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Optics Express

Optics Express

  • Editor: Andrew M. Weiner
  • Vol. 21, Iss. 8 — Apr. 22, 2013
  • pp: 10259–10268
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An interference coating of metamaterial as an ultrathin light absorber in the violet-to-infrared regime

Yi-Jun Jen, Meng-Jie Lin, Huang-Ming Wu, Hung-Sheng Liao, and Jia-Wei Dai  »View Author Affiliations


Optics Express, Vol. 21, Issue 8, pp. 10259-10268 (2013)
http://dx.doi.org/10.1364/OE.21.010259


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Abstract

A metamaterial with brief and ultrathin structure performs high efficiency in light absorption. An upright aluminum nanorod array (Al NRA) is obliquely deposited, measured, and analyzed its optical property. The Al NRA performs high efficiency of light absorption and low reflectance simultaneously. Based on the measured refractive index and impedances, the wave propagation through the Al NRA is traced to demonstrate the destructive interference that leads to antireflection. According to the analysis of wave tracing, an Al semicontinuous film with thickness of 15nm is introduced under an Al NRA with thickness of only 245nm as a brief and thin two-layered structure. The broadband and polarization-independent light absorption is measured over the violet-to-infrared regime.

© 2013 OSA

1. Introduction

Highly efficient light absorption by a nanoscale structure has been developed and discussed intensively for the past ten years [1

1. N. I. Landy, S. Sajuyigbe, J. J. Mock, D. R. Smith, and W. J. Padilla, “Perfect metamaterial absorber,” Phys. Rev. Lett. 100(20), 207402 (2008). [CrossRef] [PubMed]

3

3. Y.-F. Huang, S. Chattopadhyay, Y.-J. Jen, C.-Y. Peng, T.-A. Liu, Y.-K. Hsu, C.-L. Pan, H.-C. Lo, C.-H. Hsu, Y.-H. Chang, C.-S. Lee, K.-H. Chen, and L.-C. Chen, “Improved broadband and quasi-omnidirectional anti-reflection properties with biomimetic silicon nanostructures,” Nat. Nanotechnol. 2(12), 770–774 (2007). [CrossRef] [PubMed]

]. How to reduce the thickness of a light absorber has been an important issue. Intuitively, the surface of a strong light absorber must have low reflection and the bulk must have a high energy dissipation rate inside when it interacts with light. The optical property of a non-magnetic material is described by its refractive index. The real and imaginary parts of the refractive index are the index of refraction and the extinction coefficient, respectively. Low reflection requires the refractive index to be close to that of incident medium to enable the incident light to couple into the absorber efficiently. However, high energy dissipation in a thin material requires a high extinction coefficient, which is at odds with the need for low reflection. Although a high extinction coefficient ensures efficient dissipation of energy, it also results in strong reflection when a light wave hits the surface. One study developed a super dark array of carbon nanotubes with a refractive index of 1.026 + i0.0006 and an ultra-low reflectance of 0.045% but it was 300μm thick [4

4. Z.-P. Yang, L. Ci, J. A. Bur, S.-Y. Lin, and P. M. Ajayan, “Experimental observation of an extremely dark material made by a low-density nanotube array,” Nano Lett. 8(2), 446–451 (2008). [CrossRef] [PubMed]

]. Some other light absorbers are developed to perform antireflection and energy dissipation separately with two different parts of structure. Jing-qun Xi et al. [5

5. J.-Q. Xi, M. F. Schubert, J. K. Kim, E. F. Schubert, M. Chen, S.-Y. Lin, W. Liu, and J. A. Smart, “Optical thin-film materials with low refractive index for broadband elimination of Fresnel reflection,” Nat. Photonics 1, 176–179 (2007).

] applied oblique-angle deposition to deposit a five-layered structure on an aluminum nitride substrate. They obliquely deposited TiO2 and SiO2 to form a five-layered structure with a refractive index that varied from 2.03 to 1.05. The grade index profiles satisfied a perfectly antireflective design whose reflectance was 0.1%. After the light passed through the antireflective structure with a thickness of 0.6μm, the energy of the light was absorbed by the aluminum nitride substrate with a refractive index of approximately 2.05 + i0.00002. An average thickness of 10mm was required to absorb 95% of the energy of the incident light.

Recent research has shown that the optical property of a typical metamaterial film should take the bianisotropy into account [14

14. C. É. Kriegler, M. S. Rill, S. Linden, and M. Wegener, “Bianisotropic photonic metamaterials,” IEEE J. Sel. Top. Quantum Electron. 16(2), 367–375 (2010). [CrossRef]

, 15

15. 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]

], and an equivalent bianisotropic parameter should be added to equivalent relative permittivity and equivalent relative permeability to represent the associated optical parameters, which are equivalent refractive index and the two equivalent impedances that are associated with forward and backward propagations. The equivalent refractive index and equivalent impedances determine the fundamental optical properties of a metamaterial, such as reflectance, transmittance and aborptance. The tailored impedance dominates the reflection/transmission at the surface and the refractive index dominates the propagation and dissipation of waves in the thin film. The equivalent refractive index and equivalent impedance are mutually independent, providing strong absorption of light in a very thin film. Therefore, light can be coupled into a film that is impedance-matched to the incident medium and dissipated with high imaginary part of the refractive index.

2. Single Al NRA on glass

Three films with thicknesses d = 350nm, 520nm and 750nm were fabricated. Figure 2
Fig. 2 Top-view and cross-sectional scanning electron microscopic images for the three Al NRAs with thicknesses of (a) 350nm, (b) 520nm and (c) 750nm.
shows their top-view and cross-section scanning electron microscopic (SEM) images. The diameter w of the pillars in the films with thicknesses 350nm, 520 nm and 750 nm, are approximately 162 nm, 211 nm and 283 nm, respectively.

Figure 3
Fig. 3 Measured spectra of Tj, Rj, and Aj, j{x,y}of three NRAs. Light was normally incident when transmittance measurements were made but obliquely incident at 5 deg to the normal when reflectance measurements were made.
shows the spectra of the transmittance Tj, reflectance Rj and absorptance Aj, j{x,y} measured by using a spectrometer (Jobin Yvon iHR320). The three Al NRAs were illuminated under normally incident light. Both x-polarized and y-polarized spectra were obtained in orthogonal polarization directions. The difference of transmittance between x-polarized and y-polarized spectra is less than 0.0511, indicating that the NRA is polarization-independent when the light is normally incident. The low reflectances of the 350nm-thick, 520nm-thick and 750nm-thick films are less than 0.0134, 0.0119 and 0.0068, respectively. The transmittance (absorptance) increases (decreases) as the wavelength increases from 400m to 700nm. The high absorption extends to higher wavelengths as the thickness of the Al NRA increases. The average absorptance is proportional to the thickness of the film. The 750nm-thick film exhibits absorptance from 0.96388 at 400nm to 0.80804 at 700nm.

To understand the propagation of waves through the film, the refractive index, forward impedance, and backward impedance were derived using a walk-off interferometer [19

19. A. V. Kildishev, W. Cai, U. K. Chettiar, H.-K. Yuan, A. K. Sarychev, V. P. Drachev, and V. M. Shalaev, “Negative Refractive Index in Optics of Metal-Dielectric Composites,” J. Opt. Soc. Am. B 23(3), 423–433 (2006). [CrossRef]

] to measure the reflection and transmission coefficients of the air/film/glass structure and the reverse configuration, glass/film/air. The 520nm-thick film was adopted to make measurements because its transmission was enough to be detected. Figure 4
Fig. 4 Measured spectra of refractive index nj, forward impedance Z+,j, and backward impedance Z,j, j{x,y}.
shows the equivalent refractive index nj, forward impedance Z+,j, and backward impedanceZ,j. The forward impedance Z+,j and backward impedance Z,j are 1.01779 and −0.02288, respectively, so the reflection remains low when light interacts with the surface of the film and the light is coupled into the film efficiently. The near-unity impedance of the metamaterial film does not result in a near-unity refractive index, as it does in a nonmagnetic material, which has a reciprocal relationship between impedance and refractive index. The real part of the tailored refractive index n' is 1.07671. The extinction coefficient n" falls from 0.13771 to 0.10125 as the wavelength increases from 405nm to 690 nm, reflecting the decay of absorptance with increasing wavelength.

From the equivalent parameters n', n" and Z, the propagation of light through the film can be understood. As shown in Fig. 5
Fig. 5 The wave tracing of the Al NRA normally illuminated by light with an electric field amplitude of unity at wavelength of 690 nm.
, when the 520nm-thick film is illuminated by light with an electric field amplitude of unity and a wavelength of 690 nm, the wave penetrates the upper interface of the film with transmission coefficient 0.94131.1395. From the upper interface to the bottom interface, the wave propagates with a phase change of −50.77 and the field amplitude decays to be 0.5828. The reflection coefficient and transmission coefficient at the bottom interface are 0.1031171.9876 and 0.89800.9174, respectively. The electric field amplitude of the transmitted wave is 0.5233. The reflected wave from the bottom interface is backward-propagating with an initial field amplitude of 0.0601. When the reflected wave returns to the upper interface, the transmitted wave with an amplitude of 0.0515 interferes with first-order reflected wave destructively. The initial amplitude of the reflected wave from the upper interface is 0.0143, and it decays to be 0.0089 when the wave again reaches the bottom interface. The first-order transmitted wave and the first two orders of reflected waves contribute most to the transmittance and reflectance from the film, respectively.

The relative electromagnetic parameters-permittivity εj, permeability μj and equivalent bianisotropic parameter ξj, j{x,y}-can be derived from equivalent refractive index nj and equivalent impedances Z+,j and Z,j equivalently via the relationships: εj=(nj+iξj)/Z+,j, μj=(njiξj)Z+,j and ξj=inj(Z,j+Z+,j)/(Z,jZ+,j), as shown in Fig. 5. The real parts of both relative permittivi ty and relative permeability are positive. The real part and the imaginary part of the equivalent bianisotropic parameter are within the ranges (−0.0212, 0.21869) and (−0.4827, 0.2696), respectively. The real parts of ε and μ are within the ranges (1.0926, 1.6772) and (0.5195, 1.0421), respectively. The imaginary parts of ε and μ are within the ranges (0.0767, 0.2904) and (−0.0867, 0.1523), respectively. The relative permittivity and relative permeability vary in a manner similar to the variations in Figs. 6(a)
Fig. 6 Measured spectra of permittivity εj, permeability μj and bianisotropic parameter ξj,j{x,y}.
and 6(b) because, when the equivalent bianisotropic parameter is negligible, εj and μj are equal, yielding unity impedance.

The absorptance spectra of Al NRAs that are coated on a BK7 glass substrate were examined. The substrate is opaque in the visible regime from 400 nm to 700nm at angle of incidence θ[5,60] with respect to the z axis with linear polarization (p- or s-polarization). Figure 7
Fig. 7 P-polarized and s-polarized absorptances versus wavelength λ[400,700]nm and angle of incidence θ[5,60] for three Al NRAs with thicknesses of (a) (b) 350nm, (c) (d) 520nm and (e) (f) 750nm.
plots shows the absorptances, Ap and As, versus λ[400,700] nm and θ[5,60], for the 350nm-thick, 520nm-thick and 750nm-thick Al NRAs. The absorptance Ap and As have similar angular and wavelength spectra for each sample. As plotted in Figs. 7(a) and 7(b), the Ap and As of the 350nm-thick film increase with angle of incidence from θ=20 to θ=60. The absorptance decreases with increasing wavelength. As plotted in Figs. 7(c) and (d), the Ap and As of the 520nm-thick film over 0.80 continuously distribute at λ[400,554] nm and θ[5,60]. The Ap is larger than As at each wavelength and angle of incidence. The Ap and As of the 750nm-thick film present absorptance exceed 0.85 and 0.82 over the whole wavelength and angular ranges, respectively, as shown in Figs. 7(e) and (f). The minimum absorptance As occurs at the wavelength of 700nm and angle of incidenceθ=60. For a given incident angle and wavelength, the absorptance increases with thickness from 350 nm to 750 nm because the dissipation is proportional to thickness.

The Al NRA overcomes the problem of ultra-low reflectance with high absorption for a thin structure. Comparing the above results with those for a nonmagnetic optical thin film reveals that at a wavelength of 405nm, the forward impedance of Al NRA with a thickness of 520nm that yields a low R = 0.5% is 1.184 + i0.013. For a nonmagnetic thin film, such low reflection requires the equivalent refractive index to be 0.8449 + i0.0092. However, with such a refractive index, the film must be 9285nm-thick to yield an absorptance of 90.7%.

3. Al NRA/Al SCF/glass system

It has been demonstrated that semi-continuous metal films exhibit anomalous absorption and moderate reflection [20

20. K. Seal, A. K. Sarychev, H. Noh, D. A. Genov, A. Yamilov, V. M. Shalaev, Z. C. Ying, and H. Cao, “Near-Field Intensity Correlations in Semicontinuous Metal-Dielectric Films,” Phys. Rev. Lett. 94(22), 226101 (2005). [CrossRef] [PubMed]

22

22. D. A. Genov, V. M. Shalaev, and A. K. Sarychev, “Surface plasmon excitation and correlation-induced localization-delocalization transition in semicontinuous metal films,” Phys. Rev. B 72(11), 113102 (2005). [CrossRef]

]. The localized surface plasmons and localization of strongly enhanced electromagnetic fields alone the boundaries of the voids lead to broadband absorption. Such an ultra-thin film can be applied under the Al NRA to promote absorption. In the fabrication of Al SCFs, the deposition rate was maintained at 0.2 nm/s and the deposition angle was kept at θv=0°. Six Al SCFs with thicknesses of 5 nm to 30 nm were grown to observe their performance as buffer layers between NRA and glass substrate.

The transmittance, reflectance and absorptance of Al SCFs with difference thickness were measured at wavelengths from 400nm to 700nm. Figure 8
Fig. 8 Measured spectra of Tj, Rj, and Aj, j{x,y} , of Al SCFs.
plots the average transmittance, reflectance and absorptance spectra of Al SCFs that were illuminated by normally incident x-polarized and y-polarized in the visible regime. The average reflectance increases from 0.2028 to 0.5614 as the thickness of the Al SCFs increases from 5 nm to 30nm, while the average transmittance decreases from 0.4446 to 0.1406. The average absoptance is within the range 0.2980 to 0.4242. Therefore, the 15nm-thick Al SCF was utilized as the bottom layer of Al NRA to improve absorption.

Al NRAs with thicknesses of 245nm and 340nm were separately grown on a 15nm-thick SCF. Figure 9
Fig. 9 Measured spectra of Tj, Rj, and Aj, j{x,y} of Air/Al NRA(245nm)/SCF/BK7.
shows the transmittance, reflectance and absorptance spectra of the Air/Al NRA(245nm)/SCF/BK7 system that was illuminated by normally incident x-polarized and y-polarized light. The average reflectance is less than 0.1591 and the average transmittance is 0.1304. The absorptance decreases from 0.8620 at λ = 350nm to 0.6893 at λ = 1000nm. Figure 10
Fig. 10 Measured spectra of Tj, Rj, and Aj, j{x,y} of Air/Al NRA(340nm)/SCF/BK7
shows the measured transmittance, reflectance and absorptance spectra of the other system, Air/Al NRA(340nm)/SCF/BK7. The average reflectance is less than 0.0680 and the average transmittance is 0.2232. Both reflectance and transmittance increases with wavelength. The absorptance decreases from 0.8914 at λ = 350nm to 0.5679 at λ = 1000nm. Figure 11
Fig. 11 Absorptance spectra Axand Ay of the compound film composited with Al SCF and Al NRA as functions of wavelength from 350nm to 1000nm and incident of angle from 5° to 60° for the Air/Al NRA(245nm)/SCF/BK7 system.
plots the absorptances Axand Ay versus λ[350,1000] nm and θ[5,60] for system Air/Al NRA(245nm)/SCF/BK7. BothAx and Ayexceed 0.60 over the whole wavelength range, and the average value of Ax and Ayover the whole wavelength and angle ranges are 0.7613 and 0.7600, respectively. Based on the measurement results of the two Al NRAs, the absorptance of the Air/Al NRA(245nm)/SCF/BK7 is considerably extended throughout the violet-to-infrared regime.

Figure 12
Fig. 12 Absorptance spectra of A=(Ax+Ay)/2 for trapezoid fishnet structure [13] and Al NRA(245nm)/SCF coating on BK7 substrate.
compares absorptance of Air/Al NRA(245nm)/SCF/BK7 with that of the aforementioned trapezoid fishnet structure that was developed by Aydin et al. [13

13. K. Aydin, V. E. Ferry, R. M. Briggs, and H. A. Atwater, “Broadband polarization-independent resonant light absorption using ultrathin plasmonic super absorbers,” Nat Commun 2, 517 (2011). [CrossRef] [PubMed]

]. The Al NRA on SCF has an average absorptance of approximately 70.98%, which exceeds that of the fishnet structure. Our structure has a higher absorption than the trapezoid fishnet structure at wavelengths shorter than 450nm and longer than 650nm. The thicknesses of the two structures are almost equal.

4. Conclusion

Acknowledgments

This work was supported by grants from the National Taipei University of Technology, the National Science Council of the Republic of China (NSC 99-2221-E-027-043-MY3 and NSC 101-3113-P-002-021).

References and links

1.

N. I. Landy, S. Sajuyigbe, J. J. Mock, D. R. Smith, and W. J. Padilla, “Perfect metamaterial absorber,” Phys. Rev. Lett. 100(20), 207402 (2008). [CrossRef] [PubMed]

2.

V. G. Kravets, S. Neubeck, A. N. Grigorenko, and A. F. Kravets, “Plasmonic blackbody: Strong absorption of light by metal nanoparticles embedded in a dielectric matrix,” Phys. Rev. B 81(16), 165401 (2010). [CrossRef]

3.

Y.-F. Huang, S. Chattopadhyay, Y.-J. Jen, C.-Y. Peng, T.-A. Liu, Y.-K. Hsu, C.-L. Pan, H.-C. Lo, C.-H. Hsu, Y.-H. Chang, C.-S. Lee, K.-H. Chen, and L.-C. Chen, “Improved broadband and quasi-omnidirectional anti-reflection properties with biomimetic silicon nanostructures,” Nat. Nanotechnol. 2(12), 770–774 (2007). [CrossRef] [PubMed]

4.

Z.-P. Yang, L. Ci, J. A. Bur, S.-Y. Lin, and P. M. Ajayan, “Experimental observation of an extremely dark material made by a low-density nanotube array,” Nano Lett. 8(2), 446–451 (2008). [CrossRef] [PubMed]

5.

J.-Q. Xi, M. F. Schubert, J. K. Kim, E. F. Schubert, M. Chen, S.-Y. Lin, W. Liu, and J. A. Smart, “Optical thin-film materials with low refractive index for broadband elimination of Fresnel reflection,” Nat. Photonics 1, 176–179 (2007).

6.

D. Schurig, J. J. Mock, B. J. Justice, S. A. Cummer, J. B. Pendry, A. F. Starr, and D. R. Smith, “Metamaterial electromagnetic cloak at microwave frequencies,” Science 314(5801), 977–980 (2006). [CrossRef] [PubMed]

7.

W. T. Lu and S. Sridhar, “Superlens imaging theory for anisotropic nanostructured metamaterials with broadband all-angle negative refraction,” Phys. Rev. B 77(23), 233101 (2008). [CrossRef]

8.

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

9.

X. Liu, T. Starr, A. F. Starr, and W. J. Padilla, “Infrared spatial and frequency selective metamaterial with near-unity absorbance,” Phys. Rev. Lett. 104(20), 207403 (2010). [CrossRef] [PubMed]

10.

H.-T. Chen, J. Zhou, J. F. O’Hara, F. Chen, A. K. Azad, and A. J. Taylor, “Antireflection coating using metamaterials and identification of its mechanism,” Phys. Rev. Lett. 105(7), 073901 (2010). [CrossRef] [PubMed]

11.

M. G. Silveirinha, “Additional Boundary Condition for the Wire Medium,” IEEE Trans. Antenn. Propag. 54(6), 1766–1780 (2006). [CrossRef]

12.

P. A. Belov, R. Marqués, S. I. Maslovski, I. S. Nefedov, M. Silveirinha, C. R. Simovski, and S. A. Tretyakov, “Strong spatial dispersion in wire media in the very large wavelength limit,” Phys. Rev. B 67(11), 113103 (2003). [CrossRef]

13.

K. Aydin, V. E. Ferry, R. M. Briggs, and H. A. Atwater, “Broadband polarization-independent resonant light absorption using ultrathin plasmonic super absorbers,” Nat Commun 2, 517 (2011). [CrossRef] [PubMed]

14.

C. É. Kriegler, M. S. Rill, S. Linden, and M. Wegener, “Bianisotropic photonic metamaterials,” IEEE J. Sel. Top. Quantum Electron. 16(2), 367–375 (2010). [CrossRef]

15.

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.

Y.-J. Jen, C.-H. Chen, and C.-W. Yu, “Deposited metamaterial thin film with negative refractive index and permeability in the visible regime,” Opt. Lett. 36(6), 1014–1016 (2011). [CrossRef] [PubMed]

17.

A. Lakhtakia and R. Messier, Sculptured Thin Films: Nanoengineered Morphology and Optics (SPIE Press, 2005).

18.

K. Seal, M. A. Nelson, Z. C. Ying, D. Genov, A. Sarychev, and V. Shalaev, “Growth, morphology, and optical and electrical properties of semicontinuous metallic films,” Phys. Rev. B 67(3), 035318 (2003). [CrossRef]

19.

A. V. Kildishev, W. Cai, U. K. Chettiar, H.-K. Yuan, A. K. Sarychev, V. P. Drachev, and V. M. Shalaev, “Negative Refractive Index in Optics of Metal-Dielectric Composites,” J. Opt. Soc. Am. B 23(3), 423–433 (2006). [CrossRef]

20.

K. Seal, A. K. Sarychev, H. Noh, D. A. Genov, A. Yamilov, V. M. Shalaev, Z. C. Ying, and H. Cao, “Near-Field Intensity Correlations in Semicontinuous Metal-Dielectric Films,” Phys. Rev. Lett. 94(22), 226101 (2005). [CrossRef] [PubMed]

21.

U. K. Chettiar, P. Nyga, M. D. Thoreson, A. V. Kildishev, V. P. Drachev, and V. M. Shalaev, “FDTD modeling of realistic semicontinuous metal films,” Appl. Phys. B 100(1), 159–168 (2010). [CrossRef]

22.

D. A. Genov, V. M. Shalaev, and A. K. Sarychev, “Surface plasmon excitation and correlation-induced localization-delocalization transition in semicontinuous metal films,” Phys. Rev. B 72(11), 113102 (2005). [CrossRef]

OCIS Codes
(310.1620) Thin films : Interference coatings
(160.3918) Materials : Metamaterials

ToC Category:
Metamaterials

History
Original Manuscript: February 8, 2013
Revised Manuscript: April 9, 2013
Manuscript Accepted: April 12, 2013
Published: April 18, 2013

Citation
Yi-Jun Jen, Meng-Jie Lin, Huang-Ming Wu, Hung-Sheng Liao, and Jia-Wei Dai, "An interference coating of metamaterial as an ultrathin light absorber in the violet-to-infrared regime," Opt. Express 21, 10259-10268 (2013)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-21-8-10259


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References

  1. N. I. Landy, S. Sajuyigbe, J. J. Mock, D. R. Smith, and W. J. Padilla, “Perfect metamaterial absorber,” Phys. Rev. Lett.100(20), 207402 (2008). [CrossRef] [PubMed]
  2. V. G. Kravets, S. Neubeck, A. N. Grigorenko, and A. F. Kravets, “Plasmonic blackbody: Strong absorption of light by metal nanoparticles embedded in a dielectric matrix,” Phys. Rev. B81(16), 165401 (2010). [CrossRef]
  3. Y.-F. Huang, S. Chattopadhyay, Y.-J. Jen, C.-Y. Peng, T.-A. Liu, Y.-K. Hsu, C.-L. Pan, H.-C. Lo, C.-H. Hsu, Y.-H. Chang, C.-S. Lee, K.-H. Chen, and L.-C. Chen, “Improved broadband and quasi-omnidirectional anti-reflection properties with biomimetic silicon nanostructures,” Nat. Nanotechnol.2(12), 770–774 (2007). [CrossRef] [PubMed]
  4. Z.-P. Yang, L. Ci, J. A. Bur, S.-Y. Lin, and P. M. Ajayan, “Experimental observation of an extremely dark material made by a low-density nanotube array,” Nano Lett.8(2), 446–451 (2008). [CrossRef] [PubMed]
  5. J.-Q. Xi, M. F. Schubert, J. K. Kim, E. F. Schubert, M. Chen, S.-Y. Lin, W. Liu, and J. A. Smart, “Optical thin-film materials with low refractive index for broadband elimination of Fresnel reflection,” Nat. Photonics1, 176–179 (2007).
  6. D. Schurig, J. J. Mock, B. J. Justice, S. A. Cummer, J. B. Pendry, A. F. Starr, and D. R. Smith, “Metamaterial electromagnetic cloak at microwave frequencies,” Science314(5801), 977–980 (2006). [CrossRef] [PubMed]
  7. W. T. Lu and S. Sridhar, “Superlens imaging theory for anisotropic nanostructured metamaterials with broadband all-angle negative refraction,” Phys. Rev. B77(23), 233101 (2008). [CrossRef]
  8. R. A. Shelby, D. R. Smith, and S. Schultz, “Experimental verification of a negative index of refraction,” Science292(5514), 77–79 (2001). [CrossRef] [PubMed]
  9. X. Liu, T. Starr, A. F. Starr, and W. J. Padilla, “Infrared spatial and frequency selective metamaterial with near-unity absorbance,” Phys. Rev. Lett.104(20), 207403 (2010). [CrossRef] [PubMed]
  10. H.-T. Chen, J. Zhou, J. F. O’Hara, F. Chen, A. K. Azad, and A. J. Taylor, “Antireflection coating using metamaterials and identification of its mechanism,” Phys. Rev. Lett.105(7), 073901 (2010). [CrossRef] [PubMed]
  11. M. G. Silveirinha, “Additional Boundary Condition for the Wire Medium,” IEEE Trans. Antenn. Propag.54(6), 1766–1780 (2006). [CrossRef]
  12. P. A. Belov, R. Marqués, S. I. Maslovski, I. S. Nefedov, M. Silveirinha, C. R. Simovski, and S. A. Tretyakov, “Strong spatial dispersion in wire media in the very large wavelength limit,” Phys. Rev. B67(11), 113103 (2003). [CrossRef]
  13. K. Aydin, V. E. Ferry, R. M. Briggs, and H. A. Atwater, “Broadband polarization-independent resonant light absorption using ultrathin plasmonic super absorbers,” Nat Commun2, 517 (2011). [CrossRef] [PubMed]
  14. C. É. Kriegler, M. S. Rill, S. Linden, and M. Wegener, “Bianisotropic photonic metamaterials,” IEEE J. Sel. Top. Quantum Electron.16(2), 367–375 (2010). [CrossRef]
  15. 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,” Nature466(7307), 735–738 (2010). [CrossRef] [PubMed]
  16. Y.-J. Jen, C.-H. Chen, and C.-W. Yu, “Deposited metamaterial thin film with negative refractive index and permeability in the visible regime,” Opt. Lett.36(6), 1014–1016 (2011). [CrossRef] [PubMed]
  17. A. Lakhtakia and R. Messier, Sculptured Thin Films: Nanoengineered Morphology and Optics (SPIE Press, 2005).
  18. K. Seal, M. A. Nelson, Z. C. Ying, D. Genov, A. Sarychev, and V. Shalaev, “Growth, morphology, and optical and electrical properties of semicontinuous metallic films,” Phys. Rev. B67(3), 035318 (2003). [CrossRef]
  19. A. V. Kildishev, W. Cai, U. K. Chettiar, H.-K. Yuan, A. K. Sarychev, V. P. Drachev, and V. M. Shalaev, “Negative Refractive Index in Optics of Metal-Dielectric Composites,” J. Opt. Soc. Am. B23(3), 423–433 (2006). [CrossRef]
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