A thermal emitter with selective wavelength: Based on the couplingbetween photonic crystals and surface plasmon polaritons

Han-Kuei Fu,1,a� Yu-Wei Jiang,2 Ming-Wei Tsai,2 Si-Chen Lee,2 and Yang-Fang Chen1,b�

1Department of Physics, National Taiwan University, Taipei, Taiwan 106, Republic of China2Department of Electrical Engineering, Graduate Institute of Electronics Engineering, National TaiwanUniversity, Taipei, Taiwan 106, Republic of China

�Received 9 November 2008; accepted 15 December 2008; published online 3 February 2009�

A thermal emitter with selective wavelength has been demonstrated in which the dielectric layersformed one-dimensional photonic crystals are sandwiched between two Ag films. The top Ag filmis perforated periodically with hexagonal hole array. The selected thermal radiation of the photoniccrystals resonates between two metal films and surface plasmon polaritons are generated on the topAg and converted to light radiation. It is found that when leakage modes adjacent to the optic bandgap of photonic crystals meet the resonant modes of surface plasmom polaritons, an enhancedthermal emission with maximum intensity can be obtained. The hybrid photonic and plasmonicthermal emitters are selective, which should be very useful for the creation of high power infraredlight sources. © 2009 American Institute of Physics. �DOI: 10.1063/1.3074293�

I. INTRODUCTION

Photonic crystals �PCs� are structures with periodicallymodulated refractive indices.1 At certain arrangements, PCsdisplay frequency gaps forbidden for propagation of electro-magnetic waves, so-called photonic band gaps �PBGs�,which can be used to strongly localize electromagneticwaves to specific areas, to inhibit or enhance spontaneousemission, and to guide propagation of electromagnetic wavesalong certain directions at restricted frequencies. The abilityof PBGs to modulate photonic density of states and hencemodify spontaneous emission rates in PCs open tremendouspossibilities for designing thermal sources. One-dimensionalPCs, for example, can significantly suppress thermal radia-tion for a range of frequencies �in the band gap� while en-hancing radiation outside the band gap2–4 and have been ex-ploited to develop resonant-cavity enhanced5 coherentthermal emitters6–8 and omnidirectional reflectors.9–13

Surface plasmon polaritons �SPPs� are longitudinalwaves that propagate along the surface of a conductor.14–17

Metal films with two-dimensional subwavelength periodicperforated hole arrays exhibit extraordinary optical transmis-sion and have been studied and demonstrated in ourgroup.18–21 They have potential applications in a great deal ofoptoelectronic devices such as wavelength selective quantumdot infrared photodetectors22 and midinfrared narrow-bandplasmonic thermal emitters.23

In this study, we combine both exciting research fields ofPCs and SPPs and design a thermal emitter with selectivewavelength. The underlying mechanism behind this devicedemitter is based on the resonant coupling between the leak-age modes of PC and SPP eigenmodes. This finding mayopen up a new route for the future development of optoelec-tronic devices.

II. EXPERIMENT

To implement our design concept that PBG is adjacent toSPP resonant modes, the periodical SiO2 /Si multilayers werefabricated to serve as PCs. The numerical solution of PBGsderived from Maxwell equations was obtained by the trans-fer matrix method.5,11,24,25 Because of the periodical charac-teristic, the solution can be simply described by Bloch func-tion. Based on the theoretical calculation, we can thereforedesign PCs having PBGs with a desirable wavelength.

In our fabrication, we first used a thermal evaporatingsystem �ULVAC� to deposit 5 nm Cr as adhesive layer and a100 nm Ag thin film on a single polished n-type silicon sub-strate. There was also an adhesive layer of 5 nm Ti on Agfilm. A multilayer structure with periodic refractive indiceswas fabricated by e-gun evaporation because of the highmelting point of SiO2 and Si. The first layer of stacked PCswas about 700 nm SiO2 and the second layer was about 300nm Si. After five pairs of SiO2 /Si, we deposited 700 nmSiO2 as the top layer of PCs. The photoresist was spun on thePCs and the hexagonal pillar array was defined and stood byphotolithography. Then, 5 nm Cr and 100 nm Ag were de-posited and then lifted off. After that, the patterned perfo-rated Ag film was presented on top of the PCs. In order toheat the structure, we sputtered 300 nm Mo on the bottom ofSi substrate. Finally, the thermal emitter was completed afterthe preceding processes. Figure 1�a� is the typical scanningelectron image of cross section of PCs. The lattice constantof hexagonal hole array is 5 �m and the diameter of hole is2.5 �m which are shown in Fig. 1�b�. A more detailed struc-ture of our devices is plotted in Figs. 1�c� and 1�d�.

In the measurement of reflectance spectra, a Bruker IFS66 v/s system was adopted with reflecting module which canvary the incident angle from 12° to 65°. The wave numberresolution of the measurement was 8 cm−1. The measure-ment of thermal radiation spectra is collected by a PerkinElmer 2000 Fourier transform infrared spectrometer system.

a�Electronic mail: d92222013@ntu.edu.tw.b�Electronic mail: yfchen@phys.ntu.edu.tw.

JOURNAL OF APPLIED PHYSICS 105, 033505 �2009�

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III. RESULTS AND DISCUSSION

The measured reflectance spectra of PCs set at an angleof 12° was shown in Fig. 2�a�. There is a PBG between thewavelength of 3.7 and 6.4 �m, in which light is entirely

reflected. For the wavelength longer than 6.4 �m, there existseveral leakage modes showing that light is absorbed andleaks to PCs. The absorption peaks in the reflectance spec-trum of PCs give us a hint that the leakage modes can serveas good resonant cavities as well as good thermal radiatorswhen we heat the structure. The leakage modes existing inthe edge of PBG act like defect modes in which light cantravel in the PCs. They are also passbands that divide differ-ent PBGs. The splitting at large wave vectors arises from theintrinsic property of photonic band structure as demonstratedby the theoretical calculation published previously.5 The en-ergy dispersion relation of PCs obtained from reflectancespectra of varied incident angles is also shown in Fig. 2�b�.The white band from 0.19 eV �wavelength 6.4 �m� to 0.34eV �3.7 �m� is the PBG, which can be theoretically pre-dicted from the previous reports,5,9,11 and the leakage modessplit up into two modes at larger kx. Figure 2�b� is a mea-sured result obtained from various incident angles � from12° to 65° with various wavelengths � of kx= �2� /��sin �,and reflectance intensities are described in the contrast ofblack and white colors. For consistency, Fig. 4 shown belowis also obtained by the same method.

We next consider the theoretical calculation of surfaceplasmon �SP� eigenmodes. In a two-dimensional hexagonallattice, the momentum conservation law of SPs is written by

(a)

(b)

(c)

(d)

FIG. 1. �Color online� The typical scanning electron images: �a� side viewof the thermal emitter with SiO2 /Si multilayer. The thickness of SiO2 isabout 700 nm and the thickness of Si is about 300 nm. It is sandwichedbetween two 100 nm Ag films. �b� Top view of perforated hexagonal holearray with lattice constant 5 �m and the diameter of hole is 2.5 �m. �c�The sketch of our device is shown on the side view and �d� the measurementbasis of our device is plotted on the top view.

(a)

(b)

FIG. 2. �Color online� �a� Reflectance spectra of SiO2 /Si PCs measured atangle of 12°. �b� Energy dispersion relation of PCs.

033505-2 Fu et al. J. Appl. Phys. 105, 033505 �2009�

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ksp = kx + iGx + jGy , �1�

where ksp is the SP wave vector given by

�ksp� =�

c� �1�2

�1 + �2�1/2

, �2�

where � is the frequency of the SP that is excited by theincident light of frequency �, kx= �k0�sin �, �k0�=2� /� is thewave vector of the incident light, and � is the wavelength ofthe light in vacuum. � is the incident angle with respect tothe normal direction of the surface. �1 and �2 are the dielec-tric constants of the insulator SiO2 and metal Ag, respec-tively. Gx and Gy are the reciprocal lattice vectors of hex-agonal lattice with Gx= �4� /31/2a���1 /2�x+ �31/2 /2�y� andGy = �4� /31/2a���1 /2�x− �31/2 /2�y�, where a is the latticeconstant and i and j are integers. For normal incident light,�=00 and kx=0, Eq. �1� reduces to

� = a�4

3�i2 + ij + j2�−1/2� �1�2

�1 + �2�1/2

. �3�

The real parts of the dielectric constants of Ag at 6.3 �m are−1.34�103.26 For the dielectric constant of SiO2 at thiswavelength, we adopt the theoretical calculation given byCollin et al.27 which dealt with the coupling strength of SPsin a metal/dielectric/metal structure. According to their work,the equation of the effective refractive index of the trans-

verse magnetic mode in a thin dielectric layer sandwiched bytwo metal layers is given as follows:

neff = �11/2�1 +

�

�w�− �2�1/2�1 +�1

− �2�1/21/2

, �4�

where w is the thickness of intermediate dielectric layer and� is the wavelength in free space. The calculated effectiverefractive index for the SiO2 layer with a thickness of 100nm is 1.54. Therefore, the theoretical calculation for the de-generate Ag /SiO2 mode is 6.6 �m, which is close to ourexperimental value located at 6.3 �m �0.19 eV� shown inFig. 5�a�.

The hexagonal hole array has two specific directionsalong �M and �K when we measured the energy dispersionrelation of SPPs. For the �K direction, the measured reflec-

(a)

(b)

FIG. 3. Reflectance spectra of �a� the coupling between SPP modes and 100nm SiO2 epilayer denoted as device A. �b� The resonant coupling betweenleakage modes of PC and SPP modes denoted as device B.

(b)

(a)

(c)

FIG. 4. �Color online� Energy dispersion relation along �K direction for �a�device A and �b� device B. �c� is a theoretical calculation of energy disper-sion relation for SPP modes.

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tance spectra collected at 12° were shown in Figs. 3�a� and3�b�. For comparison, we have fabricated a device with thedielectric layers of PCs replaced by a thickness of 100 nmSiO2 epilayer denoted as device A, and the correspondingreflectance spectrum is shown in Fig. 3�a�. There are severalSPP modes indicated by Arabic numerals. The sinks at 6.8and 7.1 �m are degenerate �0,1� �1,1� and �0,1� �1,1�Ag /SiO2 modes that separate at 12°, respectively. The mea-surement of hybrid design device B with PCs and SPPs isshown in Fig. 3�b�. The spectrum displays an intriguing be-havior with many fine structures, arising from the couplingbetween the leakage modes of PCs shown in Fig. 2�a� andthe SPP modes shown in Fig. 3�a�.

In order to get the energy dispersion relation of SPPs, wehave measured the reflectance spectra covering the incidentangle from 12° to 65° shown in Figs. 4�a� and 4�b�. Figure4�a� displays the energy dispersion relation along �K direc-tion for device A. The dark lines show the absorption sinksarising from SPP modes. The four Ag /SiO2 modes are de-generate at about 0.19 eV and split from 12° to 65°. Figure4�b� is the energy dispersion relation along the �K directionfor device B. A lot of interference patterns such as waves canbe clearly seen. The couplings between the leakage modes ofPC and SPP modes are more complicated and split from 12°to 65°. This is because the dielectric constants are real num-bers in substrate SiO2 and Si and are complex numbers inmetal Ag. Besides, in Fig. 4�b�, we can clearly see that whenthe leakage modes of PCs overlap with SP modes, the dis-persion relation is split at the cross points such as at the �0,1��1,1� mode. We have performed a theoretical calculationbased on Eq. �3� and the result is shown in Fig. 4�c� toconfirm the energy dispersion relation diagram observed inFig. 4�a�.

The device B is heated by sending electric currentthrough the back Mo metal on Si substrate. The bottom 100nm Ag film has very small emissivity that generates verysmall blackbody radiation, yet it can completely block thebackground radiation from the Si substrate and heat the PClayers. The selected thermal radiation generated in theSiO2 /Si layers resonates between the two metal film PBGsand Ag /SiO2 SPPs are induced and then converted to lightradiation. The emission spectra are measured at differenttemperatures from 25 to 200 °C and the peak intensities aregradually simultaneously enlarged, but without changing theposition shown in Fig. 5�b�. The spectra of device A areshown in Fig. 5�a� for comparison. We can clearly see thatthe spectra of device B contain many more features thanthose of device A. The pronounced peaks appearing in thespectrum of device A can be easily understood based on thecoupling between SPPs and SiO2 epilayer if we compare thespectra between Fig. 3�a� and Fig. 5�a�. For comparison theemission spectra of the PC in the absence of the hole array,i.e., the device without top Ag film, have been performed andshown in Fig. 5�c�. The observed emission peaks do obey thereflectance spectra shown in Fig. 2�a�. We can see that with-out the top Ag film of SPP structure, the emission spectrumbehaves like a blackbody radiation, and most of energy ex-penses at long wavelengths with broad bands, which is notan efficient thermal emitter.

We now examine the complicated structures contained indevice B as shown in Fig. 5�b�. Because of the formation ofphotonic stop bands in PCs, the PBGs inhibit thermal emis-sion arising from the dielectric layers in the range of3–8 �m, including the largest PBG from 3.7 to 6.4 �m andother smaller PBGs occurring at different wavelengths di-vided by leakage modes. But, the resonant coupling betweenSPP and PBG modes can open leakage modes to serve asgood light path for thermal radiation. Comparing with thecoupling modes between SPPs and PBGs in the reflectancespectra shown in Fig. 3�b�, it is clear that there exists anexcellent one-to-one correspondence. It implies the impor-tant role in the assistance of thermal emission played byleakage modes in PBGs and SPPs. Quite interestingly, it is

(b)

(a)

(c)

FIG. 5. �Color online� Thermal emission spectra of �a� device A, �b� deviceB, and �c� device without top Ag film layer �i.e., without SPP structure�. Dotline shows the background emission spectrum at room temperature of 25 °Cbefore heating the device.

033505-4 Fu et al. J. Appl. Phys. 105, 033505 �2009�

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found that the resonant coupling between PC and SPP modesat 6.6 �m has the maximum intensity in the emission spec-tra. The evidence is also presented in Fig. 3�b�. The thermalenergy is not large enough to excite the universal maximumsink mode at 3.4 �m but can excite the local maximum sinkmode at 6.6 �m. It provides a distinctive evidence showingthat when PC leakage modes coincide with SPP modes, themaximum light radiation efficiency in a thermal emitter canbe obtained. This mechanism should be very important tocreate high efficiency thermal emitters with selective wave-length. In the comparison of devices A and B, the 6.6 �mpeak intensity in device B is less than the 6.3 �m peakintensity in device A. The main reason is that the structure ofPCs opens several emission channels around the 6.6 �mpeak. If we need a higher intensity of emission, we may heatthe device B at higher temperature without changing thepeak positions. Another possibility to improve the emissionefficiency is to put a thick defect layer on the bottom ofone-dimensional PCs and increase the thickness of one SiO2

layer upon the bottom metal film. In terms of emission effi-ciency, the defect modes in the middle of PBG are moreeffective than the leakage modes in the edge of PBG.

IV. CONCLUSIONS

In conclusion, we have demonstrated a thermal emitterwith selective emission wavelengths. The underlying mecha-nism of the deviced thermal emitter is based on the resonantcoupling between leakage modes of PCs and SPPs. It isfound that when the PC leakage mode directly meets the SPPresonant mode, the thermal radiation has the maximum effi-ciency. Therefore, through a proper design, it is possible toachieve a strong thermal emission with a desirable wave-length. Our approach may open up a new possibility to ob-tain high temperature operated, narrow bandwidth, and highpower infrared light sources.

ACKNOWLEDGMENTS

This work was supported by the Education of Ministryand the National Science Council of the Republic of China.

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