Light absorber based on nano-spheres
on a substrate reflector
Jin Dai,1 Fei Ye,2 Yiting Chen,1 Mamoun Muhammed,2 Min Qiu,1 and
Min Yan1∗
1
Optics and Photonics, School of Information and Communication Technology, KTH - Royal
Institute of Technology, Electrum 229, 164 40 Kista, Sweden
2 Functional Materials, School of Information and Communication Technology, KTH - Royal
Institute of Technology, Electrum 229, 164 40 Kista, Sweden
∗ [email protected]
Abstract:
We systematically study a type of plasmonic light absorber
based on a monolayer of gold nano-spheres with less than 30 nm in
diameters deposited on top of a continuous gold substrate. The influences
of particle size, inter-particle distance, particle-substrate spacer size etc
on the resonance are studied thoroughly with a 3D finite-element method.
We identified that the high-absorption resonance is mainly due to gap
plasmon (coupled through particle bodies) when the separation between
neighboring nano-spheres is small enough, such as close to 1 nm; at larger
particle separations, the resonance is dominated by particle dipoles (coupled
through the host dielectric). Experimentally, an absorber was fabricated
based on chemically-synthesized gold nanoparticles coated with silica shell.
The absorber shows a characteristic absorption band around 810 nm with
a maximum absorbance of approximately 90%, which agrees reasonably
well with our numerical calculation. The fabrication technique can be easily
adapted for devising efficient light absorbers of large areas.
© 2013 Optical Society of America
OCIS codes: (250.5403) Plasmonics; (160.3918) Metamaterials; (310.6628) Subwavelength
structures, nanostructures; (220.4241) Nanostructure fabrication.
References and links
1. M. L. Brongersma and P. G. Kik, Surface Plasmon Nanophotonics (Springer, P.O. Box 17, 3300 AA Dordrecht,
The Netherlands, 2007).
2. K. L. Kelly, E. Coronado, L. L. Zhao, and G. C. Schatz, “The optical properties of metal nanoparticles: The
influence of size, shape, and dielectric environment,” J. Phys. Chem. B 107, 668–677 (2003).
3. E. M. Hicks, S. Zou, G. C. Schatz, K. G. Spears, R. P. Van Duyne, L. Gunnarsson, T. Rindzevicius, B. Kasemo,
and M. Käll, “Controlling plasmon line shapes through diffractive coupling in linear arrays of cylindrical
nanoparticles fabricated by electron beam lithography,” Nano Lett. 5, 1065-1070 (2005).
4. M. Burresi, D. Diessel, D. v. Oosten, S. Linden, M. Wegener, and L. Kuipers, “Negative-index metamaterials:
Looking into the unit cell,” Nano Lett. 10, 2480-2483 (2010).
5. C. J. Brinker, Y. Lu, A. Sellinger, and H. Fan, “Evaporation-induced self-assembly: Nanostructures made easy,”
Adv. Mater. 11, 579-585 (1999).
6. X. Chen, Y. Chen, M. Yan, and M. Qiu, “Nanosecond photothermal effects in plasmonic nanostructures,” ACS
Nano 6, 2550–2557 (2012).
7. H. A. Atwater and A. Polman, “Plasmonics for improved photovoltaic devices,” Nat. Mater. 9, 205-213 (2010).
8. K. Nakayama, K. Tanabe, and H. A. Atwater, “Plasmonic nanoparticle enhanced light absorption in gaas solar
cells,” Appl. Phys. Lett. 93, 121904 (2008).
9. Y. Xiong, R. Long, D. Liu, X. Zhong, C. Wang, Z.-Y. Li, and Y. Xie, “Solar energy conversion with tunable
plasmonic nanostructures for thermoelectric devices,” Nanoscale 4, 4416-4420 (2012).
#181799 - $15.00 USD
(C) 2013 OSA
Received 14 Dec 2012; revised 3 Mar 2013; accepted 4 Mar 2013; published 11 Mar 2013
25 March 2013 / Vol. 21, No. 6 / OPTICS EXPRESS 6697
10. N. Liu, M. Mesch, T. Weiss, M. Hentschel, and H. Giessen, “Infrared perfect absorber and its application as
plasmonic sensor,” Nano Lett. 10, 2342–2348 (2010).
11. J. Hao, J. Wang, X. Liu, W. J. Padilla, L. Zhou, and M. Qiu, “High performance optical absorber based on a
plasmonic metamaterial,” Appl. Phys. Lett. 96, 251104 (2012).
12. A. Tittl, P. Mai, R. Taubert, D. Dregely, N. Liu, and H. Giessen, “Palladium-based plasmonic perfect absorber in
the visible wavelength range and its application to hydrogen sensing,” Nano Lett. 11, 4366–4369 (2011).
13. M. Yan, “Metal-insulator-metal light absorber: a continuous structure,” J. Opt. 15, 025006 (2013).
14. M. K. Hedayati, M. Javaherirahim, B. Mozooni, R. Abdelaziz, A. Tavassolizadeh, V. S. K. Chakravadhanula, V.
Zaporojtchenko, T. Strunkus, F. Faupel, and M. Elbahri, “Design of a perfect black absorber at visible frequencies
using plasmonic metamaterials,” Adv. Mater. 23, 5410–5414 (2011).
15. L. M. Liz-Marzán, M. Giersig, and P. Mulvaney, “Synthesis of nanosized gold-silica core-shell particles,” Langmuir 12, 4329–4335 (1996).
16. P. B. Johnson and R. W. Christy, “Optical constants of the noble metals,” Phys. Rev. B 6, 4370–4379 (1972).
17. H. Xu, E. J. Bjerneld, M. Käll, and L. Börjesson, “Spectroscopy of single hemoglobin molecules by surface
enhanced raman scattering,” Phys. Rev. Lett. 83, 4357–4360 (1999).
18. S. Maier, Plasmonics: Fundamentals And Applications (Springer, 2007).
19. S. Kim, J. Jin, Y.-J. Kim, I.-Y. Park, Y. Kim, and S.-W. Kim, “High-harmonic generation by resonant plasmon
field enhancement,” Nature 453, 757–760 (2008).
20. C. Ciraci, R. T. Hill, J. J. Mock, Y. Urzhumov, A. I. Fernàndez-Domı̀nguez, S. A. Maier, J. B. Pendry, A. Chilkoti,
and D. R. Smith, “Probing the ultimate limits of plasmonic enhancement,” Science 337, 1072–1074 (2012).
21. K. J. Savage, M. M. Hawkeye, R. Esteban, A. G. Borisov, J. Aizpurua, and J. J. Baumberg, “Revealing the
quantum regime in tunnelling plasmonics,” Nature 491, 574–577 (2012).
1.
Introduction
Noble metals, like copper, silver, and gold are used for mirrors since ancient time. However,
when they are patterned in subwavelength nanostructure, light reflection from such planar structure may disappear because light can be strongly coupled to the collective electron excitations
and henceforth damped through collision with lattice and surfaces. This leads to efficient plasmonic light absorbers. The collective electron oscillation excited by optical-frequency electromagnetic wave is known as localized surface plasmon resonances (LSPR) [1]. LSPR is usually
manifested as a characteristic absorption or extinction peak near to its resonant wavelength.
This resonant frequency strongly depends on the corresponding nanostructure’s size, shape,
and the surrounding environment [2]. Thanks to modern nanofabrication techniques such as
electron-beam lithography (EBL) [3], focused-ion beam milling [4], or self-assembly of colloids [5], the applications of optical properties of LSPR have been explored enormously in recent years. For example, a lot of research has focused on utilizing the energy absorption process
and photothermal effect [6] to enhance the efficiency of photovoltaic cells [7, 8] and thermoelectric devices [9]. Also, based on the resonance’s dependence on dielectric environment, Liu
et al demonstrated an absorber which functions as a plasmonic sensor for measuring refractive index variations [10]. A near-infrared absorber based on a similar geometry was presented
in [11]. The strong absorption of light achieved in [10, 11] is achieved by LSPR sustained by
a layer of metallic nanoparticles (NPs) and a planar metal surface. However, in order for such
a metal-insulator-metal (MIM) structure absorber [10, 11] to operate in the visible regime, its
top-layer NPs are required to have a lateral size smaller than 100 nm [12]. Even with EBL process, it’s a formidable task to fabricate these tiny particles, especially when it comes to fabricate
large samples. It was also shown that large-area light absorbers can be made with a thin continuous top-layer metal film instead of a layer of discrete metal particles [13]. However, such
structures have relatively angle-dependent absorption bands. Recently, a large-area plasmonic
absorber in the form of metal-dielectric nanocomposite showing almost 100% absorbance covering all the visible spectra range has been demonstrated by Hedayati et al [14]. It is fabricated
by magnetron sputtering method which is cost-effective and scalable for large area devices.
Syntheses of metal NPs through wet chemistry approaches on the other hand have been well
developed in recent years, mainly due to the advent of nanotechnology [15]. Such NPs, usually
#181799 - $15.00 USD
(C) 2013 OSA
Received 14 Dec 2012; revised 3 Mar 2013; accepted 4 Mar 2013; published 11 Mar 2013
25 March 2013 / Vol. 21, No. 6 / OPTICS EXPRESS 6698
suspended in a liquid, can be prepared in large volumes. If they are deposited on a carefully
prepared substrate, large-area absorber can be realized. Here, we study such a plasmonic absorber structure, which can be easily prepared with chemically synthesized gold nano-spheres.
We numerically found that the absorber can be engineered to absorb light with a peak absorption at any wavelength from visible to 800 nm with a maximum absorbance larger than 70%.
Additionally, the absorption is almost angle- and polarization-independent, which is desirable
for solar energy harvesting devices. We also experimentally demonstrate one of these absorbers
which has an absorption peak centered at 810 nm.
2.
Numerical investigation
t
s
h
x
y
z
Fig. 1. Schematic view of the proposed absorber.
The absorber to be developed can be modeled by the idealized structure schematically shown
in Fig. 1. A periodic array of gold nano-spheres, which are embedded in a silica layer, are
distributed in a 2D triangular lattice on a continuous gold reflector. Very importantly, the nanospheres are separated from the gold reflector by a spacer, which is a part of the silica host. The
total thickness of the silica layer is t, the spacer thickness is s, and the gold reflector thickness
is h. The interparticle separation is g, and the diameters of the spheres are d. In the following
simulations we set t = 35 nm, s = 10 nm, h = 100 nm, d = 8 nm, and g = 1.8 nm unless
otherwise specified. The 3D simulations are carried out by the finite element method using
COMSOL Multiphysics. The optical constants of gold are from data measured by Johnson and
Christy [16]; the refractive index of silicon dioxide is fixed at 1.5. The top-view of the unit cell
used in our simulations is as shown later. The top and bottom sides of the unit are covered with
perfectly matched layers; four sides are defined as periodic boundary conditions. We start with
structures consisting identical spheres; the scenario with different sphere sizes will be briefly
mentioned at the end of this section.
After extensive numerical case studies, we determined that the most critical geometrical parameter of this absorber is the particle-to-particle gap g. To show the effect of g on the resonance
of the absorber, we calculate the absorption spectra for the d = 8 nm absorber at various g values
for normal incidence case, as summarized in Fig. 2. Incoming light is polarized along x direction. All curves exhibit two distinctive absorption bands: one insensitive to g at the blue side of
the visible spectrum, and one sensitive to g at the red side of the spectrum. The absorption at
short wavelength is predominately from inherent material loss of gold due to interband electron
transitions; the absorption band at longer wavelength is due to LSPR. It is shown that a decrease
in g contributes to a red-shift of the LSPR resonant wavelength, and the amount of shift seems
to increase with the decrease of g [17]. Since the nano-spheres are closely placed, the LSPR is
#181799 - $15.00 USD
(C) 2013 OSA
Received 14 Dec 2012; revised 3 Mar 2013; accepted 4 Mar 2013; published 11 Mar 2013
25 March 2013 / Vol. 21, No. 6 / OPTICS EXPRESS 6699
1
0.9
0.8
Absorbance
0.7
0.6
0.5
0.4
0.3
g=1 nm
g=2 nm
g=3 nm
g=4 nm
g=5 nm
0.2
0.1
0
400
450
500
550
600
650
700
750
800
Wavelength (nm)
Fig. 2. Calculated absorption spectra for d = 8 nm the absorber at various interparticle
separation g at normal incidence.
certainly spectrally different from the dipole resonance originated from individual spheres. It
would be interesting though to see how the LSPR resonance peak differs from the individual
particle dipole resonance. For small spheres with diameter less than 10 nm, the phase retardation across the sphere can be neglected and quasi-static approximation can be applied. An
individual sphere has its resonant wavelength satisfying the relation Re{εAu } + 2εSiO2 = 0 [18],
which gives rise to a resonant free-space wavelength of approximately 520 nm. While from Fig.
2 it is evident that by placing the spheres in close adjacency with each other, one can achieve a
high-absorption band at a much longer wavelength.
To probe the nature of the resonance, we look into the field distribution of the absorber
at the resonance. Figures 3(a) and 3(b) show the norm of electric displacement field for the
g = 1 nm and g = 5 nm absorbers at their resonances, respectively. For the absorber with
g = 1 nm [Fig. 3(a)], we see that the field is dominantly distributed in the gap between neighboring nano-spheres. Such a highly localized field confined in a dielectric gap between two
metallic bodies usually referred to as a gap plasmon. Similar strong field enhancement in a
dielectric gap between two metal tips was well discussed in works related to bowtie nanoantenna [19]. When the interparticle gap becomes larger [Fig. 3(b)], the field strength in the gap
region weakens sharply, and the resonance is closer to resonance of individual particle dipoles.
It is understandable that at other gap values, the resonance can be an equal mixture of the two
above-mentioned resonances. It should be mentioned that as the structure has a periodicity of
only ∼ 10 nm, each gap plasmon (particle dipole) in Fig. 3 is bonded with their neighboring gap
plasmon (particle dipole). This is vital for understanding absorber structures with non-uniform
nano-sphere inclusions.
Our investigation on the effect of the gap size stops at g = 1nm. Further increase of the
LSPR resonance in wavelength by reducing g to a value less than 1 nm is possible but limited
by quantum effects [20, 21], which are out of the scope of this work.
So far we have used a fixed sphere diameter d = 8 nm. In order to see the effect of changing
d on the LSPR resonance wavelength, we simulated the absorption spectra of absorbers with a
constant g = 1 nm whereas d is varying from 6 to 12 nm. The results are summarized in Fig. 4.
It is seen that an increase in the diameter of the spheres shifts the resonant wavelength further
to a larger value, slightly outside of visible spectrum but below 800 nm. One feature worth
noticing is that when the diameter of nano-spheres gets bigger, the absorption peak becomes
lower.
Through the above analyses, we therefore conclude that by choosing g and d carefully, one
#181799 - $15.00 USD
(C) 2013 OSA
Received 14 Dec 2012; revised 3 Mar 2013; accepted 4 Mar 2013; published 11 Mar 2013
25 March 2013 / Vol. 21, No. 6 / OPTICS EXPRESS 6700
y
y
x
x
(a)
(b)
Fig. 3. Calculated norm of electric displacement field (C/m2 ) at the cut plane parallel to
the gold reflector and through the center of the nano-spheres for (a) g = 1 nm absorber at
680 nm wavelength and (b) g = 5 nm absorber with d = 8 nm at 560 nm wavelength for Ex
polarized field at normal incidence in one unit cell.
can achieve high absorption at any wavelength from visible up to ∼ 800 nm.
1
0.9
0.8
Absorbance
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0
400
d=6 nm
d=8 nm
d=10 nm
d=12 nm
450
500
550
600
650
700
750
800
Wavelength (nm)
Fig. 4. Calculated absorption spectra for g = 1 nm absorber as a function of sphere diameter
d at normal incidence.
In a recent experimental work a similar type of plasmonic absorber was fabricated [14]. The
absorbers developed have a reflector at the bottom, a dielectric spacer in the middle, and a goldnanoparticle layer on the top which is deposited through the magnetron sputtering process. The
major difference is that there are “multiple layers” of gold particles randomly distributed in the
dielectric host. In the reference it was argued that the broadband absorption of the structures
is partially attributed to the magnetic-dipole resonance sustained by the top nano-composite
and the bottom reflector, as was found in a MIM absorber structure [11]. However, through our
simulations, we didn’t see any such magnetic-dipole resonance from 400 nm to even 1800 nm.
Our analysis was also extended to absorbers consisting multiple layers of nano-spheres, and
structures with several nano-spheres sizes.
In order to clearly identify the role of the reflector, we further compare the absorption spectra
of the absorber for the structure with and without gold reflector (i.e. with air termination below
SiO2 ). In the meantime, we change the spacer thickness s between the nano-sphere layer and
the reflector. The diameters d are fixed as 8 nm, and the total thickness of the silica layer
t = s + 25 nm. As depicted by Figs. 5(a) and 5(b), the absorption spectra for structures with and
#181799 - $15.00 USD
(C) 2013 OSA
Received 14 Dec 2012; revised 3 Mar 2013; accepted 4 Mar 2013; published 11 Mar 2013
25 March 2013 / Vol. 21, No. 6 / OPTICS EXPRESS 6701
without the gold reflector change periodically with an increasing s. In both Figs. 5(a) and 5(b),
the additional spacer thickness necessary for achieving the maximum absorbance at the same
wavelength position translates into an additional phase shift of 2π for the light in the spacer
medium. Effectively the structure acts like a Fabry-Pérot Étalon with one reflector formed by
the nano-sphere layer and the other by the bottom gold reflector or just the air termination. It
is also seen that the maximum absorbance for the structure with gold reflector is about twice
of that for the structure without gold reflector. Additionally, it is interesting to notice that the
enhancement of absorption is particularly evident at the blue-side of the visible spectrum, where
the absorption is mostly due to inherent material loss of gold.
(a)
(b)
Fig. 5. Calculated absorption spectra for the t = s + 25 nm, d = 8 nm, g = 1.8 nm absorber
(a) with bottom gold reflector (b) without bottom gold reflector as a function of s at normal
incidence.
The absorption characteristics of the nano-spheres based absorber is found to be insensitive to
light incidence and almost completely independent of polarization. We performed a simulation
with the angle of incidence changed from 0◦ to 80◦. As shown in Fig. 6, the absorption profile
doesn’t change much for incident angle up to 60◦ . This is desirable for, e.g. improving the
efficiency of photovoltaic cells.
Fig. 6. Calculated absorption spectra for the d = 8 nm, g = 1.8 nm absorber as incident
angle increases from 0◦ to 80◦ .
In the discussions above, we have only considered structure with identical spheres. However, it is in general not possible to fabricate nano-spheres of uniform size in practice. Refer
#181799 - $15.00 USD
(C) 2013 OSA
Received 14 Dec 2012; revised 3 Mar 2013; accepted 4 Mar 2013; published 11 Mar 2013
25 March 2013 / Vol. 21, No. 6 / OPTICS EXPRESS 6702
to the unit cell shown in Fig. 3. Starting from a uniform-sphere structure with d = 10 nm and
g = 1 nm, we reduce the center sphere’s diameter dm to 9 nm and then to 8 nm while keeping the corner spheres intact. All sphere positions are not changed. The absorption spectra of
the three structures subject to a x-polarized normally-incident light are shown in Fig. 7. By
changing the center sphere to a smaller size, we see that a new absorption peak arises on the
left side of the main (“original”) LSPR absorption peak. The reduced center sphere diameter
effectively introduces a new particle-particle gap size; the modified geometry, according to our
previous discussion, effectively introduces a new absorption peak at smaller wavelength position. The “original” LSPR absorption peak is also shifted because the main LSPR and the
newly-introduced resonance are not de-coupled. Finally in Fig. 7 we compare the spectra between the structure with dm = 8 nm and that with all sphere diameters at 8 nm (hence g = 3 nm).
It is seen that the additional absorption peak in the nonuniform-sphere structure coincides with
the single absorption peak of the uniform-sphere structure.
1
0.9
0.8
Absorbance
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0
400
d=10 nm, g=1 nm
d =9 nm, g=1 nm
m
dm=8 nm, g=1 nm
d=8 nm, g=3 nm
450
500
550
600
650
700
750
800
Wavelength (nm)
Fig. 7. Calculated absorption spectra for dm = 9 nm, g = 1 nm and dm = 8 nm, g = 1 nm
absorber with nonuniform sphere size, in comparison with d = 10 nm, g = 1 nm and d = 8
nm, g = 3 nm absorber with identical sphere size.
3.
Experimental realization
Based on the discussions in Section 2, we designed and fabricated an absorber as depicted in
Fig. 8. The absorber was fabricated on a glass substrate, which was deposited with a 100 nmthick gold and followed by a 3 nm-thick aluminium oxide using an electron beam physical
vapor deposition. A 4 nm-thick Titanium layer was used between the gold layer and the glass
substrate as an adhesive layer. Ideally in our design, a monolayer of silica-coated gold nanospheres with gold sphere diameter at 20 nm and the silica shell thickness at 1 nm are placed on
top in a triangular lattice. Such a structure exhibits LSPR-induced absorption bands similarly
as the structure which we discussed in Section 2. The only difference is in the dielectric host.
Practically such an absorber based on core-shell nano-spheres are much easier to realize.
Gold NPs were prepared chemically via a sol-gel method, by reducing 10 mM hydrochloroauric acid (HAuCl4 ) using 20 mM ascorbic acid and 1 mM sodium borohydride
(NaBH4 ) in the presence of aqueous solution of cetyltrimethyl ammonium bromide (CTAB,
0.2 M) and silver nitrate (AgNO3 , 2 mM) at room temperature. The mixture was stirred for
one hour and kept at 25◦C overnight. To coat the gold nanoparticles with silica shell, 1 mL
of obtained suspension of gold NPs was diluted to 20 mL and the pH was tuned to ca. 12.
When the temperature of this suspension was elevated to 70◦ C , 5 µ L of tetraethyl orthosilicate
(TEOS) was added, and the solution was collected after 1 h reaction and centrifuged to obtained
#181799 - $15.00 USD
(C) 2013 OSA
Received 14 Dec 2012; revised 3 Mar 2013; accepted 4 Mar 2013; published 11 Mar 2013
25 March 2013 / Vol. 21, No. 6 / OPTICS EXPRESS 6703
11 nm
SiO2
10 nm
3 nm 100 nm
Au
z
x
Al2O3
y
Fig. 8. Schematic view of the idealized silica-coated gold NPs based absorber.
silica-coated gold NPs (Au@SiO2 ).
Fig. 9. TEM micrographs of (a) gold NPs before coating; (b) Au@SiO2 core-shell NPs.
The morphology of Au and Au@SiO2 NPs, as shown in Fig. 9, was characterized by JEM2100F field emission transmission electron microscope (TEM) operating at accelerating voltage
of 200 kV. The concentration of gold in colloidal suspensions was measured by inductively
coupled plasma atomic emission spectroscopy (ICP-AES). The concentration of gold (element)
and number of NPs in Au@SiO2 suspension are about, Au: 120 ppm (µ g/mL); NPs: 1.505 ×
1022 /mL. In addition, the Au@SiO2 NPs in aqueous solution have a dark red appearance.
Then 170 µ L Au@SiO2 are deposited on the coated substrate. After that, the sample is baked
in a vacuum oven at 70◦ C for 2 hours. The SEM micrograph of the surface of the fabricated
absorber is given in Fig. 10.
The setup for measuring reflectance is schematically shown in Fig. 11. A supercontinuum
white light source (NKT SuperK Compact) with a repetition rate of 27 KHz is connected to
a reflective collimator through a single-mode fiber. The collimated beam diameter is about
8.5 mm. The collimated beam then passes through a 600 µ m pinhole. And then the beam is
focused by an achromatic lens ( f = 45 mm) on to the sample. On the back side of the sample,
a 20× objective ( f = 200 mm) and a CCD camera monitored with a PC are used to track the
#181799 - $15.00 USD
(C) 2013 OSA
Received 14 Dec 2012; revised 3 Mar 2013; accepted 4 Mar 2013; published 11 Mar 2013
25 March 2013 / Vol. 21, No. 6 / OPTICS EXPRESS 6704
Fig. 10. SEM micrographs of the fabricated absorber.
Source
C
P
L Sample L
CCD
FH
C: collimator
L
P: pinhole
L: lens
FH: fiber holder
OSA
Fig. 11. Schematic setup for the reflectance measurement.
position of the beam on the sample. The reflected light was focused by another achromatic lens
on the fiber core to couple the reflected light into a multi-mode fiber which is connected to the
Optical Spectrum Analyzer (OSA).
45%
1
Simulation
Experiment
0.9
0.8
35%
0.7
Absorbance
40%
30%
0.6
25%
0.5
20%
0.4
15%
0.3
0.2
10%
0.1
5%
0
600
700
800
900
1000
Wavelength (nm)
(a)
1100
1200
0%
11
14
17
20
23
26
Particle size (nm)
(b)
Fig. 12. (a) Simulated and measured absorption spectra of the fabricated absorber at 15◦
angle of incidence; (b) particles sizes distribution based on 146 NPs as in Fig. 9(a).
The reflection (R) is measured using the system depicted above. The transmission can be
neglected since the bottom gold reflector is much thicker than the skin depth in the concerned
wavelength range. The absorbance (A) can be calculated by A = 1 − R. Figure 12(a) shows
the measured absorption spectrum of the fabricated sample at 15◦ incident angle. A distinctive
absorption peak around 810 nm wavelength is noticed. We also performed simulation with
the idealized structure as given in Fig. 8. The calculated curve is superimposed in Fig. 12(a).
#181799 - $15.00 USD
(C) 2013 OSA
Received 14 Dec 2012; revised 3 Mar 2013; accepted 4 Mar 2013; published 11 Mar 2013
25 March 2013 / Vol. 21, No. 6 / OPTICS EXPRESS 6705
The agreement between our simulation and experiment is reasonably good. We notice that the
measured absorption value is in general higher than the simulated one, and the absorption band
is also broader. This discrepancy should be due to the fact that the fabricated absorber is not
perfectly flat at the scale of the focus beam size, which is limited by the current NP deposition
process. The imperfect surface on one hand leads to random light scattering (therefore loss).
On the other hand it would lead to a reflected beam with a slightly different (usually larger)
shape, which in turn would cause the collected signal to be weaker. The size distribution of the
NPs sampled based on 146 NPs is seen in Fig. 12(b). The average size is slightly more than
20 nm, which can be the reason why the measured absorption peak is at a longer wavelength
than the calculated one (i.e. based on 20 nm nano-spheres). The broader absorption peak of
the measured curve can be attributed to the inhomogeneity in particle size, shape and shell
thickness, as expected.
4.
Conclusion
In conclusion, we have numerically and experimentally investigated the absorption characteristics of absorbers that are easily producible at a large scale based on chemically synthesized
gold-silica core-shell NPs. The highly efficient absorption band due to LSPR can be tailored to
any wavelength from visible to 800 nm by carefully engineering the size of the gold particle
and the separation between each particle. Our calculation shows that the high absorption band
is not due to magnetic-dipole resonance between the NPs and the bottom gold layer. However,
an inclusion of the bottom reflector, together with an appropriate spacer thickness, can indeed
drastically increase the absorption efficiency of the overall structure. We also experimentally
demonstrated an absorber based on Au@SiO2 core-shell NPs which has more than 50% average absorption from 600 nm to 900 nm. The maximum absorption value is measured at ∼ 90%
centered at 810 nm. This approach effectively avoids expensive and tedious lithographic patterning process. Therefore it can be a practical and economical way for fabrication of, e.g.
photo-thermo-electric energy conversion devices, provided that the light-induced heat can be
efficiently converted to electricity.
Acknowledgments
This work is supported by the Swedish Research Council (VR) and VR’s Linnaeus center in
Advanced Optics and Photonics (ADOPT). J. D. would like to acknowledge scholarship received under the European (Erasmus Mundus) Master of Science in Photonics programme.
#181799 - $15.00 USD
(C) 2013 OSA
Received 14 Dec 2012; revised 3 Mar 2013; accepted 4 Mar 2013; published 11 Mar 2013
25 March 2013 / Vol. 21, No. 6 / OPTICS EXPRESS 6706