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Optical Materials Express

Optical Materials Express

  • Editor: David J. Hagan
  • Vol. 1, Iss. 8 — Dec. 1, 2011
  • pp: 1548–1554
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Selective electroless silver plating of three dimensional SU-8 microstructures on silicon for metamaterials applications

Yuanjun Yan, M. Ibnur Rashad, Ee Jin Teo, Hendrix Tanoto, Jinghua Teng, and Andrew A. Bettiol  »View Author Affiliations


Optical Materials Express, Vol. 1, Issue 8, pp. 1548-1554 (2011)
http://dx.doi.org/10.1364/OME.1.001548


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Abstract

We report a method for selective silver coating of SU-8 structures on Si substrates by treating the sample with radio frequency plasma prior to electroless plating. Silver films with high conductivity of 9 × 10−8Ω.m and low surface roughness of 9 nm have been obtained. When combined with two-photon lithography, this process can be utilized for three-dimensional metamaterials applications. Unlike previous work on selective coating, our process can coat directly on SU-8 photoresist that is widely used for two-photon lithography and does not require any resin modification.

© 2011 OSA

1. Introduction

The rapid research progress made in recent years in artificially structured sub-wavelength metallic structures, or metamaterials, has been driven by the desire to make materials that possess electromagnetic properties that cannot be found in nature. The ability to directly control the effective permittivity (ε) and permeability (µ) of these materials through top down fabrication technology has opened up many potential applications such as negative refractive index [1

1. V. M. Shalaev, W. Cai, U. K. Chettiar, H.-K. Yuan, A. K. Sarychev, V. P. Drachev, and A. V. Kildishev, “Negative index of refraction in optical metamaterials,” Opt. Lett. 30(24), 3356–3358 (2005). [CrossRef] [PubMed]

], optical magnetism [2

2. S. Linden, C. Enkrich, G. Dolling, M. W. Klein, J. Zhou, T. Koschny, C. M. Soukoulis, S. Burger, F. Schmidt, and M. Wegener, “Photonic metamaterials: magnetism at optical frequencies,” IEEE J. Sel. Top. Quantum Electron. 12(6), 1097–1105 (2006). [CrossRef]

], slow light [3

3. G. Dolling, C. Enkrich, M. Wegener, C. M. Soukoulis, and S. Linden, “Simultaneous negative phase and group velocity of light in a metamaterial,” Science 312(5775), 892–894 (2006). [CrossRef] [PubMed]

], cloaking [4

4. W. S. Cai, U. K. Chettiar, A. V. Kildishev, and V. M. Shalaev, “Optical cloaking with metamaterials,” Nat. Photonics 1(4), 224–227 (2007). [CrossRef]

], superlensing [5

5. 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]

], broadband polarizers [6

6. J. K. Gansel, M. Thiel, M. S. Rill, M. Decker, K. Bade, V. Saile, G. von Freymann, S. Linden, and M. Wegener, “Gold helix photonic metamaterial as broadband circular polarizer,” Science 325(5947), 1513–1515 (2009). [CrossRef] [PubMed]

] and sensing [7

7. N. Liu, T. Weiss, M. Mesch, L. Langguth, U. Eigenthaler, M. Hirscher, C. Sönnichsen, and H. Giessen, “Planar metamaterial analogue of electromagnetically induced transparency for plasmonic sensing,” Nano Lett. 10(4), 1103–1107 (2010). [CrossRef] [PubMed]

,8

8. R. Singh, Z. Tian, J. G. Han, C. Rockstuhl, J. Q. Gu, and W. L. Zhang, “Cryogenic temperatures as a path toward high-Q terahertz metamaterials,” Appl. Phys. Lett. 96(7), 071114 (2010). [CrossRef]

]. The majority of the metamaterials that have been demonstrated thus far have been fabricated using planar lithographic techniques such as electron beam lithography or UV lithography [9

9. A. Boltasseva and V. M. Shalaev, “Fabrication of optical negative-index metamaterials: recent advances and outlook,” Metamaterials (Amst.) 2(1), 1–17 (2008). [CrossRef]

]. In an attempt to increase the interaction length between the impinging electromagnetic radiation and the metamaterial, research has recently moved towards extending fabrication technologies to the third dimension. Two notable examples are the multilayered fishnet structures fabricated by Valentine et al. [10

10. J. Valentine, S. Zhang, T. Zentgraf, E. Ulin-Avila, D. A. Genov, G. Bartal, and X. Zhang, “Three-dimensional optical metamaterial with a negative refractive index,” Nature 455(7211), 376–379 (2008). [CrossRef] [PubMed]

] and three dimensional chiral metamaterials that have been fabricated by two photon lithography (TPL) [6

6. J. K. Gansel, M. Thiel, M. S. Rill, M. Decker, K. Bade, V. Saile, G. von Freymann, S. Linden, and M. Wegener, “Gold helix photonic metamaterial as broadband circular polarizer,” Science 325(5947), 1513–1515 (2009). [CrossRef] [PubMed]

].

One limitation of the TPL technique is that it can typically only be used to pattern polymers like SU-8 and Ormocer. A metallization step is needed to convert the patterns into metallic structures for metamaterials applications. Rill et al. [21

21. M. S. R. M. S. Rill, C. Plet, M. Thiel, I. Staude, G. von Freymann, S. Linden, and M. Wegener, “Photonic metamaterials by direct laser writing and silver chemical vapour deposition,” Nat. Mater. 7(7), 543–546 (2008). [CrossRef] [PubMed]

] utilized a chemical vapor deposition (CVD) process for transferring the SU-8 structures to metal. The process however required the deposition of a SiO2 layer to protect the SU-8 template from the high temperatures necessary for deposition. To achieve conformal coating on 3D structures, a more suitable approach would be electroless plating. It involves metallization through a chemical reduction process. Ag is widely adopted as the metal of choice, especially for applications in metamaterials and plasmonics, because of its high conductivity and low absorption coefficient [22

22. S. A. Maier and H. A. Atwater, “Plasmonics: localization and guiding of electromagnetic energy in metal/dielectric structures,” J. Appl. Phys. 98(1), 011101 (2005). [CrossRef]

]. Due to the poor adhesion between metal particles and the polymer surface, additional processes are needed for surface functionalization. In addition, selective coating is important for metamaterials applications in order to prevent the substrate from an electrical short between the structures. Radke et al. [23

23. A. Radke, T. Gissibl, T. Klotzbücher, P. V. Braun, and H. Giessen, “Three-dimensional bichiral plasmonic crystals fabricated by direct laser writing and electroless silver plating,” Adv. Mater. (Deerfield Beach Fla.) 23(27), 3018–3021 (2011). [CrossRef] [PubMed]

] have used TPL and electroless Ag plating to fabricate SU-8 chiral metamaterial structures. However, due to the lack of plating selectivity, the structures had to be fabricated on post and detached from the substrate prior to optical measurements.

Therefore various pretreatment techniques have been employed to achieve coating selectivity. Formanek et al. [24

24. F. Formanek, N. Takeyasu, T. Tanaka, K. Chiyoda, A. Ishikawa, and S. Kawata, “Selective electroless plating to fabricate complex three-dimensional metallic micro/nanostructures,” Appl. Phys. Lett. 88(8), 083110 (2006). [CrossRef]

] demonstrated a number of steps needed to achieve selective electroless coating of metal on polymer instead of glass. First, it requires the production of a hydrophobic coating on glass supporting the polymer structures. Also a crucial part of the work involves the use of chemically modified photopolymerizable resin for TPP fabrication. The sample is then submerged into a tin chloride solution for activation of the polymer surface for metal reduction. Wen Dai et al. [25

25. W. Dai and W. J. Wang, “Selective metallization of cured SU-8 microstructures using electroless plating method,” Sens. Actuators A Phys. 135(1), 300–307 (2007). [CrossRef]

] applied a large UV dose to rearrange the epoxy structures of the cross-linked SU-8 surface in order to make it more adhesive to Ag. However this process is directional and cannot achieve conformal coating of 3D structures.

In this work, we demonstrate a direct approach for selective conformal coating of Ag on three-dimensional SU-8 microstructures that have been fabricated on a Si substrate. After fabricating the structures, the SU-8 surface is modified using a Radio Frequency (RF) plasma prior to electroless Ag coating. The plasma alters the SU-8 surface so that Ag nanoparticles can preferentially nucleate on the surface while leaving the Si substrate unaltered. No resin modification is required and this process can be performed on SU-8 resist that is widely available and used for two-photon lithography. The simplicity and effectiveness of this process makes it more appealing for 3D metamaterials applications.

For our experiment a 25 μm layer of SU-8 is spin-coated on a clean Si substrate. 3D chiral structures are then fabricated on the resist using our in-house TPL fabrication system. The femtosecond laser pulses used in TPL are generated by a mode-locked Ti:Sapphire oscillator (Coherent, Mira 900D) which is pumped by a 10 W 532 nm laser (Coherent,Verdi V10), with a centre wavelength of 800 nm and pulse width of approximately 150 fs. A computerized sample platform consisting of several precision stages allow the user to move in all three dimensions with nanometer precision. The chiral structures were fabricated by moving the stages in three dimensions according to a predefined path. After irradiation, the sample was post-baked at 95°C for 10 minutes before a final development and iso-propyl alcohol (IPA) rinse. After TPL, the sample was subjected to an RF plasma treatment using a plasma cleaner (Harrick Plasma PDC-32G). Since the plasma comes from the coils surrounding the sample holder, all surfaces receive the same amount of plasma dose. For electroless Ag plating, we utilized the standard Tollens’ reagent method. The plating process is conducted in a 50 mL silver nitrite (AgNO3) solution with a concentration of 0.05 mol/L kept at a temperature of 35°C to 50°C. A glucose solution (1 mL) with a concentration of 0.025mg/mL is added into the solution to reduce the Ag ions to metal particles. After 3 minutes of reaction, the sample is taken out and cleaned.

Atomic force microscopy images were used to measure the surface roughness of the coated SU-8 sample. In order to find the coated surface more easily, a Si substrate was first spin-coated with SU-8 and cut into pieces. Then the SU-8 samples were UV exposed, plasma pretreated and electroless Ag plated. The AFM scan area was set to 5.0 × 5.0μm2 and the root mean squared (RMS) surface roughness value was measured.

Fourier Transform Infrared Spectroscopy was carried out using a Bruker Vertex 80v vacuum FTIR system. The terahertz beam produced from a mercury arc lamp was discriminated into transverse electric (TE) and transverse magnetic (TM) polarization using a polyethylene FIR polarizer placed before the sample. The transmitted signals were then detected with a liquid helium cooled Si bolometer (Infrared Laboratories) at a resolution of 0.2cm−1.

2. Results and discussion

One indicator of how well the surface is coated by the electroless plating process is the coating coverage. The coating coverage, defined as the ratio of coated area to the total surface area, can be determined from the Scanning Electron Microscope (SEM) image. Figure 1
Fig. 1 Coating coverage of Si and SU-8 surfaces as a function of plasma treatment dose. Insets: SEM images of the Si and SU-8 Ag coated surface using a 2160 J plasma treatment.
shows a plot of Ag coverage as a function of applied plasma dose for a Si substrate and cross-linked SU-8 polymer. If no plasma is applied, there is almost no Ag coverage on both the Si and SU-8 surfaces after electroless Ag plating for 3 minutes. As the RF plasma dose is increased, we find that the Si and SU-8 surfaces behave differently after the treatment. By increasing the plasma dose, we see that there is hardly any silver on the Si surface. On the other hand, a significant increase in the silver coverage of SU-8 can be observed. At a dose of 420 J, 42% of the SU-8 surface has been covered by silver. As the dose increased further to 2160 J, the SU-8 surface becomes completely covered, while leaving the Si surface relatively free from silver particles. This can be seen clearly from the SEM images in the inset of Fig. 1. We measured the resistivity of the coated SU-8 surface using the four point probe method and found that the Ag coated layer of 100 nm had a value of 9 × 10−8 Ω.m. This is of the same order as the nominal value for bulk Ag which is 1.6 × 10−8 Ω.m. Ellipsometry measurements were also performed in order to compare the optical properties of the silver electroless coated SU-8 films under optimized conditions, with films of similar thickness prepared using electron beam evaporation. Both films exhibited very similar optical properties.

Surface roughness is an important parameter to consider for metamaterials applications due to its contribution to scattering loss. Optimization of the plating parameters needs to be carried out in order to minimize the surface roughness. One of the most important factors affecting the roughness is the concentration of glucose since it is responsible for the reduction of Ag ions into particles and the rate at which Ag nanoparticles are generated. By keeping all other parameters constant, we have investigated the surface roughness for three glucose concentrations (0.0125mg/mL, 0.025mg/mL and 0.0375mg/mL) and at three different plating times (60s, 90s and 120s) for a constant temperature of 45°C. AFM results plotted in Fig. 3(a)
Fig. 3 (a) RMS surface roughness measured as a function of glucose concentration; (b) RMSsurface roughness measured as a function of coating temperature, for three plating times; (c) SEM image of a coated surface under un-optimized conditions; (d) SEM image of a coated surface under optimized conditions.
show that the surface roughness increases with increasing glucose concentration, and plating time.

To demonstrate the effectiveness of the coating technique for metamaterials applications, an array of THz double split ring resonators (SRRs) was fabricated. THz metamaterials have been extensively studied in recent years [29

29. T. J. Yen, W. J. Padilla, N. Fang, D. C. Vier, D. R. Smith, J. B. Pendry, D. N. Basov, and X. Zhang, “Terahertz magnetic response from artificial materials,” Science 303(5663), 1494–1496 (2004). [CrossRef] [PubMed]

32

32. R. Singh, E. Plum, W. L. Zhang, and N. I. Zheludev, “Highly tunable optical activity in planar achiral terahertz metamaterials,” Opt. Express 18(13), 13425–13430 (2010). [CrossRef] [PubMed]

] and the THz metallic SRR structures exhibit LC resonance for incident electromagnetic waves with polarization parallel to their split gaps. To verify the performance of the coated SRRs, the structures were first fabricated in a 2 µm SU-8 layer spin-coated on a Si substrate using UV lithography, followed by plasma pretreatment and then coated with a 100 nm layer of Ag. Figure 4(a)
Fig. 4 (a) An array of 2D double split ring resonators with height 5 µm fabricated in SU-8 on Si and coated with Ag using selective electroless plating. (b) Transmission spectra for the electric field parallel and perpendicular to the SRR gap showing an LC resonance dip at 0.64 THz.
shows an SEM image of the Ag coated SU-8 SRR array, with minimal Ag deposition on the Si substrate. The FTIR spectrum, shown in Fig. 4(b), reveals the presence of an LC resonance at approximately 0.64 THz for light with the electric field polarized parallel to the SRR gap. The resonance disappears when the electric field polarization is rotated by 90°. At the resonance frequency, the skin depth of the incident light is less than the coated Ag thickness therefore the interaction between the incident THz wave and the underlying SU-8 dielectric is minimal. Simulations (not shown) reveal that at these frequencies, the difference between the resonant frequency of an SRR made from bulk metal, and one made from an SU-8 coated metal is minimal.

3. Conclusion

In conclusion, we have demonstrated a simple and effective electroless Ag plating method for conformal uniform coating of arbitrary three-dimensional SU-8 structures. High coating selectivity and good conductivity has been achieved on SU-8 over Si by a plasma pretreatment of the surface prior to plating, a feature that is crucial for metamaterials applications. An RMS surface roughness of 9 nm was obtained under optimized conditions. This compares favorably with Ag surfaces that are produced using electron beam evaporation (Typically 4-8 nm RMS depending on the substrate [33

33. H. Liu, B. Wang, E. S. P. Leong, P. Yang, Y. Zong, G. Si, J. Teng, and S. A. Maier, “Enhanced surface plasmon resonance on a smooth silver film with a seed growth layer,” ACS Nano 4(6), 3139–3146 (2010). [CrossRef] [PubMed]

]). To further improve the surface quality we are currently investigating the use of physical surface smoothing techniques [34

34. Z. Insepov, I. Yamada, and M. Sosnowski, “Sputterring and smoothing of metal surface with energetic gas cluster beams,” Mater. Chem. Phys. 54(1-3), 234–237 (1998). [CrossRef]

]. An SRR array fabricated using this technique shows optical properties similar to bulk electroplated or evaporated metals. Combined with TPL laser fabrication, this process opens up numerous possibilities for true three dimensional metamaterials structures.

Acknowledgments

This work is supported by the Singapore Ministry of Education, Academic Research Fund Tier 1 grant (R-144-000-291-112).

References and links

1.

V. M. Shalaev, W. Cai, U. K. Chettiar, H.-K. Yuan, A. K. Sarychev, V. P. Drachev, and A. V. Kildishev, “Negative index of refraction in optical metamaterials,” Opt. Lett. 30(24), 3356–3358 (2005). [CrossRef] [PubMed]

2.

S. Linden, C. Enkrich, G. Dolling, M. W. Klein, J. Zhou, T. Koschny, C. M. Soukoulis, S. Burger, F. Schmidt, and M. Wegener, “Photonic metamaterials: magnetism at optical frequencies,” IEEE J. Sel. Top. Quantum Electron. 12(6), 1097–1105 (2006). [CrossRef]

3.

G. Dolling, C. Enkrich, M. Wegener, C. M. Soukoulis, and S. Linden, “Simultaneous negative phase and group velocity of light in a metamaterial,” Science 312(5775), 892–894 (2006). [CrossRef] [PubMed]

4.

W. S. Cai, U. K. Chettiar, A. V. Kildishev, and V. M. Shalaev, “Optical cloaking with metamaterials,” Nat. Photonics 1(4), 224–227 (2007). [CrossRef]

5.

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]

6.

J. K. Gansel, M. Thiel, M. S. Rill, M. Decker, K. Bade, V. Saile, G. von Freymann, S. Linden, and M. Wegener, “Gold helix photonic metamaterial as broadband circular polarizer,” Science 325(5947), 1513–1515 (2009). [CrossRef] [PubMed]

7.

N. Liu, T. Weiss, M. Mesch, L. Langguth, U. Eigenthaler, M. Hirscher, C. Sönnichsen, and H. Giessen, “Planar metamaterial analogue of electromagnetically induced transparency for plasmonic sensing,” Nano Lett. 10(4), 1103–1107 (2010). [CrossRef] [PubMed]

8.

R. Singh, Z. Tian, J. G. Han, C. Rockstuhl, J. Q. Gu, and W. L. Zhang, “Cryogenic temperatures as a path toward high-Q terahertz metamaterials,” Appl. Phys. Lett. 96(7), 071114 (2010). [CrossRef]

9.

A. Boltasseva and V. M. Shalaev, “Fabrication of optical negative-index metamaterials: recent advances and outlook,” Metamaterials (Amst.) 2(1), 1–17 (2008). [CrossRef]

10.

J. Valentine, S. Zhang, T. Zentgraf, E. Ulin-Avila, D. A. Genov, G. Bartal, and X. Zhang, “Three-dimensional optical metamaterial with a negative refractive index,” Nature 455(7211), 376–379 (2008). [CrossRef] [PubMed]

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D. F. Tan, Y. Li, F. J. Qi, H. Yang, Q. H. Gong, X. Z. Dong, and X. M. Duan, “Reduction in feature size of two-photon polymerization using SCR500,” Appl. Phys. Lett. 90(7), 071106 (2007). [CrossRef]

13.

W. H. Teh, U. Durig, G. Salis, R. Harbers, U. Drechsler, R. F. Mahrt, C. G. Smith, and H. J. Guntherodt, “SU-8 for real three-dimensional subdiffraction-limit two-photon microfabrication,” Appl. Phys. Lett. 84(20), 4095–4097 (2004). [CrossRef]

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C. Reinhardt, R. Kiyan, S. Passinger, A. L. Stepanov, A. Ostendorf, and N. Chichkov, “Rapid laser prototyping of plasmonic components,” Appl. Phys., A Mater. Sci. Process. 89(2), 321–325 (2007). [CrossRef]

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S.-Y. Chiam, R. Singh, C. Rockstuhl, F. Lederer, W. Zhang, and A. A. Bettiol, “Analogue of electromagnetically induced transparency in a terahertz metamaterial,” Phys. Rev. B 80(15), 153103 (2009). [CrossRef]

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M. S. R. M. S. Rill, C. Plet, M. Thiel, I. Staude, G. von Freymann, S. Linden, and M. Wegener, “Photonic metamaterials by direct laser writing and silver chemical vapour deposition,” Nat. Mater. 7(7), 543–546 (2008). [CrossRef] [PubMed]

22.

S. A. Maier and H. A. Atwater, “Plasmonics: localization and guiding of electromagnetic energy in metal/dielectric structures,” J. Appl. Phys. 98(1), 011101 (2005). [CrossRef]

23.

A. Radke, T. Gissibl, T. Klotzbücher, P. V. Braun, and H. Giessen, “Three-dimensional bichiral plasmonic crystals fabricated by direct laser writing and electroless silver plating,” Adv. Mater. (Deerfield Beach Fla.) 23(27), 3018–3021 (2011). [CrossRef] [PubMed]

24.

F. Formanek, N. Takeyasu, T. Tanaka, K. Chiyoda, A. Ishikawa, and S. Kawata, “Selective electroless plating to fabricate complex three-dimensional metallic micro/nanostructures,” Appl. Phys. Lett. 88(8), 083110 (2006). [CrossRef]

25.

W. Dai and W. J. Wang, “Selective metallization of cured SU-8 microstructures using electroless plating method,” Sens. Actuators A Phys. 135(1), 300–307 (2007). [CrossRef]

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29.

T. J. Yen, W. J. Padilla, N. Fang, D. C. Vier, D. R. Smith, J. B. Pendry, D. N. Basov, and X. Zhang, “Terahertz magnetic response from artificial materials,” Science 303(5663), 1494–1496 (2004). [CrossRef] [PubMed]

30.

O. Paul, C. Imhof, B. Reinhard, R. Zengerle, and R. Beigang, “Negative index bulk metamaterial at terahertz frequencies,” Opt. Express 16(9), 6736–6744 (2008). [CrossRef] [PubMed]

31.

J. Q. Gu, J. G. Han, X. C. Lu, R. Singh, Z. Tian, Q. R. Xing, and W. L. Zhang, “A close-ring pair terahertz metamaterial resonating at normal incidence,” Opt. Express 17(22), 20307–20312 (2009). [CrossRef] [PubMed]

32.

R. Singh, E. Plum, W. L. Zhang, and N. I. Zheludev, “Highly tunable optical activity in planar achiral terahertz metamaterials,” Opt. Express 18(13), 13425–13430 (2010). [CrossRef] [PubMed]

33.

H. Liu, B. Wang, E. S. P. Leong, P. Yang, Y. Zong, G. Si, J. Teng, and S. A. Maier, “Enhanced surface plasmon resonance on a smooth silver film with a seed growth layer,” ACS Nano 4(6), 3139–3146 (2010). [CrossRef] [PubMed]

34.

Z. Insepov, I. Yamada, and M. Sosnowski, “Sputterring and smoothing of metal surface with energetic gas cluster beams,” Mater. Chem. Phys. 54(1-3), 234–237 (1998). [CrossRef]

OCIS Codes
(160.3900) Materials : Metals
(160.3918) Materials : Metamaterials

ToC Category:
Metamaterials

History
Original Manuscript: September 29, 2011
Revised Manuscript: November 9, 2011
Manuscript Accepted: November 10, 2011
Published: November 11, 2011

Citation
Yuanjun Yan, M. Ibnur Rashad, Ee Jin Teo, Hendrix Tanoto, Jinghua Teng, and Andrew A. Bettiol, "Selective electroless silver plating of three dimensional SU-8 microstructures on silicon for metamaterials applications," Opt. Mater. Express 1, 1548-1554 (2011)
http://www.opticsinfobase.org/ome/abstract.cfm?URI=ome-1-8-1548


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References

  1. V. M. Shalaev, W. Cai, U. K. Chettiar, H.-K. Yuan, A. K. Sarychev, V. P. Drachev, and A. V. Kildishev, “Negative index of refraction in optical metamaterials,” Opt. Lett.30(24), 3356–3358 (2005). [CrossRef] [PubMed]
  2. S. Linden, C. Enkrich, G. Dolling, M. W. Klein, J. Zhou, T. Koschny, C. M. Soukoulis, S. Burger, F. Schmidt, and M. Wegener, “Photonic metamaterials: magnetism at optical frequencies,” IEEE J. Sel. Top. Quantum Electron.12(6), 1097–1105 (2006). [CrossRef]
  3. G. Dolling, C. Enkrich, M. Wegener, C. M. Soukoulis, and S. Linden, “Simultaneous negative phase and group velocity of light in a metamaterial,” Science312(5775), 892–894 (2006). [CrossRef] [PubMed]
  4. W. S. Cai, U. K. Chettiar, A. V. Kildishev, and V. M. Shalaev, “Optical cloaking with metamaterials,” Nat. Photonics1(4), 224–227 (2007). [CrossRef]
  5. N. Fang, H. Lee, C. Sun, and X. Zhang, “Sub-diffraction-limited optical imaging with a silver superlens,” Science308(5721), 534–537 (2005). [CrossRef] [PubMed]
  6. J. K. Gansel, M. Thiel, M. S. Rill, M. Decker, K. Bade, V. Saile, G. von Freymann, S. Linden, and M. Wegener, “Gold helix photonic metamaterial as broadband circular polarizer,” Science325(5947), 1513–1515 (2009). [CrossRef] [PubMed]
  7. N. Liu, T. Weiss, M. Mesch, L. Langguth, U. Eigenthaler, M. Hirscher, C. Sönnichsen, and H. Giessen, “Planar metamaterial analogue of electromagnetically induced transparency for plasmonic sensing,” Nano Lett.10(4), 1103–1107 (2010). [CrossRef] [PubMed]
  8. R. Singh, Z. Tian, J. G. Han, C. Rockstuhl, J. Q. Gu, and W. L. Zhang, “Cryogenic temperatures as a path toward high-Q terahertz metamaterials,” Appl. Phys. Lett.96(7), 071114 (2010). [CrossRef]
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