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Optics Express

  • Editor: Andrew M. Weiner
  • Vol. 22, Iss. 6 — Mar. 24, 2014
  • pp: 6287–6295
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Tunable reflector with active magnetic metamaterials

Tianwei Deng, Ruifeng Huang, Ming-Chun Tang, and Peng Khiang Tan  »View Author Affiliations


Optics Express, Vol. 22, Issue 6, pp. 6287-6295 (2014)
http://dx.doi.org/10.1364/OE.22.006287


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Abstract

We placed active magnetic metamaterials on metallic surface to implement a tunable reflector with excellent agile performance. By incorporating active elements into the unit cells of the magnetic metamaterial, this active magnetic metamaterial can be tuned to switch function of the reflector among a perfect absorber, a perfect reflector and a gain reflector. This brings about DC control lines to electrically tune the active magnetic metamaterial with positive loss, zero loss and even negative loss. The design, analytical and numerical simulation methods, and experimental results of the tunable reflector are presented.

© 2014 Optical Society of America

1. Introduction

Metamaterials offer an entirely new route to design of effective material properties. These artificial materials implemented by geometric unit cells are enriching studies in the field from conventional microscopic materials. A number of metamaterials from microwave to optical frequencies [1

1. D. R. Smith, J. B. Pendry, and M. C. K. Wiltshire, “Metamaterials and negative refractive index,” Science 305(5685), 788–792 (2004). [CrossRef] [PubMed]

4

4. A. E. Nikolaenko, N. Papasimakis, A. Chipouline, F. De Angelis, E. Di Fabrizio, and N. I. Zheludev, “THz bandwidth optical switching with carbon nanotube metamaterial,” Opt. Express 20(6), 6068–6079 (2012). [CrossRef] [PubMed]

] for various applications, including cloaks [5

5. J. B. Pendry, D. Schurig, and D. R. Smith, “Controlling electromagnetic fields,” Science 312(5781), 1780–1782 (2006). [CrossRef] [PubMed]

7

7. B. Kanté, A. de Lustrac, J. M. Lourtioz, and S. N. Burokur, “Infrared cloaking based on the electric response of split ring resonators,” Opt. Express 16(12), 9191–9198 (2008). [CrossRef] [PubMed]

], absorbers [8

8. N. I. Landy, S. Sajuyigbe, J. J. Mock, D. R. Smith, and W. J. Padilla, “Perfect metamaterial absorber,” Phys. Rev. Lett. 100, 207402 (2008).

10

10. R. Alaee, M. Farhat, C. Rockstuhl, and F. Lederer, “A perfect absorber made of a graphene micro-ribbon metamaterial,” Opt. Express 20(27), 28017–28024 (2012). [CrossRef] [PubMed]

], lenses [11

11. N. Fang, H. Lee, C. Sun, and X. Zhang, “Sub-diffraction-limited optical imaging with a silver superlens,” Science 308(5721), 534–537 (2005). [CrossRef] [PubMed]

] microwave circuits [12

12. F. Capolino, Theory and Phenomena of Metamaterials (CRC Press LLC, 2009), Vol. 1.

] and antennas [13

13. Y. Dong and T. Itoh, “Metamaterial-based antennas,” Proc. IEEE 100(7), 2271–2285 (2012). [CrossRef]

], have been proposed in recent years. Although exotic electromagnetic properties have attracted more research on metamaterials, the loss factor inherently limits their engineering practice. Thus, some active media or devices were brought in for loss reduction [14

14. S. Xiao, V. P. Drachev, A. V. Kildishev, X. Ni, U. K. Chettiar, H. K. Yuan, and V. M. Shalaev, “Loss-free and active optical negative-index metamaterials,” Nature 466(7307), 735–738 (2010). [CrossRef] [PubMed]

16

16. S.A. Ramakrishna and J.B. Pendry, “Removal of absorption and increase in resolution in a near-field lens via optical gain,” Phys. Rev. B 67, 201101 (2003).

]. However, the negative refraction will disappear when the losses are compensated or significantly reduced by active media [17

17. M. I. Stockman, “Criterion for negative refraction with low optical losses from a fundamental principle of causality,” Phys. Rev. Lett . 98, 177404 (2007).

, 18

18. L. Sun, X. Yang, and J. Gao, “Loss-compensated broadband epsilon-near-zero metamaterials with gain media,” Appl. Phys. Lett . 103, 201109 (2013).

]. Alternatively, more applications of these active metamaterials as gain medium have been proposed. For example, an active metamaterial device demonstrated real-time control and manipulation of THz radiation [19

19. H. T. Chen, W. J. Padilla, J. M. O. Zide, A. C. Gossard, A. J. Taylor, and R. D. Averitt, “Active terahertz metamaterial devices,” Nature 444(7119), 597–600 (2006). [CrossRef] [PubMed]

]. The control capability is achieved by means of incorporating a Schottky diode and the transmission of THz wave is enhanced by the amount of gate bias applied to the diode. Another application can be in modifying the reflectivity of an object from a high value to a low value instantly [20

20. A. Tennant and B. Chambers, “Adaptive radar absorbing structure with PIN diode controlled active frequency selective surface,” Smart Mater. Struct. 13(1), 122–125 (2004). [CrossRef]

]. Currently, switchable reflectors/absorbers are employed to tune the reflectivity value by shifting frequencies of absorption peak [21

21. N. Mattiucci, R. Trimm, G. D’Aguanno, N. Akozbek, and M.J. Bloemer, “Tunable, narrow-band, all-metallic microwave absorber,” Appl. Phys. Lett. 101, 141115 (2012).

23

23. W. Xu and S. Sonkusale, “Microwave diode switchable metamaterial reflector/absorber,” Appl. Phys. Lett. 103, 031902 (2013).

] and these reflectors can be switched from being perfect absorber to perfect reflector. Yet, a truly smart absorber demands wider tuning capabilities [24

24. B. Chambers, “A smart radar absorber,” Smart Mater. Struct. 8(1), 64–72 (1999). [CrossRef]

]. In this paper, we describe a method to achieve reconfigurable absorptivity/reflectivity by the novel use of a controllable gain medium. We can tailor the reflectance of a metallic surface not only from negative values to zero, but also to positive ones at an instance.

2. Design, fabrication and measurement of tunable reflector

In this work, we demonstrate a tunable reflector with function of the reflector among a perfect absorber, a perfect reflector and a gain reflector. As shown in Fig. 1(a), metallic reflectors with conventional material slabs usually exhibit reduced reflectivity, while; with gain medium, amplified reflectivity in Fig. 1(b) can be realized from gain reflector, (i.e. the power of reflected wave from the reflector is stronger than the incident one).
Fig. 1 Illustration of reflectivity reduction or amplification from metallic surfaces with material slab.
The proposed tunable reflector incorporates a tunable active magnetic metamaterial as gain medium on a metallic surface (Fig. 2).
Fig. 2 (a) Tunable reflector with active magnetic metamaterial on metallic surface; (b) A unit cell of the active magnetic metamaterial under consideration.
The active magnetic metamaterial is based on conventional magnetic metamaterials of periodic split rings [1

1. D. R. Smith, J. B. Pendry, and M. C. K. Wiltshire, “Metamaterials and negative refractive index,” Science 305(5685), 788–792 (2004). [CrossRef] [PubMed]

], and active elements are loaded between two rings to constitute a unit cell [5

5. J. B. Pendry, D. Schurig, and D. R. Smith, “Controlling electromagnetic fields,” Science 312(5781), 1780–1782 (2006). [CrossRef] [PubMed]

]. The active elements provide magnitude amplification and phase shifting of flowing current from sensing ring to driven ring, and DC control lines are used to electrically tune active elements. With suitable DC control voltages, the active magnetic metamaterial exhibits properties of a gain medium with tunable effective permeability of positive loss, zero loss, and even negative loss.

The tunable reflector is implemented using PCB (printed circuit board) technology. Due to the relatively large impedance of inductance on the ring compared to the input impedance of the active elements, the current in the driven ring is limited. To increase the input voltage of the active elements, and decrease the loading impedance of the active elements, a capacitor-loaded split ring is used in active magnetic metamaterial. The dimensions of the capacitor-loaded split ring are shown in the inset of Fig. 3.
Fig. 3 One periodic unit cell of the active magnetic metamaterials. (r = 20.5, a = 1,θ = 50°, l1 = 5 and g = 0.5; units in mm)
The rings are periodically and vertically placed on the metallic surface. The total thickness of one unit cell is only 5.1cm (~0.1λ), which is electrically small. The active elements include a power amplifier (minicircuits, model: VNA-28), a voltage tunable phase shifter (minicircuits, model: JSPHS-661), and a voltage tunable attenuator (minicircuits, model: SVA-2000) to modify flowing current from sensing ring to driven ring. The fabricated reflector with six periodic units of the active magnetic metamaterials on metallic surface is shown in Fig. 4.
Fig. 4 The fabricated tunable reflector with six periodic unit cells.
The measurement configuration is shown in Fig. 5.
Fig. 5 Measurement setup for tunable reflector.
A TEM cell was used as the test fixture. Quasi-TEM wave can be excited and most of the E field and H field are horizontally and vertically distributed in the TEM cell at measured frequencies, respectively. As shown in Fig. 6, the measurement of the TEM cell loaded with six periodic units approximately simulates the free space measurement of doubly infinite periodic units with normal incident TEM waves.
Fig. 6 Tunable reflector with six units measured by using a TEM cell test fixture.

The DC power supplies of the active magnetic metamaterial have three channels, pumped by three control lines, respectively. Channel 1 is the control voltage for the phase shifters (V1) which can be varied from 0V to 12V. Channel 2 is the control voltage for the attenuators (V2) which can also be varied from 0V to 12V. Channel 3 is the supply voltage which is set to be 5V and with amplification gain of around 18dB at 600MHz. The measured reflectivity, when the DC power supplies are off, is shown in Fig. 7.
Fig. 7 Measured reflectivity/absorbance of the tunable reflector as perfect absorber when the power supplies are off.
The reflector exhibits near unity absorbance (A(ω)>99%) as perfect absorber [8

8. N. I. Landy, S. Sajuyigbe, J. J. Mock, D. R. Smith, and W. J. Padilla, “Perfect metamaterial absorber,” Phys. Rev. Lett. 100, 207402 (2008).

], where A(ω) is absorbance, R(ω) is reflectivity and A(ω) = 1-R(ω). We fixed the V2 to a certain value, and then changed V1 from 0V to 12V. Measured results are given in Fig. 8.
Fig. 8 Measured reflectivity results of active magnetic metamaterials, when V1 = 0V, 3V, 5V, 6V, 9V (i.e. phase shift≈0°, 45°, 90°, 135°, 180°) and V2 = 2V, 3V, 6V, 12V (i.e. attenuation≈13dB, 10dB, 5dB, 2dB).
It shows that the maximum reflectivity increases when the attenuator control voltage increases. In Fig. 9, at 0.625GHz, it is demonstrated that the reflectivity can be tuned from −20dB to + 15dB by carefully selecting control voltages V1 and V2.
Fig. 9 Measured reflectivity that varies from −20dB to + 15dB.
Thus, the tunable reflector can work as perfect absorber (R(ω)<-20dB), absorbance tunable absorber (−20dB<R(ω)<0dB), perfect reflector (R(ω) = 0dB) and tunable gain reflector (R(ω)>0dB).

3. Analytical simulation method for active magnetic metamaterials

The equivalent circuit of the active magnetic metamaterial unit cell is shown in Fig. 10.
Fig. 10 Equivalent circuit of the active magnetic metamaterial cell.
Because the ring of magnetic metamaterials particle is electrically small and |k|rλ where k=ωε0μ0 denotes the wavenumber in the medium, magnetic dipole moment induced in the ring by incident field can be calculated and the excited current by the incident fields on the ring can be modelled as the equivalent voltage sources Vs,inc and Vd,inc in the equivalent circuit. Vsd and Vds are the equivalent sources caused by mutual coupling. L and C are the self-inductance and self-capacitance of the rings. Zin and Zout are the input and output impedances of the power amplifier, respectively. Vin and Vctl are the input and the output voltages of the power amplifier, respectively. l1 and l2 are the equivalent cable length when the phase shift caused by the connecting cable and the power amplifier is taken into account.

The corresponding current on the sensing and driven rings can be expressed as follows:
is=Vs,incVsdjωX+Zin',id=id1+id2=Geqv(Vs,incVsd)jωX+Vd,incVdsjωX+Zout',
(1)
where Zin and Zout are the input and output, impedances of the power amplifier seen from the sensing or driven rings, respectively. The X = L/(1-ω2LC) and Geqv is the equivalent gain of the system. The active magnetic metamaterial under consideration generates the magnetic moment m = S(is + id). The polarizability in this unit cell is expressed as the following [25

25. S. A. Schelkunoff and H. T. Friis, Antennas Theory and Practice, (Wiley, 1966).

]:
amm=S(is+id)μ0Hinc(ω02ω21)1,
(2)
where ω0=1/LCis the resonant frequency of the LC circuit formed by the ring and the loaded capacitor. The relevant effective permeability of the medium composed by the proposed active magnetic metamaterial cells is:

μr=χm1=μ0ammVc1=S(is+id)HincVc(ω02ω21)11,
(3)

When the system is perfect matched and the mutual coupling between rings is negligible, the imaginary part of effective permeability can be expressed as:

μr''=μ0S2VcZ0ω2X2+Z02(2ωArω2cos(θ)+ArZ02Xsin(θ))(ω02ω21)1
(4)

The phase delay on two cables (θ=β1l1+β2l2) and the gain of amplification (Ar) dominate the sign and magnitude of the μ, namely, magnetic loss. Thus, the active metamaterial with tunable magnetic loss (especially from positive to negative value) could be realized by suitable control of the active elements according to Eq. (4).

4. Numerical simulation method for tunable reflector

In the experiment, we used six periodic units inside a TEM cell to simulate doubly infinite periodic units, as illustrated in Fig. 6. The reflectivity of the tunable reflector was calculated by numerical simulation in Fig. 12.
Fig. 12 Numerically simulated reflectivity of the tunable reflector.
It provides good prediction of the tunable reflector performance. When the attenuator voltage V2 increases (i.e. lower attenuation), the difference between numerical simulation and measurement occurs, which is due to the saturation of the power amplifiers and non-uniform performance of active elements in six unit cells.

5. Conclusion

In summary, our investigation on active tunable reflector verifies the feasibility of a reflector that is electrically tuned among a perfect absorber, a perfect reflector and a gain reflector by incorporating active magnetic metamaterials with tunable effective permeability. The effective magnetic loss of the active magnetic metamaterial is tunable from positive to negative and the reflectivity from the tunable reflector varies from −20dB to + 15dB. Furthermore, we point out that, although the tunable reflector was experimentally demonstrated at Gigahertz frequencies, it can be easily scaled to Terahertz and infrared frequencies on semiconductor or MEMS/NEMS technologies based on the analytical and numerical simulation methods presented in this paper. As a future work, benefiting from the capacitor-loaded split ring reconfigurability, frequency-agile function could be introduced by incorporating varactor diodes into the split rings. Furthermore, the designed reflector can only interact with TEM or TM wave, new tunable reflector with active electric metamaterials for incident TE waves is under consideration.

References and links

1.

D. R. Smith, J. B. Pendry, and M. C. K. Wiltshire, “Metamaterials and negative refractive index,” Science 305(5685), 788–792 (2004). [CrossRef] [PubMed]

2.

E. Ekmekci, K. Topalli, T. Akin, and G. Turhan-Sayan, “A tunable multi-band metamaterial design using micro-split SRR structures,” Opt. Express 17(18), 16046–16058 (2009). [CrossRef] [PubMed]

3.

J. Yao, Z. Liu, Y. Liu, Y. Wang, C. Sun, G. Bartal, A. M. Stacy, and X. Zhang, “Optical negative refraction in bulk metamaterials of nanowires,” Science 321(5891), 930 (2008). [CrossRef] [PubMed]

4.

A. E. Nikolaenko, N. Papasimakis, A. Chipouline, F. De Angelis, E. Di Fabrizio, and N. I. Zheludev, “THz bandwidth optical switching with carbon nanotube metamaterial,” Opt. Express 20(6), 6068–6079 (2012). [CrossRef] [PubMed]

5.

J. B. Pendry, D. Schurig, and D. R. Smith, “Controlling electromagnetic fields,” Science 312(5781), 1780–1782 (2006). [CrossRef] [PubMed]

6.

H. Chen, B. Zheng, L. Shen, H. Wang, X. Zhang, N.I. Zheludev, and B. Zhang, “Ray-optics cloaking devices for large objects in incoherent natural light,” Nat. Commun . 4, 2652 (2013).

7.

B. Kanté, A. de Lustrac, J. M. Lourtioz, and S. N. Burokur, “Infrared cloaking based on the electric response of split ring resonators,” Opt. Express 16(12), 9191–9198 (2008). [CrossRef] [PubMed]

8.

N. I. Landy, S. Sajuyigbe, J. J. Mock, D. R. Smith, and W. J. Padilla, “Perfect metamaterial absorber,” Phys. Rev. Lett. 100, 207402 (2008).

9.

H. Wang and L. Wang, “Perfect selective metamaterial solar absorber,” Opt. Express 21(S6), A1078–A1093 (2013). [CrossRef]

10.

R. Alaee, M. Farhat, C. Rockstuhl, and F. Lederer, “A perfect absorber made of a graphene micro-ribbon metamaterial,” Opt. Express 20(27), 28017–28024 (2012). [CrossRef] [PubMed]

11.

N. Fang, H. Lee, C. Sun, and X. Zhang, “Sub-diffraction-limited optical imaging with a silver superlens,” Science 308(5721), 534–537 (2005). [CrossRef] [PubMed]

12.

F. Capolino, Theory and Phenomena of Metamaterials (CRC Press LLC, 2009), Vol. 1.

13.

Y. Dong and T. Itoh, “Metamaterial-based antennas,” Proc. IEEE 100(7), 2271–2285 (2012). [CrossRef]

14.

S. Xiao, V. P. Drachev, A. V. Kildishev, X. Ni, U. K. Chettiar, H. K. Yuan, and V. M. Shalaev, “Loss-free and active optical negative-index metamaterials,” Nature 466(7307), 735–738 (2010). [CrossRef] [PubMed]

15.

Y. Yuan, B. I. Popa, and S. A. Cummer, “Zero loss magnetic metamaterials using powered active unit cells,” Opt. Express 17(18), 16135–16143 (2009). [CrossRef] [PubMed]

16.

S.A. Ramakrishna and J.B. Pendry, “Removal of absorption and increase in resolution in a near-field lens via optical gain,” Phys. Rev. B 67, 201101 (2003).

17.

M. I. Stockman, “Criterion for negative refraction with low optical losses from a fundamental principle of causality,” Phys. Rev. Lett . 98, 177404 (2007).

18.

L. Sun, X. Yang, and J. Gao, “Loss-compensated broadband epsilon-near-zero metamaterials with gain media,” Appl. Phys. Lett . 103, 201109 (2013).

19.

H. T. Chen, W. J. Padilla, J. M. O. Zide, A. C. Gossard, A. J. Taylor, and R. D. Averitt, “Active terahertz metamaterial devices,” Nature 444(7119), 597–600 (2006). [CrossRef] [PubMed]

20.

A. Tennant and B. Chambers, “Adaptive radar absorbing structure with PIN diode controlled active frequency selective surface,” Smart Mater. Struct. 13(1), 122–125 (2004). [CrossRef]

21.

N. Mattiucci, R. Trimm, G. D’Aguanno, N. Akozbek, and M.J. Bloemer, “Tunable, narrow-band, all-metallic microwave absorber,” Appl. Phys. Lett. 101, 141115 (2012).

22.

W. Zhu, Y. Huang, I. D. Rukhlenko, G. Wen, and M. Premaratne, “Configurable metamaterial absorber with pseudo wideband spectrum,” Opt. Express 20(6), 6616–6621 (2012). [CrossRef] [PubMed]

23.

W. Xu and S. Sonkusale, “Microwave diode switchable metamaterial reflector/absorber,” Appl. Phys. Lett. 103, 031902 (2013).

24.

B. Chambers, “A smart radar absorber,” Smart Mater. Struct. 8(1), 64–72 (1999). [CrossRef]

25.

S. A. Schelkunoff and H. T. Friis, Antennas Theory and Practice, (Wiley, 1966).

26.

D. M. Pozar, Microwave Engineering (Wiley, 2012), p.174.

OCIS Codes
(350.4010) Other areas of optics : Microwaves
(310.3915) Thin films : Metallic, opaque, and absorbing coatings
(160.3918) Materials : Metamaterials
(050.6624) Diffraction and gratings : Subwavelength structures

ToC Category:
Metamaterials

History
Original Manuscript: December 30, 2013
Revised Manuscript: February 20, 2014
Manuscript Accepted: February 25, 2014
Published: March 10, 2014

Citation
Tianwei Deng, Ruifeng Huang, Ming-Chun Tang, and Peng Khiang Tan, "Tunable reflector with active magnetic metamaterials," Opt. Express 22, 6287-6295 (2014)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-22-6-6287


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References

  1. D. R. Smith, J. B. Pendry, M. C. K. Wiltshire, “Metamaterials and negative refractive index,” Science 305(5685), 788–792 (2004). [CrossRef] [PubMed]
  2. E. Ekmekci, K. Topalli, T. Akin, G. Turhan-Sayan, “A tunable multi-band metamaterial design using micro-split SRR structures,” Opt. Express 17(18), 16046–16058 (2009). [CrossRef] [PubMed]
  3. J. Yao, Z. Liu, Y. Liu, Y. Wang, C. Sun, G. Bartal, A. M. Stacy, X. Zhang, “Optical negative refraction in bulk metamaterials of nanowires,” Science 321(5891), 930 (2008). [CrossRef] [PubMed]
  4. A. E. Nikolaenko, N. Papasimakis, A. Chipouline, F. De Angelis, E. Di Fabrizio, N. I. Zheludev, “THz bandwidth optical switching with carbon nanotube metamaterial,” Opt. Express 20(6), 6068–6079 (2012). [CrossRef] [PubMed]
  5. J. B. Pendry, D. Schurig, D. R. Smith, “Controlling electromagnetic fields,” Science 312(5781), 1780–1782 (2006). [CrossRef] [PubMed]
  6. H. Chen, B. Zheng, L. Shen, H. Wang, X. Zhang, N.I. Zheludev, B. Zhang, “Ray-optics cloaking devices for large objects in incoherent natural light,” Nat. Commun. 4, 2652 (2013).
  7. B. Kanté, A. de Lustrac, J. M. Lourtioz, S. N. Burokur, “Infrared cloaking based on the electric response of split ring resonators,” Opt. Express 16(12), 9191–9198 (2008). [CrossRef] [PubMed]
  8. N. I. Landy, S. Sajuyigbe, J. J. Mock, D. R. Smith, W. J. Padilla, “Perfect metamaterial absorber,” Phys. Rev. Lett. 100, 207402 (2008).
  9. H. Wang, L. Wang, “Perfect selective metamaterial solar absorber,” Opt. Express 21(S6), A1078–A1093 (2013). [CrossRef]
  10. R. Alaee, M. Farhat, C. Rockstuhl, F. Lederer, “A perfect absorber made of a graphene micro-ribbon metamaterial,” Opt. Express 20(27), 28017–28024 (2012). [CrossRef] [PubMed]
  11. N. Fang, H. Lee, C. Sun, X. Zhang, “Sub-diffraction-limited optical imaging with a silver superlens,” Science 308(5721), 534–537 (2005). [CrossRef] [PubMed]
  12. F. Capolino, Theory and Phenomena of Metamaterials (CRC Press LLC, 2009), Vol. 1.
  13. Y. Dong, T. Itoh, “Metamaterial-based antennas,” Proc. IEEE 100(7), 2271–2285 (2012). [CrossRef]
  14. S. Xiao, V. P. Drachev, A. V. Kildishev, X. Ni, U. K. Chettiar, H. K. Yuan, V. M. Shalaev, “Loss-free and active optical negative-index metamaterials,” Nature 466(7307), 735–738 (2010). [CrossRef] [PubMed]
  15. Y. Yuan, B. I. Popa, S. A. Cummer, “Zero loss magnetic metamaterials using powered active unit cells,” Opt. Express 17(18), 16135–16143 (2009). [CrossRef] [PubMed]
  16. S.A. Ramakrishna, J.B. Pendry, “Removal of absorption and increase in resolution in a near-field lens via optical gain,” Phys. Rev. B 67, 201101 (2003).
  17. M. I. Stockman, “Criterion for negative refraction with low optical losses from a fundamental principle of causality,” Phys. Rev. Lett. 98, 177404 (2007).
  18. L. Sun, X. Yang, J. Gao, “Loss-compensated broadband epsilon-near-zero metamaterials with gain media,” Appl. Phys. Lett. 103, 201109 (2013).
  19. H. T. Chen, W. J. Padilla, J. M. O. Zide, A. C. Gossard, A. J. Taylor, R. D. Averitt, “Active terahertz metamaterial devices,” Nature 444(7119), 597–600 (2006). [CrossRef] [PubMed]
  20. A. Tennant, B. Chambers, “Adaptive radar absorbing structure with PIN diode controlled active frequency selective surface,” Smart Mater. Struct. 13(1), 122–125 (2004). [CrossRef]
  21. N. Mattiucci, R. Trimm, G. D’Aguanno, N. Akozbek, M.J. Bloemer, “Tunable, narrow-band, all-metallic microwave absorber,” Appl. Phys. Lett. 101, 141115 (2012).
  22. W. Zhu, Y. Huang, I. D. Rukhlenko, G. Wen, M. Premaratne, “Configurable metamaterial absorber with pseudo wideband spectrum,” Opt. Express 20(6), 6616–6621 (2012). [CrossRef] [PubMed]
  23. W. Xu, S. Sonkusale, “Microwave diode switchable metamaterial reflector/absorber,” Appl. Phys. Lett. 103, 031902 (2013).
  24. B. Chambers, “A smart radar absorber,” Smart Mater. Struct. 8(1), 64–72 (1999). [CrossRef]
  25. S. A. Schelkunoff and H. T. Friis, Antennas Theory and Practice, (Wiley, 1966).
  26. D. M. Pozar, Microwave Engineering (Wiley, 2012), p.174.

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