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Double-sided polarization-independent
plasmonic absorber at near-infrared region
Shuowei Dai,1,3 Ding Zhao,1,3 Qiang Li,1,4 and Min Qiu1,2,*
1
2
State Key Laboratory of Modern Optical Instrumentation, Department of Optical Engineering, Zhejiang University,
Hangzhou 310027, China
School of Information and Communication Technology, Royal Institute of Technology (KTH), Electrum 229, 164 40
Kista, Sweden
3
The first two authors contributed equally to this work
4
[email protected]
*
[email protected]
Abstract: A double-sided polarization-independent plasmonic absorber is
proposed and numerically investigated. Distinct from previously studied
absorbers, it could absorb light incident from both sides of the surface
through an ultrathin three-layer metal-insulator-metal nanostructure.
Patterned metal particles are adopted instead of metal films in this absorber.
It shows a high absorbance over a wide incident-angle range at near-infrared
region. For electromagnetic waves incident from different sides of the
structure, the maximum absorption locates at different wavelengths due to
asymmetry. The effective medium theory demonstrates that the whole
structure exhibits different impedances for both top and bottom incidences.
This double-sided-absorption characteristic could lead to potential
applications in thermal emitters, sensing, etc.
©2013 Optical Society of America
OCIS codes: (250.5403) Plasmonics; (240.6680) Surface plasmons; (300.1030) Absorption;
(310.6628) Subwavelength structures, nanostructures.
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#187555 - $15.00 USDReceived 21 Mar 2013; revised 15 May 2013; accepted 16 May 2013; published 21 May 2013
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1. Introduction
Metals such as gold, silver, copper support plasmonic resonances at visible frequencies. The
plasmonic resonance strongly relies on the size, shape of the metallic structures, and the
surrounding dielectric environment [1]. By artificially designing structured geometry, the
plasmonic resonant peak could be shifted from visible frequency [2,3] to a much wider spectral
range, covering radio [4], microwave [5], terahertz [6] and infrared region [7]. Many efforts
have been dedicated to enhancing the electromagnetic resonant absorption relevant to the
inevitable intrinsic losses of metals [2,5–24]. Hence, losses actually boost the research of
plasmonic absorbers, leading to a variety of potential applications, such as thermal emitters [8],
thermal imaging [9], plasmonic sensors [23], et al. A tremendous number of absorbing
structures have been proposed, such as absorbers based on lumped elements [11],
electric-field-coupled-LC unit cells [12], dielectric strips embedded on bulk metal [13] and
three-layer metal-insulator-metal (MIM) structures [2,14–24]. Among these structures, the
three-layer MIM structures catch special attention due to their excellent electric and magnetic
responses where electromagnetic energy could be efficiently confined in the intermediate layer
[14,23]. Recently Liu et al. proposed a one-sided patterned MIM structure as a selective thermal
emitter [8]. Zoysa et al. realized conversion of broadband to narrowband thermal emission
through energy recycling [25]. However, for certain applications, such as controlling thermal
emissivity, double-sided absorber instead of single-sided absorber could be used for
wavelength conversion. During this process, the incident light with the wavelength
#187555 - $15.00 USDReceived 21 Mar 2013; revised 15 May 2013; accepted 16 May 2013; published 21 May 2013
(C) 2013 OSA
3 June 2013 | Vol. 21, No. 11 | DOI:10.1364/OE.21.013125 | OPTICS EXPRESS 13126
corresponding to the resonance from top incidence is first absorbed by the structure; then the
structure emits another wavelength alone from the bottom side. This could be certainly
achieved by mirroring the single-sided absorber structures; however, the mirroring structure
consists of at least five layers, resulting in an increased thickness and fabrication complexity.
Alternative ways of obtaining double-sided absorption would thus be of great importance.
Here, we present a double-sided polarization-independent three-layer MIM nanostructured
plasmonic absorber, which works at the near-infrared region. We show that for both top and
bottom normal incidences, the maximum absorbances could be above 85% at respective
resonances. For both transverse electric (TE) and transverse magnetic (TM) waves, this
double-sided plasmonic structure keeps highly-efficient absorption over a wide angle of
incidence. Numerical simulations provide clear details of electric and magnetic responses at
main and secondary resonant wavelengths. Furthermore, the effective medium theory is
introduced to interpret the underlying physics of the double-sided highly-efficient absorption.
2. Design, results and discussion
As illustrated in Fig. 1(a), the proposed double-sided nanostructure, which only consists of
three layers, is assumed to be deposited on a thick quartz substrate. The top and bottom layers
are made up of gold square nanoparticle arrays. A set of optimized parameters are given here as
w1 = 280 nm, w2 = 130 nm and h1 = h3 = 40 nm, where w1 (w2) denotes the width of the top
(bottom) square metallic particle, and h1 (h3) represents the thickness of the top (bottom)
metallic layer. Figure 1(b) displays the bottom view of a unit cell. The periods along both x and
y axes are described by p = 300 nm. An Al2O3 dielectric spacer separates the two metallic
layers, which forms a T-shaped pattern as shown in Fig. 1(c). The thickness of the dielectric
layer is designed to be h2 = 10 nm. The total thickness of the MIM layers is only 90 nm. The
gap-parameter d equals to 10 nm.
Fig. 1. (a) Schematic of the double-sided MIM nanostructure, the cube in red represents a unit
cell. (b) Bottom view of a unit cell. w1 (w2) denotes the width of the top (bottom) square metallic
particle, respectively. The periods along both x and y axes are described by p. (c) Side view of a
unit cell. d represents half distance of the air gap between two adjacent top metallic particles. h1,
h2, h3 denote the thickness of each layer, respectively.
Numerical simulations are performed using the finite element method (FEM) based
commercial software Comsol Multiphysics. The refractive indices of Al2O3 spacer and quartz
substrate are set as 1.75 and 1.45, respectively. The relative permittivities of gold are extracted
#187555 - $15.00 USDReceived 21 Mar 2013; revised 15 May 2013; accepted 16 May 2013; published 21 May 2013
(C) 2013 OSA
3 June 2013 | Vol. 21, No. 11 | DOI:10.1364/OE.21.013125 | OPTICS EXPRESS 13127
from the experimental data by Johnson and Christy [26], which are shown in Fig. 2. The real
and imaginary parts of the experimental permittivities are fitted by cubic spline interpolation,
and then used in the simulations. It should be mentioned that since the thickness of the substrate
is much larger (~mm scale) than the MIM layers and the quartz substrate is transparent to the
incident light at near-infrared region, the substrate thickness could be assumed to be infinite in
the simulations. All the wavelengths mentioned below are considered in vacuum.
Fig. 2. Relative permittivities of gold adopted in simulations. (a) real parts; (b) imaginary parts.
The dots indicate the experimental data by Johnson and Christy. The curves are fitted by cubic
spline interpolation.
Absorbance (%)
100
G1 top
G2 top
80
(b)
60
40
20
0
800
900
1000
1100
1200
100
Absorbance (%)
(a)
G1 bottom
G2 bottom
80
60
40
20
0
800
1300
900
Wavelength (nm)
(d)
Wavelength (nm)
1200
1100
1000
900
800
Top incidence
Bottom incidence
700
250
260
270
280
Top nanoparticle width (nm)
290
Wavelength (nm)
(c)
1000
1100
1200
1300
Wavelength (nm)
1100
1000
900
800
Top incidence
Bottom incidence
110
120
130
140
150
Bottom nanoparticle width (nm)
Fig. 3. Simulated absorption spectra under TE polarization (E//y) with different metallic
nanoparticle geometries. (a) top normal incidence; (b) bottom normal incidence. G1 refers to w1
= 280 nm, w2 = 130 nm; G2 refers to w1 = 140 nm, w2 = 270 nm. Resonant wavelength shifts as
the width of nanoparticle changes. (c) w2 = 130 nm, w1 varies; (d) w1 = 280 nm, w2 varies. Blue
squares and red circles indicate top and bottom incident cases, respectively.
Since the periods along both x and y axes are the same, such symmetric geometry yields
identical optical response for both TE and TM polarizations. For a TE-polarized plane wave
illuminating the nanostructure at normal incidence, the direction of the incident electric field is
parallel to the y-axis. The tunability of absorption is investigated via changing the width of
nanoparticles, while keeping the other parameters constant. Figures 3(a) and 3(b) present the
absorption spectra for the double-sided incidences with different metallic nanoparticle
geometries. Corresponding absorbance is calculated by the formula A = 1-R-T as a function of
wavelengths [5], where R refers to reflectance and T stands for transmittance. At w1 = 280 nm
and w2 = 130 nm, the maximum absorbance achieves 88% at λ = 1064 nm for the top incidence
and 85% at λ = 849 nm for the bottom incidence, respectively. Figure 3(c) shows a remarkable
red-shift of the resonant wavelength for both top and bottom incidences when the top
#187555 - $15.00 USDReceived 21 Mar 2013; revised 15 May 2013; accepted 16 May 2013; published 21 May 2013
(C) 2013 OSA
3 June 2013 | Vol. 21, No. 11 | DOI:10.1364/OE.21.013125 | OPTICS EXPRESS 13128
nanoparticle width is increased from 250 nm to 290 nm at w2 = 130 nm. Furthermore, there is an
approximate linear correlation between the resonant wavelength and the top nanoparticle width.
Similar results are observed when the bottom nanoparticle width is changed, as depicted in Fig.
3(d). Dashed curves in both Figs. 3(a) and 3(b) show that the resonant wavelength for the top
incidence could be shorter than that for the bottom incidence if w1 is far smaller than w2. The
absorbances from both sides exceed 75% among the cases investigated above, with one
exception at w1 = 290 nm, w2 = 130 nm. In this case, top metallic arrays almost act as a metal
film, thus dropping top-sided absorbance to 57%, while raising the bottom-sided absorbance to
90%.
Fig. 4. Absorbances as functions of wavelengths and incident angles. Schematic drawing of (a)
TE polarization (d) TM polarization for both top and bottom incidences. (b) TE, top incidence
(c) TE, bottom incidence; (e) TM, top incidence (f) TM, bottom incidence. For the bottom
incident cases, the incident angles correspond to the refractive angles in quartz at quartz/air
interface.
Given optimized parameters mentioned in Fig. 1, the angular dependences of the absorption
are illustrated in Figs. 4(a)-4(f), where absorbances are plotted as functions of wavelengths and
incident angles. Figures 4(a) and 4(d) present schematic drawings of TE and TM polarizations,
respectively. For the bottom incidence in reality, the light first propagates from air to quartz
substrate and then to the MIM nanostructure. Therefore the incident angles shown in Fig. 4(c)
and Fig. 4(f) correspond to the refractive angles in quartz at quartz/air interface. According to
the Snell’s Law, the maximum incident angle θ is 43.6°. On the one hand, with the increase of
the incident angle, the maximum absorbance could exceed 90% over a wide range of oblique
angles, regardless of the polarization and the incident direction. Take the TM polarization as an
#187555 - $15.00 USDReceived 21 Mar 2013; revised 15 May 2013; accepted 16 May 2013; published 21 May 2013
(C) 2013 OSA
3 June 2013 | Vol. 21, No. 11 | DOI:10.1364/OE.21.013125 | OPTICS EXPRESS 13129
example, when the light illuminates from the top side, 95% absorbance could be kept from 40°
to 60°. On the other hand, for the TE polarization, the resonant peak shifts when the incident
angle increases; meanwhile the top-sided absorbance reduces to 60% at 70°. While for the TM
polarization, the absorption peak is nearly independent of the incident angle. Even at 70° the
top-sided absorbance could achieve 85%. This is due to the fact that the direction of the
magnetic field is always parallel to + y axis for all the incident angles as depicted in Fig. 4(d),
thus maintaining the strength of magnetic resonance effectively [27]. Figures 4(c) and 4(f)
show similar phenomena for the bottom incidence. However, the secondary resonance for the
bottom incidence is excited almost at the same wavelength where the top-sided absorbance
reaches the maximum.
Fig. 5. (a) Three-dimensional perspective of a unit cell with a cross-section (green) at the bottom
Al2O3/Au interface for top view and two cross-sections for side view, a purple plane denotes the
middle section and a red plane is 35 nm to right, respectively. (b) Magnetic field maps on two
different cross-sections in red (first row) and green (second row). First column and second
column indicate maps at the main peak wavelengths from the top and bottom incidences,
respectively. Third column indicates maps at the secondary peak wavelength from bottom
incidence for comparison. (c) Electric field maps on two different cross-sections in purple (first
row) and red (second row) at the peak wavelengths. Graphs in (b) and (c) are plotted in
logarithmic scale, and red color corresponds to higher amplitude.
#187555 - $15.00 USDReceived 21 Mar 2013; revised 15 May 2013; accepted 16 May 2013; published 21 May 2013
(C) 2013 OSA
3 June 2013 | Vol. 21, No. 11 | DOI:10.1364/OE.21.013125 | OPTICS EXPRESS 13130
Further simulations are performed to gain more insight into the diversity of the resonant
absorption peak for both top and bottom normal incidences under TE polarization (E//y). Figure
5(a) shows a three-dimensional perspective of a unit cell. The green cross-section at the bottom
Al2O3/Au interface is used to present the amplitude of the magnetic field. The purple
cross-section is set to observe the distribution of the electric field. Another perspective for both
the electric and the magnetic responses is shown with the cross-section in red, which is 35 nm
from the right boundary of the unit cell. Figures 5(b) and 5(c) plot the corresponding colormaps
at the resonant peak wavelengths.
Figure 5(b) shows that the magnetic field is mainly confined in the areas where the top and
bottom gold particles overlaps. However, the amplitudes are different at the main resonant
wavelengths between top and bottom incidences. For the top incident case, the fundamental
mode is excited at λ = 1064 nm. While for the bottom incident case, high-order mode is strongly
excited at λ = 849 nm. In order to elucidate the feature of electric response, the spacer between
top and bottom nanoparticles (middle gap) as well as the gap between top adjacent
nanoparticles (top gap) are both investigated. This type of gaps between adjacent gold particles
acts as effective “electric field trappers”, providing remarkable surface plasmon coupling. At
the purple cross-section, the structure is symmetric for both incident directions, leading to the
electric field’s being strongly concentrated in the top gaps at both main resonant wavelengths
under both incident cases, as shown in the first row in Fig. 5(c). At the red cross-section, a
reversed “T-shaped” gap appears, which is shaped by both the middle gap and the top gap. It is
thus not surprising that, due to the asymmetry, the enhanced electric field mainly gathers in the
middle gap for the top-incident resonant case; whereas for the bottom-incident case, the electric
field almost uniformly distributes in the whole “T-shaped” gap.
It is also worth noting that the two resonances could be excited under both incident cases.
However, due to the asymmetry, field intensity distributions are significantly different for
different incidences even at the same resonant wavelength. Taking the resonant mode at λ =
1064 nm as an example (see Figs. 5(b) and 5(c)), one can observe that the field intensity for the
top incidence is much higher than that for the bottom incidence, leading to a much higher
absorption for the top incidence. Basically, one can attribute the bi-anisotropic absorbing
behavior to lacking inversion symmetry of the proposed nanostructure.
#187555 - $15.00 USDReceived 21 Mar 2013; revised 15 May 2013; accepted 16 May 2013; published 21 May 2013
(C) 2013 OSA
3 June 2013 | Vol. 21, No. 11 | DOI:10.1364/OE.21.013125 | OPTICS EXPRESS 13131
(b)
Impedance Impedance
(a)
(d)
(e)
R & T (%) R & T (%)
Index
(c)
0.6
0.4
0.2
0
1
0.8
0.6
0.4
0.2
0
8.0
6.0
4.0
2.0
0
100
80
60
40
20
0
100
80
60
40
20
0
800
Re(z+)
Im(z+)
Re(z-)
Im(z-)
Re(n)
Im(n)
R+
T+
RT900
1000
1100
1200
1300
Wavelength (nm)
Fig. 6. Retrieved equivalent parameters for impedances of (a) top normal incidence, (b) bottom
normal incidence, and (c) refractive index. The vertical dashed lines depict the wavelengths
where resonant modes are excited. Simulated spectra of reflectance (R) and transmittance (T) for
(d) top normal incidence and (e) bottom normal incidence.
To explore the underlying nature of this bi-anisotropic absorbing behavior, the equivalent
parameters are retrieved from reflection and transmission coefficients [28–31]. The
characteristic impedances Z for both top and bottom normal incidences are depicted in Figs.
6(a) and 6(b), respectively, where the vertical dashed lines at main resonant wavelengths
indicate Z+(1064nm) = 0.61 + 0.10i and Z-(849nm) = 0.57 + 0.49i. Reflectance could then be
approximately expressed by formula R(λ) = (Z(λ)-Zmedium)2 / (Z(λ) + Zmedium)2 [32]. We obtain
|R+(1064nm)| = 6% with Zair = 1 and |R-(849nm)| = 14% with Zquartz = 0.69, respectively, which show
remarkably good agreement with the simulated reflectances in Figs. 6(d) and 6(e). Meanwhile,
the imaginary parts of the refractive index at resonant wavelengths are large enough (see Fig.
6(c)) to ensure strong light absorption. These two factors together bring high absorbance at
resonance. Furthermore, bi-anisotropic equivalent model gives a clear interpretation of the
asymmetric absorption property at the same resonant wavelength. With Z+(894nm) = 0.01 + 0.14i
and Z-(1064nm) = 0.06 + 0.50i, the calculated reflectances remain high due to
impedance-mismatching, leading to low absorption efficiency.
Moreover, the proposed nanostructure could be fabricated through a bottom-up process.
The bottom gold patterns with a 3-D chromium-mask array are fabricated by electron beam
lithography and lift-off technique first. After alumina deposition, selective chromium etching
[33], only the stuffed alumina will be kept. Then the alumina is further deposited to form a
T-shaped layer. With another electron beam lithography and lift-off, the top gold patterns are
finally fabricated.
3. Summary
In conclusion, we have numerically demonstrated a double-sided polarization-independent
plasmonic absorber at near-infrared region. The proposed absorber is based on a three-layer
MIM nanostructure with an ultrathin thickness of only 90 nm. It could realize highly-efficient
absorption over a wide range of incident angles. Both magnetic and electric field maps indicate
the different resonant modes between top-sided and bottom-sided absorption, which are excited
#187555 - $15.00 USDReceived 21 Mar 2013; revised 15 May 2013; accepted 16 May 2013; published 21 May 2013
(C) 2013 OSA
3 June 2013 | Vol. 21, No. 11 | DOI:10.1364/OE.21.013125 | OPTICS EXPRESS 13132
at main resonant peak wavelengths. The presented absorber exhibits asymmetric absorption
property due to the lack of inversion symmetry. The effective medium theory is applied to
describe this bi-anisotropic absorbing behavior. The impedance-matching and large imaginary
parts of the refractive index together contribute to strong resonant absorption. Such
double-sided absorption property may provide potential applications in thermal emitters,
sensing, etc.
Acknowledgments
This work is supported by the National Natural Science Foundation of China (Grant Nos.
61275030, 61235007 and 61205030), the Opened Fund of State Key Laboratory of Advanced
Optical Communication Systems and Networks, the Opened Fund of State Key Laboratory on
Integrated Optoelectronics, the Fundamental Research Funds for the Central Universities and
the Swedish Foundation for Strategic Research (SSF) and the Swedish Research Council (VR).
We thank Xingxing Chen, Hanmo Gong, Weichun Zhang, Lijun Meng and Hang Zhao for
useful discussions.
#187555 - $15.00 USDReceived 21 Mar 2013; revised 15 May 2013; accepted 16 May 2013; published 21 May 2013
(C) 2013 OSA
3 June 2013 | Vol. 21, No. 11 | DOI:10.1364/OE.21.013125 | OPTICS EXPRESS 13133