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Optics Express

  • Editor: C. Martijn de Sterke
  • Vol. 19, Iss. 21 — Oct. 10, 2011
  • pp: 20642–20649

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Truncated spherical voids for nearly omnidirectional optical absorption

Min Wang, Chenggang Hu, Mingbo Pu, Cheng Huang, Zeyu Zhao, Qin Feng, and Xiangang Luo  »View Author Affiliations


Optics Express, Vol. 19, Issue 21, pp. 20642-20649 (2011)
http://dx.doi.org/10.1364/OE.19.020642


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Abstract

Truncated spherical voids nanostructured tungsten films are shown to have nearly perfect absorption with characteristics of broad-band, polarization-independent and wide-incidence angle in near infrared and visible regime. Through optimizing material and structural parameters, we can achieve the absorbance above 90% from 420THz to 600THz within incidence angle from 0° to 60° for TE polarization and from 450THz to 800THz within incidence angle from 0° to 75° for TM polarization. In particular, absorbance can achieve 99.9% at 550.5THz for both polarizations under normal incidence. Such strong absorption is explained using multilayer effective media theory and cavity resonance.

© 2011 OSA

1. Introduction

In recent years, the topic of perfect optical absorption has attracted intensive interest [1

V. G. Kravets, F. Schedin, and A. N. Grigorenko, “Plasmonic blackbody: Almost complete absorption of light in nanostructured metallic coatings,” Phys. Rev. B 78(20), 205405 (2008). [CrossRef]

,2

C. Hu, Z. Zhao, X. Chen, and X. Luo, “Realizing near-perfect absorption at visible frequencies,” Opt. Express 17(13), 11039–11044 (2009). [CrossRef] [PubMed]

], since the perfect absorption may play an important role in devising solar cells, biosensors, photodetectors, and thermal emitters [3

N. C. Panoiu and R. M. Osgood Jr., “Enhanced optical absorption for photovoltaics via excitation of waveguide and plasmon-polariton modes,” Opt. Lett. 32(19), 2825–2827 (2007). [CrossRef] [PubMed]

­–6

M. Diem, T. Koschny, and C. M. Soukoulis, “Wide-angle perfect absorber/thermal emitter in the terahertz regime,” Phys. Rev. B 79(3), 033101 (2009). [CrossRef]

]. Various approaches are put forward to achieve omnidirectional absorption [7

R. Esteban, M. Laroche, and J. J. Greffet, “Dielectric gratings for wide-angle, broadband absorption by thin film photovoltaic cells,” Appl. Phys. Lett. 97(22), 221111 (2010). [CrossRef]

–15

Z. B. Li, W. Y. Zhou, X. T. Kong, and J. G. Tian, “Polarization dependence and independence of near-field enhancement through a subwavelength circle hole,” Opt. Express 18(6), 5854–5860 (2010). [CrossRef] [PubMed]

]. For example, Researchers utilized a lossy core and layered dielectric spacers to construct optical black hole structure [10

E. E. Narimanov and A. V. Kildishev, “Optical Black Hole: Broadband omnidirectional light absober,” Appl. Phys. Lett. 95(4), 041106 (2009). [CrossRef]

] with different relative permittivity of each cylindrical dielectric layer, which will absorb the energy of incidence light transmitting. However, such material is difficult to fine in nature world. The author designed an infrared nearly perfect absorber with the characteristic of two resonant frequencise and wide-angle response but a narrow operating regime based on the impedance match theory. Another reprehensive approach is to use nanostructured gold films comprising periodically arranged spherical voids to achieve perfect omnidirectional absorption [12

T. V. Teperik, F. J. Garcia de Abajo, A. G. Borisov, M. Abdelsalam, P. N. Bartlett, Y. Sugawara, and J. J. Baumberg, “Omnidirectional absorption in nanostructured metal surfaces,” Nat. Photonics 2(5), 299–301 (2008). [CrossRef]

]. These voids can realize a nearly perfect absorption by inducing the existence of localized plasmons in the voids [16

T. A. Kelf, Y. Sugawara, R. M. Cole, J. J. Baumberg, M. E. Abdelsalam, S. Cintra, S. Mahajan, A. E. Russell, and P. N. Bartlett, “Localized and delocalized plasmons in metallic nanovoids,” Phys. Rev. B 74(24), 245415 (2006). [CrossRef]

,17

R. M. Cole, J. J. Baumberg, F. J. Garcia de Abajo, S. Mahajan, M. Abdelsalam, and P. N. Bartlett, “Understanding Plasmons in Nanoscale Voids,” Nano Lett. 7(7), 2094–2100 (2007). [CrossRef]

]. Such behavior is independent of the polarization of incidence wave over nearly full range of incident angles, but within a narrow operating regime (approximately from 1.16eV to 1.18eV at 80% absorbance). Periodic tungsten grating structures were also demonstrated to have ability of nearly perfect absorbing light with careful arrangement in the visible regime [18

C. H. Lin, R. L. Chern, and H. Y. Lin, “Polarization-independent broad-band nearly perfect absorbers in the visible regime,” Opt. Express 19(2), 415–424 (2011). [CrossRef] [PubMed]

]. It is worth to point out that the structure adjusts TE wave absorption with cavity resonance, and the grating, providing perfect conducting walls for TEM mode propagating inside the groove. However, the absorbance is reduced when changing the azimuth angle of the structure due to the unsymmetrical design. Otherwise, the acceptable absorbance only arises at right frequency regime within a narrow incidence angle for TE polarization. Additionally, dielectric grating [7

R. Esteban, M. Laroche, and J. J. Greffet, “Dielectric gratings for wide-angle, broadband absorption by thin film photovoltaic cells,” Appl. Phys. Lett. 97(22), 221111 (2010). [CrossRef]

,18

C. H. Lin, R. L. Chern, and H. Y. Lin, “Polarization-independent broad-band nearly perfect absorbers in the visible regime,” Opt. Express 19(2), 415–424 (2011). [CrossRef] [PubMed]

] and V-grooved structure [19

N. P. Sergeant, O. Pincon, M. Agrawal, and P. Peumans, “Solar-Selective Absorbers using Sub-Wavelength Gratings,” sergeant@stanford.edu, http://peumans-pc.stanford.edu.

,20

N. P. Sergeant, M. Agrawal, and P. Peumans, “High performance solar-selective absorbers using coated sub-wavelength gratings,” Opt. Express 18(6), 5525–5540 (2010). [CrossRef] [PubMed]

] are also used in perfect absorber design to achieve strong absorption in a broad frequency regime, but in a right azimuth angle due to the 1D arrangement.

In this paper, we use the truncated spherical nanovoids structured tungsten films to design a polarization-independent broad-band wide-incidence angle absorber in near-infrared and visible regime. The absorber is made of tungsten with positive permittivity and gentle change in near-infrared and visible regime. Additionally, we can tune the absorbance and resonant frequency by changing the dimension of the structure and filling different dielectric materials into the nanovoids. Effective medium theory and cavity resonance modes are adopted to explain the absorption behavior.

2. Structure design and simulation

Figure 1 shows the proposed structure which mainly consists of a tungsten layer emptied with compact arranged truncated spherical nanovoids. We can gain different truncated spherical nanovoids by changing the thickness t of tungsten layer. The distance d between neighboring voids, considering the reality fabrication limitation [21

M. E. Abdelsalam, P. N. Bartlett, J. J. Baumberg, and S. Coyle, “Preparation of arrays of isolated spherical cavities by self-assembly of polystyrene spheres on self-assembled pro-patterned macroporous films,” Adv. Mater. (Deerfield Beach Fla.) 16(1), 90–93 (2004). [CrossRef]

], is set to be 4 nm (for idea case, nanovoids are compact arrayed when d=0 nm, the structure is easy to collapse. Considering the condition of fabrication, we set the distance as 4 nm and find there is no significant difference between d=0 and d=4 nm). We perform the simulation using commercial software Microwave Studios by CST 2008. In the simulation, full frequency-dependent dielectric function of tungsten is adopted [22

E. D. Palik, Handbook of Optical Constants of Solids (Academic Press, San Diego, 1998).

]. The absorbance can be obtained by A=1−R−T, where R is the reflectance and T is the transmittance. Absorbance is only relative to reflectance R since the transmittance is nearly zero at the operating regime if the substrate is thick enough (In our simulation, the thickness of substrate is s=50 nm, bigger than the skin depth of tungsten within the visible regime). The black line in Fig. 2 shows the result which has two absorption peaks respectively at 350THz and 550.5THz. The red dot line in Fig. 2 shows the absorbance of tungsten layer which thickness is 200nm much larger than the skin depth of tungsten in operating regime.

Fig. 1 Schematic drawing of metal nanostructure. (a) Cross section view. (b) Top view. (c) Perspective view. The main geometric parameters are marked in (a). The red (dash) line in (b) shows the unit cell in simulation. The yellow area in (c) shows the incidence plane. Incidence angle θ and azimuth angle φ are marked in (c).
Fig. 2 Absorption spectra with r=200nm, t=314nm, d=4nm (black and solid line). The blue dash line shows the absorbance of the nanostructure surrounded by the dielectric material with ε=4. The red dot line describes the absorbance of tungsten layer (thickness is 200nm).

3. Discussion

We resort to the cavity resonance theory to explain the absorption behavior, by observing and analyzing the electric filed distribution at the absorption peaks in detail. When high frequency EM wave illuminates, the corresponding wavelength becomes smaller or closes to the hole at the surface of absorber in geometrical dimension, and the EM wave enters the nanovoids and induces Fabry-Perot resonance, as Fig. 3(a) shows. As frequency reduces, wavelength of incidence increases, and more high-order modes and energy slip away from the nanovoids, and then the intensity of electric field along x axis is weakened, as Fig. 3(b) shows. Comparing to the electric field distribution in reference [23

M. Born and E. Wolf, Principles of Optcs (Cambridge University Press, Cambirdge, 2003).

], we find that there are similar field distribution in the proposed nanostructure. And the cases when frequency is 550 THz and 797 THz show the first and second electric partial wave respectively. Further reducing the frequency, the corresponding wavelength becomes much larger than the hole at the surface of structure, cavity resonance modes nearly vanishes, as Fig. 3(c) shows. Consequently, we can regard the nanostructure as an effective media comprised of tungsten.

Fig. 3 Distribution of Ex under TM polarization respectively at (a) f=797THz, (b) f=550THz and (c) f=345THz.

We can, therefore, bring multilayer effective media theory to explain the absorption behavior in long-wavelength case. The structure can be regarded as a tungsten film embedded with a series periodically arrayed truncated spherical nanovoids. According to multilayer effective media principle, the whole nanostructure will be divided into N layers with single-layer thickness of Δt. Then, the effective permittivity of each layer can be calculated by Maxwell-Garnett theory [24

B. E. A. Saleh and M. C. Teich, Fundamentals of Photonics (John Wiley, New Jersey, 2007).

]:
εi= ε tungsten ε air(1+2f)+2 ε tungsten(1−f) ε air(1−f)+ ε tungsten(2+f),
(1)
where f= V air/ V0 is the filling fraction of air, V0=2r×2 3r×Δt, and V air=2×πr '2. Subsequently, we simulate the absorbance of multilayer model at low frequency regime through MATLAB, and compare the results with that of nanovoids as shown in Fig. 4 . The curves are well overlapped between 150THz to 400THz, and become dissimilar at higher frequency due to the closer dimension of incident wavelength and the void. But in this case, the effective media theory is not appropriate any more. The absorbance curve presents an acute deterioration below 200THz. Comparing with the structure refered in [25

E. Rephaeli and S. Fan, “Tungsten black absorber for solar light with wide angular operation range,” Appl. Phys. Lett. 92(21), 211107 (2008). [CrossRef]

], we find that although the structures are different, the two cases have the similar absorption behavior around 200THz. they have the common feature that part of the structure along z-axis direction has a high metallic ratio resulting in impedance mismatching between structure and free space. Such shortage can be covered by filling a surrounded dielectric. As shown in Fig. 2, the blue dash line describes the absorbance of the nanostructure which has the same structural parameters, but filling the dielectric material with the permittivity ε=4. It is obvious that the absorbance below f=200 THz is enhanced. In other cases, the high filling fraction of air can also improve the absorbance below 200THz [26

H. Sai and H. Yugami, “Thermophotovoltaic generation with selective radiators based on tungsten surface gratings,” Appl. Phys. Lett. 85(16), 3399 (2004). [CrossRef]

].

Fig. 4 Absorbance calculated by multilayer effective media theory. The insert shows the process visually.

Figure 5 shows the absorbance as a function of frequency and incidence angle separately under (a) TE and (b) TM polarizations. The absorbance of TE polarization is different from that of TM polarization at 350 THz since the reflectivities are different at each incidence angle for TE and TM polarization according Fresnel’s law [23

M. Born and E. Wolf, Principles of Optcs (Cambridge University Press, Cambirdge, 2003).

]. The absorbance is related to the reflectivity which is a function of incidence angle and frequency. The reflectivities of TM polarization are much smaller than that of TE polarization when oblique incidence is adopted, so most rays come into the tungsten layer and are absorbed for TM polarization. The absorption peaks around 550 THz and 800 THz are independent of polarization since they are respectively caused by the first-order and second-order cavity resonance mode. In addition, the structure has a better absorption near 700 THz at wide-angle, although the higher-order resonant modes still slip away from the voids. At the case of TM polarization, the component of electric field existing along perpendicular direction of voids can induce an additional electric filed distribution in the voids and electric charge collection at the rim of the structure, resulting in an obvious improvement of the wide-angle absorbance when compared with TE polarization.

Fig. 5 Absorbance as a function of incidence angle and frequency under (a) TE polarization and (b) TM polarizations.

Absorbance as a function of azimuth angle and frequency is plotted in Fig. 6 . We can obviously see that the absorption peak around 550THz is independent of azimuth angle due to cavity resonance. That means the absorbance is independent of the polarization at the normal incidence, and the structure have a steady absorption for the EM waves which frequencies are approximately from 550THz to 650THz.

Fig. 6 Absorbance as a function of azimuth angle and frequency at 20° incident angle for (a) TE polarization and (b) TM polarizations.

In order to well understand how parameters influence the absorbance of the structure, we define the relative thickness as t′=t/(2×r), and observe absorption spectra at different r and d, as Fig. 7 shows. Figure 6(a) describes the absorbance changing with the radius of nanovoids while keeping t′=0.785 and d=4nm unchanged. As the radius increases, the absorption peak around 350THz is enhanced a little, but absorption frequency is not changed. Due to the thickness t and the radius r change synchronously, effective permittivity of each layer is invariable, and so the absorption frequency is unchanged. This validates the multilayer effective theory in our simulation. Other resonance frequencies have a red shift as Fig. 7(a) shows, obeying the common rule: the resonant frequency makes a red shift as the dimensions of the structure increase. Figure 7(b) describes the absorbance as a function of distance d while keeping r=200nm and t′=0.785 unchanged. It is obvious that d has a little effect on resonance frequency, but intensively affects the absorbance and bandwidth. At a low frequency, the proportion of metal in the effective media increases as the distance d becomes bigger. This breaks the matching of the media and results in a degenerate absorption. At a high frequency, owing to the increase of distance d, the interaction effect of adjacent nanovoids becomes weaker, so the bandwidth of absorption becomes narrower. Additionally, it indirectly reflects the influence of error in fabrication at a certain extent.

Fig. 7 Dependence of absorbance on (a) the radius of voids r and (b) distance d.

To realize the tunable absorption with the proposed absorber in visible regime, we simulate the absorber filled with porous silicon in nanovoids which permittivity is dependent of frequency [22

E. D. Palik, Handbook of Optical Constants of Solids (Academic Press, San Diego, 1998).

]. Figure 8 shows the absorbance as a function of incidence angle and frequency of the improved absorber. For TE polarization, the absorbance is well adjusted at θ= 55o, and the whole absorption enhances at the operating regime. So does for TM polarization, especially at a high frequency (absorbance is about 80% at θ= 80o). The reasons for enhancing absorption after filling dielectric materical are: 1, compared to air, the porous silicon brings additional lossy source at operating regime; 2, the effective wavelength in voids filled with dielectric material becomes shorter. Therefore, more cavity resonance modes can be supported.

Fig. 8 Absorbance of improved absorber as a function of incidence angle and frequency for (a) TE polarization and (b) TM polarizations.

Figure 9 shows the absorbance as a function of azimuth angle and frequency under normal incidence when the absorber is filled with porous silicon. The absorbance are various for different polarization EM waves because the effective permittivity is variational along different radial directions at the right frequency regime. Even though absorbance of the filled absorber is dependent of azimuth angle, it is still above 80% at the operating frequency regime.

Fig. 9 Absorbance as a function of azimuth angle and frequency in normal incident case, 0° for TM polarization and 90° for TE polarization.

4. Conclusions

In conclusion, we have shown that truncated spherical voids nanostructured tungsten films have nearly perfect absorption with characteristics of broad-band, polarization-independent and wide-incidence angle. In particular, the absorbance is all above 90% from 305THz to 653THz (λ = 459.4nm~983.6nm) for TE and TM polarizations under normal incidence. The main features, broad-band and wide-angle absorption, result from the property of choosed metal and designed structure. The absorbance is greatly enhanced compared to the tungsten film. In addition, because the voids of structure can be filled, the absorber can be applied in many devices, such as solar cells and sensors.

Acknowledgments

This work was supported by the Nation Basic Research Program (973) of China under Grant No. 2011CB301800.

References and links

1.

V. G. Kravets, F. Schedin, and A. N. Grigorenko, “Plasmonic blackbody: Almost complete absorption of light in nanostructured metallic coatings,” Phys. Rev. B 78(20), 205405 (2008). [CrossRef]

2.

C. Hu, Z. Zhao, X. Chen, and X. Luo, “Realizing near-perfect absorption at visible frequencies,” Opt. Express 17(13), 11039–11044 (2009). [CrossRef] [PubMed]

3.

N. C. Panoiu and R. M. Osgood Jr., “Enhanced optical absorption for photovoltaics via excitation of waveguide and plasmon-polariton modes,” Opt. Lett. 32(19), 2825–2827 (2007). [CrossRef] [PubMed]

4.

B. Sepulveda, L.G. Carrascossa, D. Regarots, M. A. Otte, D. Farina, and L.M. Lechuga, “Surfance Plasmon Resonance Biosensors for highly sensitive detection in real samples,” Proc. SPIE 7397, 1–11 (2009).

5.

J. Rosenberg, R. V. Shenoi, T. E. Vandervelde, S. Krishna, and O. Painter, “A multispectral and polarization selective surface-plasmon resonant midinfrared detector,” Appl. Phys. Lett. 95(16), 161101 (2009). [CrossRef]

6.

M. Diem, T. Koschny, and C. M. Soukoulis, “Wide-angle perfect absorber/thermal emitter in the terahertz regime,” Phys. Rev. B 79(3), 033101 (2009). [CrossRef]

7.

R. Esteban, M. Laroche, and J. J. Greffet, “Dielectric gratings for wide-angle, broadband absorption by thin film photovoltaic cells,” Appl. Phys. Lett. 97(22), 221111 (2010). [CrossRef]

8.

J. Hao, J. Wang, X. Liu, W. J. Padilla, L. Zhou, and M. Qiu, “High performance optical absorber based in a plasmonic metamaterial,” Appl. Phys. Lett. 96(25), 251104 (2010). [CrossRef]

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.

E. E. Narimanov and A. V. Kildishev, “Optical Black Hole: Broadband omnidirectional light absober,” Appl. Phys. Lett. 95(4), 041106 (2009). [CrossRef]

11.

M. Pu, C. Hu, M. Wang, C. Huang, Z. Zhao, C. Wang, Q. Feng, and X. Luo, “Design principles for infrared wide-angle perfect absorber based on plasmonic structure,” Opt. Express 19(18), 17413–17420 (2011). [CrossRef]

12.

T. V. Teperik, F. J. Garcia de Abajo, A. G. Borisov, M. Abdelsalam, P. N. Bartlett, Y. Sugawara, and J. J. Baumberg, “Omnidirectional absorption in nanostructured metal surfaces,” Nat. Photonics 2(5), 299–301 (2008). [CrossRef]

13.

E. Popov, S. Enoch, and N. Bonod, “Absorption of light by extremely shallow metallic gratings: metamaterial behavior,” Opt. Express 17(8), 6770–6781 (2009). [CrossRef] [PubMed]

14.

N. P. Sergeant, O. Pincon, M. Agrawal, and P. Peumans, “Design of wide-angle solar-selective absorbers using aperiodic metal-dielectric stacks,” Opt. Express 17(25), 22800–22812 (2009). [CrossRef] [PubMed]

15.

Z. B. Li, W. Y. Zhou, X. T. Kong, and J. G. Tian, “Polarization dependence and independence of near-field enhancement through a subwavelength circle hole,” Opt. Express 18(6), 5854–5860 (2010). [CrossRef] [PubMed]

16.

T. A. Kelf, Y. Sugawara, R. M. Cole, J. J. Baumberg, M. E. Abdelsalam, S. Cintra, S. Mahajan, A. E. Russell, and P. N. Bartlett, “Localized and delocalized plasmons in metallic nanovoids,” Phys. Rev. B 74(24), 245415 (2006). [CrossRef]

17.

R. M. Cole, J. J. Baumberg, F. J. Garcia de Abajo, S. Mahajan, M. Abdelsalam, and P. N. Bartlett, “Understanding Plasmons in Nanoscale Voids,” Nano Lett. 7(7), 2094–2100 (2007). [CrossRef]

18.

C. H. Lin, R. L. Chern, and H. Y. Lin, “Polarization-independent broad-band nearly perfect absorbers in the visible regime,” Opt. Express 19(2), 415–424 (2011). [CrossRef] [PubMed]

19.

N. P. Sergeant, O. Pincon, M. Agrawal, and P. Peumans, “Solar-Selective Absorbers using Sub-Wavelength Gratings,” sergeant@stanford.edu, http://peumans-pc.stanford.edu.

20.

N. P. Sergeant, M. Agrawal, and P. Peumans, “High performance solar-selective absorbers using coated sub-wavelength gratings,” Opt. Express 18(6), 5525–5540 (2010). [CrossRef] [PubMed]

21.

M. E. Abdelsalam, P. N. Bartlett, J. J. Baumberg, and S. Coyle, “Preparation of arrays of isolated spherical cavities by self-assembly of polystyrene spheres on self-assembled pro-patterned macroporous films,” Adv. Mater. (Deerfield Beach Fla.) 16(1), 90–93 (2004). [CrossRef]

22.

E. D. Palik, Handbook of Optical Constants of Solids (Academic Press, San Diego, 1998).

23.

M. Born and E. Wolf, Principles of Optcs (Cambridge University Press, Cambirdge, 2003).

24.

B. E. A. Saleh and M. C. Teich, Fundamentals of Photonics (John Wiley, New Jersey, 2007).

25.

E. Rephaeli and S. Fan, “Tungsten black absorber for solar light with wide angular operation range,” Appl. Phys. Lett. 92(21), 211107 (2008). [CrossRef]

26.

H. Sai and H. Yugami, “Thermophotovoltaic generation with selective radiators based on tungsten surface gratings,” Appl. Phys. Lett. 85(16), 3399 (2004). [CrossRef]

OCIS Codes
(160.0160) Materials : Materials
(260.3910) Physical optics : Metal optics

ToC Category:
Physical Optics

History
Original Manuscript: August 31, 2011
Manuscript Accepted: September 2, 2011
Published: October 3, 2011

Citation
Min Wang, Chenggang Hu, Mingbo Pu, Cheng Huang, Zeyu Zhao, Qin Feng, and Xiangang Luo, "Truncated spherical voids for nearly omnidirectional optical absorption," Opt. Express 19, 20642-20649 (2011)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-19-21-20642


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References

  1. V. G. Kravets, F. Schedin, and A. N. Grigorenko, “Plasmonic blackbody: Almost complete absorption of light in nanostructured metallic coatings,” Phys. Rev. B78(20), 205405 (2008). [CrossRef]
  2. C. Hu, Z. Zhao, X. Chen, and X. Luo, “Realizing near-perfect absorption at visible frequencies,” Opt. Express17(13), 11039–11044 (2009). [CrossRef] [PubMed]
  3. N. C. Panoiu and R. M. Osgood., “Enhanced optical absorption for photovoltaics via excitation of waveguide and plasmon-polariton modes,” Opt. Lett.32(19), 2825–2827 (2007). [CrossRef] [PubMed]
  4. B. Sepulveda, L.G. Carrascossa, D. Regarots, M. A. Otte, D. Farina, and L.M. Lechuga, “Surfance Plasmon Resonance Biosensors for highly sensitive detection in real samples,” Proc. SPIE7397, 1–11 (2009).
  5. J. Rosenberg, R. V. Shenoi, T. E. Vandervelde, S. Krishna, and O. Painter, “A multispectral and polarization selective surface-plasmon resonant midinfrared detector,” Appl. Phys. Lett.95(16), 161101 (2009). [CrossRef]
  6. M. Diem, T. Koschny, and C. M. Soukoulis, “Wide-angle perfect absorber/thermal emitter in the terahertz regime,” Phys. Rev. B79(3), 033101 (2009). [CrossRef]
  7. R. Esteban, M. Laroche, and J. J. Greffet, “Dielectric gratings for wide-angle, broadband absorption by thin film photovoltaic cells,” Appl. Phys. Lett.97(22), 221111 (2010). [CrossRef]
  8. J. Hao, J. Wang, X. Liu, W. J. Padilla, L. Zhou, and M. Qiu, “High performance optical absorber based in a plasmonic metamaterial,” Appl. Phys. Lett.96(25), 251104 (2010). [CrossRef]
  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. E. E. Narimanov and A. V. Kildishev, “Optical Black Hole: Broadband omnidirectional light absober,” Appl. Phys. Lett.95(4), 041106 (2009). [CrossRef]
  11. M. Pu, C. Hu, M. Wang, C. Huang, Z. Zhao, C. Wang, Q. Feng, and X. Luo, “Design principles for infrared wide-angle perfect absorber based on plasmonic structure,” Opt. Express19(18), 17413–17420 (2011). [CrossRef]
  12. T. V. Teperik, F. J. Garcia de Abajo, A. G. Borisov, M. Abdelsalam, P. N. Bartlett, Y. Sugawara, and J. J. Baumberg, “Omnidirectional absorption in nanostructured metal surfaces,” Nat. Photonics2(5), 299–301 (2008). [CrossRef]
  13. E. Popov, S. Enoch, and N. Bonod, “Absorption of light by extremely shallow metallic gratings: metamaterial behavior,” Opt. Express17(8), 6770–6781 (2009). [CrossRef] [PubMed]
  14. N. P. Sergeant, O. Pincon, M. Agrawal, and P. Peumans, “Design of wide-angle solar-selective absorbers using aperiodic metal-dielectric stacks,” Opt. Express17(25), 22800–22812 (2009). [CrossRef] [PubMed]
  15. Z. B. Li, W. Y. Zhou, X. T. Kong, and J. G. Tian, “Polarization dependence and independence of near-field enhancement through a subwavelength circle hole,” Opt. Express18(6), 5854–5860 (2010). [CrossRef] [PubMed]
  16. T. A. Kelf, Y. Sugawara, R. M. Cole, J. J. Baumberg, M. E. Abdelsalam, S. Cintra, S. Mahajan, A. E. Russell, and P. N. Bartlett, “Localized and delocalized plasmons in metallic nanovoids,” Phys. Rev. B74(24), 245415 (2006). [CrossRef]
  17. R. M. Cole, J. J. Baumberg, F. J. Garcia de Abajo, S. Mahajan, M. Abdelsalam, and P. N. Bartlett, “Understanding Plasmons in Nanoscale Voids,” Nano Lett.7(7), 2094–2100 (2007). [CrossRef]
  18. C. H. Lin, R. L. Chern, and H. Y. Lin, “Polarization-independent broad-band nearly perfect absorbers in the visible regime,” Opt. Express19(2), 415–424 (2011). [CrossRef] [PubMed]
  19. N. P. Sergeant, O. Pincon, M. Agrawal, and P. Peumans, “Solar-Selective Absorbers using Sub-Wavelength Gratings,” sergeant@stanford.edu, http://peumans-pc.stanford.edu .
  20. N. P. Sergeant, M. Agrawal, and P. Peumans, “High performance solar-selective absorbers using coated sub-wavelength gratings,” Opt. Express18(6), 5525–5540 (2010). [CrossRef] [PubMed]
  21. M. E. Abdelsalam, P. N. Bartlett, J. J. Baumberg, and S. Coyle, “Preparation of arrays of isolated spherical cavities by self-assembly of polystyrene spheres on self-assembled pro-patterned macroporous films,” Adv. Mater. (Deerfield Beach Fla.)16(1), 90–93 (2004). [CrossRef]
  22. E. D. Palik, Handbook of Optical Constants of Solids (Academic Press, San Diego, 1998).
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