Ultra-narrow-band light dissipation by a stack of lamellar silver and aluminaDing Zhao, Lijun Meng, Hanmo Gong, Xingxing Chen, Yiting Chen, Min Yan, Qiang Li, and Min Qiu Citation: Applied Physics Letters 104, 221107 (2014); doi: 10.1063/1.4881267 View online: http://dx.doi.org/10.1063/1.4881267 View Table of Contents: http://scitation.aip.org/content/aip/journal/apl/104/22?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Optical Salisbury screen with design-tunable resonant absorption bands J. Appl. Phys. 115, 193103 (2014); 10.1063/1.4876117 Anisotropic permittivity of ultra-thin crystalline Au films: Impacts on the plasmonic response of metasurfaces Appl. Phys. Lett. 103, 091106 (2013); 10.1063/1.4819770 Realization of an extraordinary transmission window for a seamless Ag film based on metal-insulator-metalstructures Appl. Phys. Lett. 102, 201109 (2013); 10.1063/1.4807734 Multiple enhanced transmission bands through compound periodic array of rectangular holes J. Appl. Phys. 106, 093108 (2009); 10.1063/1.3254248 Optimization of the reflectivity of magnetron sputter deposited silver films J. Vac. Sci. Technol. A 18, 1632 (2000); 10.1116/1.582397

This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP:

183.157.160.40 On: Mon, 08 Dec 2014 03:41:51

Ultra-narrow-band light dissipation by a stack of lamellar silver and alumina

Ding Zhao,1 Lijun Meng,1 Hanmo Gong,1 Xingxing Chen,1 Yiting Chen,2 Min Yan,2

Qiang Li,1 and Min Qiu1,2,a)

1State Key Laboratory of Modern Optical Instrumentation, Department of Optical Engineering,Zhejiang University, Hangzhou 310027, China2School of Information and Communication Technology, KTH Royal Institute of Technology, Electrum 229,16440 Kista, Sweden

(Received 19 February 2014; accepted 21 May 2014; published online 3 June 2014)

An ultra-narrow band absorber consisting of continuous silver and alumina films is investigated.

Owing to Fabry–P�erot resonance and silver’s inherent loss, an ultra-narrow spectral range of light

can be entirely trapped in the structure. By varying thicknesses of metallic and dielectric films,

absorption peak shifts in visible and near-infrared regions. When two such metal-insulator-metal

stacks are cascaded, experimental results show that an ultra-narrow absorption bandwidth of

7 nm is achieved, though theoretical results give that of 2 nm. Features of high-efficiency and

ultra-narrow band absorption have huge potential in optical filtering, thermal emitter design, etc.VC 2014 AIP Publishing LLC. [http://dx.doi.org/10.1063/1.4881267]

Optical perfect absorbers (OPAs) have gained extensive

interest and research efforts over the past decade due to their

ability to absorb nearly all incident radiation and eliminate

transmission, reflection, and scattering of light at specific

frequencies.1–18 According to the classification of absorbing

bandwidth, OPAs can be categorized into two types: narrow-

band absorbers and broadband absorbers. Generally, broad-

band absorbers rely either on materials whose large intrinsic

losses are nearly frequency independent or on a superposi-

tion of multiple spectrally close packed absorption bands. It

has been reported that the wide bandwidth could cover from

1 lm to 14 lm with near-unity absorbance by a two-

dimensional pyramidal shape structure.12 However, in many

practical applications, such as optical sensing3,4 and thermal

emission,13–16 it is necessary to make OPAs operate only

within a narrow spectral range. Hence, narrowband absorbers

are also of great importance, which have attracted consider-

able attentions. For example, Hao et al.6 proposed a high

performance OPA made of metallic nanoparticles and a con-

tinuous metallic film, separated by an insulation material,

where FWHM (full width at half maximum) of the absorp-

tion band was less than 50 nm. Polyakov et al.17 presented a

subwavelength groove nanocavity whose absorption band-

width approximated to 30 nm. Sharon et al.11 demonstrated a

grating-waveguide structure composed of a dielectric wave-

guide superimposed on metal with a narrow absorption band-

width of 0.1 nm. Recently, a type of lithography-free light

absorber consisting of multilayered metallic and dielectric

films exhibited a narrow absorption bandwidth in the near-

infrared region.18 The maximum absorption could achieve

95.4% with the FWHM of 33 nm. But the peak absorbance

was only 32%, when the bandwidth was tuned to 15 nm.

This absorber can hardly simultaneously meet two require-

ments of high absorption (such as more than 90%) and nar-

row bandwidth (such as less than 10 nm). In this Letter, by

introducing silver into the metal layer, nearly total absorp-

tion with the FWHM of less than 5 nm can be realized.

Moreover, the tunability of the absorption peak is more mod-

erate than that in the previous structures.

The resonant-cavity enhanced metal-insulator-metal

(MIM) absorber structure consists of two flat continuous Ag

films sandwiching a dielectric spacer as illustrated in

Fig. 1(a). The thickness of top metal layer is thin, enabling

light to both transmit and be reflected. While the bottom is

more than 100 nm, which is thick enough to prevent light

from penetrating. The transfer matrix method19 is introduced

to calculate the reflection and transmission coefficients using

boundary conditions at each interface (the details of deriva-

tion are presented in the supplementary material22). Here, the

refractive index of alumina is taken to be 1.75. The relative

permittivity of Ag is modeled using the Drude model, i.e.,

e(x)¼ e1�xp2/(x2þ ixC0), fitting the experimental data

by Johnson and Christy.20 Detailed parameter values21 are as

follows, e1¼ 5.0, xp¼ 9.2159 eV, and C0¼ 0.0212 eV. A

set of optimized structure parameters for near-unity absorb-

ance are d¼ 50 nm and h¼ 150 nm, where d and h represent

the thicknesses of top metallic and middle dielectric films,

respectively. The calculated reflectance R, transmittance T, and

absorbance A of proposed structure for normal incident light

are shown in Fig. 1(b). Notably, the MIM absorber reflects

most of incident light, only an ultra-narrow spectral range of

light is almost entirely confined in the structure, owing to the

Fabry–P�erot (FP) resonance and the metal’s inherent loss. An

enlarged plot around the resonant wavelength is displayed in

the inset, where the peak absorbance reaches above 97% at

686.4 nm with the absorption bandwidth being less than 5 nm.

For this type of absorber, its absorbance is typically

highly angular dependent due to the FP resonance.

Calculations are performed to verify this effect with the opti-

mal dimensions. Absorbance as functions of incident angles

and wavelengths for two different polarized waves are shown

in Figs. 1(c) and 1(d). An obvious blue-shift of the absorp-

tion peak is exhibited with the increasing of the incident

angle. For the p-polarized wave [Fig. 1(c)], the absorption

resonance occurs around 672 nm at 30�, and it moves to

643 nm as the incident angle is up to 60�. However, whena)Electronic mail: minqiu@zju.edu.cn

0003-6951/2014/104(22)/221107/4/$30.00 VC 2014 AIP Publishing LLC104, 221107-1

APPLIED PHYSICS LETTERS 104, 221107 (2014)

This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP:

183.157.160.40 On: Mon, 08 Dec 2014 03:41:51

the electric field is perpendicular to the incident plane

(s-polarized wave), the resonance wavelength varies more

sharply and the shift range becomes wider. It is also obtained

here that the larger angle of the incidence, the narrower

FWHM of the absorption band. Furthermore, the absorbance

remains high even if the oblique incident angle is very large

for both cases. All these features could be utilized in angular

sensing and polarization detection.

Generally speaking, for a lossless FP resonator, the

phase difference at half maximum is Dd¼ 2(1�R0)/R01/2,

where R0 represents reflectivity of a given mirror. It indicates

that FWHM becomes narrower when R0 increases. This con-

clusion remains valid for a low loss condition (Dd0 /Dd),

resulting in a narrower band of absorption when top metal

slab gets thicker (larger reflectivity). The coupling strength

between the cavity mode and the external incident light can

be simultaneously regulated by the thickness of the top metal

layer, leading to a trend of peak absorbance increasing and

then decreasing. As shown in Fig. 2, the FWHMs of the

absorption spectra are projected on the bottom plane of the

plot as green bars, while the peak absorbance is projected to

the side plane as an open circle. Based on the optimal param-

eters mentioned above, the absorption peak shifts slowly

towards short wavelength direction and the bandwidth nar-

rows gradually with the thickness d growing from 30 nm to

65 nm. This trend is similar to that reported in Ref. 18.

However, the absorption bands are much narrower here. It is

mainly attributed to the lower intrinsic loss of Ag than that

of Au, which exhibits better performance on wavelength

selection based on the FP resonating.

Further studies about the dependence of absorption on

spacer thickness are presented in Fig. 3(a). The resonance

wavelength based on a FP cavity is equivalent to twice of the

cavity length for the fundamental mode, and it thus shifts to

a longer wavelength with increasing dielectric thickness, yet

the FWHM is nearly invariable. The thickness of top Ag film

is kept to 45 nm. The resonant absorption wavelength could

be accurately tuned over a broad spectral range by control-

ling the spacer thickness. However, a high order resonance

peak would appear when the increment of h is nearly half an

effective wavelength. The corresponding result is shown in

Fig. 3(b), where the spacer thickness is 345 nm, leading to

appearance of two resonance peaks. Compared with the

green curve (h¼ 150 nm) in Fig. 3(a), the bandwidth of

absorption near 685 nm in Fig. 3(b) is narrower (less than

3 nm). The following equation is found, Dh¼ 195 nm

¼ kres/2n (kres¼ 685 nm and n¼ 1.75), which is a typical

feature of a FP resonator. Therefore, it is expected that nar-

rower absorption bandwidth could be obtained when the

spacer thickness rises at integer times Dh growth, but more

redundant resonance peaks would be inevitably introduced

in the meantime.

An alternative to narrow the absorption bandwidth is to

cascade more FP resonators together. For the two-cavity

structure, the key is the efficient coupling between two

FIG. 1. (a) Schematic of the MIM absorber structure. (b) Reflection, trans-

mission, and absorption spectra under normal incidence. The inset shows an

enlarged plot around the resonance wavelength. Absorbance as a function of

the incident angle and wavelength for (c) p-polarized and (d) s-polarized

incident radiations.

FIG. 2. Influence of the top metallic layer thickness d on absorption (red

curves). The FWHMs of the absorption spectra are presented by green bars.

Black crosses indicate the location of peak wavelengths. The maximum ab-

sorbance corresponding to the thickness d is projected onto one side plane of

the plot, as shown by an open blue circle.

FIG. 3. (a) Absorbance as a function of wavelength and the thickness of

middle dielectric layer. (b) A high order resonance peak with a narrower

(less than 3 nm) absorption bandwidth.

221107-2 Zhao et al. Appl. Phys. Lett. 104, 221107 (2014)

This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP:

183.157.160.40 On: Mon, 08 Dec 2014 03:41:51

individual cavities. It is found that the absorption band of a

single-cavity structure splits into two bands, and the separa-

tion between the two bands increases with decreasing center

metal thickness. The thickness of top metal layer mainly

affects the peak absorption and the bandwidth, while it has

little effect on the peak wavelength. According to these char-

acteristics, a sharp absorption peak near 600 nm can be

obtained by appropriate thicknesses of both top and center

metal layers. The reflection spectrum of two cascaded FP

resonators and its geometrical sketch is shown in Fig. 4(a).

The thicknesses from top to bottom are 60 nm, 150 nm,

10 nm, and 150 nm, respectively. Two resonance peaks

appear whose locations are approximately 200 nm apart with

no overlap. The bandwidth of the primary absorption peak

has been dramatically reduced to 2 nm.

Correspondingly, reflection measurements are conducted

to verify the calculated results. Measured reflection spectrum

and corresponding sectional view of this multi-layered struc-

ture are presented in Fig. 4(b). The metal and dielectric layers

were deposited on a glass substrate by electron-beam evapo-

ration method. With the assist of focus ion beam milling tech-

nique, each layer could be clearly observed. Benefiting from

large-area uniform surface of the sample, it can be directly

illuminated by an unfocused Gaussian beam from supercon-

tinuum source through a pin hole, to ensure that the incident

light is collimated. The spot size on sample surface is around

2 mm. Two resonance peaks in the experiment are consistent

with those in the calculation, and the primary peak locates at

610 nm with an absorption bandwidth of 7 nm. It is worth

nothing that the bandwidths have been broadened in the ex-

perimental result, due to the deviation between optical con-

stants of Ag used in the calculation and actual values,

flatness, and surface roughness of the film.

In order to reveal the underlying physics behind the two

absorption peaks, electromagnetic field distributions for

these resonant modes are investigated. Figure 5(a) illustrates

normalized amplitude of electric field (Ez) inside the

absorber. Both anti-symmetric mode (623 nm) and symmet-

ric mode (860 nm) are supported by this structure. As shown

in Fig. 5(b), the proportion of jEj inside all three metal layers

at 860 nm is more than that at 623 nm. It gives rise to a larger

loss ratio and thus a broader absorption bandwidth at

860 nm, which has been evidenced in eigenfrequency analy-

sis by commercial software COMSOL MULTIPHYSICS.

Another interesting phenomenon is that, as seen from the

heat distribution map in Fig. 5(c), most of heating is gener-

ated in the top and bottom metal films at 623 nm, while it is

in the middle layer at 860 nm. This can be easily understood

from Fig. 5(b) that the electric field is much stronger in in-

tensity inside the middle layer at 860 nm, while it is close to

zero in the middle at 623 nm.

Moreover, reflection spectra of the two cascaded FP res-

onators under oblique incidence are also measured. In Fig. 6,

it is found that high absorption is persistent with varying

incident angles and polarizations. Both for p-polarized and

s-polarized light illuminations, reflection peaks exhibit blue-

shift feature and reflection minima first decline to 10% at

10� and then start to grow with incident angles increasing;

meanwhile, FWHMs stay around 7 nm. A similar trend as

shown in Fig. 1, that the resonance wavelength moves faster

under s-polarization than under p-polarization could also be

seen, especially for large incident angles.

In summary, our work demonstrates a high-efficiency

polarization-sensitive absorber based on multilayered metal-

lic and dielectric films, whose absorption bandwidth is

ultra-narrow. Theoretical results reveal that the near-unity

absorption peak could be tuned from visible to near-infrared

regions by increasing the spacer thickness. According to the

characteristics of FP resonators, the absorption bandwidth of

the structure would be further reduced by controlling the

distance between two metallic layers or cascading more FP

resonators together. Since the fabrication of this structure

does not need patterning step, it can be easily obtained

simply using E-beam evaporation method. The feature of

FIG. 4. Reflectance of the resonant-cavity enhanced absorber containing

two cascaded FP resonators. (a) Schematic diagram and calculated reflec-

tance of the proposed structure. The bandwidth of the primary peak is

reduced to 2 nm. (b) Measured reflection spectrum and corresponding sec-

tional view of the multi-layered structure. The scale bar is 200 nm.

FIG. 5. (a) Normalized amplitude of

electric field Ez, (b) total electric field

E, and (c) resistive heating for normal

incident light at 623 nm and 860 nm.

221107-3 Zhao et al. Appl. Phys. Lett. 104, 221107 (2014)

This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP:

183.157.160.40 On: Mon, 08 Dec 2014 03:41:51

ultra-narrow band absorption would have applications in

nanophotonic devices.

The authors acknowledge the support by the National

Natural Science Foundation of China (Grants Nos.

61275030, 61205030, and 61235007), Qianjiang River

Fellow Fund of Zhejiang Province, the Opened Fund of State

Key Laboratory of Advanced Optical Communication

Systems and Networks, the Fundamental Research Funds for

the Central Universities, Doctoral Fund of Ministry of

Education of China (Grant No. 20120101120128), and the

Swedish Research Council (VR).

1N. I. Landy, S. Sajuyigbe, J. J. Mock, D. R. Smith, and W. J. Padilla, Phys.

Rev. Lett. 100, 207402 (2008).2J. Hao, J. Wang, X. Liu, W. J. Padilla, L. Zhou, and M. Qiu, Appl. Phys.

Lett. 96, 251104 (2010).3N. Liu, M. Mesch, T. Weiss, M. Hentschel, and H. Giessen, Nano Lett. 10,

2342 (2010).4A. Tittl, P. Mai, R. Taubert, D. Dregely, N. Liu, and H. Giessen, Nano

Lett. 11, 4366 (2011).5K. Aydin, V. E. Ferry, R. M. Briggs, and H. A. Atwater, Nat. Commun. 2,

517 (2011).6J. Hao, L. Zhou, and M. Qiu, Phys. Rev. B 83, 165107 (2011).7C. M. Watts, X. Liu, and W. J. Padilla, Adv. Mater. 24, OP98 (2012).8L. Meng, D. Zhao, Q. Li, and M. Qiu, Opt. Express 21(S1), A111 (2013).9X. Chen, H. Gong, S. Dai, D. Zhao, Y. Yang, Q. Li, and M. Qiu, Opt. Lett.

38, 2247 (2013).10S. Dai, D. Zhao, Q. Li, and M. Qiu, Opt. Express 21, 13125 (2013).11A. Sharon, G. Glasberg, D. Rosenblatt, and A. A. Friesem, J. Opt. Soc.

Am. A 14, 588 (1997).12Q. Liang, T. Wang, Z. Lu, Q. Sun, Y. Fu, and W. Yu, Adv. Opt. Mater. 1,

43 (2013).13I. Celanovic, D. Perreault, and J. Kassakian, Phys. Rev. B 72, 075127

(2005).14B. J. Lee, L. P. Wang, and Z. M. Zhang, Opt. Express 16, 11328 (2008).15X. Liu, T. Tyler, T. Starr, A. F. Starr, N. M. Jokerst, and W. J. Padilla,

Phys. Rev. Lett. 107, 045901 (2011).16L. Meng, D. Zhao, Z. Ruan, Q. Li, Y. Yang, and M. Qiu, Opt. Lett. 39,

1137 (2014).17A. Polyakov, K. F. Thompson, S. D. Dhuey, D. L. Olynick, S. Cabrini,

P. J. Schuck, and H. A. Padmore, Sci. Rep. 2, 933 (2012).18M. Yan, J. Opt. 15, 025006 (2013).19P. Yeh, Optical Waves in Layered Media (John Wiley & Sons, New York,

1988).20P. B. Johnson and R. W. Christy, Phys. Rev. B 6, 4370 (1972).21U. K. Chettiar, R. F. Garcia, S. A. Maier, and N. Engheta, Opt. Express

20, 16104 (2012).22See supplementary material at http://dx.doi.org/10.1063/1.4881267 for

detailed derivation of reflection and transmission coefficients.

FIG. 6. Measured reflectance of the two cascaded FP resonators under

oblique incidence. (a) p-polarized light illumination and (b) s-polarized light

illumination.

221107-4 Zhao et al. Appl. Phys. Lett. 104, 221107 (2014)

This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP:

183.157.160.40 On: Mon, 08 Dec 2014 03:41:51