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Single-band high absorption and coupling between localized surface plasmons modes in a metamaterials absorber
Optical Materials 72 (2017) 283e288
Contents lists available at ScienceDirect
Optical Materials journal homepage: www.elsevier.com/locate/optmat
Single-band high absorption and coupling between localized surface plasmons modes in a metamaterials absorber Min Zhong*, Shui Jie Liu, Bang Li Xu, Jie Wang, Hua Qing Huang Hezhou University, Hezhou 542899, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 4 April 2017 Received in revised form 21 May 2017 Accepted 9 June 2017
In this paper, we design and simulate a metamaterials absorbers based on the resonance of the local surface plasmon (LSP) mode. The damping constant of gold layer is optimized in simulations to eliminate the effect of the inappropriate material parameters on the electromagnetic properties of the proposed metamaterial absorber. The horizontal distance between two metal particles is optimized in simulations and a perfect absorption resonance peak is achieved due to the strong coupling of LSP modes. A new absorption peak is obtained when the horizontal distance is 0 nm. The vertical distance between the new metal particles and the bottom metal layer is reduced, which leads to the absorption peak reduce based on the reduction of the intensity of LSP modes. A new absorption peak is obtained when the new metallic particle and the bottom gold layer form a whole structure. © 2017 Elsevier B.V. All rights reserved.
OCIS codes: 160.3918 160.4236 260.1180 260.3910 160.5298 Keywords: Metamaterials Absorbers Absorption peak
1. Introduction The metamaterial is a kind of artificially prepared material that has many properties that are not natural [1e6]. Because of its unique properties, metamaterials have been developed and applied with many fields [7e10]. In many applications, metamaterial absorbers attract the attention of many researchers [11e15]. There are many literature indicate that the high absorption of light in a thin metamaterial absorber is very particularly desirable and important for applications, such as of high sensitivity imaging, photodetectors, plasmonic sensing, microbolometers, surface enhanced Raman spectroscopy, etc [16e19]. Recently, the design and fabrication of terahertz (THz) regions metamaterial absorbers attract much researcher's attention [20e23]. Many researcher's efforts have focused on different properties, such as: the polarization of electromagnetic waves, or the insensitive to the incident angle, or the extending the resonance absorption to shorter wavelengths [24e27]. Yongzhi Cheng et al [28] proposed and simulated a polarization-insensitive and wide-angle metamaterial absorber,
* Corresponding author. E-mail address: [email protected] (M. Zhong). http://dx.doi.org/10.1016/j.optmat.2017.06.019 0925-3467/© 2017 Elsevier B.V. All rights reserved.
which can easy modulate its electromagnetic properties based on a pump beam. Moreover, an infrared non-planar plasmonic metamaterials absorber is also simulated verification, which can be applied in refractive index sensing [29]. Govind Dayal et al [30] numerical study of a multi-band metamaterial absorber based on stacking different combinations of metal-dielectric disks. Han Xiong et al [31] theory analyze the physical mechanism of absorption peaks based on a reflection theory. Moreover, the high absorption can be obtained through many structural design strategies, such as: structured metallic surfaces [32,33], and microcavities [34], which is based on the resonance of the localized surface plasmon (LSP) mode [34]. The LSP modes resonance receives a great deal of interest [35,36]. Moreover, the property of the LSP modes can be modulated through changing the designed structure. Many experimental and theoretical literature indicate that the LSP mode shows coupling with surface plasmon polariton (SPP) mode in a structure consisting of a metal particle array and a metallic film [37e39]. These coupling effects between LSP and SPP leads to these structures useful for applying in SERS, fluorescence extraction, bio-sensors [40,41]. However, few of researchers focus on the coupling effect between LSP modes on the absorption property of metamaterial absorber. Therefore, it is important to modulate the absorption property of the designed absorber based
284
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on the resonance of LSP modes. In this paper, a composite structure metamaterial absorber is designed and simulated. Due to the resonance of LSP modes, an absorption peak is obtained. A high performance absorption resonance is achieved when the horizontal distance between two metallic particles is optimized. The field distributions corresponding to the maximum absorption peak is investigated to explore the physical mechanism. It is found that the coupling intensity between LSP modes can be enhanced through optimizing the horizontal distance between two metallic particles, which leads to the maximum absorption peak. A new absorption peak is obtained when two metallic particles touch together and form a new metal particle. The absorption rate is reduced with the vertical distance between the new metallic particle and the bottom gold layer reducing. Another new absorption peak is achieved due to two new LSP modes are excited near edges of the new bottom metallic layer. 2. Structural design and optimization 2.1. Structural design and theoretical model The proposed metamaterial absorber consists of three functional parts: a bottom gold layer, which works as an electromagnetic wave eliminator. A media layer on the bottom gold layer. A dual metallic particles array is embedded in the media layer, works as an electromagnetic resonator, as shown in Fig. 1(a-b). The commercially software Ansoft's HFSS 11.0 is applied in simulations. In Fig. 1(a-b), the lattice constant of the proposed unite cell is given by “P”, the gold layer thick is set to be “h”, the media layer thick is given by “H”. Dimensional parameters of the proposed unite cell are shown in Table 1. The transmission of the proposed unite cell closes to zero due to the bottom metal layer is thick enough. Therefore, the absorption of the proposed unite cell is achieved as: A(f) ¼ 1-R(f)
Table 1 Dimensional parameters of the proposed unite cell. Parameter
P
L
w
r
h
H
D
Value (nm)
600
280
100
30
30
400
230
be optimized. A reported literature indicates that the damping constant of gold layer in simulations is lower than that in real system, due to the surface scattering and the grain boundary effects of gold film [45]. Therefore, it is reasonable that optimizing the damping constant of gold layer to eliminate the effect of the inappropriate material parameter on the electromagnetic properties of the proposed metamaterial absorber. Fig. 2 shows the calculated absorption spectrum of the proposed metamaterial absorber under different times of damping constants of gold layer. When the 1.0 time of the damping constants (1.0*uc ¼ 4:08 1013 s1 in the simulation) of gold layer is used in simulation, an absorption peak is obtained at 630 nm, the maximum absorption rate reaches to 48%, as shown in Fig. 2. For the 2.5times of the damping constants, the maximum absorption rate reaches to 74%, as shown in Fig. 2. However, for the 4.0times of the damping constants, the maximum absorption rate is reduced to 53%, as shown in Fig. 2. It is obviously that the high performance absorption peak is obtained at 630 nm when the 2.5times of the damping constants of gold layer in simulation is used. These simulated results reveal that the 2.5times of the damping constants of gold layer is the most optimization parameter in simulating the proposed structure in this paper. Thence, the damping constants of gold layer is adopted 2.5time in the following simulations.
(1)
where, the A(f) is the simulated absorption rates, while the R(f) is the simulated reflection rates. In simulations, gold layers follow the Drude mode with the damping constant uc ¼ 4:08 1013 s1 , and the plasma frequency gD ¼ 9 1013 s1 [42]. The simulated electromagnetic wave incident to the proposed unite cell in air. Ideal electric boundaries and magnetic boundaries are applied on normal to the x-axis and y-axis [43]. The dielectric constant of SiO2 layer is set by 2.105 [44]. 2.2. Parameter optimization To reveal the physical mechanism of the proposed metamaterial absorber, the material parameter of gold layers in simulations should Fig. 2. Simulated absorption spectra with different damping constants.
Fig. 1. (a) Top view of the proposed unit cell on the xoy plane; (b) Side view of the proposed unit cell on the xoz plane. The yellow part is gold layer, the gray part is media layer, (c) Simulated absorption and reflectivity spectra. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
M. Zhong et al. / Optical Materials 72 (2017) 283e288
Fig. 3. Absorbance spectra under with different structures.
285
Fig. 5. Simulated absorption spectrum with different horizontal distance L.
Moreover, the resonance intensity of LSP modes is obviously higher than that of two pure metallic particle arrays, which leads to the maximum absorption rate of the proposed unite cell is higher.
3. Simulation results and discussion 3.1. Physical mechanism Due to the proposed unite cell is a composite structure with two metallic particles embedded in media layer, the absorption spectrum of pure metallic particle array is also calculated, as shown in Fig. 3. The black curve is the absorption spectrum of the proposed unite cell. The red curve represents the calculated results of one pure metallic particle array, while the blue curve represents the other pure metallic particle array. It can be found that two distinct absorption peaks are obtained at 670 nm and 690 nm with different pure metallic particle arrays, respectively. When the result of two pure metallic particles taking as a whole unite cell, a higher absorption peak is obtained. The resonance wavelengthes of two pure metallic particle arrays are both higher than that of the proposed unite cell, as shown in Fig. 3. To reveal the physical mechanism of the proposed unite cell, electric field intensity distributions are calculated at resonance wavelengthes, as shown in Fig. 4. For the one pure metallic particle array, a local surface plasmon (LSP) mode is excited on the left edge of the metallic particle, which leads to the absorption peak at 670 nm, as shown Fig. 4(a). For the other pure metallic particle array, a different LSP mode is excited on the right edge of the metallic particle, which leads to the absorption peak at 690 nm, as shown Fig. 4(b). While for the proposed unite cell, LSP modes are excited effectively on edges of two metallic particles, which leads to the absorption peak at 630 nm, as shown Fig. 4(c).
3.2. Horizontal distance (L) optimization To enhance the absorption rate of the proposed unite cell, the horizontal distance L between two metallic particles is optimized. Fig. 5 shows the simulated absorption spectrum of the proposed unite cell with different L. When two metallic particles close to each other (L ¼ 210 nm), the maximum absorption rate is increased to 88%, see the red curve in Fig. 5. When the L ¼ 70 nm, the absorption peak is increased to the maximum rate 99%, a perfect absorption resonance is obtained, see the blue curve in Fig. 5. However, for L ¼ 0 nm, the absorption peak is reduced to 94%. Moreover, the resonance absorption peak is shifted to shorter wavelength, as shown in Fig. 5. To reveal the electromagnetic resonance behavior behind the absorption spectrum in Fig. 5, the corresponding electric field intensity distributions are simulated, as shown in Fig. 6. It can be found that the resonance intensity of LSP modes is increased when two metallic particles close to each other, as shown in Fig. 6(b). Moreover, these LSP modes are interacted and coupled together. For the L ¼ 70 nm, two metallic particles close more to each other, resonance intensity of LSP modes and coupled intensity are more increased, which lead to the perfect absorption in Fig. 5. When the L ¼ 0 nm, two metallic particles touch with each other and constitute of a whole structure. A new metallic particle is
Fig. 4. Simulated electric field distributions with different structures.
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Fig. 6. Simulated electric field distributions with different horizontal distance L.
obtained in the proposed structure on the same level, as shown in Fig. 6(d). The volume of the new formed metal particle is twice of the original metal particles. It can be found that the interacted and coupled between LSP modes is disappear, which results in the peak at 630 nm disappearance [46]. However, two new LSP modes are excited on edges of the new metal particle, which results in the new absorption peak at 604 nm in Fig. 5, as shown in Fig. 6.
3.3. Vertical distance (D) optimization In order to further study the relationship between the LSP
resonance mode and the absorption spectrum of the proposed unite cell, the vertical distance D between the new metal particle and the bottom gold layer is optimized. Fig. 7 shows the simulated absorption spectrum with different vertical distance D. For the D ¼ 110 nm, the absorption peak is reduced from 94% to 77%, see the red curve in Fig. 7. For the D ¼ 30 nm, the absorption peak is reduced to 58%, see the blue curve in Fig. 7. While for the D ¼ 0 nm, the absorption peak is abnormally reduced to 26%, as shown in Fig. 7. To reveal the physical mechanism that resulting the reduction of the absorption peak in Fig. 7, the corresponding electric field intensity distributions are simulated, as shown in Fig. 8. When the new metal particle closes to the bottom gold layer, the resonance intensity of LSP modes is reduced obviously, as shown in Fig. 8(b). For the D ¼ 30 nm, the resonance intensity of LSP modes is weaker than that in Fig. 8(b), which leads to the absorption peak reduce in Fig. 7, as shown in Fig. 8(c). While the D ¼ 0 nm, the metal particle is touch with the bottom gold layer, and form a new bottom gold layer, which results in the peak at 604 nm disappearance [46]. Two new LSP modes are excited, as shown in Fig. 8(d), which results in the new absorption peak at 565 nm in Fig. 7. 4. Conclusion
Fig. 7. Simulated absorption spectrum with different vertical distance (D).
In conclusion, a LSP mode resonance metamaterial absorber is designed and simulated. It is found that the a strong absorption resonance (74%) is achieved for 2.5 times damping constant of bulk gold. The horizontal distance (L) and vertical distance (D) are both optimized in simulations. It is found that a perfect absorption resonance peak is achieved with the L reducing due to the strong coupling between LSP modes. When two metallic particles touch with each other, two new LSP modes are excited and leads to a new
M. Zhong et al. / Optical Materials 72 (2017) 283e288
287
Fig. 8. Simulated electric field distributions with different vertical distance (D).
absorption resonance peak. The new absorption peak is reduced when the D is reduced obviously. Two new LSP modes are excited when the new metallic particle and the bottom gold layer form a whole structure, which results in a new small absorption resonance peak at 565 nm. The proposed LSP resonance can be applied in emitters, sensing, SERS substrates. Acknowledgments This research was supported by Doctor's scientific research foundation of Hezhou University (HZUBS201503), Promotion of the Basic Ability of Young and Middle-aged Teachers in Universities project of Guangxi Province (KY2016YB453), Mathematical support autonomous discipline project of Hezhou University (2016HZXYSX01), and Innovation and entrepreneurship students project of Hezhou University (201611838018). References [1] W. Cai, U.K. Chettiar, A.V. Kildishev, V.M. Shalaev, Optical cloaking with metamaterials, Nat. Phot. 1 (2007) 224e227. [2] N. Engheta, A. Alù, Plasmonic materials in transparency and cloaking problems: mechanism, robustness, and physical insights, Opt. Express 15 (2007) 3318e3332. [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] 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) 977e979. [5] R.A. Shelby, D.R. Smith, S. Schultz, Experimental verification of a negative index of refraction, Science 292 (2001) 77e79. [6] S. Lim, C. Caloz, T. Itoh, Metamaterial-based electronically controlled transmission-line structure as a novel leaky-wave antenna with tunable radiation angle and beamwidth, IEEE Trans. Microw. Theory Techol. 52 (2004) 161e173. [7] R. Alaee, M. Farhat, C. Rockstuhl, F. Lederer, A perfect absorber made of a
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Contents lists available at ScienceDirect
Optical Materials journal homepage: www.elsevier.com/locate/optmat
Single-band high absorption and coupling between localized surface plasmons modes in a metamaterials absorber Min Zhong*, Shui Jie Liu, Bang Li Xu, Jie Wang, Hua Qing Huang Hezhou University, Hezhou 542899, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 4 April 2017 Received in revised form 21 May 2017 Accepted 9 June 2017
In this paper, we design and simulate a metamaterials absorbers based on the resonance of the local surface plasmon (LSP) mode. The damping constant of gold layer is optimized in simulations to eliminate the effect of the inappropriate material parameters on the electromagnetic properties of the proposed metamaterial absorber. The horizontal distance between two metal particles is optimized in simulations and a perfect absorption resonance peak is achieved due to the strong coupling of LSP modes. A new absorption peak is obtained when the horizontal distance is 0 nm. The vertical distance between the new metal particles and the bottom metal layer is reduced, which leads to the absorption peak reduce based on the reduction of the intensity of LSP modes. A new absorption peak is obtained when the new metallic particle and the bottom gold layer form a whole structure. © 2017 Elsevier B.V. All rights reserved.
OCIS codes: 160.3918 160.4236 260.1180 260.3910 160.5298 Keywords: Metamaterials Absorbers Absorption peak
1. Introduction The metamaterial is a kind of artificially prepared material that has many properties that are not natural [1e6]. Because of its unique properties, metamaterials have been developed and applied with many fields [7e10]. In many applications, metamaterial absorbers attract the attention of many researchers [11e15]. There are many literature indicate that the high absorption of light in a thin metamaterial absorber is very particularly desirable and important for applications, such as of high sensitivity imaging, photodetectors, plasmonic sensing, microbolometers, surface enhanced Raman spectroscopy, etc [16e19]. Recently, the design and fabrication of terahertz (THz) regions metamaterial absorbers attract much researcher's attention [20e23]. Many researcher's efforts have focused on different properties, such as: the polarization of electromagnetic waves, or the insensitive to the incident angle, or the extending the resonance absorption to shorter wavelengths [24e27]. Yongzhi Cheng et al [28] proposed and simulated a polarization-insensitive and wide-angle metamaterial absorber,
* Corresponding author. E-mail address: [email protected] (M. Zhong). http://dx.doi.org/10.1016/j.optmat.2017.06.019 0925-3467/© 2017 Elsevier B.V. All rights reserved.
which can easy modulate its electromagnetic properties based on a pump beam. Moreover, an infrared non-planar plasmonic metamaterials absorber is also simulated verification, which can be applied in refractive index sensing [29]. Govind Dayal et al [30] numerical study of a multi-band metamaterial absorber based on stacking different combinations of metal-dielectric disks. Han Xiong et al [31] theory analyze the physical mechanism of absorption peaks based on a reflection theory. Moreover, the high absorption can be obtained through many structural design strategies, such as: structured metallic surfaces [32,33], and microcavities [34], which is based on the resonance of the localized surface plasmon (LSP) mode [34]. The LSP modes resonance receives a great deal of interest [35,36]. Moreover, the property of the LSP modes can be modulated through changing the designed structure. Many experimental and theoretical literature indicate that the LSP mode shows coupling with surface plasmon polariton (SPP) mode in a structure consisting of a metal particle array and a metallic film [37e39]. These coupling effects between LSP and SPP leads to these structures useful for applying in SERS, fluorescence extraction, bio-sensors [40,41]. However, few of researchers focus on the coupling effect between LSP modes on the absorption property of metamaterial absorber. Therefore, it is important to modulate the absorption property of the designed absorber based
284
M. Zhong et al. / Optical Materials 72 (2017) 283e288
on the resonance of LSP modes. In this paper, a composite structure metamaterial absorber is designed and simulated. Due to the resonance of LSP modes, an absorption peak is obtained. A high performance absorption resonance is achieved when the horizontal distance between two metallic particles is optimized. The field distributions corresponding to the maximum absorption peak is investigated to explore the physical mechanism. It is found that the coupling intensity between LSP modes can be enhanced through optimizing the horizontal distance between two metallic particles, which leads to the maximum absorption peak. A new absorption peak is obtained when two metallic particles touch together and form a new metal particle. The absorption rate is reduced with the vertical distance between the new metallic particle and the bottom gold layer reducing. Another new absorption peak is achieved due to two new LSP modes are excited near edges of the new bottom metallic layer. 2. Structural design and optimization 2.1. Structural design and theoretical model The proposed metamaterial absorber consists of three functional parts: a bottom gold layer, which works as an electromagnetic wave eliminator. A media layer on the bottom gold layer. A dual metallic particles array is embedded in the media layer, works as an electromagnetic resonator, as shown in Fig. 1(a-b). The commercially software Ansoft's HFSS 11.0 is applied in simulations. In Fig. 1(a-b), the lattice constant of the proposed unite cell is given by “P”, the gold layer thick is set to be “h”, the media layer thick is given by “H”. Dimensional parameters of the proposed unite cell are shown in Table 1. The transmission of the proposed unite cell closes to zero due to the bottom metal layer is thick enough. Therefore, the absorption of the proposed unite cell is achieved as: A(f) ¼ 1-R(f)
Table 1 Dimensional parameters of the proposed unite cell. Parameter
P
L
w
r
h
H
D
Value (nm)
600
280
100
30
30
400
230
be optimized. A reported literature indicates that the damping constant of gold layer in simulations is lower than that in real system, due to the surface scattering and the grain boundary effects of gold film [45]. Therefore, it is reasonable that optimizing the damping constant of gold layer to eliminate the effect of the inappropriate material parameter on the electromagnetic properties of the proposed metamaterial absorber. Fig. 2 shows the calculated absorption spectrum of the proposed metamaterial absorber under different times of damping constants of gold layer. When the 1.0 time of the damping constants (1.0*uc ¼ 4:08 1013 s1 in the simulation) of gold layer is used in simulation, an absorption peak is obtained at 630 nm, the maximum absorption rate reaches to 48%, as shown in Fig. 2. For the 2.5times of the damping constants, the maximum absorption rate reaches to 74%, as shown in Fig. 2. However, for the 4.0times of the damping constants, the maximum absorption rate is reduced to 53%, as shown in Fig. 2. It is obviously that the high performance absorption peak is obtained at 630 nm when the 2.5times of the damping constants of gold layer in simulation is used. These simulated results reveal that the 2.5times of the damping constants of gold layer is the most optimization parameter in simulating the proposed structure in this paper. Thence, the damping constants of gold layer is adopted 2.5time in the following simulations.
(1)
where, the A(f) is the simulated absorption rates, while the R(f) is the simulated reflection rates. In simulations, gold layers follow the Drude mode with the damping constant uc ¼ 4:08 1013 s1 , and the plasma frequency gD ¼ 9 1013 s1 [42]. The simulated electromagnetic wave incident to the proposed unite cell in air. Ideal electric boundaries and magnetic boundaries are applied on normal to the x-axis and y-axis [43]. The dielectric constant of SiO2 layer is set by 2.105 [44]. 2.2. Parameter optimization To reveal the physical mechanism of the proposed metamaterial absorber, the material parameter of gold layers in simulations should Fig. 2. Simulated absorption spectra with different damping constants.
Fig. 1. (a) Top view of the proposed unit cell on the xoy plane; (b) Side view of the proposed unit cell on the xoz plane. The yellow part is gold layer, the gray part is media layer, (c) Simulated absorption and reflectivity spectra. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
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Fig. 3. Absorbance spectra under with different structures.
285
Fig. 5. Simulated absorption spectrum with different horizontal distance L.
Moreover, the resonance intensity of LSP modes is obviously higher than that of two pure metallic particle arrays, which leads to the maximum absorption rate of the proposed unite cell is higher.
3. Simulation results and discussion 3.1. Physical mechanism Due to the proposed unite cell is a composite structure with two metallic particles embedded in media layer, the absorption spectrum of pure metallic particle array is also calculated, as shown in Fig. 3. The black curve is the absorption spectrum of the proposed unite cell. The red curve represents the calculated results of one pure metallic particle array, while the blue curve represents the other pure metallic particle array. It can be found that two distinct absorption peaks are obtained at 670 nm and 690 nm with different pure metallic particle arrays, respectively. When the result of two pure metallic particles taking as a whole unite cell, a higher absorption peak is obtained. The resonance wavelengthes of two pure metallic particle arrays are both higher than that of the proposed unite cell, as shown in Fig. 3. To reveal the physical mechanism of the proposed unite cell, electric field intensity distributions are calculated at resonance wavelengthes, as shown in Fig. 4. For the one pure metallic particle array, a local surface plasmon (LSP) mode is excited on the left edge of the metallic particle, which leads to the absorption peak at 670 nm, as shown Fig. 4(a). For the other pure metallic particle array, a different LSP mode is excited on the right edge of the metallic particle, which leads to the absorption peak at 690 nm, as shown Fig. 4(b). While for the proposed unite cell, LSP modes are excited effectively on edges of two metallic particles, which leads to the absorption peak at 630 nm, as shown Fig. 4(c).
3.2. Horizontal distance (L) optimization To enhance the absorption rate of the proposed unite cell, the horizontal distance L between two metallic particles is optimized. Fig. 5 shows the simulated absorption spectrum of the proposed unite cell with different L. When two metallic particles close to each other (L ¼ 210 nm), the maximum absorption rate is increased to 88%, see the red curve in Fig. 5. When the L ¼ 70 nm, the absorption peak is increased to the maximum rate 99%, a perfect absorption resonance is obtained, see the blue curve in Fig. 5. However, for L ¼ 0 nm, the absorption peak is reduced to 94%. Moreover, the resonance absorption peak is shifted to shorter wavelength, as shown in Fig. 5. To reveal the electromagnetic resonance behavior behind the absorption spectrum in Fig. 5, the corresponding electric field intensity distributions are simulated, as shown in Fig. 6. It can be found that the resonance intensity of LSP modes is increased when two metallic particles close to each other, as shown in Fig. 6(b). Moreover, these LSP modes are interacted and coupled together. For the L ¼ 70 nm, two metallic particles close more to each other, resonance intensity of LSP modes and coupled intensity are more increased, which lead to the perfect absorption in Fig. 5. When the L ¼ 0 nm, two metallic particles touch with each other and constitute of a whole structure. A new metallic particle is
Fig. 4. Simulated electric field distributions with different structures.
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Fig. 6. Simulated electric field distributions with different horizontal distance L.
obtained in the proposed structure on the same level, as shown in Fig. 6(d). The volume of the new formed metal particle is twice of the original metal particles. It can be found that the interacted and coupled between LSP modes is disappear, which results in the peak at 630 nm disappearance [46]. However, two new LSP modes are excited on edges of the new metal particle, which results in the new absorption peak at 604 nm in Fig. 5, as shown in Fig. 6.
3.3. Vertical distance (D) optimization In order to further study the relationship between the LSP
resonance mode and the absorption spectrum of the proposed unite cell, the vertical distance D between the new metal particle and the bottom gold layer is optimized. Fig. 7 shows the simulated absorption spectrum with different vertical distance D. For the D ¼ 110 nm, the absorption peak is reduced from 94% to 77%, see the red curve in Fig. 7. For the D ¼ 30 nm, the absorption peak is reduced to 58%, see the blue curve in Fig. 7. While for the D ¼ 0 nm, the absorption peak is abnormally reduced to 26%, as shown in Fig. 7. To reveal the physical mechanism that resulting the reduction of the absorption peak in Fig. 7, the corresponding electric field intensity distributions are simulated, as shown in Fig. 8. When the new metal particle closes to the bottom gold layer, the resonance intensity of LSP modes is reduced obviously, as shown in Fig. 8(b). For the D ¼ 30 nm, the resonance intensity of LSP modes is weaker than that in Fig. 8(b), which leads to the absorption peak reduce in Fig. 7, as shown in Fig. 8(c). While the D ¼ 0 nm, the metal particle is touch with the bottom gold layer, and form a new bottom gold layer, which results in the peak at 604 nm disappearance [46]. Two new LSP modes are excited, as shown in Fig. 8(d), which results in the new absorption peak at 565 nm in Fig. 7. 4. Conclusion
Fig. 7. Simulated absorption spectrum with different vertical distance (D).
In conclusion, a LSP mode resonance metamaterial absorber is designed and simulated. It is found that the a strong absorption resonance (74%) is achieved for 2.5 times damping constant of bulk gold. The horizontal distance (L) and vertical distance (D) are both optimized in simulations. It is found that a perfect absorption resonance peak is achieved with the L reducing due to the strong coupling between LSP modes. When two metallic particles touch with each other, two new LSP modes are excited and leads to a new
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Fig. 8. Simulated electric field distributions with different vertical distance (D).
absorption resonance peak. The new absorption peak is reduced when the D is reduced obviously. Two new LSP modes are excited when the new metallic particle and the bottom gold layer form a whole structure, which results in a new small absorption resonance peak at 565 nm. The proposed LSP resonance can be applied in emitters, sensing, SERS substrates. Acknowledgments This research was supported by Doctor's scientific research foundation of Hezhou University (HZUBS201503), Promotion of the Basic Ability of Young and Middle-aged Teachers in Universities project of Guangxi Province (KY2016YB453), Mathematical support autonomous discipline project of Hezhou University (2016HZXYSX01), and Innovation and entrepreneurship students project of Hezhou University (201611838018). References [1] W. Cai, U.K. Chettiar, A.V. Kildishev, V.M. Shalaev, Optical cloaking with metamaterials, Nat. Phot. 1 (2007) 224e227. [2] N. Engheta, A. Alù, Plasmonic materials in transparency and cloaking problems: mechanism, robustness, and physical insights, Opt. Express 15 (2007) 3318e3332. [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] 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) 977e979. [5] R.A. Shelby, D.R. Smith, S. Schultz, Experimental verification of a negative index of refraction, Science 292 (2001) 77e79. [6] S. Lim, C. Caloz, T. Itoh, Metamaterial-based electronically controlled transmission-line structure as a novel leaky-wave antenna with tunable radiation angle and beamwidth, IEEE Trans. Microw. Theory Techol. 52 (2004) 161e173. [7] R. Alaee, M. Farhat, C. Rockstuhl, F. Lederer, A perfect absorber made of a
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