreated as an efficient light source.

3. The Basic Principle of Thermal Radiation Control by a Refractory Metasurface There are two ways to suppress IR light from incandescent lamps: (i) use of an optical filter coated on a bulb and (ii) thermal radiation control of a filament. For optical filters, dielectric multilayers are used for short-pass (IR rejection) optical filters. Although short-pass optical filters are used for commercially available halogen light bulbs, the shapes of blubs are limited to elliptical, and the transparency of the bulb is reduced due to coloring of the dielectric multilayers, resulting in a reduction of the beauty of incandescent light bulbs. In contrast, using thermal radiation control, we can modify the thermal radiation spectrum for a filament directly by forming nanostructures on it. In this method, IR light is suppressed, and visible light is enhanced from a filament directly, resulting in a significant improvement in the luminous efficacy. Spectral radiant intensity I (λ,T) [W m 2 m 1 sr 1] of blackbody radiation per area and per solid bb · − · − · − angle at temperature T and wavelength λ is given by Equation (1):

2hc2 1 Ibb(λ, T) = , (1) λ5 ehc/λkBT 1 − where c is the speed of light, h is the Planck constant, and kB is the Boltzmann constant. The thermal radiation spectrum from a real surface can be calculated by the product of Ibb(λ,T) and spectral emissivity ε(λ). Hence, we can control the radiation spectrum artificially by specifying ε(λ). This is the basic principle of thermal radiation control. In a microcavity lamp, ε(λ) can be controlled by a microcavity array formed on the surface of a refractory metal filament. Figure3a shows a schematic view of a cuboidal hole microcavity array. Such a cuboidal hole behaves as an open-end cavity for optical electromagnetic (EM) fields, and they are confined inside the hole. Contrary to the Waymouth hypothesis, previous experimental studies in the IR range demonstrated that a microcavity enhances thermal radiation at specific wavelengths by resonance instead of suppression by the cut-off effect [6,7]. The resonant wavelength of the microcavity (Figure3a) is given by Equation (2):

2 λ = q , (2)  2  n 2  2 nx + y + nz a a 2d where nx, ny = 0, 1, 2, 3 ... are mode numbers of the x- or y- (horizontal) direction, respectively, and nz = 0, 1, 3, 5 ... is the mode number of the z- (vertical) direction, a and d are the width in the x-y direction and the depth of the cuboidal cavity, respectively [5,6]. Photonics 2019, 6, 105 5 of 20 Photonics 2019, 6, x FOR PEER REVIEW 5 of 20

Figure 3. PrinciplePrinciple of of thermal thermal radiation radiation control control by by a ametasurface: metasurface: (a ()a an) an array array of of a amicrocavity microcavity on on a refractorya refractory metal metal filament, filament, (b ()b resonant) resonant modes modes for for nn == 1,1, 3, 3, and and 5 5 inside inside a a microcavity with perfect conductor walls, and ( c)) the thermal radiation spectrum spectrum can can be be controlled by the product of spectral emissivity of the metasurface and Planck’s law. Figure3b shows typical resonant modes in the cavity. In principle, such resonant modes Figure 3b shows typical resonant modes in the cavity. In principle, such resonant modes can can enhance absorption at the resonant wavelengths. According to Kirchhoff’s law, such resonant enhance absorption at the resonant wavelengths. According to Kirchhoff’s law, such resonant absorption increases the emissivity of opaque materials in thermal radiation. On a metallic surface, absorption increases the emissivity of opaque materials in thermal radiation. On a metallic surface, emissivity is very low at non-resonant wavelengths, as shown in Figure3c, resulting in a steep resonant emissivity is very low at non-resonant wavelengths, as shown in Figure 3c, resulting in a steep enhancement of the spectral emissivity ε(λ). Hence, the total radiation spectrum can be controlled resonant enhancement of the spectral emissivity ε(λ). Hence, the total radiation spectrum can be by ε(λ). controlled by ε(λ). 4. Thermal Radiation Control by a Microcavity Array 4. Thermal Radiation Control by a Microcavity Array 4.1. Fabrication by Nanoimprint 4.1. Fabrication by Nanoimprint To build a microcavity lamp, we fabricated microcavity array structures on a refractory metal substrate,To build cut a itmicrocavity into filament lamp, strips we fabricated then inserted microcavity them into array incandescent structures on light a refractory bulbs. Inmetal the substrate,nanofabrication cut it process,into filament we used strips a nanoimprint then inserted method them tointo fabricate incandescent microcavity light arraybulbs. patterns In the nanofabricationlooking forward process, to a mass-production we used a nanoimprint process. The method details to of fabricate the fabrication microcavity process array are shownpatterns in lookingAppendix forwardA. to a mass-production process. The details of the fabrication process are shown in AppendixFigure A.4a shows a photograph of a 20 20 mm polished tantalum (Ta) substrate with a thickness × of 100Figureµm, on 4a whichshows microcavity a photograph structures of a 20 × are 20 formed.mm polished The structures tantalum were(Ta) substrate fabricated with on a a single thickness side ofor 100 both µm, sides on of which the substrate. microcavity Figure structures4b shows are a scanning formed. ion The microscope structures (SIM) were image fabricated of the on structures. a single sideThe patternor both sizessides of of the the mold substrate. are width Figurea = 3004b shows nm, depth a scanningd = ~200 ion nm, microscope and period (SIM)P = 600 image nm .of From the structures.Figure4b it The is confirmed pattern sizes that of a 350-nm-squaredthe mold are width cuboidal a = 300 microcavity nm, depth withd = ~200P = nm,600 and nm period formed P on = 600 the nm.substrate. From TheFigure depth 4b ofit is the confirmed cavity is estimated that a 350-nm-squared to be ~280 nm bycuboidal the slanted microcavity angle of with the SIM P = image600 nm at formed on the substrate. The depth of the cavity is estimated to be ~280 nm by the slanted angle of 30◦. The measured depth is shallower than the designed depth of 500 nm. This is because the depth theis limited SIM image by the at di 30°.fferences The measured in the dry depth etching is shallo rate betweenwer than the the Cr designed mask and depth the Taof substrate.500 nm. This After is becausefabricating the the depth pattern, is limited the substrate by the differences was cut into in strips the dry (length: etching 20 mm,rate between width: 500 theµ m)Cr usingmask aand dicing the Tasaw. substrate. A single After strip wasfabricating placed intothe twopattern, holding the subs stemstrate to form was acut filament into strips by welding (length: into 20 amm, bulb width: made 500 µm) using a dicing saw. A single strip was placed into two holding stems to form a filament by of Pyrex glass (borosilicate glass). Inert gases (75% Ar and 25% N2) were put into the bulb. As a result, welding into a bulb made of Pyrex glass (borosilicate glass). Inert gases (75% Ar and 25% N2) were put into the bulb. As a result, we prepared two types of light bulbs with a structure on both sides and a single side. We also prepared a light bulb with a flat filament for reference.

Photonics 2019, 6, 105 6 of 20

we prepared two types of light bulbs with a structure on both sides and a single side. We also prepared PhotonicsaPhotonics light bulb 20192019,, 6with6,, xx FORFOR a flat PEERPEER filament REVIEWREVIEW for reference. 66 ofof 2020

FigureFigure 4.4. MicrocavityMicrocavityMicrocavity filament:filament: filament: ((aa)) 20-mm-squared (20-mm-squareda) 20-mm-squared TaTa substratesubstrate Ta substrate andand itsits and splitsplit its intointo split stripsstrips into usingusing strips aa using dicing-dicing- a sawdicing-sawsaw processprocess process andand ((bb)) andscanningscanning (b) scanning ionion microscopemicroscope ion microscope (SIM)(SIM) imageimage (SIM) ofof image thethe microcavity.microcavity. of the microcavity. TheThe horizontalhorizontal The horizontal scalescale barbar isscaleis 350350 bar nm.nm. is 350 nm. Figure5a shows a photograph of a microcavity lamp prototype. The fact that rainbow colors are FigureFigure 5a5a showsshows aa photographphotograph ofof aa microcavitymicrocavity lamplamp prototype.prototype. TheThe factfact thatthat rainbowrainbow colorscolors areare seen on the filament shows that periodic structures are formed successfully on the filament. As shown seenseen onon thethe filamentfilament showsshows thatthat periodicperiodic structurstructureses areare formedformed successfullysuccessfully onon thethe filament.filament. AsAs shownshown in Figure5b, the light bulb was set into an E26 socket, and it emitted visible light from the filament by inin FigureFigure 5b,5b, thethe lightlight bulbbulb waswas setset intointo anan E26E26 socket,socket, andand itit emittedemitted visiblevisible lightlight fromfrom thethe filamentfilament connecting an electric power supply. byby connectingconnecting anan electricelectric powerpower supply.supply.

FigureFigure 5. 5. AA prototypeprototype of of the the microcavity microcavity lamp: lamp: (( aa)) turningturning off ooffff andand ( (bb))) on. on.on. 4.2. Measurements 4.2.4.2. MeasurementsMeasurements Measurements of thermal radiation spectra were performed using an integrating sphere for MeasurementsMeasurements ofof thermalthermal radiationradiation spectraspectra werewere performedperformed usingusing anan integratingintegrating spheresphere forfor collecting the total luminous flux. A light bulb with a microcavity filament was set into the integrating collectingcollecting thethe totaltotal luminousluminous flux.flux. AA lightlight bulbbulb withwith aa microcavitymicrocavity filamentfilament waswas setset intointo thethe integratingintegrating sphere (LMS-200, Labsphere, Inc., North Sutton, NH, USA) with a diameter of 25 cm. The radiation spheresphere (LMS-200,(LMS-200, Labsphere,Labsphere, Inc.,Inc., NorthNorth Sutton,Sutton, NH,NH, USA)USA) withwith aa diameterdiameter ofof 2525 cm.cm. TheThe radiationradiation spectra were measured using a fiber multichannel spectrometer (QE65Pro, Ocean Optics, Inc., Largo, spectraspectra werewere measuredmeasured usingusing aa fiberfiber multichannelmultichannel spspectrometerectrometer (QE65Pro,(QE65Pro, OceanOcean Optics,Optics, Inc.,Inc., Largo,Largo, FL, USA) over the wavelength range of 500–1100 nm. A voltage source was used to heat the filament FL,FL, USA)USA) overover thethe wavelengthwavelength rangerange ofof 500–1100500–1100 nmnm.. AA voltagevoltage sourcesource waswas usedused toto heatheat thethe filamentfilament under a constant DC voltage of 1.5 V, where the two-terminal resistance of the light bulb was ~0.1 Ω at underunder aa constantconstant DCDC voltagevoltage ofof 1.51.5 V,V, wherewhere thethe twtwo-terminalo-terminal resistanceresistance ofof thethe lightlight bulbbulb waswas ~0.1~0.1 ΩΩ room temperature. Next, the light bulb with a flat filament (without a microcavity) was measured atat roomroom temperature.temperature. Next,Next, thethe lightlight bulbbulb withwith aa flatflat filamentfilament (without(without aa microcavity)microcavity) waswas measuredmeasured under the same conditions as the reference. All measurements were done under a constant electric underunder thethe samesame conditionsconditions asas thethe reference.reference. AllAll memeasurementsasurements werewere donedone underunder aa constantconstant electricelectric power of 7.9 W. powerpower ofof 7.97.9 W.W. WeWe notenote thatthat thethe conditionsconditions forfor measuringmeasuring thermalthermal radiationradiation spectraspectra shouldshould bebe identicalidentical forfor allall samples.samples. TwoTwo measurementmeasurement conditioconditionsns areare standard:standard: (i)(i) constantconstant temperaturetemperature modemode andand (ii)(ii) constantconstant powerpower mode.mode. InIn constantconstant temperaturetemperature mode,mode, thethe radiationradiation spectraspectra areare measured,measured, maintainingmaintaining thethe samesame filamentfilament temperaturetemperature forfor allall samples.samples. InIn coconstantnstant powerpower mode,mode, radiationradiation spectraspectra areare measuredmeasured

Photonics 2019, 6, 105 7 of 20

We note that the conditions for measuring thermal radiation spectra should be identical for all samples.Photonics 2019 Two, 6, x measurement FOR PEER REVIEW conditions are standard: (i) constant temperature mode and (ii) constant7 of 20 power mode. In constant temperature mode, the radiation spectra are measured, maintaining the samemaintaining filament constant temperature electric for power all samples. to heat In a constant filament. power Since mode,it is difficult radiation to spectradirectly are measure measured the maintainingtemperature constantof a filament electric inside power a light to bulb, heat awe filament. used constant Since itpower is di ffimodecult tohere. directly measure the temperature of a filament inside a light bulb, we used constant power mode here. 4.3. Results and Discussion 4.3. Results and Discussion Figure 6 shows the results of the thermal radiation spectra of the total flux from two light bulbs: a lightFigure bulb6 withshows a two-si the resultsded microcavity of the thermal filament radiation (Φc( spectraλ)) and of one the with total a flux flat fromfilament two ( lightΦF(λ bulbs:)) [33]. aWe light can bulb clearly with see a that two-sided the total microcavity flux of the filamentmicrocav (ityΦc (filamentλ)) and oneis higher with athan flat the filament flat filament. (ΦF(λ)) This [33]. Wesuggests can clearly that the see emissivity that the total of fluxthe microcavity of the microcavity filament filament increases is higher compared than thewith flat the filament. flat filament This suggestsbecause the that temperature the emissivity of both of the filaments microcavity was filamentalmost identical increases du comparede to the use with of thethe flat same filament input becausepower. the temperature of both filaments was almost identical due to the use of the same input power.

Figure 6. Thermal radiation spectra of the total flux from a microcavity surface (solid line) and Figure 6. Thermal radiation spectra of the total flux from a microcavity surface (solid line) and flat flat surface (dotted line). The ratio of total flux (solid red line) is also plotted, representing the surface (dotted line). The ratio of total flux (solid red line) is also plotted, representing the enhancement factor. enhancement factor.

To analyze the enhancement mechanism, we plotted the enhancement factor defined as Φc(λ)/Φf(λ), To analyze the enhancement mechanism, we plotted the enhancement factor defined as as shown in Figure6. In the enhancement factor plot, we observe a single broad peak at ~700 nm. The Φc(λ)/Φf(λ), as shown in Figure 6. In the enhancement factor plot, we observe a single broad peak at enhancement factor physically means relative spectral emissivity, which is defined as the ratio of the ~700 nm. The enhancement factor physically means relative spectral emissivity, which is defined as spectral emissivity of a microcavity surface to that of a flat surface: i.e., εc(λ)/εf(λ), where εc(λ) and the ratio of the spectral emissivity of a microcavity surface to that of a flat surface: i.e., εc(λ)/εf(λ), εf(λ) are spectral emissivities at λ of the microcavity and the flat surface, respectively. where εc(λ) and εf(λ) are spectral emissivities at λ of the microcavity and the flat surface, respectively. 4.4. Simulated Results 4.4. Simulated Results To analyze the enhancement effect quantitatively, we performed numerical calculations on the spectralTo analyze absorptivity the enhancement for the microcavity effect filamentquantitatively versus, we the performed depth of the numerica cavity usingl calculations a commercially on the availablespectral absorptivity numerical simulator for the microcavity and the rigorous filament coupled-wave versus the depth analysis of the (RCWA) cavity methodusing a (Dicommerciallyffract Mod, RSoftavailable Inc.). numerical simulator and the rigorous coupled-wave analysis (RCWA) method (Diffract Mod,Figure RSoft7 Inc.).a shows the spectral map, α(λ,d), of the calculated absorptivity versus the depth, d, of the cavity.Figure We see 7a thatshowsα(λ the,d) hasspectral a peak map, at ~600–900 α(λ,d), of nm the withcalculated an α value absorptivity of 0.9, which versus then the decreasesdepth, d, toof the cavity. We see that α(λ,d) has a peak at ~600–900 nm with an α value of 0.9, which then decreases to ~0.1 at λ > 1.0 µm. These peaks in absorptivity are attributed to the resonant modes inside a single microcavity, and the rapid decrease is due to the cut-off effect in the cavity. However, no peak structure is observed in the measured spectrum, Φc(λ), as shown in Figure 6.

Photonics 2019, 6, 105 8 of 20

~0.1Photonics at λ 2019> 1.0, 6, µx m.FOR These PEER REVIEW peaksin absorptivity are attributed to the resonant modes inside a single8 of 20 microcavity, and the rapid decrease is due to the cut-off effect in the cavity. However, no peak structure is observed in the measured spectrum, Φc(λ), as shown in Figure6.

Figure 7. Simulated spectral maps: (a) spectral absorptivity/emissivity of a Ta microcavity metasurface toFigure the depth7. Simulated of a microcavity spectral maps: with w (a=) 350spectral nm andabsorptivity/emissivityP = 600 nm and of (b )a relative Ta microcavity spectral absorptivitymetasurface/emissivity to the depth for of the a flatmicrocavity surface of with Ta. w = 350 nm and P = 600 nm and (b) relative spectral absorptivity/emissivity for the flat surface of Ta. To compare the simulation to the experimental results, we calculated the relative spectral absorptivity,To compare which isthe equal simulation to the relative to the spectral experimental emissivity, results,εc(λ)/ εwef(λ ),calculated from Kirchho theff ’srelative law. By spectral taking theabsorptivity, ratio of absorptivity which is equal to the to flat the surface, relative we spectral obtain theemissivity, relative spectralεc(λ)/εf( absorptivityλ), from Kirchhoff’s/emissivity law. map By αtaking(λ,d)/α (λthe,0) shownratio inof Figureabsorptivity7b. Note to that the even flat a flat surface, Ta surface we has obtain moderate the broadrelative absorption spectral ofabsorptivity/emissivityα = ~0.5 at λ < 0.6 µm. map We α see(λ, thatd)/α( absorptionλ,0) shown enhancementin Figure 7b. occursNote that at~800 even nm, a flat and Ta the surface relative has absorptivitymoderate broad increases absorption as the of cavity α = ~0.5 depth at λ increases, < 0.6 µm. asWe shown see that in absorption Figure7b. Atenhancement a sample depth occurs of at d~800= 280 nm, nm andin Figurethe relative4b, the absorptivity peak position increases for the as relative the cavity emissivity depth increases, is at ~700–900 as shown nm (Figurein Figure7b). 7b. ThisAt a is sample consistent depth with of thed = 280 peak nm position in Figure of the 4b, experiment the peak position in Figure for6 .the Thus, relative the broad emissivity peak observedis at ~700– for900 the nm relative (Figure emissivity 7b). This is attributedconsistent towith the the microcavity peak position effect. of the experiment in Figure 6. Thus, the broadIf wepeak can observed fabricate for a sutheffi relativeciently deepemissivi microcavityty is attributed with d to> the500 microcavity nm, we expect effect. that the thermal radiationIf we spectrum can fabricate will havea suffic a narroweriently deep resonant microcavity peak atwith ~850 d > nm 500 and nm, its we relative expect absorptivity that the thermal will increaseradiation to spectrum five, as seen will in have Figure a7 narrowerb. However, resonant increasing peak the at cavity~850 nm depth and further its relative is di ffiabsorptivitycult due to thewill limitincrease of the to dry five, etching as seen process in Figure using 7b. refractory However, metals. increasing As a the Ta substratecavity depth is a hardfurther material is difficult compared due to withthe limit Si, the of etching the dry contrast etchingratio process between using the refractory resist mask metals. and As the a Ta Ta substrate substrate is is not a suhardfficient material for deepercompared etching with (see Si, Appendixthe etchingA ).contrast Besides, ratio it is challengingbetween the to resist control mask the and absorptivity the Ta substrate in the visible is not rangesufficient because for typicaldeeper refractoryetching (see metals Appendix such as A). Ta, Beside Mo, ands, it W is are challenging “dielectric” to fromcontrol the the negative absorptivity value ofin the the dielectric, visible range resulting because in absorption typical refractory in the visible metals spectrum such as Ta, (see Mo, Appendix and WB are)[ 41“dielectric”]. Actually, from even the a flatnegative Ta surface value (d of= the0) hasdielectric, an absorption resulting of inα absorpti= ~0.5 atonλ in= the0.4–0.7 visibleµm, spectrum as shown (see in Appendix Figure7a. B) This [41]. meansActually, that even these a metalsflat Ta aresurface far from (d = 0) perfect has an conductors absorption in of the α visible= ~0.5 at range. λ = 0.4–0.7 Hence, µm, a new as shown kind of in metasurfaceFigure 7a. This is needed means beyond that these the performancemetals are far of from a microcavity perfect conductors array to enhance in the visible further range. the emissivity Hence, a innew the kind visible of metasurface spectrum. Plasmonic is needed materialsbeyond the and perf itsormance metasurfaces of a microcavity are needed array beyond to enhance conventional further refractorythe emissivity metals in to the control visible the spectrum. thermal radiationPlasmonic spectra materials in the and visible its metasurfaces range. are needed beyond conventional refractory metals to control the thermal radiation spectra in the visible range. 5. Thermal Radiation Control by a Refractory Plasmonic Metasurface 5. Thermal Radiation Control by a Refractory Plasmonic Metasurface 5.1. Thermal Radiation Control by Plasmonic Cavities 5.1. AsThermal described Radiation in Section Control4, weby Plasmonic achieved Cavities thermal radiation control using a microcavity filament in the visibleAs described range. As in aSection next step, 4, we we achieved propose thermal a new kind radiation of filament control using using a a plasmonic microcavity metasurface, filament in asthe illustrated visible range. in Figure As a next8. Figure step, we8 shows propose the a conceptnew kind of of a filament plasmonic using metasurface a plasmonic where metasurface, thick microcavitiesas illustrated on in the Figure refractory 8. Figure metal 8 are shows replaced the byconcept very thin of MDMa plasmonic plasmonic metasurface cavities. Awhere plasmonic thick microcavities on the refractory metal are replaced by very thin MDM plasmonic cavities. A plasmonic resonator is very thin (<<λ) compared with the wavelength while a microcavity needs a deep trench structure on the order of the controlled optical wavelength (~λ). Since the thickness of the metasurface

Photonics 2019, 6, 105 9 of 20 resonator is very thin (<<λ) compared with the wavelength while a microcavity needs a deep trench structurePhotonics 2019 on, 6 the, x FOR order PEER of theREVIEW controlled optical wavelength (~λ). Since the thickness of the metasurface9 of 20 is much smaller than the wavelength, the heat capacity is small and is compatible with a planar fabricationis much smaller process. than However, the wavelength, the melting the point heat of capa conventionalcity is small plasmonic and is compatible metals such with as Ag a and planar Au arefabrication not high process. enough However, for thermal the radiation melting control point of in theconventional visible spectrum. plasmonic metals such as Ag and Au are not high enough for thermal radiation control in the visible spectrum.

Figure 8. Refractory metasurface (a) from a microcavity array to (b) a plasmonic cavity array. Figure 8. Refractory metasurface (a) from a microcavity array to (b) a plasmonic cavity array. In recent years, nitride ceramics such as titanium nitride (TiN) have been proposed and studied as newIn plasmonic recent years, materials nitride operating ceramicsat such higher as titanium temperatures nitride ( T(TiN> 1500) have K been [42–45 proposed]). Melting and points studied of typicalas new plasmonic materials operating and nitride at ceramicshigher temperatures are summarized (T > 1500 in Table K [42–45]).1. The melting Melting points points of of thesetypical materials plasmonic are materials similar to and conventional nitride ceramics refractory are metals, summarized and the in permittivity Table 1. The of melting these materials points of is negativethese materials in the visibleare similar range. to Hence,conventional those arerefractory called “refractory metals, and plasmonic the permittivity materials.” of these materials is negative in the visible range. Hence, those are called “refractory plasmonic materials.” Table 1. Refractory metals and refractory plasmonic materials in order of its melting point. Table 1. Refractory metals and refractory plasmonic materials in order of its melting point. Material Melting Point (K) Permittivity in Visible Range Material Melting Point (K) Permittivity in Visible Range AgAg 1235 1235 ND ND AuAu 1337 1337 ND ND SiO2 SiO2 19831983 D D MoMo 2896 2896 D D HfO2 HfO2 30313031 D D TiNTiN 3203 3203 ND ND TaTa 3290 3290 D/ND D/ND HfNHfN 3607 3607 ND ND WW 3695 3695 D D 1 ND:ND: Negative Negative Dielectric;Dielectric; D: D: Dielectric. Dielectric. In this study, we used hafnium nitride (HfN) since the melting point of HfN is higher than that of TiNIn thisand study, it is the we usedsame hafnium order as nitride W. The (HfN) most since crucial the melting property point of ofnitride HfN isceramics higher than is that that its of TiNpermittivity and it is theis negative same order in asthe W. visible The most range crucial like property the noble of nitridemetals. ceramics Such a isfeature that its is permittivity useful for isplasmonic negative materials. in the visible The range spectral like permittivity the noble metals. of conventional Such a feature and isplasmonic useful for refractory plasmonic metals materials. are Theshown spectral in Appendix permittivity C. If of we conventional realize plasmonic and plasmonic metasurfaces refractory using metals plasmonic are shown refractory in Appendix materialsC. Ifinstead we realize of noble plasmonic metals, metasurfaceswe can control using the thermal plasmonic radiation refractory spectra materials more precisely instead of and noble obtain metals, higher we canQ-value control of the the plasmonic thermal radiation cavity than spectra that more of the precisely microcavity. and obtain higher Q-value of the plasmonic cavityFigure than that9 shows of the a microcavity.schematic of a cross-sectional view of a refractory MDM metasurface, where the Fabry–PérotFigure9 shows (FP) a plasmonic schematic ofresonator a cross-sectional disk type viewbased of on a refractoryHfN are arranged MDM metasurface, in a periodic where array. the Fabry–Pérot (FP) plasmonic resonator disk type based on HfN are arranged in a periodic array. The diameter d, the period P of the resonator, the gap thickness in the dielectric layer, Tg, and the top The diameter d, the period P of the resonator, the gap thickness in the dielectric layer, Tg, and the top metal layer (HfN) Td are shown in Figure 9. We note that the dielectric layer should be selected in metal layer (HfN) T are shown in Figure9. We note that the dielectric layer should be selected in accordance with the doperating temperature T as such that HfO2 for T > 2000 K or SiO2 for T < 2000 K. accordance with the operating temperature T as such that HfO2 for T > 2000 K or SiO2 for T < 2000 K.

Photonics 2019, 6, x FOR PEER REVIEW 10 of 20 Photonics 2019, 6, 105 10 of 20

Photonics 2019, 6, x FOR PEER REVIEW 10 of 20

Figure 9. A schematic and cross-sectional view of a metal-dielectric-metal (MDM) metasurface based

on hafnium nitride (HfN). Figure 9. A schematic and cross-sectional view of a metal-dielectric-metal (MDM) metasurface based Figure 9. A schematic and cross-sectional view of a metal-dielectric-metal (MDM) metasurface based on hafnium nitride (HfN). Toon confirmhafnium nitridethe efficiency (HfN). of thermal radiation control by this refractory plasmonic metasurface, we Tocalculated confirm the etheoreticalfficiency of radiation thermal radiation spectrum control of the by MDM this refractory metasurface plasmonic and compared metasurface, it to we a To confirm the efficiency of thermal radiation control by this refractory plasmonic metasurface, calculatedblackbody the surface theoretical under radiation the condition spectrum that ofboth the radiation MDM metasurface powers are and identical, compared i.e., ita constant to a blackbody power we calculated the theoretical radiation spectrum of the MDM metasurface and compared it to a surfacemode as under described the condition in Section that 4.2. both Figure radiation 10 show powerss the are simulated identical, results i.e., a constantfor the thermal power moderadiation as blackbody surface under the condition that both radiation powers are identical, i.e., a constant power describedspectrum inobtained Section from 4.2. Figurethe metasurface 10 shows composed the simulated of HfN results and for HfO the2 at thermal T = 2500 radiation K (red line) spectrum with d mode as described in Section 4.2. Figure 10 shows the simulated results for the thermal radiation obtained= 40 nm, fromP = 80 the nm, metasurface Tg = 60 nm, composed and Td = of20 HfNnm. andHere, HfO we2 observeat T = 2500 that K the (red highest line) withpowerd = radiated40 nm, spectrum obtained from the metasurface composed of HfN and HfO2 at T = 2500 K (red line) with d Pfrom= 80 the nm ,metasurfaceTg = 60 nm, is and focusedTd = 20on nm. the Here,resonant we pe observeak at ~700 that thenm highestwith a full-width power radiated half-maximum from the = 40 nm, P = 80 nm, Tg = 60 nm, and Td = 20 nm. Here, we observe that the highest power radiated metasurface(FWHM) value is focused of 571 nm on thedue resonant to the plasmonic peak at ~700 reso nmnance with in aan full-width FP resonator half-maximum disk. From Figure (FWHM) 10, from the metasurface is focused on the resonant peak at ~700 nm with a full-width half-maximum valuethe equivalent of 571 nm power due to thefrom plasmonic the metasurface resonance at T in = an 2500 FP resonatorK corresponds disk. Fromto the Figurepower 10 from, the a equivalent blackbody (FWHM) value of 571 nm due to the plasmonic resonance in an FP resonator disk. From Figure 10, powerat T = from1777 theK. This metasurface means that at T radiated= 2500 K power corresponds from the to metasurface the power from at T a = blackbody 2500 K equals at T =that1777 from K. the equivalent power from the metasurface at T = 2500 K corresponds to the power from a blackbody Thisthe blackbody means that at radiated only T = power 1777 K. from According the metasurface to the Stefan–Boltzmann at T = 2500 K equals law, the that efficiency from the is blackbody improved at T = 1777 K. This means that radiated power from the metasurface at T = 2500 K equals that from atby only a factorT = 1777 of (2500/1700) K. According4 = 3.9; to thei.e., Stefan–Boltzmann the metasurface is law, 3.9 thetimes effi moreciency efficient is improved than the by ablackbody factor of the blackbody at only T = 1777 K. According to the Stefan–Boltzmann law, the efficiency is improved (2500from/1700) the viewpoint4 = 3.9; i.e., of the power metasurface consumption. is 3.9 times Additionally, more efficient from than Figure the blackbody 10, the radiation from the intensity viewpoint at by a factor of (2500/1700)4 = 3.9; i.e., the metasurface is 3.9 times more efficient than the blackbody ofthe power plasmonic consumption. resonant Additionally, wavelength fromis more Figure than 10 10, the times radiation greater intensity than that at theof the plasmonic blackbody resonant at T = from the viewpoint of power consumption. Additionally, from Figure 10, the radiation intensity at wavelength1777 K. is more than 10 times greater than that of the blackbody at T = 1777 K. the plasmonic resonant wavelength is more than 10 times greater than that of the blackbody at T = 1777 K.

FigureFigure 10. 10.Simulated Simulated thermal thermal radiation radiation spectra spectra in in constant constant power power mode: mode: radiation radiation spectra spectra from from the the MDM metasurface (red line) composed of HfN and HfO at T = 2500 K with d = 40 nm, P = 80 nm, MDM metasurface (red line) composed of HfN and HfO22 at T = 2500 K with d = 40 nm, P = 80 nm, Tg T Figure= 60 nm, 10. TSimulated= 20 nm, thermal and the radiation reference spectra blackbody in consta (bluent line) power at T =mode:1777 radiation K. spectra from the =g 60 nm, Td =d 20 nm, and the reference blackbody (blue line) at T = 1777 K. MDM metasurface (red line) composed of HfN and HfO2 at T = 2500 K with d = 40 nm, P = 80 nm, Tg 5.2. Fabrication 5.2. Fabrication= 60 nm, T d = 20 nm, and the reference blackbody (blue line) at T = 1777 K. To demonstrate the thermal radiation control by a refractory plasmonic metasurface, we fabricated MDM5.2. FabricationTo metasurfaces demonstrate based the on thermal HfN. In radiation this study, control we designed by a therefractory metasurface plasmonic to be a metasurface, perfect absorber we infabricated the mid-IR MDM range metasurfaces (~4 µm) instead based of on in HfN. the visibleIn thisas study, the first we stepdesigned towards the themetasurface fabrication to ofbe aa perfectTo absorber demonstrate in the the mid-IR thermal range radiation (~4 µm) controlinstead ofby ina therefractory visible asplasmonic the first stepmetasurface, towards thewe fabricated MDM metasurfaces based on HfN. In this study, we designed the metasurface to be a

perfect absorber in the mid-IR range (~4 µm) instead of in the visible as the first step towards the

PhotonicsPhotonics2019 2019, ,6 6,, 105 x FOR PEER REVIEW 11 ofof 2020

fabrication of a “plasmonic” thermal radiation light source. To design and optimize the size “plasmonic”parameters for thermal a perfect radiation absorber light operating source. Toat ~4 design µm, we and performed optimize thenumerical size parameters simulations for ausing perfect the absorbercommercially operating available at ~4 µfinite-differencem, we performed time-domain numerical simulations method (FDTD) using thesoftware commercially (Lumerical available Inc., finite-diVancouver,fference BC, time-domainCanada) for the method metasurface (FDTD) softwarecomposed (Lumerical of HfN and Inc., SiO Vancouver,2 as shown BC, in Appendix Canada) for F. theFrom metasurface Figure A6, composed the designed of HfN value and SiOof diameter2 as shown d in= Appendix1.2 µm wasF. Fromdetermined Figure A6for, theachieving designed the valueabsorption of diameter peak ofd =4 µm.1.2 µ m was determined for achieving the absorption peak of 4 µm. FigureFigure 11 11 shows shows thethe MDMMDM metasurfacemetasurface samplesample withwith d = 1.141.14 µm,µm, PP == 2.02.0 µm,µm, TTg g= =130130 nm, nm, and and Td T= 200= 200 nm. nm. The The metasurface metasurface was was fabricated fabricated on on a a15 15 × 1515 mm mm square quartz substratesubstrate usingusing RFRF d × sputteringsputtering andand electronelectron beam (EB) lithography. lithography. The The details details of of the the fabrication fabrication process process are are described described in inAppendix Appendix D.D The. The SEM SEM image image of of meta-atoms meta-atoms is is shown shown in in Figure Figure 11c.11c. Additionally, Additionally, we we fabricated fabricated a ablackbody blackbody reference reference sample sample by by spraying spraying a a blac blackbodykbody spray spray (TA410KS, Ichinen TASCO Co.,Co., Ltd.,Ltd., Osaka,Osaka, Japan)Japan) toto thethe 1515 × 1515 mmmm squaresquare quartzquartz substrate,substrate, asas shownshown inin FigureFigure 11 11a.a. ThisThis blackbodyblackbody × referencereference has has an an average average absorptivity absorptivity of ofα α~0.989 ~0.989 atatλ λ= = 3–103–10 µµm.m.

FigureFigure 11.11. RefractoryRefractory MDMMDM metasurfacemetasurface andand blackbodyblackbody reference:reference: ((aa)) aa photographphotograph ofof thethe blackbodyblackbody 2 referencereference sample, sample, ( b(b)) the the MDM MDM metasurface metasurface sample sample composed composed of of HfN HfN and and SiO SiO2 on ona aquartz quartzsubstrate substrate withwithd d= = 1.141.14 µµm,m, PP == 2.0 µµm,m, TTg == 130130 nm, nm, and and TTd = =200200 nm. nm. The The patterned patterned area area is is10 10 × 10 10mm, mm, and and (c) d × (cSEM) SEM image image of ofplasmonic plasmonic resonators. resonators.

5.3.5.3. MeasurementsMeasurements BeforeBefore wewe measuredmeasured thethe thermalthermal radiation,radiation, wewe measuredmeasured thethe spectralspectral reflectivityreflectivity RR((λλ)) ofof thethe metasurfacemetasurface atat λλ == 3–123–12 µµmm usingusing aa confocalconfocal infraredinfrared microscopemicroscope (HYPERION2000,(HYPERION2000, BrukerBruker Inc.,Inc., Billerica,Billerica, MA,MA, USA)USA) andand aa FourierFourier transformtransform infraredinfrared (FTIR)(FTIR) spectrometerspectrometer (VERTEX 70v,70v, BrukerBruker Inc.,Inc., Billerica,Billerica, MA,MA, USA)USA) atat roomroom temperature.temperature. IRIR lightlight partiallypartially shieldedshielded byby slitsslits waswas focusedfocused onon aa samplesample through an 15 (NA: 0.4) Schwarzschild objective lens. The reflected light from the sample was through an ××15 (NA: 0.4) Schwarzschild objective lens. The reflected light from the sample was correctedcorrected through through the the objective objective using using a detector a detector and converted and converted through through a Fourier a transformFourier transform to calculate to reflectivity. Spectral absorptivity, A(λ), can be calculated by A(λ) = 1 R(λ) if the sample is opaque. calculate reflectivity. Spectral absorptivity, A(λ), can be calculated by− A(λ) = 1 − R(λ) if the sample is opaque.The thermal radiation spectra were measured using an FTIR spectrometer (FT/IR 6000, JASCO Co., Tokyo,The thermal Japan) radiation at λ = 3–12 spectraµm. were The measured setup of the using thermal an FTIR radiation spectrometer measurement (FT/IR 6000, is shown JASCO in AppendixCo., Tokyo,E. ToJapan) avoid at oxidation λ = 3–12 of µm. the surface,The setup the of sample the therma was setl radiation in a vacuum measurement chamber connected is shown to in theAppendix FTIR spectrometer E. To avoid andoxidation heated of on the a ceramicsurface, heater.the sample A DC was power set in supply a vacuum was usedchamber to control connected the temperature.to the FTIR spectrometer The temperature and washeated measured on a cerami by ac thermocouple heater. A DC placed power on supply the surface was used of the to sample. control Thethe radiationtemperature. spectra The weretemperature measured was for measured both the metasurfaceby a thermocouple and the placed blackbody on the sample surface at 573of the K. Hence,sample. all The measurements radiation spectra were were done measured under the constantfor both the temperature metasurface of 573and K. the blackbody sample at 573 K. Hence, all measurements were done under the constant temperature of 573 K. 5.4. Results and Discussion 5.4. ResultsThe measured and Discussion absorptivity spectrum of the MDM metasurface at room temperature is shown µ in FigureThe 12measureda. We observed absorptivity a single spectrum resonant of the peak MDM at 4.11 metasurfacem. To identify at room the temperature physical origin is shown of the in peak,Figure we 12a. calculated We observed the spectral a single absorptivity resonant peak (see at the 4.11 dashed µm. To line identify in Figure the physical12a) and origin field distribution of the peak, bywe the calculated FDTD method. the spectral The absorptivity measured spectrum (see the isdash in gooded line agreement in Figure with12a) theand simulatedfield distribution spectrum. by Figurethe FDTD12bshows method. a cross-sectional The measured view spectrum of the spatialis in good distribution agreement of thewith electric the simulated field normal spectrum. to the Figure 12b shows a cross-sectional view of the spatial distribution of the electric field normal to the

Photonics 2019, 6, 105 12 of 20 Photonics 2019, 6, x FOR PEER REVIEW 12 of 20 incident electric field field at 4.11 µµmm around a meta-atommeta-atom (plasmonic cavity). Here, we can confirm confirm that the gap plasmon is excited to anan FPFP resonantresonant modemode betweenbetween twotwo metalmetal layers.layers. Hence, the peak in absorptivity around 4 4 µmµm is attributed to to the pl plasmonicasmonic resonance resonance inside inside a a single single plasmonic plasmonic cavity. cavity. Note that the resonant peak position is robust againstagainst incident angle for bothboth p- and s-polarizations as shown in Appendix GG.. TheThe measuredmeasured FWHMFWHM ofof thethe peakpeak (~2(~2 µm)µm) is higher than the simulated value (~1.5 µm)µm) while the peak position and the pe peakak value are red-shifted slightly and and decreased, respectively. The The difference difference in in the FWHM is attributed to the unexpected loss increase in the real materials. The difference difference in the peak value is prob probablyably due to to the the off-axial off-axial arrangement arrangement of the incident incident light through the Schwarzschild objective lens lens of the infrared microscope. From From this this measurement, measurement, we were able to confirmconfirm that the sample was correctly fabricated and operating as designed for a perfect absorber.

Figure 12. Absorptivity spectra and electric field field distri distributionbution of the MDM metasurface composed of

HfN and SiO 2 withwith PP == 2.02.0 µm,µm, d d= =1.141.14 µm,µm, TgT =g 130= 130 nm, nm, and and Td =T 200d = nm:200 nm:(a) measured (a) measured (solid (solid line) line)and simulatedand simulated (dashed (dashed line) line) absorptivity absorptivity spectra spectra at atroom room temperature, temperature, and and (b (b) )normalized normalized electric electric field field distribution around the meta-atom for the resonance at 4.11 µµm.m.

Next, we performed a thermal radiation experiment.experiment. Figure Figure 13a13a shows the thermal radiation spectra at 573 K for thethe MDMMDM metasurfacemetasurface and referencereference blackbodyblackbody sample.sample. We We observe that the µ radiation intensity is suppressed at λ > 5 µmm compared with the blackbody level. Such suppression µ is caused by the lower absorptivity (emissivity) at λ > 55 µm,m, as seen in Figure 12a.12a. Additionally, we can derive the spectral emissivity, ε (λ), of the metasurface from Figu Figurere 13a.13a. From From Kirchhoff’s Kirchhoff’s law, law, this must be equal to α (λ) shown in Figure 12a12a if the temperature of a sample is identical. Figure 13b13b shows the measured spectral emissivity at 573 K of the MDM metasurface. The resonant peak value µ µ of ε = ~1 at at 4.1 4.1 µmm with FWHM of ~3 µmm is obtained fromfrom Figure 1313a.a. This is consistent with the simulated results for absorptivity in Figure 12a12a (see also the solid line in FigureFigure 1313b).b). These results suggest that perfect absorption/emission absorption/emission occurred as designed, and the cavity loss was increased due to the temperature increase. The The FWHM FWHM of of the the measur measureded peak actually is broader than the calculated µ result of ~1.5 µm.m. This This is is evidence evidence of of the the loss loss increase increase caused by the thermal effect. effect. Finally, we note that the measured radiation spectrum in Figure 13a is not an intrinsic radiation spectrum, but it includes the transmission function of the optical system in the spectrometer (see AppendixE). Hence, it is necessary to separate it out so we can estimate the intrinsic radiation spectrum from the sample. Since we obtained ε (λ), as shown in Figure 13b, we can determine the intrinsic radiation spectrum of the sample by calculating the product of ε (λ) and Planck’s law (Equation (1)). Figure 14 shows the presumed spectrum of the intrinsic radiation as well as the blackbody radiation at 573 K. It is clearly observed that thermal radiation from the metasurface is significantly suppressed at longer wavelength region at λ > 5 µm while the surface temperature of the sample is 573 K. Here, we

Photonics 2019, 6, x FOR PEER REVIEW 13 of 20

Photonics 2019, 6, 105 13 of 20 point out a crucial fact that the area under the spectral curve of the metasurface is much smaller than that of the blackbody. This indicates that the radiative power from the metasurface is significantly suppressed compared with the blackbody resulting in the achievement of an efficient IR emitter, i.e., we are able to heat a sample quite efficiently by a small amount of power. Such behavior is typical for constant-temperature-modePhotonics 2019, 6, x FOR PEER REVIEW measurements, which is different from the constant power mode. 13 of 20 Figure 13. Experimental thermal radiation spectra for the MDM metasurface: (a) thermal radiation spectra for the MDM metasurface (red line) and blackbody reference sample (solid line) at 573 K. (b) Experimental spectral emissivity at 573 K (red line) derived from (a) and simulated absorptivity at room temperature (solid line).

Finally, we note that the measured radiation spectrum in Figure 13a is not an intrinsic radiation spectrum, but it includes the transmission function of the optical system in the spectrometer (see Appendix E). Hence, it is necessary to separate it out so we can estimate the intrinsic radiation spectrum from the sample. Since we obtained ε (λ), as shown in Figure 13b, we can determine the intrinsic radiation spectrum of the sample by calculating the product of ε (λ) and Planck’s law (Equation (1)). Figure 14 shows the presumed spectrum of the intrinsic radiation as well as the blackbody radiation at 573 K. It is clearly observed that thermal radiation from the metasurface is significantly suppressed at longer wavelength region at λ > 5 µm while the surface temperature of the sample is 573 K. Here, we point out a crucial fact that the area under the spectral curve of the metasurface is much smaller than that of the blackbody. This indicates that the radiative power from the metasurface is significantly suppressed compared with the blackbody resulting in the achievementFigureFigure 13.13. of ExperimentalExperimental an efficient thermal thermalIR emitter, radiationradiation i.e., wespectraspectra are forablefor thethe to MDM MDMheat metasurface:ametasurface: sample quite ((aa)) thermalefficientlythermal radiationradiation by a small amountspectraspectra of power. forfor the MDM MDMSuch metasurfacebehavior metasurface is (redtypical (red line) line) for and and constant-temperature-mode blac blackbodykbody reference reference sample sample (solid measurements, (solid line) line) at 573 at 573 K. which ( K.b) is (b) Experimental spectral emissivity at 573 K (red line) derived from (a) and simulated absorptivity at differentExperimental from the spectral constant emissivity power mode. at 573 K (red line) derived from (a) and simulated absorptivity at roomroom temperaturetemperature (solid(solid line).line).

Finally, we note that the measured radiation spectrum in Figure 13a is not an intrinsic radiation spectrum, but it includes the transmission function of the optical system in the spectrometer (see Appendix E). Hence, it is necessary to separate it out so we can estimate the intrinsic radiation spectrum from the sample. Since we obtained ε (λ), as shown in Figure 13b, we can determine the intrinsic radiation spectrum of the sample by calculating the product of ε (λ) and Planck’s law (Equation (1)). Figure 14 shows the presumed spectrum of the intrinsic radiation as well as the blackbody radiation at 573 K. It is clearly observed that thermal radiation from the metasurface is significantly suppressed at longer wavelength region at λ > 5 µm while the surface temperature of the sample is 573 K. Here, we point out a crucial fact that the area under the spectral curve of the metasurface is much smaller than that of the blackbody. This indicates that the radiative power from the metasurface is significantly suppressed compared with the blackbody resulting in the achievement of an efficient IR emitter, i.e., we are able to heat a sample quite efficiently by a small amount of power. Such behavior is typical for constant-temperature-mode measurements, which is different from the constant power mode. FigureFigure 14.14. CalculatedCalculated thermal radiation spectra at 573573 K:K: TheThe radiationradiation spectrumspectrum fromfrom thethe MDMMDM metasurfacemetasurface (red(red line)line) isis calculatedcalculatedfrom fromthe themeasured measuredemissivity emissivity shown shown in in FigureFigure 13 13b.b. TheThe theoreticaltheoretical blackbodyblackbody radiationradiation spectrumspectrum (Equation(Equation (1))(1)) atat573 573K K(solid (solidline) line) is is plotted plotted for for reference. reference. 6. Conclusions

We fabricated a prototype of microcavity lamp by a nanoimprint method that is suitable for mass production and demonstrated to control visible-light spectrum using a refractory metasurface made of Ta with an optical microcavity implemented into an incandescent light bulb. It was confirmed that thermal radiation intensity from the microcavity filament was increased 1.8 times compared to the flat filament under the constant power input. Then, we fabricated and demonstrated the thermal radiation control in mid-IR range by using an MDM plasmonic metasurface composed of a refractory plasmonic cavity made of HfN. A single narrow resonant peak was observed at designed wavelength

Figure 14. Calculated thermal radiation spectra at 573 K: The radiation spectrum from the MDM metasurface (red line) is calculated from the measured emissivity shown in Figure 13b. The theoretical blackbody radiation spectrum (Equation (1)) at 573 K (solid line) is plotted for reference.

Photonics 2019, 6, 105 14 of 20 as well as the suppression of thermal radiation in wide mid-IR range under the condition of constant surface temperature. We revaluated a thermal radiation light source as an efficient light source from the perspective of energy conversion. For a future energy-saving society, it is vital to reconsider thermal radiation sources as energy-saving technology.

Author Contributions: J.T. conceived the idea of incandescent light bulbs based on refractory metasurface. H.T. and A.K. performed the numerical simulations. H.T. and K.K. performed the experiments. J.T. analyzed the experimental data and wrote the initial draft of the manuscript. J.T. supervised the project. All the authors discussed the results and contributed to the writing of the manuscript. Funding: This research was funded in part by the Photonics Advanced Research Center (PARC) from the Ministry of Education, Culture, Sports, Science and Technology, Japan (MEXT) and the JSPS Core-to-Core Program, and A. Advanced Research Networks (Advanced Nanophotonics in the Emerging Fields of Nano-imaging, Spectroscopy, Nonlinear Optics, Plasmonics/Metamaterials, and Devices). Acknowledgments: We would like to thank Yosuke Ueba and Yusuke Nagasaki for useful discussions. A part of this work was supported by the “Nanotechnology Platform Project (Nanotechnology Open Facilities in Osaka University)” of the Ministry of Education, Culture, Sports, Science, and Technology, Japan [No.: F-17-OS-0011, S-17-OS-0011]. Conflicts of Interest: The authors declare no conflicts of interest. The funders had no role in the design of the study, in the collection, analyses, or interpretation of data; in the writing of the manuscript, or in the decision to publish the results.

Appendix A The microcavity filaments were fabricated by a nanoimprint process suitable for mass production as described below. Figure A1 shows the fabrication process of a microcavity array onto a Ta substrate. At first, a 3-inch Si wafer (thickness 380 µm) was prepared, and the electron beam (EB) resist (ZEP520A-7) spin-coated to a thickness of 300 nm using a spinner for 60 s at 4000 rpm. It was then baked for 5 min at 180 C. After that, a 20 20 mm square microcavity array pattern was drawn onto the EB resist using a ◦ × 50-kV electron beam (EB) lithography system (F5112+VD01; ADVANTEST Co., Tokyo, Japan). After developing the resist, the Si wafer was dry-etched by an inductively coupled plasma (ICP) etching machine (EIS-700, ELIONIX Inc., Tokyo, Japan) to transfer the pattern to the substrate, resulting in a Si master mold with 300 300 nm square holes, 300 nm wall width, and approximately 200 nm × depth. Then, the master mold was duplicated onto a photocrosslinkable resin film under heat and high-pressure conditions, resulting in an intermediate resin membrane (IRM). These IRMs are used for mass production in the future so the master mold can be preserved. Next, we prepared polished Ta substrates (20 20 mm squares with a thickness of 100 µm) for × making filaments. A 30-nm-thickness Cr layer was deposited on the Ta substrate, and a photoresist (MUR-XR2-150, Maruzen Petrochemical Co., Ltd., Tokyo, Japan) was spin-coated on the substrate to a thickness of 215 nm by using a spinner at 3000 rpm. Then, the sample was placed in a vacuum chamber, and the IRM placed on a quartz cylinder was pressed onto the resist under UV light irradiation, resulting in stamping the pattern onto the resist residing on the substrate with the Cr layer. Since the IRM and the cylinder are transparent, it can be used as a template to transfer a pattern to a photoresist under UV irradiation. After removing the IRM, the Ta substrate was dry-etched through the resist and the Cr mask film using an electron cyclotron resonance (ECR) ion shower machine (EIS-200ER, ELIONIX Inc., Tokyo, Japan) and ICP etching machine (EIS-700, ELIONIX Inc., Tokyo, Japan). The etching depths are 3.2 nm for the Cr layer and 18 nm for the Ta substrate. Finally, the resist pattern was transferred to the Ta substrate, as shown in Figure4b. The resulting cavity size was wider than the mold (350 350 nm × square holes, 250-nm wall width, and ~280-nm depth). Note that such a widening effect was caused by the dry-etching process and was calibrated by designing the EB lithography process. We fabricated three kinds of Ta substrate: (i) the microcavity on a single side, (ii) the microcavity on both sides, and (iii) a plane without patterning for reference. In the case of (i), we performed the Photonics 2019, 6, x FOR PEER REVIEW 15 of 20

After removing the IRM, the Ta substrate was dry-etched through the resist and the Cr mask film using an electron cyclotron resonance (ECR) ion shower machine (EIS-200ER, ELIONIX Inc., Tokyo, Japan) and ICP etching machine (EIS-700, ELIONIX Inc., Tokyo, Japan). The etching depths are 3.2 nm for the Cr layer and 18 nm for the Ta substrate. Finally, the resist pattern was transferred to the Ta substrate, as shown in Figure 4b. The resulting cavity size was wider than the mold (350 × Photonics3502019 nm, 6 ,square 105 holes, 250-nm wall width, and ~280-nm depth). Note that such a widening effect was15 of 20 caused by the dry-etching process and was calibrated by designing the EB lithography process. We fabricated three kinds of Ta substrate: (i) the microcavity on a single side, (ii) the microcavity processon once. both sides, For (ii), and we (iii) repeated a plane without the process patterning twice. for Finally, reference. the In Ta the substrates case of (i), were we performed cut into narrowthe 500-µmprocess strips once. as shown For (ii), in we Figure repeated4a. the process twice. Finally, the Ta substrates were cut into narrow 500-µm strips as shown in Figure 4a.

FigureFigure A1. A1.Nanoimprint Nanoimprint process process for for fabricating fabricating microcavity filaments. filaments.

AppendixAppendix B B FigureFigure A2 shows A2 shows the relativethe relative permittivity permittivity spectra spectra ofof conventional refractory refractory metals metals (W, (W,Ta, and Ta, and Mo) [41Mo)]. [41]. The The real real part part of of thethe permittivity for for W and W andMo are Mo both are positive both positive in the visible in the range visible (λ < 0.8 range (λ < 0.8µm).µm The). The real real part part of the of thepermittivity permittivity of Ta ofswitches Ta switches from negative from negative to positive to positive below 0.6 below µm. The 0.6 µm. imaginary part of the permittivity of Ta is ~1/2 that of W and Mo. PhotonicsThe imaginary 2019, 6, x partFOR PEER of the REVIEW permittivity of Ta is ~1/2 that of W and Mo. 16 of 20

Figure A2. Spectral relative permittivities of conventional refractory metals (W, Ta, and Mo): (a) real Figure A2. Spectral relative permittivities of conventional refractory metals (W, Ta, and Mo): (a) real and (b) imaginary part of the relative permittivity [41]. and (b) imaginary part of the relative permittivity [41].

Appendix C Figure A3 shows the relative permittivity spectra of conventional plasmonic metals (Au, Ag, and Al) [41], and refractory plasmonic materials (TiN and HfN) [43]. The real part of the permittivity of HfN is negative within the entire visible range and is very close to that of Au at λ < 0.6 µm. The imaginary part of the permittivity of HfN is ~1/2 that of TiN in the visible range.

Figure A3. Spectral relative permittivity of conventional plasmonic metals (Au, Ag, and Al) [41] and refractory plasmonic materials (TiN and HfN) [43]: (a) real and (b) imaginary part of the relative permittivities.

Appendix D Figure A4 shows the fabrication process for the refractory MDM metasurface. First, a HfN layer and a 130-nm-thick SiO2 layer were deposited on a quartz substrate (15 × 15 mm) using an RF sputtering system (SVC-700LRF, SANYU Electron, Tokyo, Japan). In fabricating the HfN layer, we used an HfN target (Toshima Manufacturing Co., Ltd., Saitama, Japan) under an Ar gas flow rate of 25 sccm at 2 × 10−4 Pa. Next, hexamethyldisilazane (HMDS) was spin-coated using a spinner for 90 s at 5000 rpm. Then, a photoresist (TSMR-8900) was spin-coated to a thickness of 700 nm using a spinner for 90 s at 5000 rpm. The metasurface patterns were exposed using a mask-less UV lithography system (DL-1000, Nanosystem Solutions Inc., Tokyo, Japan) then developed. Next, a 200-

Photonics 2019, 6, x FOR PEER REVIEW 16 of 20

Photonics 2019, 6, 105 16 of 20 Figure A2. Spectral relative permittivities of conventional refractory metals (W, Ta, and Mo): (a) real and (b) imaginary part of the relative permittivity [41]. Appendix C Appendix C Figure A3 shows the relative permittivity spectra of conventional plasmonic metals (Au, Ag, and Figure A3 shows the relative permittivity spectra of conventional plasmonic metals (Au, Ag, and Al) [41], and refractory plasmonic materials (TiN and HfN) [43]. The real part of the permittivity of HfN Al) [41], and refractory plasmonic materials (TiN and HfN) [43]. The real part of the permittivity of is negative within the entire visible range and is very close to that of Au at λ < 0.6 µm. The imaginary HfN is negative within the entire visible range and is very close to that of Au at λ < 0.6 µm. The part ofimaginary the permittivity part of the of permittivity HfN is ~1/ 2of that HfN of is TiN~1/2 inthat the of visibleTiN in the range. visible range.

FigureFigure A3. A3.Spectral Spectral relative relative permittivity permittivity of of convention conventionalal plasmonic plasmonic metals metals (Au, Ag, (Au, and Ag, Al) and[41] and Al) [41] and refractoryrefractory plasmonic plasmonic materials materials (TiN (TiN and andHfN)HfN) [43]: (a [43) real]: (anda) real (b) imaginary and (b) imaginary part of the partrelative of the permittivities. relative permittivities.

AppendixAppendix D D Figure A4 shows the fabrication process for the refractory MDM metasurface. First, a HfN layer Figure A4 shows the fabrication process for the refractory MDM metasurface. First, a HfN and a 130-nm-thick SiO2 layer were deposited on a quartz substrate (15 × 15 mm) using an RF layer and a 130-nm-thick SiO layer were deposited on a quartz substrate (15 15 mm) using an RF sputtering system (SVC-700LRF,2 SANYU Electron, Tokyo, Japan). In fabricating× the HfN layer, we sputteringused an system HfN target (SVC-700LRF, (Toshima Manu SANYUfacturing Electron, Co., Ltd., Tokyo, Saitama, Japan). Japan) In under fabricating an Ar gas the flow HfN rate layer, of we used an25 sccm HfN at target 2 × 10− (Toshima4 Pa. Next, Manufacturinghexamethyldisilazane Co., (HMDS) Ltd., Saitama, was spin-coated Japan) under using a an spinner Ar gas for flow 90 s rate 4 of 25at sccm 5000 at rpm. 2 Then,10− Pa.a photoresist Next, hexamethyldisilazane (TSMR-8900) was spin-coated (HMDS) to wasa thickness spin-coated of 700 usingnm using a spinner a × for 90spinner s at 5000 for rpm.90 s at Then, 5000 arpm. photoresist The metasurface (TSMR-8900) patterns was were spin-coated exposed using to a thicknessa mask-less of 700UV nm usinglithography a spinner forsystem 90 s (DL-1000, at 5000 rpm. Nanosystem The metasurface Solutions Inc., patterns Tokyo, wereJapan) exposed then developed. using a Next, mask-less a 200- UV lithography system (DL-1000, Nanosystem Solutions Inc., Tokyo, Japan) then developed. Next, a Photonics 2019, 6, x FOR PEER REVIEW 17 of 20 200-nm-thick HfN layer was deposited by RF sputtering. Finally, the HfN layer was lifted off using N-methyl-2-pyrrolidonenm-thick HfN layer (NMP). was deposited by RF sputtering. Finally, the HfN layer was lifted off using N- methyl-2-pyrrolidone (NMP).

FigureFigure A4. FabricatingA4. Fabricating the the refractoryrefractory MDM MDM metasurface. metasurface.

Appendix E Figure A5 shows the experimental setup for measuring the thermal radiation spectrum. The optical system was placed in a vacuum chamber connected to an FTIR spectrometer (FT/IR 6000, JASCO Co., Tokyo, Japan) through a tunnel tube. The vacuum chamber and the FTIR were pumped to 2.0 × 102 Pa and 1.4 × 102 Pa, respectively. The spectral resolution was set to 4 cm−1, and a DLATGS detector was used for the measurement. The device is shown in Figure 12 and was placed on a micro- ceramic heater (MS-1000, Sakaguchi E.H VOC Corp., Tokyo, Japan). The temperature of the sample was measured by a K-type sheath thermocouple (T350251H, Sakaguchi E.H VOC Corp., Tokyo, Japan) placed on the surface of the sample. The measurements were performed at 573 K.

Figure A5. Experimental setup for measuring the thermal radiation spectrum. The sample is set on a ceramic heater in a vacuum chamber that is connected to the FTIR spectrometer through a tunnel tube.

Appendix F Figure A6 shows simulated spectral absorptivity to the diameter d of an MDM metasurface composed of HfN and SiO2 (see Figure 9) with P = 2.0 µm, Tg = 130 nm, and Td = 200 nm. The peak

Photonics 2019, 6, x FOR PEER REVIEW 17 of 20

nm-thick HfN layer was deposited by RF sputtering. Finally, the HfN layer was lifted off using N- methyl-2-pyrrolidone (NMP).

Photonics 2019, 6, 105 17 of 20

Appendix E Figure A4. Fabricating the refractory MDM metasurface.

FigureAppendix A5 Eshows the experimental setup for measuring the thermal radiation spectrum. The optical systemFigure wasA5 shows placed the in experimental a vacuum chamber setup for connectedmeasuring the to anthermal FTIR radiation spectrometer spectrum. (FT /IRThe 6000, JASCOoptical Co., Tokyo,system Japan)was placed through in a avacuum tunnel chamber tube. The connected vacuum to chamber an FTIR and spectrometer the FTIR were(FT/IR pumped 6000, to 2.0 10JASCO2 Pa andCo., Tokyo, 1.4 10 Japan)2 Pa, through respectively. a tunnel The tube. spectral The vacuum resolution chamber was and set tothe 4 FTIR cm 1were, and pumped a DLATGS × × − detectorto 2.0 was × 10 used2 Pa and for 1.4 the × 10 measurement.2 Pa, respectively. The The device spectral is resolution shown in was Figure set to 412 cm and−1, and was a DLATGS placed on a micro-ceramicdetector was heater used for (MS-1000, the measurement. Sakaguchi The E.H device VOC is shown Corp., in Tokyo, Figure Japan).12 and was The placed temperature on a micro- of the sampleceramic was measured heater (MS-1000, by a K-type Sakagu sheathchi E.H thermocouple VOC Corp., Tokyo, (T350251H, Japan). Sakaguchi The temperature E.H VOC of the Corp., sample Tokyo, Japan)was placed measured on the by surface a K-type of theshea sample.th thermocouple The measurements (T350251H, wereSakaguchi performed E.H VOC at 573Corp., K. Tokyo, Japan) placed on the surface of the sample. The measurements were performed at 573 K.

FigureFigure A5. Experimental A5. Experimental setup setup for for measuring measuring the the thermalthermal radiation radiation spectrum. spectrum. The The sample sample is set is on set a on a ceramicceramic heater heater in a vacuumin a vacuum chamber chamber that that is connected is connected to theto the FTIR FTIR spectrometer spectrometer through through a a tunnel tunnel tube. tube. Appendix F Appendix F PhotonicsFigure 2019 , A66, x FORshows PEER simulated REVIEW spectral absorptivity to the diameter d of an MDM metasurface18 of 20 composedFigure of HfN A6 and shows SiO simulated2 (see Figure spectral9) with absorptivityP = 2.0 µ tom, theTg diameter= 130 nm, d andof anT MDMd = 200 metasurface nm. The peak positionpositioncomposed ofof absorptionabsorption of HfN andcausedcaused SiO 2byby (see gapgap Figure plasmonplasmon 9) with modemode P = 2.0 inin theµm,the circularcircularTg = 130 cavitycavitynm, and cancan Td bebe= 200 changedchanged nm. The fromfrom peak 3.03.0 toto 7.07.0 µµmm byby changingchanging dd..

Figure A6. Simulated spectral absorptivity/emissivity map to the diameter of an MDM metasurface Figure A6. Simulated spectral absorptivity/emissivity map to the diameter of an MDM metasurface composed of HfN and SiO2 with P = 2.0 µm, Tg = 130 nm, and Td = 200 nm. composed of HfN and SiO2 with P = 2.0 µm, Tg = 130 nm, and Td = 200 nm.

Appendix G Figure A7 shows simulated spectral absorptivity to the incident angle to an MDM metasurface composed of HfN and SiO2 (see Figure 9). The single absorption peak caused by gap plasmon mode in the circular cavity is observed at ~4.5 µm for both polarizations, which is not strongly dependent on incident angle. The strong angle-dependent steep absorption is caused by diffraction at 2.5–4.0 µm only for p-polarization as shown in (a). Note that the designed value of d = 1.2 µm is slightly greater than that of the experimental value (d = 1.14 µm).

Figure A7. Simulated spectral absorptivity/emissivity maps to the incident angle to an MDM

metasurface composed of HfN and SiO2 with P = 2.0 µm, d = 1.2 µm, Tg = 130 nm, and Td = 200 nm: (a) p-polarization and (b) s-polarization.

References

1. Takahara, J.; Ueba, Y.; Nagatsuma, T. Thermal radiation control by microcavity and ecological incandescent lamps. Jpn. J. Opt. 2010, 39, 482–488.

Photonics 2019, 6, x FOR PEER REVIEW 18 of 20 position of absorption caused by gap plasmon mode in the circular cavity can be changed from 3.0 to 7.0 µm by changing d.

PhotonicsFigure2019 ,A6.6, 105 Simulated spectral absorptivity/emissivity map to the diameter of an MDM metasurface18 of 20 composed of HfN and SiO2 with P = 2.0 µm, Tg = 130 nm, and Td = 200 nm.

Appendix G Figure A7 shows simulated spectral absorptivity to the incident angle to anan MDMMDM metasurfacemetasurface composed ofof HfNHfN andand SiOSiO22(see (see Figure Figure9 ).9). The The single single absorption absorption peak peak caused caused by by gap gap plasmon plasmon mode mode in thein the circular circular cavity cavity is observedis observed at ~4.5at ~4.5µm µm for for both both polarizations, polarizations, which which is not is not strongly strongly dependent dependent on incidenton incident angle. angle. The The strong strong angle-dependent angle-dependent steep steep absorption absorption is caused is caused by dibyff ractiondiffraction at 2.5–4.0 at 2.5–4.0µm onlyµm only for p-polarization for p-polarization as shown as shown in (a). in Note (a). thatNote the that designed the designed value ofvalued = 1.2of dµ m= 1.2 is slightlyµm is slightly greater thangreater that than of thethat experimental of the experimental value (d value= 1.14 (dµ =m). 1.14 µm).

Figure A7.A7. SimulatedSimulated spectral spectral absorptivity/emissivity absorptivity/emissivity maps maps to to the the incident incident angle to an MDM metasurface composed of HfN and SiO with P = 2.0 µm, d = 1.2 µm, T = 130 nm, and T = 200 nm: metasurface composed of HfN and SiO2 with P = 2.0 µm, d = 1.2 µm, Tg =g 130 nm, and Td = d200 nm: (a) (p-polarizationa) p-polarization and and (b) (s-polarization.b) s-polarization.

References 1. Takahara, J.; Ueba, Y.; Nagatsuma, T. Thermal radiation control by microcavity and ecological incandescent 1. Takahara, J.; Ueba, Y.; Nagatsuma, T. Thermal radiation control by microcavity and ecological incandescent lamps. Jpn. J. Opt. 2010, 39, 482–488. lamps. Jpn. J. Opt. 2010, 39, 482–488. 2. Takahara, J.; Ueba, Y. Thermal Infrared Emitters by Plasmonic Metasurface. Proc. SPIE 2013, 8818, 88180X.

3. Waymouth, J.F. Where will the next generation of lamps come from? J. Illum. Energy Inst. Jpn. 1990, 74, 800. [CrossRef] 4. Waymouth, J.F. Optical Light Source Devices. U.S. Patent 5,079,473, 7 January 1992. 5. Hesketh, P.J.; Zemalr, J.N.; Gebhart, B. Organ pipe radiant modes of periodic micromachined silicon surface. Nature 1986, 324, 549–551. [CrossRef][PubMed] 6. Maruyama, S.; Kashiwa, T.; Yugami, H.; Esashi, M. Thermal radiation from two-dimensionally confined modes in microcavities. Appl. Phys. Lett. 2001, 79, 1393–1395. [CrossRef] 7. Kusunoki, F.; Kohama, T.; Hiroshima, T.; Fukumoto, S.; Takahara, J.; Kobayashi, T. Narrow-Band Thermal radiation with Low Directivity by Resonant Modes inside Tungsten Microcavities. Jpn. J. Appl. Phys. 2004, 43, 5253–5258. [CrossRef] 8. Lin, S.Y.; Fleming, J.G.; Chow, E.; Bur, J.; Choi, K.K.; Goldberg, A. Enhancement and suppression of thermal emission by a three-dimensional photonic crystal. Phys. Rev. 2000, 62, R2243–R2246. [CrossRef] 9. Zoysa, M.D.; Asano, T.; Mochizuki, K.; Oskooi, A.; Inoue, T.; Noda, S. Conversion of broadband to narrowband thermal emission through energy recycling. Nat. Photonics 2012, 6, 535–539. [CrossRef] 10. Ikeda, K.; Miyazaki, H.T.; Kasaya, T.; Yamamoto, K.; Inoue, Y.; Fujimura, K.; Kanakugi, T.; Okada, M.; Hatada, K.; Kitagawa, S. Controlled thermal emission of polarized infrared waves from arrayed plasmon nanocavities. Appl. Phys. Lett. 2008, 92, 021117. [CrossRef] 11. Miyazaki, H.; Ikeda, K.; Kasaya, T.; Yamamoto, K.; Inoue, Y.; Fujimura, K.; Kanakugi, T.; Okada, M.; Hatade, K.; Kitagawa, S. Thermal emission of two-color polarized infrared waves from integrated plasmon cavities. Appl. Phys. Lett. 2008, 92, 141114. [CrossRef] Photonics 2019, 6, 105 19 of 20

12. Schuller, J.A.; Taubner, T.; Brongersma, M.L. Optical antenna thermal emitters. Nat. Photonics 2009, 3, 658–661. [CrossRef] 13. Greffet, J.J.; Carminati, R.; Joulain, K.; Mulet, J.P.; Mainguy, S.; Chen, Y. Coherent emission of light by thermal sources. Nature 2002, 416, 61–62. [CrossRef][PubMed] 14. Kusunoki, F.J.; Kobayashi, T. Qualitative change of resonant peaks in thermal emission from periodic array of microcavities. Electron. Lett. 2003, 39, 23–24. [CrossRef] 15. Biener, G.; Dahan, N.; Niv, A.; Kleiner, V.; Hasmana, E. Highly coherent thermal emission obtained by plasmonic bandgap structures. Appl. Phys. Lett. 2008, 92, 081913. [CrossRef] 16. Ueba, Y.; Takahara, J.; Nagatsuma, T. Thermal radiation control in the terahertz region using the spoof surface plasmon mode. Opt. Lett. 2011, 36, 909–911. [CrossRef] 17. Yang, Z.Y.; Ishii, S.; Yokoyama, T.; Dao, T.D.; Sun, M.-G.; Nagao, T.; Chen, K.-P. Tamm plasmon selective thermal emitters. Opt. Lett. 2016, 41, 4453–4456. [CrossRef] 18. Landy, N.I.; Sajuyigbe, S.; Mock, J.J.; Smith, D.R.; Padilla, W.J. Perfect Metamaterial Absorber. Phys. Rev. Lett. 2008, 100, 207402. [CrossRef] 19. Watts, C.M.; Liu, X.; Padilla, W.J. Metamaterial Electromagnetic Wave Absorbers. Adv. Mater. 2012, 24, OP98. [CrossRef] 20. Lee, B.J.; Wang, L.P.; Zhang, Z.M. Coherent thermal emission by excitation of magnetic polaritons between periodic strips and a metallic film. Opt. Express 2008, 16, 11328–11336. [CrossRef] 21. Puscau, I.; Schaich, W.L. Narrow-band, tunable infrared emission from arrays of microstrip patches. Appl. Phys. Lett. 2008, 92, 233102. [CrossRef] 22. Liu, X.; Starr, T.; Starr, A.F.; Padilla, W.J. Infrared Spatial and Frequency Selective Metamaterial with Near-Unity Absorbance. Phys. Rev. Lett. 2010, 104, 207403. [CrossRef][PubMed] 23. Liu, X.; Tyler, T.; Starr, T.; Starr, A.F.; Jokerst, N.M.; Padilla, W.J. Taming the Blackbody with Infrared Metamaterials as Selective Thermal Emitters. Phys. Rev. Lett. 2011, 107, 045901. [CrossRef][PubMed] 24. Nielsen, M.G.; Pors, A.; Albrektsen, O.; Bozhevolnyi, S.I. Efficient absorption of visible radiation by gap plasmon resonators. Opt. Express 2012, 20, 13311–13319. [CrossRef][PubMed] 25. Ueba, Y.; Takahara, J. Spectral control of thermal radiation by metasurface with a sprit-ring resonator. Appl. Phys. Express 2012, 5, 122001–122003. [CrossRef] 26. Ito, K.; Toshiyoshi, H.; Iizuka, H. Densely-tiled metal-insulator-metal metamaterial resonators with quasi-monochromatic thermal emission. Opt. Express 2016, 24, 12803–12811. [CrossRef] 27. Yokoyama, T.; Dao, T.D.; Chen, K.; Ishii, S.; Sugavaneshwar, R.P.; Kitajima, M.; Nagao, T. Spectrally Selective

Mid-Infrared Thermal Emission from Molybdenum Plasmonic Metamaterial Operated up to 1000 ◦C. Adv. Opt. Mat. 2016, 4, 1987. [CrossRef] 28. Miyazaki, H.T.; Kasaya, T.; Iwanaga, M.; Choi, B.; Sugimoto, Y.; Sakoda, K. Dual-band infrared metasurface

thermal emitter for CO2 sensing. Appl. Phys. Lett. 2014, 105, 121107. [CrossRef] 29. Matsuno, Y.; Sakurai, A. Perfect infrared absorber and emitter based on a large-area metasurface. Opt. Mat. Express 2017, 7, 618–626. [CrossRef] 30. Ogawa, S.; Kimata, M. Metal-Insulator-Metal-Based Plasmonic Metamaterial Absorbers at Visible and Infrared Wavelengths: A Review. Materials 2018, 11, 458. [CrossRef] 31. Baranov, D.G.; Xiao, Y.; Nechepurenko, I.A.; Krasnok, A.; Alù, A.; Kats, M.A. Nanophotonic engineering of far-field thermal emitters. Nat. Mater. 2019, 18, 920–930. [CrossRef] 32. Sai, H.; Kanamori, Y.; Yugami, H. High-temperature resistive surface grating for spectral control of thermal radiation. Appl. Phys. Lett. 2003, 82, 1685–1687. [CrossRef] 33. Takahara, J. Renaissance of incandescent light bulbs through nanotechnology. Oyo Buturi 2015, 84, 564–567. 34. Takahara, J.; Kimino, K. Visible Spectral Control of Incandescent Light Bulbs by Microcavity Filament. In Proceedings of the Abstracts of 15th International Symposium on the Science and Technology of Lighting (LS15), Kyoto, Japan, 22–27 May 2016. 23P-LL1. 35. Ilic, O.; Bermel, P.; Chen, G.; Joannopoulos, J.D.; Celanovic, I.; Soljaˇci´c,M. Tailoring high-temperature radiation and the resurrection of the incandescent source. Nat. Nanotech. 2016, 11, 320–324. [CrossRef] [PubMed] 36. Shimogaki, K.; Iwai, I.; Yoshiike, H. The Luminous Efficacy of an Incandescent Lamp Which has an Infrared Reflective Film. J. Illum. Eng. Inst. Jpn. 1984, 68, 72–76. [CrossRef] Photonics 2019, 6, 105 20 of 20

37. The Illuminating Engineering Institute of Japan (IEI-J). Lighting Handbook (Compact Version); Ohmsha: Tokyo, Japan, 2006; Volume 3 2-1-2. 38. IEI-J. Lighting Databook (New Edition); Ohmsha: Tokyo, Japan, 1968; Volume 3-1-2. 39. IEI-J. Lighting Handbook (Compact Version); Ohmsha: Tokyo, Japan, 2006; Volume 3 2-2-2. 40. IEI-J. 34th Textbook of Lighting Basic Course (New Edition); IEI-J: Tokyo, Japan, 2013; pp. 2–23. 41. Palik, E.D. Handbook of Optical Constants of Solids; Academic Press: San Diego, CA, USA, 1985. 42. Boltasseva, A.; Atwater, H.A. Low-Loss Plasmonic Materials. Science 2011, 331, 290–291. [CrossRef] 43. Naik, G.V.; Shalaev, V.M.; Boltasseva, A. Alternative Plasmonic Materials: Beyond Gold and Silver. Adv. Mater. 2013, 25, 3264–3294. [CrossRef] 44. Guler, U.; Boltasseva, A.; Shalaev, V.M. Refractory Plasmonics. Science 2014, 344, 263–264. [CrossRef] 45. Kumar, M.; Umezawa, N.; Ishii, S.; Nagao, T. Examining the Performance of Refractory Conductive Ceramics as Plasmonic Materials: A Theoretical Approach. ACS Photonics 2015, 3, 43–50. [CrossRef]

© 2019 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/).