- Email: [email protected]
Broadband tunable absorber for terahertz waves based on isotropic silicon metasurfaces
Materials Letters 234 (2019) 138–141
Contents lists available at ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/mlblue
Broadband tunable absorber for terahertz waves based on isotropic silicon metasurfaces Zhengyong Song ⇑, Zhisheng Wang, Maoliang Wei Institute of Electromagnetics and Acoustics, Department of Electronic Science, Xiamen University, Xiamen 361005, China
a r t i c l e
i n f o
Article history: Received 30 July 2018 Received in revised form 30 August 2018 Accepted 15 September 2018 Available online 17 September 2018 Keywords: Optical materials and properties Microstructure Metasurface Absorber Terahertz
a b s t r a c t A broadband tunable absorber is studied based on the silicon metasurface at terahertz frequencies. By tuning the conductivity of silicon, absorptance is more than 90% from 0.497 THz to 1.045 THz with central frequency of 0.771 THz when the conductivity is equal to 2500 S=m. Simulated results show that absorptance peak can be modulated from 1% to 100% when the conductivity continuously changes from 0 S=m to2500 S=m. The central-symmetry structure leads to polarization independence. Our design may be used as optoelectronic modulator, tunable detector, and terahertz switch. Ó 2018 Elsevier B.V. All rights reserved.
1. Introduction In the past decade, artificially engineered subwavelength materials-metamaterials (MMs) have rendered us numerous opportunities to obtain effect and phenomenon that could not be easily accessed in natural materials. Now it leads to many demonstrations of interesting properties such as extraordinary optical transmission [1,2], polarization rotator [3–5], and perfect absorber [6–19]. In the field of MM, there have been tremendous efforts on absorber in recent years, because it has crucial roles in many different applications such as stealth device [6], solar cell [7], thermal imaging [8], and strongly reduced radar cross section of fighter plane and warship [9]. Several conventional metals have been proposed to realize electromagnetic absorption based on localized surface plasmon resonance of metal nanoparticle, such as silver [10], gold [11], chromium [12], nickel [13], titanium [14], and aluminium [15]. These absorbers seem expensive in price, and it is not easy to change absorption behavior once the sample is fabricated. For tunable absorber, insert of active device is always an important method and is able to provide the degrees of freedom for tuning. The variable systems are able to achieve impedance matching surface in some specific frequency bands. In order to achieve tunable absorption at different frequencies, some devices are frequently added, such as variable resistor for microwave [16] and graphene [17,18] and vanadium oxide [19] for terahertz ⇑ Corresponding author. E-mail address: [email protected] (Z. Song). https://doi.org/10.1016/j.matlet.2018.09.084 0167-577X/Ó 2018 Elsevier B.V. All rights reserved.
wave. Although graphene absorber has good performance, it would take long calculation time in simulation and have complicated fabrication procedures. Doped silicon (Si) with small resistivity can be considered as a highly lossy material in terahertz region and can be easily processed using standard lithography technology. In this work, a tunable absorber is reported based on doped Si metasurfaces. The relation between doping concentration and conductivity is r / nev , where n, e, and v are electron concentration, electronic charge, and electron mobility. Because of mature micro-and nanofabrication techniques of Si, our design is not difficult to realize under the condition of different conductivities. 2. Calculations and discussions of the results The schematic view of the proposed Si absorber is shown in Fig. 1. The basic unit cell of the designed absorber consists of one cross air hole in Si with thickness of 5.2 lm and a gold ground plane with thickness of 0.2 lm which is enough to prevent any transmission. The Si patches are separated by a 46 lm silica (SiO2 ) dielectric spacer from the background gold plane. The design parameters as labeled in Fig. 1(a) are as follows: w =70 lm, l =173 lm, and P =175 lm. Electromagnetic simulations are performed through a commercial software package (Comsol Multiphysics). The S parameters (S11 and S21 ) are simulated by frequency domain solver. In simulation, unit cell boundary conditions in x and y directions and open boundary conditions in z direction are used. The dielectric spacer is constructed by SiO2 with relative permittivity of 3.8 [20,21]. The permittivity of gold is
Z. Song et al. / Materials Letters 234 (2019) 138–141
139
Fig. 1. (a) Three-dimensional geometry of the designed absorber consisting of periodical air cross hole in Si film (orange), a silica spacer (cyan), and the gold film (yellow). The whole system is deposited on the silica (cyan) substrate. (b) Simulated absorptance spectra as a function of the conductivity of Si and frequency under normal incidence. The conductivity is varied from 0 S=m to 2500 S=m in a step of 250 S=m.
described by Drude model
xp ¼ 1:37 1016 rad=S
eAu ¼ 1 xðxxþip CÞ with plasma frequency
and
2
collision
frequency
C ¼ 1:2
1014 rad=S [22]. The conductivity of Si can be ranged from 0 S=m to 2500 S=m [23,24]. The absorptive efficiency of the proposed absorber can be defined as A ¼ 1 R T ¼ 1 jS11 j2 jS21 j2 , where A, R, and T are absorptance, reflectance, and transmittance, respectively. Due to the gold background plane, transmission (S21 ) is zero across the entire frequency range. Thus absorptance can be calculated by A ¼ 1 jS21 j2 . Simulation results of absorptance are shown in
Fig. 1(b). In order to determine the optimum conductivity, absorptance is simulated while varying the conductivity from 0 S=m to 2500 S=m. Fig. 1(b) depicts simulated absorptance of the proposed absorber with different conductivities under normal incidence. It can be observed that absorptance is larger than 90% between 0.497 THz and 1.045 THz when conductivity is 2500 S=m. Working intensity can be dynamically modulated from 1% to 100%. The results tell that absorption peak of the proposed structure can be tuned by adjusting the conductivity of Si through controlling the bias voltage. In order to further understand the physical origin of absorption, S-parameters are calculated and then the corresponding real and
Fig. 2. Retrieved effective physical parameters (a) permittivity, (b) permeability, (c) refractive index, and (d) impedance in the case of perfect absorption whenr ¼ 2500 S=m.
140
Z. Song et al. / Materials Letters 234 (2019) 138–141
Fig. 3. The dependences of simulated absorptance on polarization angle (a) and oblique incidence of TE (b) and TM (c) polarizations when absorptance is represented by different colors.
Fig. 4. Simulated results of absorptance as a function of frequency and thickness of Si (a) and SiO2 (b) when
imaginary parts of the effective impedance are shown in Fig. 2. The effective impedance is calculated according to the formula ffi qffiffiffi rffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ð1þS Þ2 S2 Z ¼ le ¼ ð1S11 Þ2 S21 2 , where S11 and S21 are the scattering param11
21
eters of reflection and transmission, respectively. From Fig. 2(d), when the frequency changes from 0.5 THz to 1.05 THz, both the real and imaginary parts are almost matched to the corresponding vacuum values of one and zero. Due to impedance matching to vacuum, the proposed absorber has a low reflectance leading to the behavior of highly-efficient broadband absorption. The performance of polarization-independent absorption is proved in Fig. 3(a). The rotation symmetry of the system leads to a polarization-independent behavior. To investigate whether the design has good absorptance for oblique incidence, the influence on absorption performance for oblique incidence is numerically studied. Fig. 3(b) shows simulated absorptance spectra of absorber under different oblique incidence of transverse electronic (TE) polarization. Absorption performance does not change for oblique
angles up to 45 . Even when incident angle increases to 60 , it can still reach 80%. So wide-angle absorption is obviously observed
within the angle of 60 . While for oblique incidence of transverse magnetic (TM) polarization in Fig. 3(c), absorption center moves to lower frequency as incident angle increases, and absorption peak becomes narrower. Even when incident angle increases to
r ¼ 2500 S=m. The normalized
r ¼ 2500 S=m.
60 , the absorber can still get over 90% absorptance. The side lobe around 1.1 THz is mainly caused by the variation of normal component of electric filed. It is necessary to discuss the influence of structure parameters on the performance of absorption. We simulate absorptance with different thicknesses of Si and SiO2 . The corresponding results are shown in Fig. 4. From Fig. 4(a), it is seen that keeping other structural parameters of absorber unchanged, absorptance fluctuates obviously when the thickness of Si changes from 1:0 lm to 10 lm with a step of 1:0 lm. Fig. 4(b) shows the impact of absorptance with different thicknesses of SiO2 , while other geometric parameters are fixed. Absorption band gradually moves to lower frequencies as the thickness of SiO2 increases. This is mainly because the effective cavity length correspondingly increases, and the ideal absorptance is obtained around 46 lm. The above results verify the role of geometrical structures in improving the performance of absorber. 3. Conclusions To summarize, Si metasurface is proposed to obtain wideband absorption at terahertz frequencies. Simulated results demonstrate that the proposed absorber achieves broadband absorption with the efficiency more than 90% in the broadband frequency range
Z. Song et al. / Materials Letters 234 (2019) 138–141
from 0.497 THz to 1.045 THz when the conductivity of Si is equal to 2500 S=m. The absorptance average level is very good for TE- and
TM- polarized waves with an incident angle up to 30 . The proposed design may find various applications in detector, energy harvesting, and tunable sensor. Acknowledgements This work was supported by the National Natural Science Foundation of China [grant number 11504305]. References [1] T.W. Ebbesen, H.J. Lezec, H.F. Ghaemi, T. Thio, P.A. Wolff, Nature 391 (1998) 667–669. [2] Z. Song, Z. Gao, Y. Zhang, B. Zhang, EPL 106 (2014) 27005. [3] N.K. Grady, J.E. Heyes, D.R. Chowdhury, Y. Zeng, M.T. Reiten, A.K. Azad, et al., Science 340 (2013) 1304–1307. [4] L. Zhang, Z. Song, Q.H. Liu, EPL 111 (2015) 27001. [5] Z. Song, Q. Chu, X. Shen, Q.H. Liu, Front. Phys. 13 (2018) 137803. [6] J.B. Pendry, D. Schurig, D.R. Smith, Science 312 (2006) 1780–1782. [7] P. Rufangura, C. Sabah, J. Alloys Compd. 680 (2016) 473–479.
141
[8] N.I. Landy, C.M. Bingham, T. Tyler, N. Jokerst, D.R. Smith, W.J. Padilla, Phys. Rev. B 79 (2009) 125104. [9] A. Tennant, B. Chambers, Smart Mater. Struct. 13 (2004) 122–125. [10] K. Aydin, V.E. Ferry, R.M. Briggs, H.A. Atwater, Nat. Commun. 2 (2011) 517. [11] J.M. Hao, J. Wang, X.L. Liu, W.J. Padilla, L. Zhou, M. Qiu, Appl. Phys. Lett. 96 (2010) 251104. [12] H. Deng, L. Stan, D.A. Czaplewski, J. Gao, X. Yang, Opt. Express 25 (2017) 28295–28304. [13] M. Luo, S. Shen, L. Zhou, S. Wu, Y. Zhou, L. Chen, Opt. Express 25 (2017) 16715– 16724. [14] L. Lei, S. Li, H. Huang, K. Tao, P. Xu, Opt. Express 26 (2018) 5686–5693. [15] L. Cong, P. Pitchappa, C. Lee, R. Singh, Adv. Mater. 29 (2017) 1700733. [16] T.T. Nguyen, S. Lim, Appl. Phys. Lett. 112 (2018) 021605. [17] H. Xiong, Y.B. Wu, J. Dong, M.C. Tang, Y.N. Jiang, X.P. Zeng, Opt. Express 26 (2018) 1681–1688. [18] C. Qu, S. Ma, J. Hao, M. Qiu, X. Li, S. Xiao, et al., Phys. Rev. Lett. 115 (2015) 235503. [19] Z. Song, K. Wang, J. Li, Q.H. Liu, Opt. Express 26 (2018) 7148–7154. [20] M. Naftaly, R.E. Miles, J. Appl. Phys. 102 (2007) 043517. [21] R. Malureanu, M. Zalkovskij, Z.Y. Song, C. Gritti, A. Andryieuski, Q. He, et al., Opt. Express 20 (2012) 22770–22782. [22] N. Liu, L. Langguth, T. Weiss, J. Kästel, M. Fleischhauer, T. Pfau, et al., Nat. Mater. 8 (2009) 758–762. [23] M. Manjappa, Y.K. Srivastava, L. Cong, I. Al-Naib, R. Singh, Adv. Mater. 29 (2017) 1603355. [24] M. Gupta, Y.K. Srivastava, R. Singh, Adv. Mater. 30 (2018) 1704845.
Contents lists available at ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/mlblue
Broadband tunable absorber for terahertz waves based on isotropic silicon metasurfaces Zhengyong Song ⇑, Zhisheng Wang, Maoliang Wei Institute of Electromagnetics and Acoustics, Department of Electronic Science, Xiamen University, Xiamen 361005, China
a r t i c l e
i n f o
Article history: Received 30 July 2018 Received in revised form 30 August 2018 Accepted 15 September 2018 Available online 17 September 2018 Keywords: Optical materials and properties Microstructure Metasurface Absorber Terahertz
a b s t r a c t A broadband tunable absorber is studied based on the silicon metasurface at terahertz frequencies. By tuning the conductivity of silicon, absorptance is more than 90% from 0.497 THz to 1.045 THz with central frequency of 0.771 THz when the conductivity is equal to 2500 S=m. Simulated results show that absorptance peak can be modulated from 1% to 100% when the conductivity continuously changes from 0 S=m to2500 S=m. The central-symmetry structure leads to polarization independence. Our design may be used as optoelectronic modulator, tunable detector, and terahertz switch. Ó 2018 Elsevier B.V. All rights reserved.
1. Introduction In the past decade, artificially engineered subwavelength materials-metamaterials (MMs) have rendered us numerous opportunities to obtain effect and phenomenon that could not be easily accessed in natural materials. Now it leads to many demonstrations of interesting properties such as extraordinary optical transmission [1,2], polarization rotator [3–5], and perfect absorber [6–19]. In the field of MM, there have been tremendous efforts on absorber in recent years, because it has crucial roles in many different applications such as stealth device [6], solar cell [7], thermal imaging [8], and strongly reduced radar cross section of fighter plane and warship [9]. Several conventional metals have been proposed to realize electromagnetic absorption based on localized surface plasmon resonance of metal nanoparticle, such as silver [10], gold [11], chromium [12], nickel [13], titanium [14], and aluminium [15]. These absorbers seem expensive in price, and it is not easy to change absorption behavior once the sample is fabricated. For tunable absorber, insert of active device is always an important method and is able to provide the degrees of freedom for tuning. The variable systems are able to achieve impedance matching surface in some specific frequency bands. In order to achieve tunable absorption at different frequencies, some devices are frequently added, such as variable resistor for microwave [16] and graphene [17,18] and vanadium oxide [19] for terahertz ⇑ Corresponding author. E-mail address: [email protected] (Z. Song). https://doi.org/10.1016/j.matlet.2018.09.084 0167-577X/Ó 2018 Elsevier B.V. All rights reserved.
wave. Although graphene absorber has good performance, it would take long calculation time in simulation and have complicated fabrication procedures. Doped silicon (Si) with small resistivity can be considered as a highly lossy material in terahertz region and can be easily processed using standard lithography technology. In this work, a tunable absorber is reported based on doped Si metasurfaces. The relation between doping concentration and conductivity is r / nev , where n, e, and v are electron concentration, electronic charge, and electron mobility. Because of mature micro-and nanofabrication techniques of Si, our design is not difficult to realize under the condition of different conductivities. 2. Calculations and discussions of the results The schematic view of the proposed Si absorber is shown in Fig. 1. The basic unit cell of the designed absorber consists of one cross air hole in Si with thickness of 5.2 lm and a gold ground plane with thickness of 0.2 lm which is enough to prevent any transmission. The Si patches are separated by a 46 lm silica (SiO2 ) dielectric spacer from the background gold plane. The design parameters as labeled in Fig. 1(a) are as follows: w =70 lm, l =173 lm, and P =175 lm. Electromagnetic simulations are performed through a commercial software package (Comsol Multiphysics). The S parameters (S11 and S21 ) are simulated by frequency domain solver. In simulation, unit cell boundary conditions in x and y directions and open boundary conditions in z direction are used. The dielectric spacer is constructed by SiO2 with relative permittivity of 3.8 [20,21]. The permittivity of gold is
Z. Song et al. / Materials Letters 234 (2019) 138–141
139
Fig. 1. (a) Three-dimensional geometry of the designed absorber consisting of periodical air cross hole in Si film (orange), a silica spacer (cyan), and the gold film (yellow). The whole system is deposited on the silica (cyan) substrate. (b) Simulated absorptance spectra as a function of the conductivity of Si and frequency under normal incidence. The conductivity is varied from 0 S=m to 2500 S=m in a step of 250 S=m.
described by Drude model
xp ¼ 1:37 1016 rad=S
eAu ¼ 1 xðxxþip CÞ with plasma frequency
and
2
collision
frequency
C ¼ 1:2
1014 rad=S [22]. The conductivity of Si can be ranged from 0 S=m to 2500 S=m [23,24]. The absorptive efficiency of the proposed absorber can be defined as A ¼ 1 R T ¼ 1 jS11 j2 jS21 j2 , where A, R, and T are absorptance, reflectance, and transmittance, respectively. Due to the gold background plane, transmission (S21 ) is zero across the entire frequency range. Thus absorptance can be calculated by A ¼ 1 jS21 j2 . Simulation results of absorptance are shown in
Fig. 1(b). In order to determine the optimum conductivity, absorptance is simulated while varying the conductivity from 0 S=m to 2500 S=m. Fig. 1(b) depicts simulated absorptance of the proposed absorber with different conductivities under normal incidence. It can be observed that absorptance is larger than 90% between 0.497 THz and 1.045 THz when conductivity is 2500 S=m. Working intensity can be dynamically modulated from 1% to 100%. The results tell that absorption peak of the proposed structure can be tuned by adjusting the conductivity of Si through controlling the bias voltage. In order to further understand the physical origin of absorption, S-parameters are calculated and then the corresponding real and
Fig. 2. Retrieved effective physical parameters (a) permittivity, (b) permeability, (c) refractive index, and (d) impedance in the case of perfect absorption whenr ¼ 2500 S=m.
140
Z. Song et al. / Materials Letters 234 (2019) 138–141
Fig. 3. The dependences of simulated absorptance on polarization angle (a) and oblique incidence of TE (b) and TM (c) polarizations when absorptance is represented by different colors.
Fig. 4. Simulated results of absorptance as a function of frequency and thickness of Si (a) and SiO2 (b) when
imaginary parts of the effective impedance are shown in Fig. 2. The effective impedance is calculated according to the formula ffi qffiffiffi rffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ð1þS Þ2 S2 Z ¼ le ¼ ð1S11 Þ2 S21 2 , where S11 and S21 are the scattering param11
21
eters of reflection and transmission, respectively. From Fig. 2(d), when the frequency changes from 0.5 THz to 1.05 THz, both the real and imaginary parts are almost matched to the corresponding vacuum values of one and zero. Due to impedance matching to vacuum, the proposed absorber has a low reflectance leading to the behavior of highly-efficient broadband absorption. The performance of polarization-independent absorption is proved in Fig. 3(a). The rotation symmetry of the system leads to a polarization-independent behavior. To investigate whether the design has good absorptance for oblique incidence, the influence on absorption performance for oblique incidence is numerically studied. Fig. 3(b) shows simulated absorptance spectra of absorber under different oblique incidence of transverse electronic (TE) polarization. Absorption performance does not change for oblique
angles up to 45 . Even when incident angle increases to 60 , it can still reach 80%. So wide-angle absorption is obviously observed
within the angle of 60 . While for oblique incidence of transverse magnetic (TM) polarization in Fig. 3(c), absorption center moves to lower frequency as incident angle increases, and absorption peak becomes narrower. Even when incident angle increases to
r ¼ 2500 S=m. The normalized
r ¼ 2500 S=m.
60 , the absorber can still get over 90% absorptance. The side lobe around 1.1 THz is mainly caused by the variation of normal component of electric filed. It is necessary to discuss the influence of structure parameters on the performance of absorption. We simulate absorptance with different thicknesses of Si and SiO2 . The corresponding results are shown in Fig. 4. From Fig. 4(a), it is seen that keeping other structural parameters of absorber unchanged, absorptance fluctuates obviously when the thickness of Si changes from 1:0 lm to 10 lm with a step of 1:0 lm. Fig. 4(b) shows the impact of absorptance with different thicknesses of SiO2 , while other geometric parameters are fixed. Absorption band gradually moves to lower frequencies as the thickness of SiO2 increases. This is mainly because the effective cavity length correspondingly increases, and the ideal absorptance is obtained around 46 lm. The above results verify the role of geometrical structures in improving the performance of absorber. 3. Conclusions To summarize, Si metasurface is proposed to obtain wideband absorption at terahertz frequencies. Simulated results demonstrate that the proposed absorber achieves broadband absorption with the efficiency more than 90% in the broadband frequency range
Z. Song et al. / Materials Letters 234 (2019) 138–141
from 0.497 THz to 1.045 THz when the conductivity of Si is equal to 2500 S=m. The absorptance average level is very good for TE- and
TM- polarized waves with an incident angle up to 30 . The proposed design may find various applications in detector, energy harvesting, and tunable sensor. Acknowledgements This work was supported by the National Natural Science Foundation of China [grant number 11504305]. References [1] T.W. Ebbesen, H.J. Lezec, H.F. Ghaemi, T. Thio, P.A. Wolff, Nature 391 (1998) 667–669. [2] Z. Song, Z. Gao, Y. Zhang, B. Zhang, EPL 106 (2014) 27005. [3] N.K. Grady, J.E. Heyes, D.R. Chowdhury, Y. Zeng, M.T. Reiten, A.K. Azad, et al., Science 340 (2013) 1304–1307. [4] L. Zhang, Z. Song, Q.H. Liu, EPL 111 (2015) 27001. [5] Z. Song, Q. Chu, X. Shen, Q.H. Liu, Front. Phys. 13 (2018) 137803. [6] J.B. Pendry, D. Schurig, D.R. Smith, Science 312 (2006) 1780–1782. [7] P. Rufangura, C. Sabah, J. Alloys Compd. 680 (2016) 473–479.
141
[8] N.I. Landy, C.M. Bingham, T. Tyler, N. Jokerst, D.R. Smith, W.J. Padilla, Phys. Rev. B 79 (2009) 125104. [9] A. Tennant, B. Chambers, Smart Mater. Struct. 13 (2004) 122–125. [10] K. Aydin, V.E. Ferry, R.M. Briggs, H.A. Atwater, Nat. Commun. 2 (2011) 517. [11] J.M. Hao, J. Wang, X.L. Liu, W.J. Padilla, L. Zhou, M. Qiu, Appl. Phys. Lett. 96 (2010) 251104. [12] H. Deng, L. Stan, D.A. Czaplewski, J. Gao, X. Yang, Opt. Express 25 (2017) 28295–28304. [13] M. Luo, S. Shen, L. Zhou, S. Wu, Y. Zhou, L. Chen, Opt. Express 25 (2017) 16715– 16724. [14] L. Lei, S. Li, H. Huang, K. Tao, P. Xu, Opt. Express 26 (2018) 5686–5693. [15] L. Cong, P. Pitchappa, C. Lee, R. Singh, Adv. Mater. 29 (2017) 1700733. [16] T.T. Nguyen, S. Lim, Appl. Phys. Lett. 112 (2018) 021605. [17] H. Xiong, Y.B. Wu, J. Dong, M.C. Tang, Y.N. Jiang, X.P. Zeng, Opt. Express 26 (2018) 1681–1688. [18] C. Qu, S. Ma, J. Hao, M. Qiu, X. Li, S. Xiao, et al., Phys. Rev. Lett. 115 (2015) 235503. [19] Z. Song, K. Wang, J. Li, Q.H. Liu, Opt. Express 26 (2018) 7148–7154. [20] M. Naftaly, R.E. Miles, J. Appl. Phys. 102 (2007) 043517. [21] R. Malureanu, M. Zalkovskij, Z.Y. Song, C. Gritti, A. Andryieuski, Q. He, et al., Opt. Express 20 (2012) 22770–22782. [22] N. Liu, L. Langguth, T. Weiss, J. Kästel, M. Fleischhauer, T. Pfau, et al., Nat. Mater. 8 (2009) 758–762. [23] M. Manjappa, Y.K. Srivastava, L. Cong, I. Al-Naib, R. Singh, Adv. Mater. 29 (2017) 1603355. [24] M. Gupta, Y.K. Srivastava, R. Singh, Adv. Mater. 30 (2018) 1704845.