Thin-film, wide-angle, design-tunable, selective absorber ...rep/conf_pubs/ConfPubs2013/DSSjanardan2013.pdf · Thin-film, wide-angle, design-tunable, selective absorber from near

Thin-film, wide-angle, design-tunable, selective absorber from near UV to far infrared

Janardan Natha, Douglas Maukonena, Evan Smitha, Pedro Figueiredoa, Guy Zummoa, Deep Panjwania, Robert E. Pealea, Glenn Boremanb, Justin W. Clearyc, Kurt Eyinkd

aDepartment of Physics, University of Central Florida, Orlando FL 32816 bDepartment of Physics and Optical Science, University of North Carolina at Charlotte, Charlotte,

NC 28223 cAir Force Research Laboratory, Sensors Directorate, Wright Patterson AFB OH 45433

dAir Force Research Laboratory, Materials and Manufacturing Directorate, Wright Patterson AFB OH 45433

ABSTRACT

We experimentally demonstrate a structured thin film that selectively absorbs incident electromagnetic waves in discrete bands, which by design occur in any chosen range from near UV to far infrared. The structure consists of conducting islands separated from a conducting plane by a dielectric layer. By changing dimensions and materials, we have achieved broad absorption resonances centered at 0.36, 1.1, 14, and 53 microns wavelength. Angle-dependent specular reflectivity spectra are measured using UV-visible or Fourier spectrometers. The peak absorption ranges from 85 to 98%. The absorption resonances are explained using the model of an LCR resonant circuit created by coupling between dipolar plasma resonance in the surface structures and their image dipoles in the ground plane. The resonance wavelength is proportional to the dielectric permittivity and to the linear dimension of the surface structures. These absorbers have application to thermal detectors of electromagnetic radiation.

Keywords: Selective absorbers, metamaterial absorber, plasmonics, near-IR, Long wave infra-red (LWIR), far-IR.

1. INTRODUCTION

Wavelength-selective absorbers have a wide range of applications including microbolometers 1, thermal imaging 2, 3, coherent thermal emitters 4, solar cells 5, thermal photovoltaic solar energy conversion 6,7 and refractive index sensing 8,9. Resonant plasmonic and metamaterial structures are being investigated widely and are reported to produce perfect absorption up to 99% in the visible 10, near- and mid- infrared 11, 12, and terahertz frequencies13. The resonance frequency depends on the size of the structures14. The resonant absorption typically has bandwidth less than 12% of the center frequency14. Both experimental and numerical studies suggest that 99.99% absorption can be achieved with these kinds of absorbers in any frequency range, from optical to terahertz 3, 4, 10, 13. The resonance absorption occurs by excitation of localized magnetic and electric dipoles10, 13. Structures may be designed to be polarization independent and omnidirectional12 - 15. These absorbers have been studied both by simulations and experiments. Analytic theory has been less extensively applied, though it promises simple design rules and an understanding of underlying physics. In this paper we study these absorbers experimentally and interpret the spectra in terms of a simple analytic model. Fabrication is by standard photolithography and is compatible with MEMS processing. Silicon dioxide is the chosen dielectric for compatibility with such processing, although silicon dioxide has strong absorption and dispersion near 10 micron wavelengths.

2. THEORITICAL CONSIDERATIONS

Figure 1(a) presents a schematic of the considered structure. Surface conductors are taken to be squares arranged in a checkerboard pattern. The squares are situated on top of a dielectric layer, which itself is supported by a metal ground plane. Gold is taken as the conductor material and silicon dioxide as the dielectric spacer. Simulations 12, 13, 16 demonstrate excitation of fundamental and higher order resonances and corresponding absorption of the driving fields. In the case of the fundamental, the incident electric field excites an electric dipole in the gold square. An image dipole is excited in the gold film under the spacer material. The driven current sloshing back and forth between the ends of the

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th and withoutce (black curvuares are presen

D RESULTS

ngth region. ift-off (Fig. 1.aThen, SiO2 wam evaporation.hy, e-beam evadeposition of Sopy. Samples w

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dielectric, which occur as a sharp peak at 6.2 microns, a partially resolved doublet at 8 and 8.7 microns, a stronger partially resolved doublet at 9.7 and 10.7 microns, and a clear band at 12.5 microns. Significantly, the oxide reference sample lacks any significant absorption bands in the wavelength range 12-20 microns.

Figure 4. LWIR (left) and far-IR (right) reflectance spectra. Black lines are spectra from samples without squares. Black dots represent theoretically calculated resonances. The peak LWIR and far-IR absorptances are 98% and 95%, respectively.

When the squares are added to the surface of the LWIR sample, the spectrum changes significantly. A new strong band appears at 7 microns. The unresolved bands 8 and 8.7 microns both deepen, but their relative strength changes dramatically. The band at 9.7 microns actually gets weaker, while its partner at 10.7 microns becomes much stronger with a shoulder appearing at 11.2 microns. Most significantly, a very strong and broad band appears at 14.3 microns, where there was no absorption before. Symbols indicate the wavelength positions of the three fundamental resonances determined graphically in Fig. 2. There is fair agreement for the predicted resonances =14.26 and =7.73 microns and the observed bands at 14.3 and 8 microns, The predicted =9.07 micron band does not account for the reduced absorption at 9.7 microns, unless for some reason this resonance has actually blue shifted in contrast to the other two. The weakening of absorption strength at 9.7 micron due to the reduction in the absorption by SiO2 is caused by shadowing from the gold squares. None of the predicted resonances corresponds to the broad band at 6.9 microns, but we note that this wavelength is about half that of the observed band at 14.3 micron and may be a harmonic.

Figure 5. Reflectivity spectra for several far-IR samples for (left) different dielectric thicknesses at constant gold thickness, and (left) different gold square thicknesses at constant dielectric thickness.

In Figure 4 (right), the reflectivity spectrum for the far-IR sample without squares confirms the absence of significant absorption for SiO2 in the wavelength range 30-80 microns. When squares are added, a strong band with 95% peak absorption appears centered at 53 microns wavelength. According to Eq. 2, the resonance should appear at =62 micron wavelength using = 2 for SiO2

18. Ref [18] reported indices of SiO2 from different papers, and it varies in the range of 1.96 to 2.10 in 50-60 micron range. It is reported that the refractive index and absorption index varies with deposition





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Figusamislan

This work wattending thiStudies.

[1] Mauskopwith Si3N4 m[2] Parsons, ATechnol. A 6[3] Liu, X., SUnity Absorb

t) presents refl% for 360 nm wth samples, whble in terms of he resonance ues of 360 nm

spectra of samnown SiO2 absofor different ad the absorptio

perimentally dregions for a spositions of thethe dielectric to 50 degrees

spheric transmi

ure 7. (a) Refmples with gold

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pf, P. D., Bockmicromesh absoA. D. and Pedd

6, 1686–1689 (Starr, T., Starr,bance,” Phys. R

lectivity spectrwavelength. Thhile the relativethe gold-islandwavelengths fand 1.134μm.

mples without gorption band at

angles of incidon blue shifts so

4

emonstrated sturface compose resonances aris required for

s. More than ~8ission window.

flectivity spectd nano-islandsctivity spectra

n part by a grare provided by

k, J. J., Del Caorbers,” Appl. der, D. J., “Thi1988). , A. F. and PadRev. Lett. 104,

ra for the nearhe absorption be widths at LWd size distributfrom equation

gold nano-islant ~296 nm 21. F

dence from 20 omewhat as th

4. SUMMAR

trong design-tused of gold squre predicted wir the perfect ab80% of absorp. This can be u

tra in near-UVs, while thinnewith varying a

ACKNOW

ant from the Flthe UCF Stud

REF

stillo, H., HolzOpt. 36, 765–7in-film infrared

dilla, W. J., “In, 1-4, (2010).

r-UV and nearband width rel

WIR and far-IRions compared2, are =348

nds are represeFigure 7 (rightto 50 degreesis angle is incr

RY AND CO

unable absorptuares or islandith reasonable bsorption to ocption is achieveuseful for therm

V and near-IRer lines represangle of inciden

WLEDGEME

orida High Tecdent Governme

FERENCES

zapfel, W. L. a771 (1997). d absorber stru

nfrared Spatial

r-IR samples. Flative to the reR wavelengthsd with the mon8 nm and 1.24

ented by thinnet) present reflecs. Strong absoreased.

ONCLUSION

tion bands in tds separated froaccuracy using

ccur. This absoed in the range

mal detectors w

R. Thick linessent spectra fronce, as indicate

ENTS

chnology Corrent Association

and Lange, A.

uctures for adva

l and Frequenc

For the near Uesonance centes were about 2no-disperse squ49 μm, in goo

er lines in figurctivity spectra rption is susta

N

the near-UV, nom a gold plang a simple anaorption is sustae of 6-16 micro

working in this

s represent refom samples wed in deg in the

ridor (I-4) progn and the UCF

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anced thermal

cy Selective M

UV sample, aber wavelength i0%. The diffe

uares. From theod agreement

re 7, and both for near-UV a

ained to wide

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flectivity spectwithout the gole legend.

gram. Travel fF Division of G

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Metamaterial wi

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samples and near-angle of

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Thin-film, wide-angle, design-tunable, selective absorber ...rep/conf_pubs/ConfPubs2013/DSSjanardan2013.pdf · Thin-film, wide-angle, design-tunable, selective absorber from near