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Experimental and simulated study of a composite structure metamaterial absorber
Optical Materials 73 (2017) 111e118
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Optical Materials journal homepage: www.elsevier.com/locate/optmat
Experimental and simulated study of a composite structure metamaterial absorber Shengyong Li*, Xiaochuan Ai, Ronghua Wu, Jiajun Chen Naval Univ. of Engineering, Wuhan 430033, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 10 June 2017 Received in revised form 26 July 2017 Accepted 2 August 2017
In this paper, a high performance metamaterial absorber is designed and experimental studied. Measured results indicate that a perfect absorption band and a short-wavelength absorption peak are achieved in the near-infrared spectrum. Current strength distributions reveal that the absorption band is excited by the cavity resonance. And electric field distributions show that the short-wavelength absorption peak is excited by the horizontal coupled of localized surface plasmon (LSP) modes near hole edges. On the one hand, the absorption property of the measured metamaterial absorber can be enhanced through optimizing the structural parameters (a, w, and H). On the other hand, the absorption property is sensitive to the change of refractive index of environmental medias. A sensing scheme is proposed for refractive index detecting based on the figure of merit (FOM) value. Measured results indicate that the proposed sensing scheme can achieve high FOM value with different environmental medias (water, glucose solution). © 2017 Published by Elsevier B.V.
OCIS codes: 160.3918 160.4236 260.1180 260.3910 160.5298 Keywords: Metamaterials Absorption band Impedance match
1. Introduction Artificially prepared electromagnetic metamaterial has drawn a surge of attention over past decade due to its unique electromagnetic properties, such as negative refractive index, negative permittivity, negative permeability, etc. Since the negative refractive index is experimental demonstrated by Smith et al. [1], a large number of novel study on electromagnetic metamaterials are proposed in a wide range of applications, include optical black hole, sub-diffraction imaging, wavelength selective blackbody emitters, invisibility cloaking [2e8], and many more. To characterize these electromagnetic metamaterial devices, an effective medium technique [9,10] is usually adopted (including complex refractive, permittivity and permeability). Among these functional devices, metamaterial absorber has received the widespread attention [11e15]. Many studies prove that plasmonic nanostructure metamaterial absorbers are highly desirable for many
* Corresponding author. E-mail address: [email protected] (S. Li). http://dx.doi.org/10.1016/j.optmat.2017.08.002 0925-3467/© 2017 Published by Elsevier B.V.
practical applications with one or more perfect absorption bands, for instance, metamaterial absorbers [16,17], photodetectors [18] photovoltaic cells [19], and optical imaging devices [20]. In order to design and manufacture a perfect metamaterial absorber, the plasmonic nanostructure is always arranged by simultaneously minimizing the reflectance between the top metallic layer of the absorber and the free air layer, while eliminating the transmittance through maximizing the material loss of the absorber [21e25]. In these metamaterial absorbers, metallic/dielectric/ metallic (MDM) structural absorbers demonstrate the potential value with perfect absorption bands, wide-angle incidence, and almost completely polarization insensitivity. The MDM structural absorber usually consists of a top periodically patterned metallic layer which serves as an electric resonator, an intermediate dielectric layer, and a thick enough bottom metal layer that serves as an optical mirror to reduce the transmittance. To date, perfect MDM metamaterial absorbers have been researched in a wide range, including single-band [26], dual-band [27], and triple-band [28] absorptions. Many single-band MDM metamaterial absorbers [26,29] are proposed in the near-infrared wavelength range, which exhibit excellent absorption properties, such as near-perfect
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absorption peak, wide-angle incidence, and polarization insensitivity. These metamaterial absorbers always operate at a narrow wavelength band. This fact greatly limits their practical applications in many devices, such as imaging and spectroscopic detection [30]. Therefore, single-band near-infrared metamaterial absorber with a wide and flat absorption band is in demand. On the other hand, many physical mechanisms are proposed in understanding the property of metamaterial absorbers, such as destructive interference mechanism, strong antisymmetric surface plasmons coupling, cavity resonance and electrical resonance [31e33]. However, few researchers focus on the role of the horizontal coupling of localized surface plasmon (LSP) modes on the property of metamaterial absorbers. In the previous work [34], the transmission of metamaterial is reduced and resonance frequency is shifted to lower frequency (longer wavelength) due to the intensity of the horizontal coupling LSP modes increasing. It is also worth studying the effects of the horizontal coupling LSP modes. In this paper, a composite patterned metamaterial absorber is suggested and manufactured. Measured results indicate that the designed single-band absorber reveals perfect absorption property in near-infrared range. A short-wavelength absorption peak is achieved due to the horizontal coupling LSP modes. The absorption property of the measured absorber can be modulated through adjusting dimension parameters, which results in the proposed absorber more attracting in designing and manufacturing near-
infrared perfect metamaterial devices. The possibility of application of the measured absorber in sensing field is studied based on the figure of merit (FOM). 2. Simulation methods and experimental details 2.1. Simulation methods The suggested metamaterial absorber is simulated through adopting a business Software Ansofts HFSS 13.0. As shown in Fig. 1(a-b), the proposed metamaterial absorber consists of three functional layers: a top patterned metal plane layer with a compound air hole array, a dielectric layer, and a metal plane layer which plays as the bottom reflector. The metal layers are selected as silver layers. The incident wave is assumed to be normally to the top surface of the proposed metamaterial absorber. In the unit cell, the periodicity is given by “P”, the thickness of metal layer and dielectric layers are set as “h” and “H”. Two 160 nm thick metal layers are separated by a 480 nm thick dielectric layer, which results in a total metamaterial thickness of 0:8mm, as shown in Fig. 1. Geometric parameters are shown in Table 1. The silver layer is thick enough that results in the electromagnetic transmission closing to zero (TðlÞz0). Therefore, the simulated absorption of the designed MDM metamaterial absorber can be calculated as AðlÞ ¼ 1 RðlÞ. Here, AðlÞ is the wavelength-dependent absorption rate, and RðlÞ is
Fig. 1. (a) Top view of the unit cell on the xoy plane. (b) Side view of the unit cell on the xoz plane (the cross section of the unit cell). The yellow part is metal layer, the gray part is dielectric layer, the dark blue part is substrate. (c) Measured absorption, reflection, and transmission spectra. (d) photograph of samples. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
S. Li et al. / Optical Materials 73 (2017) 111e118 Table 1 Structural parameters of the proposed absorber. Parameter
P
w
a
b
h
H
Value (mm)
22
12
1.2
1.8
0.16
0.48
the wavelength-dependent reflection rate. The silver layer follows the Drude model:
εðuÞ ¼ 1 Here,
gD ¼ 9
u2
u2p iugD
up ¼ 1:37 1016 s1
(1) is
the
plasma
frequency,
1013 s1
is the collision frequency [35]. In this paper, the dielectric layer is the SiO2 layer [36]. In simulation, the business Software Ansofts HFSS 13.0 is utilized to research the relationship between the current intensity distribution and the resonant wavelength. In simulations, periodic boundary conditions are applied to the four sidewalls [37]. 2.2. Experimental details
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are fabricated as following: first, choose a piece of cleaned glass as a substrate. A silver layer (0:16mm-thick) is evaporated on the glass substrate at a rate of 1.8 Å s1 by using the electron beam evaporation. Then, a dielectric layer (0:48mm-thick) is evaporated on the first silver layer at a rate of 2.3 Å s1 by also using the electron beam evaporation. Another silver layer (0:16mm-thick) is also evaporated on the dielectric layer through the same method. Finally, the proposed composite structure holes array are patterned in the top silver layer by using a focused ion beam system, as shown in Fig. 1(d). The measured absorption spectra of the sample is shown in Fig. 1(c). A short-wavelength absorption peak (the absorption magnitude is 61%) and an absorption band (the average absorption is 96.4% and the band width is 240 nm) are obtained. The manufactured area of our samples is around 2:0 2:0mm2 . The samples is take pictures by Leica DM2700 M (Using black and white contrast mode, contrast ratio is 45%), as shown in Fig. 1(d). To exploit the physical origin of the absorption band in Fig. 1(c), current strength distributions of the resonant mode (the central wavelength of the absorption band is l ¼ 2390 nm) is elucidated [38]. Fig. 2(a) and (b) present the amplitude and vector of current distributions in top and bottom silver layers under the wave at
To validate the proposed metamaterial absorber, the samples
Fig. 2. Cavity resonance, at the central wavelength l ¼ 2390nm of the absorption band (a ¼ 1:2mm,H ¼ 0:48mm): (a) surface current distribution in the top silver layer, (b) simulated surface current distribution in the bottom silver layer. (c) simulated absorption, reflection, and transmission spectra.
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addition, the proposed metamaterial absorber shows a zero reflection and transmission characteristics, which indicates a broad range of potential applications. Outside the absorption band, real parts of m and ε are imbalanced, which results in the resonance absorption reducing. On the other hand, imaginary parts are shown in Fig. 3(b). Imaginary parts of the m and ε are minimized in the absorption band, which indicates that the proposed metamaterial absorber reveals low reflection loss, as shown in Fig. 3(b). 3. Study on geometrical variation After revealing the physical phenomenon of the absorption property in Fig. 1(c) by near-field profiles, the metamaterial absorber is further measured through optimizing structural parameters (a, w, H). More of experiments are carried out: the first set of experiments achieved through varying the width of sub-holes (a) while fixed structural parameters (w, H), the second set is varied w while fixed structural parameters (a, H), the third set is varied the H while fixed structural parameters (a, w). 3.1. Effect of a, w, and H
Fig. 3. Effective permeability, permittivity, and impedance of the designed metamaterial absorber with absorption spectra (a ¼ 1:2mm,H ¼ 0:48mm).
normal incidence. Fig. 2(c) shows the simulated absorption, reflection, and transmission spectra of the designed metamaterial absorber. It can be found that a near perfect absorption band is also achieved. These simulated results are agreed well with measured results in Fig. 1(c), which indicates that simulations are effective. Apparently, at the wavelength l ¼ 2390 nm, the calculated current displacement vectors in the top silver layer is opposite to that in the bottom silver layer. As shown in Fig. 2(a) and (b), these anti-parallel currents displace circulating currents between top and bottom silver layers, which indicates that the absorption band is excited by the cavity mode resonance [33]. Effective parameters, including the permeability (m), the permittivity (ε), and the impedance (z), are fundamental parameters, which are used to characterize electromagnetic responses of metamaterial. To facilitate the description of electromagnetic properties of the designed metamaterial absorber, effective parameters of the designed metamaterial absorber are calculated [9,10] by adopting the S parameter retrieval method. As shown in Fig. 3, the real part of m reveals a magnetic resonant mode, which results in an anti-resonance of the real part of ε [39]. Meanwhile, the effective impedance z can be given by:
sffiffiffiffiffiffiffiffiffi mðlÞ zZo Z¼ εðlÞ
Fig. 4 shows the absorbance as a function of a and wavelength at normally incident wave with keeping w ¼ 12mm and H ¼ 0:48mm. In the first set of experiments, a is increased from a ¼ 1:2mm to a ¼ 1:5mm. Obviously, the resonant absorption band reveals a shifted to longer wavelength. Meanwhile, an increase of a results in the band width initially (a ¼ 1:2mm, the average absorption is 96.4% and the band width is 240 nm) increasing, reaching a maximum band width (a ¼ 1:4mm, 99.1% and 430 nm), and then decreasing (a ¼ 1:5mm, 92.2% and 295 nm). It can be deduced that the impedance matching is partly defined by a. A higher impedance matching condition between the top silver layer and the air interface is achieved with a increasing. For the maximize absorption band width (a ¼ 1:4mm,99.1% and 430 nm) case, the absorption of the designed metamaterial absorber close to 100% which implies that a perfect impedance matching condition is obtained [41]. Therefore, when a > 1:4mm, the perfect impedance matching condition is invalid which leads to the band width and the average absorption reduce, as shown Fig. 4(a). On the other hand, the shortwavelength absorption peak shifts to shorter wavelength, and the absorption is reduced from 61% to 51%, see the arrow in Fig. 4(a). The second set of experiments is carried out by varying w while fixed structural parameters (a ¼ 1:2mm,H ¼ 0:48mm), as shown in
(2)
In the absorption band, real parts of m and ε are almost balanced (it is obviously that the slop of the real parts of m and ε are similar). Moreover, real parts of m and ε cross zero at the central wavelength of the absorption band at l ¼ 2390nm, which results in the impedance match achieving between the metamaterial absorber and the free space around the wavelength according to Equation (2). On the one hand, Ding et al. reported that the reflection can be minimized based on the impedance match achieving [40]. In
Fig. 4. Measured absorption spectra of the designed metamaterial absorber with different structural parameters.
S. Li et al. / Optical Materials 73 (2017) 111e118
Fig. 4(b). Similarly, an increase in the w results in the band width initially (w ¼ 12mm, 96.4% and 240 nm) increasing, reaching a maximum band width (w ¼ 13mm, 99.6% and 465 nm), and then decreasing (w ¼ 13:5mm, 86.8% and 441 nm). The absorption band shows a shifted to longer wavelength, too. The short-wavelength peak shifts to shorter wavelength and is reduced from 61% to 47%, see the arrow in Fig. 4(b). It can be deduced that the structural parameter w also defines the impedance matching condition of the designed metamaterial absorber. Hence, the appropriate structural design with impedance matched top metallic layer surface is highly desired in exploiting perfect metamaterial absorber. In order to confirm the impedance matching condition, the effective impedance is retrieved with a or w increasing. The physical mechanism of the short-wavelength peak would be discussed in the following contents. Fig. 5 shows the effective impedance spectra with different optimal structural parameters (a, w). It is obviously that the effective impedance is close to 1 in resonance absorption bands based on optimizing structural parameters (a, or w), as shown in Fig. 5(a-b). This phenomenon is similar to the results in Fig. 3(a). These retrieved results indicate that the impedance matching condition can be modulated by optimizing structural parameters. Therefore, the absorption can be maximized in the resonant band. The third set of experiments is carried out with the dielectric layer thickness (H) increasing while other parameters unchanged, as shown in Fig. 6. The cavity resonant band shows a shifted to longer wavelength with H increasing from 0:48mm to 0:54mm. And the band width is increased form 240 nm to 413 nm obviously, which is similar to the reported work [33]. And the average absorption is enhanced from 96.4% to 97.3%. This is mainly owing to that the thickness of dielectric layer also plays a key role in revealing the high absorption property [42e44]. It should be noted that the electromagnetic wave is partially multiple reflected from the air-spacer interface with a phase shifted between the top and the bottom silver layers. When the electromagnetic wave is incident on the designed structure, multiple reflections from top and bottom layers would constructively interfere with each other on the surface of the top silver layer. Therefore, the whole multiple reflections will destructively interfere to minimize. The increasing of H leads to the phase shifted increase to achieve a new ideal interference condition to constructively interfere the whole multiple reflections. Therefore, the cavity resonant band shows a shifted to longer wavelength, as shown in Fig. 6. When H > 540 nm, the multiple reflections destructively interfering will be damaged due to the impedance matching condition is invalid. Therefore, the average absorption would be reduced and the bandwidth shrink down. However, the resonant wavelength of the short-wavelength absorption peak almost unchanged with H increasing. One can deduce that the physical mechanism of the short-wavelength absorption peak is independent of the dielectric layer. This will be verified in the following content. These measured results in Figs. 4 and 6 confirm that these structural parameters (a, w, H) mainly define the impedance matching condition of the designed metamaterial absorber. 3.2. Effect of angle of incidence (q) The absorption performance of the proposed metamaterial absorber is measured in the fourth of experiments. In this paper, the angle of incidence can be defined as the angle between the electromagnetic wave vector and the z-axis of the coordinates. In practical applications, most of the electromagnetic waves need to be absorbed, which may contain arbitrarily angled components. Fig. 7 shows the absorbance as a function of angle of incidence and wavelength. In the fourth set of experiments, the designed
115
Fig. 5. Effective impedance of the designed metamaterial absorber with absorption spectra.
Fig. 6. Measured absorption spectra of the designed metamaterial absorber with different dielectric layer thickness (H).
structure is measured for normal, 30 , 45 , and 60 angles of incidence. The absorption bandwidth of the designed structure is reduced from 240 nm to 80 nm. As illustrated in Fig. 7, the value of the short-wavelength peak almost remains unchanged with q up to 60 . Moreover, the absorption band shows a shifted to shorter wavelength as q increasing, which is mainly consisted with the reported single-band MDM absorber [29], in which the resonant wavelength is shifted to shorter in the TM case.
3.3. Discussion on the short-wavelength absorption peak To obtain a cohesive picture of the absorptive characteristics of the short-wavelength peak at l ¼ 2103nm in Fig. 1(c), the patterned of electric field distributions are computed in xoz plane (the cross section of the unit cell), as shown in Fig. 8. Two calculation wavelengths are selected in simulations around the resonant wavelength of the absorption peak, such as at 2040 nm and 2150 nm. It is obviously that only two localized surface plasmon (LSP) modes can be observed on the hole edges at both selected calculation wavelengths, as shown in Fig. 8(a, c). For the resonant wavelength of the absorption peak at 2390 nm, two LSP modes are excited on the holes edges. Moreover, the coupled phenomena between these LSP
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w increasing, which results in a decrease in horizontal coupling strength of LSP modes. Therefore, the absorption of the resonant peak is reduced in Fig. 4(a and b). However, the coupling strength is almost unchanged with the H increasing in Fig. 6. This phenomenon is similar to the previous work [34], where the resonant transmission peak is shifted to lower frequency (longer wavelength) in the case of the coupling strength increased. In other words, the resonant transmission peak will shift to higher frequency (shorter wavelength) when the coupling strength is reduced, which is consistent with these results in Fig. 4(a and b). These results in Figs. 4 and 8 indicate that the coupling of horizontal LSP modes plays an important role in designing metamaterial absorber. 3.4. Discussion on the application of the proposed metamaterial absorber
Fig. 7. Measured absorption spectra of the designed metamaterial absorber with different angle of incidence (q): (a) 0 , (b) 30 , (c) 45 , (d) 60 .
In practical applications, metamaterial absorber is always used in different surrounding mediums. The absorption performance of the metamaterial absorber will be disturbed by the variation of the refractive index of surrounding mediums, which are surrounded on the metamaterial absorber surface in the application process. These influences are mainly reflected in that absorption rate reduction, resonance wavelength deviation, bandwidth shrink, and so on. Usually, metamaterial absorbers are designed and optimized to reduce the influence of surrounding mediums. However, the influence of surrounding mediums is also can be a key factor for the application of the proposed metamaterial absorber. This is based on the refractive index of surrounding mediums directly impacts the absorption property of the metamaterial absorber. In this paper, the proposed metamaterial absorber is applied as a sensor in practice through taking the advantage of variations in surrounding mediums, which can reflect the real-time change in environment. Fig. 9(a) shows the absorption spectra with different surrounding mediums. The first measured surrounding medium on the surface of the metamaterial absorber is air, a perfect absorption band is obtained, as shown in Figs. 1(c) and 9(a). When the air layer is instead by the water layer on the surface of the metamaterial absorber in experiments, the average of the absorption band is reduced to 91%, and the band width is reduced obviously. The third measured surrounding medium is the glucose solution, the average absorption is reduced to 85%. The absorption band shows a shifted to shorter wavelength. In order to reflect the change of the surrounding medium and access corresponding data conveniently, the figure of merit (FOM) is used to in this paper:
dIðlÞ IðlÞ FOM ¼ dnðlÞ max In the above formula, the
Fig. 8. Electric field distributions in xoz plane, correspond to the resonance wavelength at (a)2040 nm, (b)2103 nm, (c)2150 nm, respectively.
modes [41] are observed, as shown in Fig. 8(b), which implies that the absorption peak in Fig. 1(c) is excited by the horizontal coupled of LSP modes. This can be adopted to understand that the absorption peak is reduced with a or w increased, see the arrow in Fig. 4(a and b), while the absorption peak is almost unchanged with H increased, seen the arrow in Fig. 6. It is because that the horizontal distance is increased between two horizontal LSP modes with a or
(3) dIðlÞ dnðlÞ
is relative variation of the ab-
sorption rate that is resulted by the change of the refractive index dnðlÞ. The IðlÞ is the absorption rate of that where the FOM is maximum [45]. Fig. 9(b) shows the FOM spectra with different surrounding mediums. For the water surrounding medium, the maximum FOM value is 92. And for the glucose solution surrounding medium, the maximum FOM value is reduced to 83. In simulations, the refractive indices of the water and the glucose solution are 1.312 and 1.322, respectively. From these results, it can be found that the proposed metamaterial absorber can sense the real-time surrounding medium change. 4. Conclusion In
conclusion,
a
single-band
metamaterial
absorber
is
S. Li et al. / Optical Materials 73 (2017) 111e118
117
Fig. 9. (a) Measured spectra with different surrounding mediums. (b) FOM spectra with different surrounding mediums.
experimental showed and simulation analyzed. Measured results indicate that the single-band is excited by the cavity resonance and the short-wavelength absorption peak is excited by the coupling of LSP modes on holes edges. The absorption property can be modulated by optimizing structural parameters (w, a, H). Moreover, the designed structure shows a high absorb performance when the angle of incidence (q) reaches to 60 . The proposed metamaterial absorber can be applied in detecting and sensing the variation of the surrounding medium. This kind of metamaterial absorber is easy to be designed and fabricated through electron-beam lithography or focus-ion-beam milling and vacuum coating equipment, which could be practical applied in filters, absorber, and detection. Acknowledgments This research was supported by National Natural Science Foundation of China (41406047, 51409252, 51209210, 41306036). References [1] R.A. Shelby, D.R. Smith, S. Schultz, Experimental verification of a negative index of refraction, Science 292 (2001) 77e79. [2] D. Schurig, J.J. Mock, B.J. Justice, S.A. Cummer, J.B. Pendry, A.F. Starr, D.R. Smith, Metamaterial electromagnetic cloak at microwave frequencies, Science 314 (2006) 977e980. [3] Y.T. Wang, B.H. Cheng, Y.Z. Ho, Y.C. Lan, P.G. Luan, D.P. Tsai, Gain-assisted hybrid-superlens hyperlens for nano imaging, Opt. Express 20 (2012) 22953e22960. [4] Q. Cheng, T.J. Cui, W.X. Jiang, B.G. Cai, An omnidirectional electromagnetic absorber made of metamaterials, New J. Phys. 12 (063006) (2010). [5] F. Alves, B. Kearney, D. Grbovic, G. Karunasiri, Narrowband terahertz emitters using metamaterial films, Opt. Express 20 (2012) 21025e21032. [6] B.H. Cheng, Y.C. Lan, D.P. Tsai, Breaking optical diffraction limitation using optical hybrid-super-hyperlens with radially polarized light, Opt. Express 21 (2013) 14898e14906. [7] Kaisar Khan, Khaled Mnaymneh, Hazem Awad, Imad Hasan, Trevor Hall, Optical waves in photonic crystal meta-materials, J. Appl. Phys. A 117 (2) (2014) 629e634. [8] Kaisar Khan, M. Mahmood, Anjan Biswas, Milivoj Belic, Nonlinear pulse propagation in optical Metamaterials, J. Comput. Theor. Nanosci. 12 (2015) 4837e4841. [9] A. Mary, S.G. Rodrigo, F.J. Garcia-Vidal, L. Martin-Moreno, Theory of negativerefractive-index response of double-fishnet structures, Phys. Rev. Lett. 101 (2008) 103902e103905. [10] D.R. Smith, D.C. Vier, T. Koschny, C.M. Soukoulis, Electromagnetic parameter retrieval from inhomogeneous metamaterials, Phys. Rev. E 71 (2005) 036617e036627.
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Optical Materials journal homepage: www.elsevier.com/locate/optmat
Experimental and simulated study of a composite structure metamaterial absorber Shengyong Li*, Xiaochuan Ai, Ronghua Wu, Jiajun Chen Naval Univ. of Engineering, Wuhan 430033, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 10 June 2017 Received in revised form 26 July 2017 Accepted 2 August 2017
In this paper, a high performance metamaterial absorber is designed and experimental studied. Measured results indicate that a perfect absorption band and a short-wavelength absorption peak are achieved in the near-infrared spectrum. Current strength distributions reveal that the absorption band is excited by the cavity resonance. And electric field distributions show that the short-wavelength absorption peak is excited by the horizontal coupled of localized surface plasmon (LSP) modes near hole edges. On the one hand, the absorption property of the measured metamaterial absorber can be enhanced through optimizing the structural parameters (a, w, and H). On the other hand, the absorption property is sensitive to the change of refractive index of environmental medias. A sensing scheme is proposed for refractive index detecting based on the figure of merit (FOM) value. Measured results indicate that the proposed sensing scheme can achieve high FOM value with different environmental medias (water, glucose solution). © 2017 Published by Elsevier B.V.
OCIS codes: 160.3918 160.4236 260.1180 260.3910 160.5298 Keywords: Metamaterials Absorption band Impedance match
1. Introduction Artificially prepared electromagnetic metamaterial has drawn a surge of attention over past decade due to its unique electromagnetic properties, such as negative refractive index, negative permittivity, negative permeability, etc. Since the negative refractive index is experimental demonstrated by Smith et al. [1], a large number of novel study on electromagnetic metamaterials are proposed in a wide range of applications, include optical black hole, sub-diffraction imaging, wavelength selective blackbody emitters, invisibility cloaking [2e8], and many more. To characterize these electromagnetic metamaterial devices, an effective medium technique [9,10] is usually adopted (including complex refractive, permittivity and permeability). Among these functional devices, metamaterial absorber has received the widespread attention [11e15]. Many studies prove that plasmonic nanostructure metamaterial absorbers are highly desirable for many
* Corresponding author. E-mail address: [email protected] (S. Li). http://dx.doi.org/10.1016/j.optmat.2017.08.002 0925-3467/© 2017 Published by Elsevier B.V.
practical applications with one or more perfect absorption bands, for instance, metamaterial absorbers [16,17], photodetectors [18] photovoltaic cells [19], and optical imaging devices [20]. In order to design and manufacture a perfect metamaterial absorber, the plasmonic nanostructure is always arranged by simultaneously minimizing the reflectance between the top metallic layer of the absorber and the free air layer, while eliminating the transmittance through maximizing the material loss of the absorber [21e25]. In these metamaterial absorbers, metallic/dielectric/ metallic (MDM) structural absorbers demonstrate the potential value with perfect absorption bands, wide-angle incidence, and almost completely polarization insensitivity. The MDM structural absorber usually consists of a top periodically patterned metallic layer which serves as an electric resonator, an intermediate dielectric layer, and a thick enough bottom metal layer that serves as an optical mirror to reduce the transmittance. To date, perfect MDM metamaterial absorbers have been researched in a wide range, including single-band [26], dual-band [27], and triple-band [28] absorptions. Many single-band MDM metamaterial absorbers [26,29] are proposed in the near-infrared wavelength range, which exhibit excellent absorption properties, such as near-perfect
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absorption peak, wide-angle incidence, and polarization insensitivity. These metamaterial absorbers always operate at a narrow wavelength band. This fact greatly limits their practical applications in many devices, such as imaging and spectroscopic detection [30]. Therefore, single-band near-infrared metamaterial absorber with a wide and flat absorption band is in demand. On the other hand, many physical mechanisms are proposed in understanding the property of metamaterial absorbers, such as destructive interference mechanism, strong antisymmetric surface plasmons coupling, cavity resonance and electrical resonance [31e33]. However, few researchers focus on the role of the horizontal coupling of localized surface plasmon (LSP) modes on the property of metamaterial absorbers. In the previous work [34], the transmission of metamaterial is reduced and resonance frequency is shifted to lower frequency (longer wavelength) due to the intensity of the horizontal coupling LSP modes increasing. It is also worth studying the effects of the horizontal coupling LSP modes. In this paper, a composite patterned metamaterial absorber is suggested and manufactured. Measured results indicate that the designed single-band absorber reveals perfect absorption property in near-infrared range. A short-wavelength absorption peak is achieved due to the horizontal coupling LSP modes. The absorption property of the measured absorber can be modulated through adjusting dimension parameters, which results in the proposed absorber more attracting in designing and manufacturing near-
infrared perfect metamaterial devices. The possibility of application of the measured absorber in sensing field is studied based on the figure of merit (FOM). 2. Simulation methods and experimental details 2.1. Simulation methods The suggested metamaterial absorber is simulated through adopting a business Software Ansofts HFSS 13.0. As shown in Fig. 1(a-b), the proposed metamaterial absorber consists of three functional layers: a top patterned metal plane layer with a compound air hole array, a dielectric layer, and a metal plane layer which plays as the bottom reflector. The metal layers are selected as silver layers. The incident wave is assumed to be normally to the top surface of the proposed metamaterial absorber. In the unit cell, the periodicity is given by “P”, the thickness of metal layer and dielectric layers are set as “h” and “H”. Two 160 nm thick metal layers are separated by a 480 nm thick dielectric layer, which results in a total metamaterial thickness of 0:8mm, as shown in Fig. 1. Geometric parameters are shown in Table 1. The silver layer is thick enough that results in the electromagnetic transmission closing to zero (TðlÞz0). Therefore, the simulated absorption of the designed MDM metamaterial absorber can be calculated as AðlÞ ¼ 1 RðlÞ. Here, AðlÞ is the wavelength-dependent absorption rate, and RðlÞ is
Fig. 1. (a) Top view of the unit cell on the xoy plane. (b) Side view of the unit cell on the xoz plane (the cross section of the unit cell). The yellow part is metal layer, the gray part is dielectric layer, the dark blue part is substrate. (c) Measured absorption, reflection, and transmission spectra. (d) photograph of samples. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
S. Li et al. / Optical Materials 73 (2017) 111e118 Table 1 Structural parameters of the proposed absorber. Parameter
P
w
a
b
h
H
Value (mm)
22
12
1.2
1.8
0.16
0.48
the wavelength-dependent reflection rate. The silver layer follows the Drude model:
εðuÞ ¼ 1 Here,
gD ¼ 9
u2
u2p iugD
up ¼ 1:37 1016 s1
(1) is
the
plasma
frequency,
1013 s1
is the collision frequency [35]. In this paper, the dielectric layer is the SiO2 layer [36]. In simulation, the business Software Ansofts HFSS 13.0 is utilized to research the relationship between the current intensity distribution and the resonant wavelength. In simulations, periodic boundary conditions are applied to the four sidewalls [37]. 2.2. Experimental details
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are fabricated as following: first, choose a piece of cleaned glass as a substrate. A silver layer (0:16mm-thick) is evaporated on the glass substrate at a rate of 1.8 Å s1 by using the electron beam evaporation. Then, a dielectric layer (0:48mm-thick) is evaporated on the first silver layer at a rate of 2.3 Å s1 by also using the electron beam evaporation. Another silver layer (0:16mm-thick) is also evaporated on the dielectric layer through the same method. Finally, the proposed composite structure holes array are patterned in the top silver layer by using a focused ion beam system, as shown in Fig. 1(d). The measured absorption spectra of the sample is shown in Fig. 1(c). A short-wavelength absorption peak (the absorption magnitude is 61%) and an absorption band (the average absorption is 96.4% and the band width is 240 nm) are obtained. The manufactured area of our samples is around 2:0 2:0mm2 . The samples is take pictures by Leica DM2700 M (Using black and white contrast mode, contrast ratio is 45%), as shown in Fig. 1(d). To exploit the physical origin of the absorption band in Fig. 1(c), current strength distributions of the resonant mode (the central wavelength of the absorption band is l ¼ 2390 nm) is elucidated [38]. Fig. 2(a) and (b) present the amplitude and vector of current distributions in top and bottom silver layers under the wave at
To validate the proposed metamaterial absorber, the samples
Fig. 2. Cavity resonance, at the central wavelength l ¼ 2390nm of the absorption band (a ¼ 1:2mm,H ¼ 0:48mm): (a) surface current distribution in the top silver layer, (b) simulated surface current distribution in the bottom silver layer. (c) simulated absorption, reflection, and transmission spectra.
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addition, the proposed metamaterial absorber shows a zero reflection and transmission characteristics, which indicates a broad range of potential applications. Outside the absorption band, real parts of m and ε are imbalanced, which results in the resonance absorption reducing. On the other hand, imaginary parts are shown in Fig. 3(b). Imaginary parts of the m and ε are minimized in the absorption band, which indicates that the proposed metamaterial absorber reveals low reflection loss, as shown in Fig. 3(b). 3. Study on geometrical variation After revealing the physical phenomenon of the absorption property in Fig. 1(c) by near-field profiles, the metamaterial absorber is further measured through optimizing structural parameters (a, w, H). More of experiments are carried out: the first set of experiments achieved through varying the width of sub-holes (a) while fixed structural parameters (w, H), the second set is varied w while fixed structural parameters (a, H), the third set is varied the H while fixed structural parameters (a, w). 3.1. Effect of a, w, and H
Fig. 3. Effective permeability, permittivity, and impedance of the designed metamaterial absorber with absorption spectra (a ¼ 1:2mm,H ¼ 0:48mm).
normal incidence. Fig. 2(c) shows the simulated absorption, reflection, and transmission spectra of the designed metamaterial absorber. It can be found that a near perfect absorption band is also achieved. These simulated results are agreed well with measured results in Fig. 1(c), which indicates that simulations are effective. Apparently, at the wavelength l ¼ 2390 nm, the calculated current displacement vectors in the top silver layer is opposite to that in the bottom silver layer. As shown in Fig. 2(a) and (b), these anti-parallel currents displace circulating currents between top and bottom silver layers, which indicates that the absorption band is excited by the cavity mode resonance [33]. Effective parameters, including the permeability (m), the permittivity (ε), and the impedance (z), are fundamental parameters, which are used to characterize electromagnetic responses of metamaterial. To facilitate the description of electromagnetic properties of the designed metamaterial absorber, effective parameters of the designed metamaterial absorber are calculated [9,10] by adopting the S parameter retrieval method. As shown in Fig. 3, the real part of m reveals a magnetic resonant mode, which results in an anti-resonance of the real part of ε [39]. Meanwhile, the effective impedance z can be given by:
sffiffiffiffiffiffiffiffiffi mðlÞ zZo Z¼ εðlÞ
Fig. 4 shows the absorbance as a function of a and wavelength at normally incident wave with keeping w ¼ 12mm and H ¼ 0:48mm. In the first set of experiments, a is increased from a ¼ 1:2mm to a ¼ 1:5mm. Obviously, the resonant absorption band reveals a shifted to longer wavelength. Meanwhile, an increase of a results in the band width initially (a ¼ 1:2mm, the average absorption is 96.4% and the band width is 240 nm) increasing, reaching a maximum band width (a ¼ 1:4mm, 99.1% and 430 nm), and then decreasing (a ¼ 1:5mm, 92.2% and 295 nm). It can be deduced that the impedance matching is partly defined by a. A higher impedance matching condition between the top silver layer and the air interface is achieved with a increasing. For the maximize absorption band width (a ¼ 1:4mm,99.1% and 430 nm) case, the absorption of the designed metamaterial absorber close to 100% which implies that a perfect impedance matching condition is obtained [41]. Therefore, when a > 1:4mm, the perfect impedance matching condition is invalid which leads to the band width and the average absorption reduce, as shown Fig. 4(a). On the other hand, the shortwavelength absorption peak shifts to shorter wavelength, and the absorption is reduced from 61% to 51%, see the arrow in Fig. 4(a). The second set of experiments is carried out by varying w while fixed structural parameters (a ¼ 1:2mm,H ¼ 0:48mm), as shown in
(2)
In the absorption band, real parts of m and ε are almost balanced (it is obviously that the slop of the real parts of m and ε are similar). Moreover, real parts of m and ε cross zero at the central wavelength of the absorption band at l ¼ 2390nm, which results in the impedance match achieving between the metamaterial absorber and the free space around the wavelength according to Equation (2). On the one hand, Ding et al. reported that the reflection can be minimized based on the impedance match achieving [40]. In
Fig. 4. Measured absorption spectra of the designed metamaterial absorber with different structural parameters.
S. Li et al. / Optical Materials 73 (2017) 111e118
Fig. 4(b). Similarly, an increase in the w results in the band width initially (w ¼ 12mm, 96.4% and 240 nm) increasing, reaching a maximum band width (w ¼ 13mm, 99.6% and 465 nm), and then decreasing (w ¼ 13:5mm, 86.8% and 441 nm). The absorption band shows a shifted to longer wavelength, too. The short-wavelength peak shifts to shorter wavelength and is reduced from 61% to 47%, see the arrow in Fig. 4(b). It can be deduced that the structural parameter w also defines the impedance matching condition of the designed metamaterial absorber. Hence, the appropriate structural design with impedance matched top metallic layer surface is highly desired in exploiting perfect metamaterial absorber. In order to confirm the impedance matching condition, the effective impedance is retrieved with a or w increasing. The physical mechanism of the short-wavelength peak would be discussed in the following contents. Fig. 5 shows the effective impedance spectra with different optimal structural parameters (a, w). It is obviously that the effective impedance is close to 1 in resonance absorption bands based on optimizing structural parameters (a, or w), as shown in Fig. 5(a-b). This phenomenon is similar to the results in Fig. 3(a). These retrieved results indicate that the impedance matching condition can be modulated by optimizing structural parameters. Therefore, the absorption can be maximized in the resonant band. The third set of experiments is carried out with the dielectric layer thickness (H) increasing while other parameters unchanged, as shown in Fig. 6. The cavity resonant band shows a shifted to longer wavelength with H increasing from 0:48mm to 0:54mm. And the band width is increased form 240 nm to 413 nm obviously, which is similar to the reported work [33]. And the average absorption is enhanced from 96.4% to 97.3%. This is mainly owing to that the thickness of dielectric layer also plays a key role in revealing the high absorption property [42e44]. It should be noted that the electromagnetic wave is partially multiple reflected from the air-spacer interface with a phase shifted between the top and the bottom silver layers. When the electromagnetic wave is incident on the designed structure, multiple reflections from top and bottom layers would constructively interfere with each other on the surface of the top silver layer. Therefore, the whole multiple reflections will destructively interfere to minimize. The increasing of H leads to the phase shifted increase to achieve a new ideal interference condition to constructively interfere the whole multiple reflections. Therefore, the cavity resonant band shows a shifted to longer wavelength, as shown in Fig. 6. When H > 540 nm, the multiple reflections destructively interfering will be damaged due to the impedance matching condition is invalid. Therefore, the average absorption would be reduced and the bandwidth shrink down. However, the resonant wavelength of the short-wavelength absorption peak almost unchanged with H increasing. One can deduce that the physical mechanism of the short-wavelength absorption peak is independent of the dielectric layer. This will be verified in the following content. These measured results in Figs. 4 and 6 confirm that these structural parameters (a, w, H) mainly define the impedance matching condition of the designed metamaterial absorber. 3.2. Effect of angle of incidence (q) The absorption performance of the proposed metamaterial absorber is measured in the fourth of experiments. In this paper, the angle of incidence can be defined as the angle between the electromagnetic wave vector and the z-axis of the coordinates. In practical applications, most of the electromagnetic waves need to be absorbed, which may contain arbitrarily angled components. Fig. 7 shows the absorbance as a function of angle of incidence and wavelength. In the fourth set of experiments, the designed
115
Fig. 5. Effective impedance of the designed metamaterial absorber with absorption spectra.
Fig. 6. Measured absorption spectra of the designed metamaterial absorber with different dielectric layer thickness (H).
structure is measured for normal, 30 , 45 , and 60 angles of incidence. The absorption bandwidth of the designed structure is reduced from 240 nm to 80 nm. As illustrated in Fig. 7, the value of the short-wavelength peak almost remains unchanged with q up to 60 . Moreover, the absorption band shows a shifted to shorter wavelength as q increasing, which is mainly consisted with the reported single-band MDM absorber [29], in which the resonant wavelength is shifted to shorter in the TM case.
3.3. Discussion on the short-wavelength absorption peak To obtain a cohesive picture of the absorptive characteristics of the short-wavelength peak at l ¼ 2103nm in Fig. 1(c), the patterned of electric field distributions are computed in xoz plane (the cross section of the unit cell), as shown in Fig. 8. Two calculation wavelengths are selected in simulations around the resonant wavelength of the absorption peak, such as at 2040 nm and 2150 nm. It is obviously that only two localized surface plasmon (LSP) modes can be observed on the hole edges at both selected calculation wavelengths, as shown in Fig. 8(a, c). For the resonant wavelength of the absorption peak at 2390 nm, two LSP modes are excited on the holes edges. Moreover, the coupled phenomena between these LSP
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w increasing, which results in a decrease in horizontal coupling strength of LSP modes. Therefore, the absorption of the resonant peak is reduced in Fig. 4(a and b). However, the coupling strength is almost unchanged with the H increasing in Fig. 6. This phenomenon is similar to the previous work [34], where the resonant transmission peak is shifted to lower frequency (longer wavelength) in the case of the coupling strength increased. In other words, the resonant transmission peak will shift to higher frequency (shorter wavelength) when the coupling strength is reduced, which is consistent with these results in Fig. 4(a and b). These results in Figs. 4 and 8 indicate that the coupling of horizontal LSP modes plays an important role in designing metamaterial absorber. 3.4. Discussion on the application of the proposed metamaterial absorber
Fig. 7. Measured absorption spectra of the designed metamaterial absorber with different angle of incidence (q): (a) 0 , (b) 30 , (c) 45 , (d) 60 .
In practical applications, metamaterial absorber is always used in different surrounding mediums. The absorption performance of the metamaterial absorber will be disturbed by the variation of the refractive index of surrounding mediums, which are surrounded on the metamaterial absorber surface in the application process. These influences are mainly reflected in that absorption rate reduction, resonance wavelength deviation, bandwidth shrink, and so on. Usually, metamaterial absorbers are designed and optimized to reduce the influence of surrounding mediums. However, the influence of surrounding mediums is also can be a key factor for the application of the proposed metamaterial absorber. This is based on the refractive index of surrounding mediums directly impacts the absorption property of the metamaterial absorber. In this paper, the proposed metamaterial absorber is applied as a sensor in practice through taking the advantage of variations in surrounding mediums, which can reflect the real-time change in environment. Fig. 9(a) shows the absorption spectra with different surrounding mediums. The first measured surrounding medium on the surface of the metamaterial absorber is air, a perfect absorption band is obtained, as shown in Figs. 1(c) and 9(a). When the air layer is instead by the water layer on the surface of the metamaterial absorber in experiments, the average of the absorption band is reduced to 91%, and the band width is reduced obviously. The third measured surrounding medium is the glucose solution, the average absorption is reduced to 85%. The absorption band shows a shifted to shorter wavelength. In order to reflect the change of the surrounding medium and access corresponding data conveniently, the figure of merit (FOM) is used to in this paper:
dIðlÞ IðlÞ FOM ¼ dnðlÞ max In the above formula, the
Fig. 8. Electric field distributions in xoz plane, correspond to the resonance wavelength at (a)2040 nm, (b)2103 nm, (c)2150 nm, respectively.
modes [41] are observed, as shown in Fig. 8(b), which implies that the absorption peak in Fig. 1(c) is excited by the horizontal coupled of LSP modes. This can be adopted to understand that the absorption peak is reduced with a or w increased, see the arrow in Fig. 4(a and b), while the absorption peak is almost unchanged with H increased, seen the arrow in Fig. 6. It is because that the horizontal distance is increased between two horizontal LSP modes with a or
(3) dIðlÞ dnðlÞ
is relative variation of the ab-
sorption rate that is resulted by the change of the refractive index dnðlÞ. The IðlÞ is the absorption rate of that where the FOM is maximum [45]. Fig. 9(b) shows the FOM spectra with different surrounding mediums. For the water surrounding medium, the maximum FOM value is 92. And for the glucose solution surrounding medium, the maximum FOM value is reduced to 83. In simulations, the refractive indices of the water and the glucose solution are 1.312 and 1.322, respectively. From these results, it can be found that the proposed metamaterial absorber can sense the real-time surrounding medium change. 4. Conclusion In
conclusion,
a
single-band
metamaterial
absorber
is
S. Li et al. / Optical Materials 73 (2017) 111e118
117
Fig. 9. (a) Measured spectra with different surrounding mediums. (b) FOM spectra with different surrounding mediums.
experimental showed and simulation analyzed. Measured results indicate that the single-band is excited by the cavity resonance and the short-wavelength absorption peak is excited by the coupling of LSP modes on holes edges. The absorption property can be modulated by optimizing structural parameters (w, a, H). Moreover, the designed structure shows a high absorb performance when the angle of incidence (q) reaches to 60 . The proposed metamaterial absorber can be applied in detecting and sensing the variation of the surrounding medium. This kind of metamaterial absorber is easy to be designed and fabricated through electron-beam lithography or focus-ion-beam milling and vacuum coating equipment, which could be practical applied in filters, absorber, and detection. Acknowledgments This research was supported by National Natural Science Foundation of China (41406047, 51409252, 51209210, 41306036). References [1] R.A. Shelby, D.R. Smith, S. Schultz, Experimental verification of a negative index of refraction, Science 292 (2001) 77e79. [2] D. Schurig, J.J. Mock, B.J. Justice, S.A. Cummer, J.B. Pendry, A.F. Starr, D.R. Smith, Metamaterial electromagnetic cloak at microwave frequencies, Science 314 (2006) 977e980. [3] Y.T. Wang, B.H. Cheng, Y.Z. Ho, Y.C. Lan, P.G. Luan, D.P. Tsai, Gain-assisted hybrid-superlens hyperlens for nano imaging, Opt. Express 20 (2012) 22953e22960. [4] Q. Cheng, T.J. Cui, W.X. Jiang, B.G. Cai, An omnidirectional electromagnetic absorber made of metamaterials, New J. Phys. 12 (063006) (2010). [5] F. Alves, B. Kearney, D. Grbovic, G. Karunasiri, Narrowband terahertz emitters using metamaterial films, Opt. Express 20 (2012) 21025e21032. [6] B.H. Cheng, Y.C. Lan, D.P. Tsai, Breaking optical diffraction limitation using optical hybrid-super-hyperlens with radially polarized light, Opt. Express 21 (2013) 14898e14906. [7] Kaisar Khan, Khaled Mnaymneh, Hazem Awad, Imad Hasan, Trevor Hall, Optical waves in photonic crystal meta-materials, J. Appl. Phys. A 117 (2) (2014) 629e634. [8] Kaisar Khan, M. Mahmood, Anjan Biswas, Milivoj Belic, Nonlinear pulse propagation in optical Metamaterials, J. Comput. Theor. Nanosci. 12 (2015) 4837e4841. [9] A. Mary, S.G. Rodrigo, F.J. Garcia-Vidal, L. Martin-Moreno, Theory of negativerefractive-index response of double-fishnet structures, Phys. Rev. Lett. 101 (2008) 103902e103905. [10] D.R. Smith, D.C. Vier, T. Koschny, C.M. Soukoulis, Electromagnetic parameter retrieval from inhomogeneous metamaterials, Phys. Rev. E 71 (2005) 036617e036627.
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