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Optics Express

Optics Express

  • Editor: C. Martijn de Sterke
  • Vol. 19, Iss. 11 — May. 23, 2011
  • pp: 10679–10685
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Tailored resonator coupling for modifying the terahertz metamaterial response

Dibakar Roy Chowdhury, Ranjan Singh, Matthew Reiten, Jiangfeng Zhou, Antoinette J. Taylor, and John F. O’Hara  »View Author Affiliations


Optics Express, Vol. 19, Issue 11, pp. 10679-10685 (2011)
http://dx.doi.org/10.1364/OE.19.010679


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Abstract

We experimentally and numerically study the nature of coupling between laterally paired terahertz metamaterial split-ring resonators. Coupling is shown to modify the inductive–capacitive (LC) resonances resulting in either red or blue-shifting. Results indicate that tuning of the electric and magnetic coupling parameters may be accomplished not by changing the orientation or density of SRRs, but by a design modification at the unit cell level. These experiments illustrate additional degrees of freedom in tuning the electromagnetic response, which offers a path to more robust metamaterial designs.

© 2011 OSA

1. Introduction

Recent developments in metamaterials (MMs) have demonstrated the possibility of manipulating light in compact and highly integrated micro and nanoscale structures [1

1. J. B. Pendry, A. Holden, D. Robbins, and W. Stewart, “Magnetism from conductors and enhanced nonlinear phenomena,” IEEE Trans. Microw. Theory Tech. 47(11), 2075–2084 (1999).

5

5. 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). [PubMed]

]. These MM-based devices hold great promise for realizing next-generation photonic technologies applicable to the microwave, terahertz and the optical regimes. The operation of most MM devices relies on strategies employed to control the fundamental resonances in MM oscillators, which are often termed as split ring resonators (SRRs) [5

5. 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). [PubMed]

8

8. V. A. Fedotov, M. Rose, S. L. Prosvirnin, N. Papasimakis, and N. I. Zheludev, “Sharp trapped-mode resonances in planar metamaterials with a broken structural symmetry,” Phys. Rev. Lett. 99(14), 147401 (2007). [PubMed]

]. These resonances accompany the high electric or magnetic polarizability of SRRs, resulting in extreme values of the electric permittivity (ε) and magnetic permeability (μ), hallmark properties of MMs. Recent work has studied lateral coupling between neighboring SRRs and its effect on these resonances [9

9. R. Singh, C. Rockstuhl, F. Lederer, and W. Zhang, “The impact of nearest neighbor interaction on the resonances in terahertz metamaterial,” Appl. Phys. Lett. 94(2), 021116 (2009).

11

11. N. Feth, M. König, M. Husnik, K. Stannigel, J. Niegemann, K. Busch, M. Wegener, and S. Linden, “Electromagnetic interaction of split-ring resonators: The role of separation and relative orientation,” Opt. Express 18(7), 6545–6554 (2010). [PubMed]

]. Recent work in microwave MMs has shown that the relative orientation between coupled SRRs determines the nature of coupling [12

12. O. Sydoruk, E. Tatartschuk, E. Shamonina, and L. Solymar, “Analytical formulation for the resonant frequency of split rings,” J. Appl. Phys. 105(1), 014903 (2009).

,13

13. F. Hesmer, E. Tatartschuk, O. Zhuromskyy, A. A. Radkovskaya, M. Shamonin, T. Hao, C. J. Stevens, G. Faulkner, D. J. Edwards, and E. Shamonina, “Coupling mechanism of split ring resonators: theory and experiment,” Phys. Status Solidi 244(4), 1170–1175 (2007) (b).

]. Effects like resonance line narrowing, resonance mode splitting [14

14. R. Singh, C. Rockstuhl, F. Lederer, and W. Zhang, “Coupling between a dark and bright eigenmode in a terahertz metamaterial,” Phys. Rev. B 79(8), 085111 (2009).

,15

15. N. Liu, S. Kaiser, and H. Giessen, “Magnetoinductive and electroinductive coupling in plasmonic metamaterial molecules,” Adv. Mater. 20(23), 4521–4525 (2008).

] and a classical analogue of electromagnetically induced transparency [16

16. S. Zhang, D. A. Genov, Y. Wang, M. Liu, and X. Zhang, “Plasmon-induced transparency in metamaterials,” Phys. Rev. Lett. 101(4), 047401 (2008). [PubMed]

] were observed in coupled SRRs. Most of these effects were the result of changing the density and/or orientation of coupled SRRs. In one case, the SRRs were also modified by the addition of extra gaps [10

10. R. S. Penciu, K. Aydin, M. Kafesaki, Th. Koschny, E. Ozbay, E. N. Economou, and C. M. Soukoulis, “Multi-gap individual and coupled split-ring resonator structures,” Opt. Express 16(22), 18131–18144 (2008). [PubMed]

]. While these results nicely illustrated the increased complexity of coupled SRRs in MMs, there remains much opportunity to better understand the deliberate tuning of SRR coupling for maximum practical benefit. In terms of MM functionality, increased coupling leads to both opportunities and complications. For example, increasing the SRR density generally strengthens the response of a MM. But it also strengthens coupling effects, which may lead to superradiant damping, actually reducing the range of available ε and μ [17

17. I. Sersic, M. Frimmer, E. Verhagen, and A. F. Koenderink, “Electric and magnetic dipole coupling in near-infrared split-ring metamaterial arrays,” Phys. Rev. Lett. 103(21), 213902 (2009).

]. Thus we become compelled to investigate more comprehensive MM design strategies, ones that simultaneously leverage the isolated SRR response as well as its coupling to neighbors. In this work, we show that coupling can be strongly modified by selective design at the unit cell level. In particular we show that opposing coupling effects may be balanced such that the isolated SRR response is approached, despite extremely close proximity between SRRs. We utilize a combination of numerical simulations and terahertz frequency measurements in this work. These offer a convenient platform for such studies since sample fabrication is straightforward, and terahertz time-domain spectroscopy (THz-TDS) provides a phase-coherent characterization technique.

2. Experiments

Five sets of metamaterial samples were fabricated on an undoped GaAs wafer as shown in Figs. 1(a)-(f)
Fig. 1 Optical microscope images of the fabricated MM samples. (a) through (c) show the symmetric gap to gap coupled SRRs, MM1 to MM3, (d) and (e) show the SRRs in which the gaps of the split rings were displaced in opposite directions, (f) shows the MM5 array. The dimensions of the unit cells are indicated in the figure. The unit cell periodicity for all samples is identical.
. In samples MM1, MM2 and MM3 the distance between the two SRRs in the unit cell, s, was varied from 2 to 6 to 10 µm. In sample array MM4 and MM5, s is fixed at 2 μm but the SRRs gaps are oppositely offset by d from the center. The unit cell periodicity (or equivalently the SRR density) in all MM samples was kept constant at Px = 90 µm and Py = 52 µm. All samples were fabricated using photolithography, followed by e-beam deposition of 10 nm of titanium, and then 200 nm of gold. Optical images of the SRRs are shown in Fig. 1 along with detailed geometrical parameters. The samples were characterized in transmission using THz-TDS in the confocal geometry [18

18. D. Grischkowsky, S. Keiding, M. Exter, and C. Fattinger, “Far-infrared time-domain spectroscopy with terahertz beams of dielectrics and semiconductors,” J. Opt. Soc. Am. B 7(10), 2006 (1990).

]. The incident THz beam was linearly polarized with the electric field oriented parallel to the gap-bearing side, as in Fig. 1(f). The MM samples and a piece of bare GaAs identical to the MM substrates were all measured in the time domain, the last to serve as a transmission reference. All measurements were done at room temperature and in a dry atmosphere to mitigate water absorption. The time-domain data was converted into frequency-dependent amplitude and phase spectra via Fourier transformation. Transmission spectra (S21) normalized to the bare GaAs substrate were obtained for all the samples. Figure 2(a)
Fig. 2 Measured (a) and simulated (b) transmission magnitudes for the MM1 to MM3 samples when the gap is fixed at the center of the arm but the inter-ring separation in the unit cell is changed. The cyan line in figure b defines the position of the resonance for the isolated SRR at around 0.46 THz.
shows the measured spectra for samples MM1, MM2 and MM3.

As the SRR spacing is reduced, we observe a 25 GHz blue shift in the LC resonance from MM1 to MM2 and an additional 35 GHz blue shift from MM2 to MM3. Only a small change is observed in the depth of the resonant transmission dip, along with a minor increase in the resonance quality factor (Q). Figure 2(b) shows the corresponding simulated data for samples MM1, MM2 and MM3 as well as the resonance frequency of the isolated ring (0.46 THz). All the measurement features have been reproduced well in the simulated spectrum. Figure 3(a)
Fig. 3 Measured (a) and simulated (b) transmission magnitudes for the coupled SRRs MM3 to MM5 when the gaps are displaced from the center of the arm but the inter-ring separation is fixed at 2 μm.
shows the response of samples MM3, MM4, and MM5. As the SRR gaps are displaced 4 µm from sample MM3 to MM4, the resonance red shifts 40 GHz and the transmitted field magnitude increases from 0.12 to 0.22. With the additional 6 µm offset in MM5 the resonance red shifts 30 GHz further and the field magnitude increases to 0.42. Thus, in this set of measurements we observe both red shifting and weakening of the response. The simulation data in Fig. 3(b) are in reasonable agreement with our measurements.

3. Discussion

To understand these results, we note that the LC resonance mode arises from electric currents oscillating around the full circumference of the SRR loop. These are excited by the incident electric field due to the asymmetry of the SRR [19

19. N. Katsarakis, T. Koschny, M. Kafesaki, E. N. Economou, and C. M. Soukoulis, “Electric coupling to the magnetic resonance of split ring resonators,” Appl. Phys. Lett. 84(15), 2943 (2004).

]. The position of the anticipated LC resonance frequency can usually be described by a simple expression, ωLC=1/LCwhere L and C are the effective inductance and capacitance of the individual SRRs. In the presence of coupling between neighboring resonators the situation is more complex.

However, the electric fields within the SRR pairs clearly go through significant changes for the various coupled resonators. The numerically simulated electric field distributions at the LC resonance for three cases – MM1, MM3 and MM5 – are shown in the Figs. 4(a)
Fig. 4 The simulated electric field distributions for the coupled SRRs MM1 (a), MM3 (b) and MM5 (c). Arrows indicate the direction of the electric field while arrow color and size indicate field strength. Red and green arrows correspond to the magnitude of strong and weak fields, respectively. The polarity of the induced electronic charges on the SRR arms is indicated in the figure. The contribution from fringing capacitance is stronger in MM1 (a) compared to MM3 (b) due to the similar polarity of the induced charges in the adjacent arms of the coupled SRRs. With the displacement of the gaps (MM5), an additional capacitive contribution arises in the central part of the adjacent gap bearing arms of the coupled SRRs (c).
, 4(b) and 4(c), respectively. When the SRRs are far apart and SRR gaps are aligned, as in case of MM1, fringing fields around the gap-bearing side contribute significantly to the overall capacitance of the individual resonators. As the separation between SRRs is reduced these fringing fields increasingly couple the individual responses of the rings. Coulomb repulsion occurs through the fringing fields between like charges in the paired rings, thereby opposing charge accumulation near the gap.

Previously it was observed that coupling effects arising in SRR arrays prevent the macroscopic metamaterial response from being a simple function of unit cell polarizability and density [17

17. I. Sersic, M. Frimmer, E. Verhagen, and A. F. Koenderink, “Electric and magnetic dipole coupling in near-infrared split-ring metamaterial arrays,” Phys. Rev. Lett. 103(21), 213902 (2009).

]. Thus high-density, high-polarizability unit cells may not create the strongest metamaterial response once coupling is accounted for [22

22. R. Singh, C. Rockstuhl, and W. Zhang, “Strong influence of packing density in terahertz metamaterials,” Appl. Phys. Lett. 97(24), 241108 (2010).

]. Our experiments show that the metamaterial unit cell may be engineered to create coupling effects that enhance, reverse, or compensate responses due to other design features, such as orientation or packing density. Of course this adds complication to metamaterial design but it also suggests a more comprehensive metamaterial design path where coupling works in concert with the high polarizability of unit cells, allowing metamaterials to achieve larger macroscopic effective parameters. In addition, it suggests a method to use coupling to stabilize a metamaterial resonance frequency against design variances. Here, properly designed unit cells may use coupling to compensate the functional effects of other design features when design variances occur, such is the case with fabrication tolerances. This would be particularly useful in high-frequency metamaterials where fabrication consistency is much more difficult.

To illustrate, if an uncoupled SRR array was undercut during fabrication it is expected to reduce the ring capacitance and drive up the resonance frequency. With a properly designed unit cell, this same undercut could simultaneously cause an opposite effect through the SRR coupling, thereby preserving the original resonance frequency as much as possible. This idea was tested by simulating an uncoupled SRR array and an array of coupled SRRs very similar to MM3. Undercuts of both 0.5 um and 1.0 um were simulated. Figure 5a
Fig. 5 Numerical simulations of single (a) and symmetric gap coupled (b) SRR systems. In both of the systems of SRRs, 0 µm, 0.5 µm and 1.0 µm undercut of the metallization is taken into considerations. In the case of a single SRR design, the 1.0 µm undercut blue shifts the resonance by 16 GHz (a). The same undercut shifts the resonance only 6 GHz for a coupled SRR unit cell similar to MM3 (b).
shows that a 1.0 um undercut on the uncoupled array results in a 16 GHz blue shift to the resonance, a shift of 3.5% of the intended resonance. In the coupled SRRs, undercuts from 0 to 1 um resulted in a (worst case) 6 GHz red shift, which is within about 1.1% of the intended resonance. Additional design iterations could likely improve this.

4. Summary

In summary, we report passive tuning of the strength and frequency of the LC resonance by exploiting the lateral coupling mechanism in terahertz MMs. Due to the changes in fringing capacitance between the resonators, a blue shift or red shift of the resonance is observed depending upon the spacing between SRR gaps and the placement of the SRR gaps. Resonance strength appears to be independently affected in the asymmetric gap case due to weaker excitation from the external electric field. All the measured features were reproduced well by the numerical calculations and explained in terms of electric dipole coupling effects. The work shows how coupling may be designed in parallel with the polarizability and densities of the unit cells in order to more comprehensively or robustly design the macroscopic metamaterial response.

Acknowledgments

We gratefully acknowledge the support of the U.S. Department of Energy through the LANL/LDRD Program for this work. We gratefully acknowledge the cleanroom facilities of Center for Integrated NanoTechnologies (CINT) located at Sandia National Laboratory for the fabrication of the metamaterial samples.

References and links

1.

J. B. Pendry, A. Holden, D. Robbins, and W. Stewart, “Magnetism from conductors and enhanced nonlinear phenomena,” IEEE Trans. Microw. Theory Tech. 47(11), 2075–2084 (1999).

2.

R. A. Shelby, D. R. Smith, and S. Schultz, “Experimental verification of a negative index of refraction,” Science 292(5514), 77–79 (2001). [PubMed]

3.

S. Linden, C. Enkrich, M. Wegener, J. Zhou, T. Koschny, and C. M. Soukoulis, “Magnetic response of metamaterials at 100 terahertz,” Science 306(5700), 1351–1353 (2004). [PubMed]

4.

C. M. Soukoulis, S. Linden, and M. Wegener, “Physics. Negative refractive index at optical wavelengths,” Science 315(5808), 47–49 (2007). [PubMed]

5.

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). [PubMed]

6.

J. F. O’Hara, R. Singh, I. Brener, E. Smirnova, J. Han, A. J. Taylor, and W. Zhang, “Thin-film sensing with planar terahertz metamaterials: sensitivity and limitations,” Opt. Express 16(3), 1786–1795 (2008). [PubMed]

7.

R. Singh, E. Smirnova, A. J. Taylor, J. F. O’Hara, and W. Zhang, “Optically thin terahertz metamaterials,” Opt. Express 16(9), 6537–6543 (2008). [PubMed]

8.

V. A. Fedotov, M. Rose, S. L. Prosvirnin, N. Papasimakis, and N. I. Zheludev, “Sharp trapped-mode resonances in planar metamaterials with a broken structural symmetry,” Phys. Rev. Lett. 99(14), 147401 (2007). [PubMed]

9.

R. Singh, C. Rockstuhl, F. Lederer, and W. Zhang, “The impact of nearest neighbor interaction on the resonances in terahertz metamaterial,” Appl. Phys. Lett. 94(2), 021116 (2009).

10.

R. S. Penciu, K. Aydin, M. Kafesaki, Th. Koschny, E. Ozbay, E. N. Economou, and C. M. Soukoulis, “Multi-gap individual and coupled split-ring resonator structures,” Opt. Express 16(22), 18131–18144 (2008). [PubMed]

11.

N. Feth, M. König, M. Husnik, K. Stannigel, J. Niegemann, K. Busch, M. Wegener, and S. Linden, “Electromagnetic interaction of split-ring resonators: The role of separation and relative orientation,” Opt. Express 18(7), 6545–6554 (2010). [PubMed]

12.

O. Sydoruk, E. Tatartschuk, E. Shamonina, and L. Solymar, “Analytical formulation for the resonant frequency of split rings,” J. Appl. Phys. 105(1), 014903 (2009).

13.

F. Hesmer, E. Tatartschuk, O. Zhuromskyy, A. A. Radkovskaya, M. Shamonin, T. Hao, C. J. Stevens, G. Faulkner, D. J. Edwards, and E. Shamonina, “Coupling mechanism of split ring resonators: theory and experiment,” Phys. Status Solidi 244(4), 1170–1175 (2007) (b).

14.

R. Singh, C. Rockstuhl, F. Lederer, and W. Zhang, “Coupling between a dark and bright eigenmode in a terahertz metamaterial,” Phys. Rev. B 79(8), 085111 (2009).

15.

N. Liu, S. Kaiser, and H. Giessen, “Magnetoinductive and electroinductive coupling in plasmonic metamaterial molecules,” Adv. Mater. 20(23), 4521–4525 (2008).

16.

S. Zhang, D. A. Genov, Y. Wang, M. Liu, and X. Zhang, “Plasmon-induced transparency in metamaterials,” Phys. Rev. Lett. 101(4), 047401 (2008). [PubMed]

17.

I. Sersic, M. Frimmer, E. Verhagen, and A. F. Koenderink, “Electric and magnetic dipole coupling in near-infrared split-ring metamaterial arrays,” Phys. Rev. Lett. 103(21), 213902 (2009).

18.

D. Grischkowsky, S. Keiding, M. Exter, and C. Fattinger, “Far-infrared time-domain spectroscopy with terahertz beams of dielectrics and semiconductors,” J. Opt. Soc. Am. B 7(10), 2006 (1990).

19.

N. Katsarakis, T. Koschny, M. Kafesaki, E. N. Economou, and C. M. Soukoulis, “Electric coupling to the magnetic resonance of split ring resonators,” Appl. Phys. Lett. 84(15), 2943 (2004).

20.

Computer Simulation Technology (CST), Darmstadt, Germany.

21.

R. Singh, I. A. I. Al-Naib, M. Koch, and W. Zhang, “Asymmetric planar terahertz metamaterials,” Opt. Express 18(12), 13044–13050 (2010). [PubMed]

22.

R. Singh, C. Rockstuhl, and W. Zhang, “Strong influence of packing density in terahertz metamaterials,” Appl. Phys. Lett. 97(24), 241108 (2010).

OCIS Codes
(260.5740) Physical optics : Resonance
(160.3918) Materials : Metamaterials

ToC Category:
Metamaterials

History
Original Manuscript: March 25, 2011
Revised Manuscript: May 11, 2011
Manuscript Accepted: May 11, 2011
Published: May 16, 2011

Citation
Dibakar Roy Chowdhury, Ranjan Singh, Matthew Reiten, Jiangfeng Zhou, Antoinette J. Taylor, and John F. O’Hara, "Tailored resonator coupling for modifying the terahertz metamaterial response," Opt. Express 19, 10679-10685 (2011)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-19-11-10679


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References

  1. J. B. Pendry, A. Holden, D. Robbins, and W. Stewart, “Magnetism from conductors and enhanced nonlinear phenomena,” IEEE Trans. Microw. Theory Tech. 47(11), 2075–2084 (1999).
  2. R. A. Shelby, D. R. Smith, and S. Schultz, “Experimental verification of a negative index of refraction,” Science 292(5514), 77–79 (2001). [PubMed]
  3. S. Linden, C. Enkrich, M. Wegener, J. Zhou, T. Koschny, and C. M. Soukoulis, “Magnetic response of metamaterials at 100 terahertz,” Science 306(5700), 1351–1353 (2004). [PubMed]
  4. C. M. Soukoulis, S. Linden, and M. Wegener, “Physics. Negative refractive index at optical wavelengths,” Science 315(5808), 47–49 (2007). [PubMed]
  5. 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). [PubMed]
  6. J. F. O’Hara, R. Singh, I. Brener, E. Smirnova, J. Han, A. J. Taylor, and W. Zhang, “Thin-film sensing with planar terahertz metamaterials: sensitivity and limitations,” Opt. Express 16(3), 1786–1795 (2008). [PubMed]
  7. R. Singh, E. Smirnova, A. J. Taylor, J. F. O’Hara, and W. Zhang, “Optically thin terahertz metamaterials,” Opt. Express 16(9), 6537–6543 (2008). [PubMed]
  8. V. A. Fedotov, M. Rose, S. L. Prosvirnin, N. Papasimakis, and N. I. Zheludev, “Sharp trapped-mode resonances in planar metamaterials with a broken structural symmetry,” Phys. Rev. Lett. 99(14), 147401 (2007). [PubMed]
  9. R. Singh, C. Rockstuhl, F. Lederer, and W. Zhang, “The impact of nearest neighbor interaction on the resonances in terahertz metamaterial,” Appl. Phys. Lett. 94(2), 021116 (2009).
  10. R. S. Penciu, K. Aydin, M. Kafesaki, Th. Koschny, E. Ozbay, E. N. Economou, and C. M. Soukoulis, “Multi-gap individual and coupled split-ring resonator structures,” Opt. Express 16(22), 18131–18144 (2008). [PubMed]
  11. N. Feth, M. König, M. Husnik, K. Stannigel, J. Niegemann, K. Busch, M. Wegener, and S. Linden, “Electromagnetic interaction of split-ring resonators: The role of separation and relative orientation,” Opt. Express 18(7), 6545–6554 (2010). [PubMed]
  12. O. Sydoruk, E. Tatartschuk, E. Shamonina, and L. Solymar, “Analytical formulation for the resonant frequency of split rings,” J. Appl. Phys. 105(1), 014903 (2009).
  13. F. Hesmer, E. Tatartschuk, O. Zhuromskyy, A. A. Radkovskaya, M. Shamonin, T. Hao, C. J. Stevens, G. Faulkner, D. J. Edwards, and E. Shamonina, “Coupling mechanism of split ring resonators: theory and experiment,” Phys. Status Solidi 244(4), 1170–1175 (2007) (b).
  14. R. Singh, C. Rockstuhl, F. Lederer, and W. Zhang, “Coupling between a dark and bright eigenmode in a terahertz metamaterial,” Phys. Rev. B 79(8), 085111 (2009).
  15. N. Liu, S. Kaiser, and H. Giessen, “Magnetoinductive and electroinductive coupling in plasmonic metamaterial molecules,” Adv. Mater. 20(23), 4521–4525 (2008).
  16. S. Zhang, D. A. Genov, Y. Wang, M. Liu, and X. Zhang, “Plasmon-induced transparency in metamaterials,” Phys. Rev. Lett. 101(4), 047401 (2008). [PubMed]
  17. I. Sersic, M. Frimmer, E. Verhagen, and A. F. Koenderink, “Electric and magnetic dipole coupling in near-infrared split-ring metamaterial arrays,” Phys. Rev. Lett. 103(21), 213902 (2009).
  18. D. Grischkowsky, S. Keiding, M. Exter, and C. Fattinger, “Far-infrared time-domain spectroscopy with terahertz beams of dielectrics and semiconductors,” J. Opt. Soc. Am. B 7(10), 2006 (1990).
  19. N. Katsarakis, T. Koschny, M. Kafesaki, E. N. Economou, and C. M. Soukoulis, “Electric coupling to the magnetic resonance of split ring resonators,” Appl. Phys. Lett. 84(15), 2943 (2004).
  20. Computer Simulation Technology (CST), Darmstadt, Germany.
  21. R. Singh, I. A. I. Al-Naib, M. Koch, and W. Zhang, “Asymmetric planar terahertz metamaterials,” Opt. Express 18(12), 13044–13050 (2010). [PubMed]
  22. R. Singh, C. Rockstuhl, and W. Zhang, “Strong influence of packing density in terahertz metamaterials,” Appl. Phys. Lett. 97(24), 241108 (2010).

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