Double-sided polarization-independent plasmonic absorber at near-infrared region

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. References and links 1. S. V. Gaponenko, Introduction to Nanophotonics (Cambridge University, 2010), Chap. 6. 2. K. Aydin, V. E. Ferry, R. M. Briggs, and H. A. Atwater, “Broadband polarization-independent resonant light absorption using ultrathin plasmonic super absorbers,” Nat Commun 2, 517 (2011). 3. Y. H. Su, Y. F. Ke, S. L. Cai, and Q. Y. Yao, “Surface plasmon resonance of layer-by-layer gold nanoparticles induced photoelectric current in environmentally-friendly plasmon-sensitized solar cell,” Light Sci. Appl. 1(6), e14 (2012). 4. M. C. K. Wiltshire, J. B. Pendry, I. R. Young, D. J. Larkman, D. J. Gilderdale, and J. V. 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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][6][7][8][9][10][11][12][13][14][15][16][17][18][19][20][21][22][23][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][15][16][17][18][19][20][21][22][23][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 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.

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 w 1 = 280 nm, w 2 = 130 nm and h 1 = h 3 = 40 nm, where w 1 (w 2 ) denotes the width of the top (bottom) square metallic particle, and h 1 (h 3 ) 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 Al 2 O 3 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 h 2 = 10 nm.The total thickness of the MIM layers is only 90 nm.The gap-parameter d equals to 10 nm. 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.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 w 1 = 280 nm and w 2 = 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 nanoparticle width is increased from 250 nm to 290 nm at w 2 = 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 w 1 is far smaller than w 2 .The absorbances from both sides exceed 75% among the cases investigated above, with one exception at w 1 = 290 nm, w 2 = 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%.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 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.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 Al 2 O 3 /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.To explore the underlying nature of this bi-anisotropic absorbing behavior, the equivalent parameters are retrieved from reflection and transmission coefficients [28][29][30][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) -Z medium ) 2 / (Z (λ) + Z medium ) 2 [32].We obtain |R +(1064nm) | = 6% with Z air = 1 and |R -(849nm) | = 14% with Z quartz = 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.

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 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.

Fig. 1 .
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.w 1 (w 2 ) 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.h 1 , h 2 , h 3 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 Al 2 O 3 spacer and quartz substrate are set as 1.75 and 1.45, respectively.The relative permittivities of gold are extracted

Fig. 2 .Fig. 3 .
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.
(c) shows a remarkable red-shift of the resonant wavelength for both top and bottom incidences when the top #187555 -$15.00USDReceived 21 Mar 2013; revised 15 May 2013; accepted 16 May 2013; published 21 May 2013 (C) 2013 OSA

Fig. 4 .
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.

Fig. 5 .
Fig. 5. (a) Three-dimensional perspective of a unit cell with a cross-section (green) at the bottom Al 2 O 3 /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.

#Fig. 6 .
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.