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Optics Express

Optics Express

  • Editor: Andrew M. Weiner
  • Vol. 21, Iss. 3 — Feb. 11, 2013
  • pp: 2942–2952
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Efficient and broadband quarter-wave plates by gap-plasmon resonators

Anders Pors and Sergey I. Bozhevolnyi  »View Author Affiliations


Optics Express, Vol. 21, Issue 3, pp. 2942-2952 (2013)
http://dx.doi.org/10.1364/OE.21.002942


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Abstract

We demonstrate numerically that metal-insulator-metal (MIM) configurations in which the top metal layer consists of a periodic arrangement of nanobricks, thus facilitating gap-surface plasmon resonances, can be designed to function as efficient and broadband quarter-wave plates in reflection by a proper choice of geometrical parameters. Using gold as the metal, we demonstrate quarter-wave plate behavior at λ ≃ 800 nm with an operation bandwidth of 160 nm, conversion efficiency of 82%, and angle of linear polarization fixed throughout the entire bandwidth. This work also includes a detailed analytical and numerical study of the optical properties and underlying physics of structured MIM configurations.

© 2013 OSA

1. Introduction

The possibility to control the polarization state of light is of paramount importance in modern optics as many phenomena are polarization sensitive. For example, many spectroscopic and microscopy applications rely on the control and knowledge of polarization in order to extract as much information as possible from the sample under study. The general attempt, however, to miniaturize photonics to the nanoscale also leads to the demand of nanometer-sized wave retarders, especially half- and quarter-wave plates that together enable the complete exploration of all polarization states.

Conventional wave retarders can be constructed from uniaxial (birefringent) crystalline materials, such as quartz, featuring two different refractive indexes, which results in a phase difference between components of the light with polarization along the two axes as the wave travels through the crystal [1

1. F. L. Pedrotti and L. S. Pedrotti, Introduction to Optics (Prentice Hall, 1993), 2nd ed.

]. By appropriately choosing the crystal thickness, it is possible to construct half- and quarter-wave plates. An alternative method to create birefringence, known as form birefringence, consists in periodically texturing on a subwavelength scale non-birefringent material, so that TE and TM waves experience different refractive indexes [2

2. L. H. Cescato, E. Gluch, and N. Streibl, “Holographic quarterwave plates,” Appl. Opt. 29, 3286–3290 (1990). [CrossRef] [PubMed]

5

5. W. Yu, A. Mizutani, H. Kikuta, and T. Konishi, “Reduced wavelength-dependent quarter-wave plate fabricated by a multilayered subwavelength structure,” Appl. Opt. 45, 2601–2606 (2006). [CrossRef] [PubMed]

]. In relation to an effective medium approach, this is equivalent to designing anisotropic metamaterials. Whichever approach used, both types of birefringence usually only show moderate differences in the two refractive indexes of ∼ 0.1 − 0.2 at optical wavelengths [1

1. F. L. Pedrotti and L. S. Pedrotti, Introduction to Optics (Prentice Hall, 1993), 2nd ed.

, 2

2. L. H. Cescato, E. Gluch, and N. Streibl, “Holographic quarterwave plates,” Appl. Opt. 29, 3286–3290 (1990). [CrossRef] [PubMed]

], which amounts in quite bulky wave plates with thicknesses larger than the wavelength.

In this paper, we design at the free-space wavelength λ ≃ 800 nm an efficient (reflectance of ≃ 82 %) and broadband (≃ 160 nm) quarter-wave plate in reflection based on a metal-insulator-metal (MIM) configuration in which the top metal layer is periodically structured according to the DED principle. Such MIM structures with nanometer-sized spacers are known to facilitate gap surface plasmon (GSP) resonances originating from efficient reflection of the GSP at the boundaries of the top layer structure [19

19. J. Jung, T. Søndergaard, and S. I. Bozhevolnyi, “Greens function surface integral equation method for theoretical analysis of scatterers close to a metal interface,” Phys. Rev. B 77, 245310 (2008). [CrossRef]

21

21. D. K. Gramotnev, A. Pors, M. Willatzen, and S. I. Bozhevolnyi, “Gap-plasmon nanoantennas and bowtie resonators,” Phys. Rev. B 85, 045434 (2012). [CrossRef]

]. The structured MIM configuration may also be viewed as a metasurface separated by a dielectric spacer to the metal back reflector [22

22. D. L. Markovich, A. Andryieuski, M. Zalkovskij, R. Malureanu, and A. V. Lavrinenko, “Metamaterial polarization converter analysis: limits of performance,” (2012). ArXiv:1209.0095v1.

]. This interpretation of the lower metal film/substrate as a reflector originates from the low frequency regime, where Ohmic losses are negligible and, consequently, all impinging light is reflected. In the visible and infrared regime, on the other hand, the periodically structured MIM configuration is in fact mainly used as efficient absorbers [23

23. N. Liu, M. Mesch, T. Weiss, M. Hentschel, and H. Giessen, “Infrared perfect absorber and its application as plasmonic sensor,” Nano Lett. 10, 2342–2348 (2010). [CrossRef] [PubMed]

28

28. M. G. Nielsen, A. Pors, O. Albrektsen, and S. I. Bozhevolnyi, “Efficient absorption of visible radiation by gap plasmon resonators,” Opt. Express 20, 13311–13319 (2012). [CrossRef] [PubMed]

] or thermal emitters [29

29. I. Puscasu and W. L. Schaich, “Narrow-band, tunable infrared emission from arrays of microstrip patches,” Appl. Phys. Lett. 92, 233102 (2008). [CrossRef]

], which results from the GSP resonance and the associated strong field confinement in the spacer. In this work, however, we demonstrate that structured MIM configurations can remain reflective at the GSP resonance by a proper choice of spacer thickness and unit cell size; a result that paves the way for efficient wave plates in reflection. It should be mentioned that structured MIM configurations have previously been demonstrated to work as half-wave plates at visible wavelengths [8

8. J. Hao, Q. Ren, Z. An, X. Huang, Z. Chen, M. Qiu, and L. Zhou, “Optical metamaterial for polarization control,” Phys. Rev. A 80, 023807 (2009). [CrossRef]

], although that work mainly focuses on the possibility of polarization conversion, leaving out any considerations on how to achieve high reflection and a broadband response. Also, it has previously been demonstrated how the polarization of light can be efficiently manipulated in a transmission geometry consisting of layered metasurfaces [30

30. W. Sun, Q. He, J. Hao, and L. Zhou, “A transparent metamaterial to manipulate electromagnetic wave polarizations,” Opt. Lett. 36, 927–929 (2011). [CrossRef] [PubMed]

]. That work, however, is limited to the microwave regime where Ohmic losses in metals are negligible.

This paper begins with a discussion of the effect of a metallic back reflector on the reflection coefficient of a metasurface and emphasizes important design considerations (Sec. 2). The metasurface is, for simplicity, considered to be made of prolate spheroids described by their point-dipole polarizabilities in the modified long-wavelength approximation (MLWA) [31

31. E. J. Zeman and G. C. Schatz, “An accurate electromagnetic theory study of surface enhancement factors for Ag, Au, Cu, Li, Na, Al, Ga, In, Zn, and Cd,” J. Phys. Chem. 91, 634–643 (1987). [CrossRef]

]. Section 3 presents detailed full-wave calculations of the optical properties of structured MIM configurations for which the top metal layer consists of periodically arranged gold nanobricks [see Fig. 2(b)]. In Sec. 4, the figures of merit of the nanobrick MIM quarter-wave plate is presented and discussed. Conclusions are given in Sec. 5.

Fig. 1 (a) Sketch of a three-medium system with a metasurface positioned at the interface between medium 1 and 2. The incident plane wave propagates normal to the interfaces in the z-direction with polarization along the major axis of the prolate spheroids. The reflection coefficient in Eq. (4) corresponds to ‘r’ in the drawing. (b) Amplitude and (c) phase of the reflection coefficient for three different cases: (I) No metasurface and medium 3 is assumed to be gold [equivalent to Z3 = ZAu and αii = 0 in Eq. (4)], (II) Structured MIM configuration with medium 3 as gold [equivalent to Z3 = ZAu in Eq. (4)], (III) Metasurface in homogeneous surroundings [equivalent to Z3 = ZAir in Eq. (4)]. The other parameters are: medium 1 and 2 are assumed to be air, the prolate gold spheroids have major and minor axes 71 nm and 12 nm, respectively, the period is Λ = 230 nm, thickness of medium 2 is ts = 150 nm, and the gold permittivity is described by interpolated experimental data [33].
Fig. 2 (a) Resonance wavelength λres as a function of the strip width w for the continuous-layer GSP resonator depicted in the inset. In the calculations, the metal strip and substrate are assumed to be gold and the spacer is SiO2 with refractive index ns = 1.45. The height of the strip is h = 50 nm. (b) Sketch of the structured MIM unit cell used in the design of a quarter-wave plate in reflection. The metal parts are assumed to be gold, the spacer is SiO2, and the upper dielectric is air. In this work, the nanobrick dimensions are fixed at t = 50 nm, Lx = 138 nm, Ly = 105 nm, and the corners are rounded with a radius of 5 nm.

2. The effect of a back reflector on a metasurface: Design considerations

Before we discuss the optical properties of nanobrick MIM configurations, which are structures suited for fabrication with standard electron beam lithography, it is instructive to consider the analytically simpler problem of a metasurface of prolate metal spheroids described by their point-dipole polarizability αii in close proximity to a metal back reflector. For notational simplicity and congruence with subsequent sections, the setup is termed a structured MIM configuration, though it does not possess GSP resonances which are characteristic of MIM structures.

Within MLWA, the polarizability of the prolate spheroids is defined as
αii=αii01ik36παii0k24πaiiαii0,
(1)
where αii0 are the diagonal elements of the electrostatic polarizability tensor (see, e.g., Ref. [32

32. C. F. Bohren and D. R. Huffman, Absorption and scattering of light by small particles (Wiley, Weinheim, 1983).

]), k is the wave number in the surrounding medium, and aii is the radius (major or minor axis) relevant for the given αii. The term proportional to k3 arises from radiation damping, while the term proportional to k2 takes into account the depolarization of the radiation due to the finite size of the spheroid.

In the following, we consider a three-layer medium for which the metasurface, which is optically thin, is placed at the boundary between medium 1 and 2 [see Fig. 1(a)]. Each bulk medium is characterized by the refractive index ni and wave impedance Zi. Following the transmission-line approach [34

34. Y. Zhao, N. Engheta, and A. Alú, “Homogenization of plasmonic metasurfaces modeled as transmission-line loads,” Metamaterials 5, 90–96 (2011). [CrossRef]

], the metasurface is modeled as a homogeneous current sheet with the boundary conditions
z^×(E1E2)=0,z^×(H1H2)=Ks,
(2)
where Ei and Hi are the electric and magnetic fields in medium i, respectively, is the unit vector in the z-direction, and Ks is the induced surface current equal to [time convention: exp(−iωt)]
Ks=iωα¯¯effΛ2(z^×E1).
(3)
Here, α̿eff is the effective diagonal 2 × 2 polarizability tensor taking into account the effect from all other spheroids and the inhomogeneity of the surrounding medium, and Λ is the unit cell period (assuming a square unit cell).

Without loss of generality, we assume the axes of the prolate spheroids to be oriented along the coordinate axes so that the 2 × 2 reflection matrix r̿ only contains nonzero diagonal elements. Furthermore, we assume a normal incident plane wave propagating along the z-direction [Fig. 1(a)], which by matching the fields at the two interfaces results in the following reflection coefficients
rii(ω)=eik0n22ts[1iΛ2(Z1+Z2)αiieffωZ1Z2]Z2Z3Z2+Z31iΛ2(Z1Z2)αiieffωZ1Z2eik0n22ts[1iΛ2(Z1Z2)αiieffωZ1Z2]Z2Z3Z2+Z31iΛ2(Z1+Z2)αiieffωZ1Z2,
(4)
where the subscript ii indicates diagonal elements of r̿. To keep things simple, we let medium 1 and 2 be identical and approximate αiieff with αii from Eq. (1), which is a reasonable approximation for not too dense packing of spheroids and too small distance ts to medium 3 (the latter requirement can be circumvented to some extend by using the image dipole approximation [35

35. T. Søndergaard and S. I. Bozhevolnyi, “Vectorial model for multiple scattering by surface nanoparticles via surface polariton-to-polariton interactions,” Phys. Rev. B 67, 165405 (2003). [CrossRef]

]). The reflection coefficient is displayed in Figs. 1(b) and 1(c) when medium 1 and 2 are assumed to be air, the prolate gold spheroids have major and minor axes 71 nm and 12 nm, respectively, the incident field is polarized along the major axis, the period is Λ = 230 nm, thickness of spacer medium (medium 2) is ts = 150 nm, and the gold permittivity is described by experimental data [33

33. P. B. Johnson and R. W. Christy, “Optical constants of the noble metals,” Phys. Rev. B 6, 4370–4379 (1972). [CrossRef]

]. Three different cases are studied: (I) Reflection from gold substrate, i.e., no metasurface (black solid line), (II) Structured MIM configuration with medium 3 as gold (red dashed line), (III) Metasurface surrounded by air (blue dashed-dotted line).

It is evident from Fig. 1(b) that case (I) corresponds to simple reflection from a gold substrate, showing almost unity reflection for wavelengths above 600 nm. The phase of the reflection coefficient [Fig. 1(c)], however, displays a much stronger variation than what can be expected from the dispersion of gold. The additional phase shift arises from the term exp(ik0n22ts), which accounts for phase retardation in the spacer layer, because we have chosen the reference plane at z = 0 in order to facilitate comparison with metasurfaces. Case (II) and (III) show, as expected, complementary behavior, where the MIM configuration demonstrates a strong dip in reflection at the resonance at λ ∼ 800 nm; the property that makes such structures relevant as plasmonic absorbers. If we study the phase of the reflection coefficient, it is clear that both case (II) and (III) demonstrate a strong variation in the phase across the resonance interval due to the change in phase of the polarizability by 180°. Interesting, though, the MIM configuration experiences a much stronger phase variation, showing a slope of ≃ −3.7 degrees/nm at the resonance compared to ≃ −1.2 degrees/nm for the isolated metasurface [case (III)], which owes to the additional phase manipulation possible with a dielectric spacer and metal back reflector. Consequently, the design of structured MIM wave plates with the DED concept requires, in general, a smaller detuning than the equivalent metasurface without back reflector. Moreover, the structured MIM configuration also allows the design of wave plates whose retardation is not limited to the 180°-change that can be achieved for metasurfaces without back reflector.

As a final comment, it should be noted that the analytical calculations presented in Fig. 1 agree well with full-wave simulations. Simulations also show that strong phase variation still exists at spacer thicknesses of ts ∼ 50 nm. Summarizing, the above analytical example of a structured MIM configuration demonstrates the improved possibility to control the phase of the reflected light with, in general, a steeper slope at the resonance compared to an isolated metasurface. The example, however, also illustrates the main problem that reflection is low at the resonance. Accordingly, an efficient wave plate based on the structured MIM configuration must employ a scattering setup that minimizes the dip in reflection; an issue that is overcome in the following section.

3. The structured metal-insulator-metal configuration

It is well-known that two-dimensional metal strips (i.e., metal films of finite width) in a homogeneous dielectric environment possess retardation-based resonances whose resonance wavelengths scale (almost) linearly with the width of the strip [36

36. G. Della Valle, T. Søndergaard, and S. I. Bozhevolnyi, “Plasmon-polariton nano-strip resonators: from visible to infra-red,” Opt. Express 16, 6867–6876 (2008). [CrossRef] [PubMed]

]. If such a strip is supported by a thin dielectric spacer and a metal substrate [see Inset of Fig. 2(a)], a continuous-layer gap-plasmon resonator is created in which resonances are standing GSP waves that can be controlled by the width of the metal strip [20

20. M. G. Nielsen, D. K. Gramotnev, A. Pors, O. Albrektsen, and S. I. Bozhevolnyi, “Continuous layer gap plasmon resonators,” Opt. Express 19, 19310–19322 (2011). [CrossRef] [PubMed]

]. The term ‘continuous-layer’ refers to the fact that truncation is carried out only for the top layer of the MIM configuration. As shown in Fig. 2(a), such a configuration also displays an almost linear dependence of the resonance wavelength λres with respect to the width of the strip w. The slope of the curve (dλres/dw) is influenced by the thickness of the spacer layer ts, however, in any case it is easy to control the resonance wavelength in the visible and near-infrared regime.

Based on the excellent resonance characteristics of continuous-layer GSP resonators, we have chosen to employ the three-dimensional analog structure as the unit cell of our structured MIM wave plate [Fig. 2(b)]. The structured top metal layer consists of gold nanobricks with lengths Lx and Ly, and height t which are separated by a distance ts to the gold substrate. The spacer medium is assumed to be SiO2 with refractive index ns = 1.45, while the upper medium is taken to be air. One should note that by selecting Lx ≠ = Ly we obtain a structure with two detuned orthogonal GSP resonances. We have chosen the lower metal part to be of infinite extend for simplicity, though it should be emphasized that a gold film of ∼ 50 nm thickness would produce almost similar results, thus keeping the overall thickness of the wave plate subwavelength. The unit cell is chosen to be square with period Λ.

Fig. 3 Reflection as a function of wavelength and unit cell period for the nanobrick MIM configuration in Fig. 2(b). The nanobrick parameters are t = 50 nm, Lx = 138 nm, Ly = 105 nm, and the incident wave is normal to the surface and x-polarized. Green dashed curve, excitation wavelength of (1,0) SPP; Blue dashed-dotted curve, excitation wavelength of (1,1) SPP; magenta dotted curve, resonance wavelength of GSP mode for isolated structure (Λ → ∞). (a) ts = 20 nm, (b) ts = 50 nm.

The origin of the reflection dips can also be verified by a simple inspection of the mode profile at the three different reflection dips. As an example, Fig. 4 displays the H-field of the three modes for Λ = 700 nm in the xz-plane (y = 0) and in the xy-plane in the center of the spacer layer. The GSP mode shows strong H-field enhancement in the spacer [Fig. 4(a)], which is characteristic of the fundamental GSP mode due to the anti-symmetric current distribution in the two metal parts. Furthermore, the strong localization of the H-field to the region below the nanobrick [Fig. 4(d)] confirms the localized nature of the mode. It is worth noting that the fundamental GSP mode possesses an electrically driven magnetic response [38

38. S. I. Bozhevolnyi and T. Søndergaard, “General properties of slow-plasmon resonant nanostructures: nano-antennas and resonators,” Opt. Express 15, 10869–10877 (2007). [CrossRef] [PubMed]

], which is why the mode is known as the magnetic resonance in the metamaterial community [39

39. G. Dolling, C. Enkrich, M. Wegener, J. F. Zhou, C. M. Soukoulis, and S. Linden, “Cut-wire pairs and plate pairs as magnetic atoms for optical metamaterials,” Opt. Lett. 30, 3198–3200 (2005). [CrossRef] [PubMed]

, 40

40. W. Cai, U. K. Chettiar, H.-K. Yuan, V. C. de Silva, A. V. Kildishev, V. P. Drachev, and V. M. Shalaev, “Metamagnetics with rainbow colors,” Opt. Express 15, 3333–3341 (2007). [CrossRef] [PubMed]

]. Additionally, the fundamental GSP mode is conceptually similar to resonances associated with high-impedance surfaces at microwave frequencies [41

41. D. Sievenpiper, L. Zhang, R. F. Jimenez Broas, N. G. Alexópolous, and E. Yablonovitch, “High-impedance electromagnetic surfaces with a forbidden frequency band,” IEEE Trans. Microwave Theory Tech. 47, 2059–2074 (1999). [CrossRef]

, 42

42. J. M. Hao, L. Zhou, and C. T. Chan, “An effective-medium model for high-impedance surfaces,” Appl. Phys. A 87, 281–284 (2007). [CrossRef]

]. The two SPP modes do not show the same degree of enhancement and localization. For example, both SPP modes demonstrate a H-field on top of the nanobrick that is comparable or higher than the field in the spacer [Figs. 4(b) and 4(c)], which is a signature of an extended mode. Furthermore, the fields in the spacer layer represent interference patterns; Fig. 4(e) corresponds to interference of two waves with wave vectors kx = ±2π/Λ [in Fig. 4(e) the nanobrick perturbs the field below it], which fits in with the (1,0) SPP mode, whereas Fig. 4(f) results from interference of four waves with wave vectors (kx, ky) = (±1, ±1)2π/Λ, i.e., the (1,1) SPP mode.

Fig. 4 (a)–(c) Magnitude of the H-field relative to H-field of the incident wave in the xz-plane. (d)–(f) The dominant Hy-component relative to the H-field of the incident wave in the xy-plane in the center of the spacer (the images display the entire unit cell). (a,d) GSP mode at λ = 840 nm, (b,e) (1,0) SPP mode at λ = 700 nm, (c,f) (1,1) SPP mode at λ = 606 nm. The nontrivial parameters are ts = 50 nm and Λ = 700 nm.

Retuning to Fig. 3, it is clear that the reflection depends strongly on the period as well as on the thickness of the spacer layer. In relation to the latter parameter, the GSP mode shows an increased confinement of the field in the spacer for decreasing spacer thickness, which results in a stronger reflection dip as less light is scattered to the far-field (i.e., the GSP is efficiently reflected from the nanobrick boundaries). The situation is different for the SPP modes, because the SPPs become less confined to the metal-dielectric interface for decreasing ts, hereby resulting in reduced propagation losses and less pronounced dips in the reflection spectra. In relation to the effect of the unit cell period, the excitation of SPP modes are by the grating coupling directly dictated by Λ. Consequently, SPP modes do not exist for subwavelength periods. The GSP mode exists, on the other hand, for any unit cell period, however, the mode is still influenced by Λ. This is especially apparent for small periods where the mode starts to red shift and broaden with a simultaneous decrease in the reflection dip. The behavior is ascribed to the near-field interaction with neighboring nanobricks, thus modifying the optical properties of the isolated nanobrick MIM configuration.

In the design of a quarter-wave plate in reflection, several conclusions can be drawn from Fig. 3. The demand of a subwavelength period to facilitate a metasurface is in fine line with the request to avoid SPP modes at the design wavelength in order not to complicate things unnecessarily. Furthermore, subwavelength periods weaken the reflection dip of the GSP mode, which makes it possible to design efficient wave plates. At first, one might think that the broadening of the GSP mode at subwavelength periods may lead to nanobrick designs with relatively large Lx/Ly-ratio (i.e., detuning) in order to create a 90° phase difference between the two orthogonal GSP resonances. The dramatic change in reflection phase at resonance for structured MIM configurations [see Fig. 1(c)], however, ensures a modest Lx/Ly-ratio of ∼ 1.3 for the quarter-wave plate design (Sec. 4). Finally, the spacer thickness ts should be chosen sufficiently large so as to ensure sufficiently strong near-field interactions (leading to high reflectivity) due to GSP mode extending beyond the nanobrick [20

20. M. G. Nielsen, D. K. Gramotnev, A. Pors, O. Albrektsen, and S. I. Bozhevolnyi, “Continuous layer gap plasmon resonators,” Opt. Express 19, 19310–19322 (2011). [CrossRef] [PubMed]

] at reasonably large lattice constants that are desirable from the fabrication viewpoint.

4. Quarter-wave plate in reflection

In the preceding sections, we have discussed the effect of a metal back reflector on the reflection coefficient of a metasurface, and we have thoroughly studied the optical properties of the nanobrick MIM configuration, emphasizing parameter regions for which an efficient quarter-wave plate in reflection can be constructed. In this section, we demonstrate a wave plate design at the wavelength λ ≃ 800 nm using the nanobrick dimensions of Sec. 3 with the spacer thickness ts = 50 nm and period Λ = 240 nm. The corresponding reflection coefficients for normal incident x- and y-polarized waves are shown in Fig. 5(a). It is seen that for both polarizations the reflection coefficient remains high at |rii| ∼ 0.9 for λ > 700 nm without any pronounced feature of the GSP resonances. Despite this fact, it is evident that the phase difference between the two complex reflection coefficients ΔΦ experiences a maximum equal to ≃ 90° at λ = 800 nm, i.e., the configuration shows quarter-wave plate operation.

Fig. 5 (a) Reflection coefficients for normal incident x- and y-polarized light for the nanobrick MIM configuration in Fig. 2(b) with ts = 50 nm and Λ = 240 nm. Here, ΔΦ = arg(rxx/ryy). (b) Reflectance and absorption for normal incident circularly-polarized light. The cyan-shaded area depicts the operation bandwidth. (c) Degree and angle of linear polarization (DoLP and AoLP, respectively) for normal incident left and right circularly polarized light (LCP and RCP, respectively). AoLP is measured from the x-axis.

Following the discussion in Ref. [9

9. Y. Zhao and A. Alú, “Manipulating light polarization with ultrathin plasmonic metasurfaces,” Phys. Rev. B 84, 205428 (2011). [CrossRef]

], the bandwidth of operation can appropriately be defined by ±10° change in ΔΦ, which results in a bandwidth of 160 nm corresponding to the wavelength interval 720–880 nm, [see Fig. 5(a)]. The bandwidth is also depicted in Figs. 5(b) and 5(c) by the cyan-colored areas. Although not pursued in this work, it is worth noting that the bandwidth can be further increased by deliberately designing the quarter-wave plate to show ΔΦ = 100° at the design wavelength.

Figure 5(b) depicts the reflectance and absorption for circularly polarized incident light (the configuration is insensitive to the handedness of the light as it does not possess optical activity), and it is seen that the reflectance is only weakly increasing in the operation bandwidth with an average reflectance of ≃ 82%. This value achieved for a realistic metal with losses included is due to the presence of the metal back reflector, and it is a considerable improvement compared to ∼ 50% theoretically predicted for a single lossless isolated metasurface [9

9. Y. Zhao and A. Alú, “Manipulating light polarization with ultrathin plasmonic metasurfaces,” Phys. Rev. B 84, 205428 (2011). [CrossRef]

]. The reflectance is expected to increase even further with the use of silver instead of gold.

An alternative way to describe the figure of merit of the quarter-wave plate is through the degree of linear polarization (DoLP) defined by the Stokes parameters DoLP=s12+s22/s0, where s0=|Exr|2+|Eyr|2, s1=|Exr|2|Eyr|2, and s2=Exr(Eyr)*+Eyr(Exr)*[32

32. C. F. Bohren and D. R. Huffman, Absorption and scattering of light by small particles (Wiley, Weinheim, 1983).

]. Here, ‘*’ means complex conjugate and the superscript ‘r’ refers to the reflected electric field. The DoLP of the reflected light is displayed in Fig. 5(c) for circularly polarized incident light, showing a value close to unity in the entire operation bandwidth (DoLP > 0.96), which is equivalent to reflected light with linear polarization. Another important parameter is the angle of linear polarization (AoLP), which describes the polarization angle of the linearly reflected light with respect to the x-axis for incident light with circular polarization. From Fig. 5(c) it is seen that the angle of polarization is almost constant at ±45°, where the sign depends on the handedness of the incident wave (the variation in AoLP is less than 2° within the bandwidth). This insensitivity of AoLP to the wavelength is a direct manifestation of the weak dispersion in the amplitude of the reflection coefficients [see Fig. 5(a)]. From a practical point of view, the weak wavelength dependence in AoLP is a desirable property, as it is equivalent to fixed fast and slow axes of a conventional quarter-wave plate.

5. Conclusion

In conclusion, we have numerically considered realizable MIM configurations in which the top metal layer consists of a periodic arrangement of nanobricks. Although such structures typically are used as efficient absorbers due to the properties of GSP resonances, we demonstrate that by choosing a subwavelength period and avoid strong coupling between the nanobricks and the metal substrate it is possible to keep the structure reflective at the GSP resonance. Accordingly, efficient quarter-wave plates in reflection can be designed by a proper ratio between the sides of the nanobrick, which ensures a phase difference of 90° for polarization along the two main axes. As an example, we demonstrate quarter-wave plate behavior at λ ≃ 800 nm with an operation bandwidth of 160 nm. Despite using gold as the metal, the configuration converts 82% of the incoming light with an angle of linear polarization fixed throughout the entire bandwidth. By replacing the metal substrate by a thin metal film (∼ 50 nm), we believe that an attractive subwavelength-thin polarization converter in reflection can be constructed and implemented in nanoscale photonic circuits.

Finally, we believe that the presented detailed analytical and numerical discussion of the optical properties and underlying physics of structured MIM configurations are not only helpful in the design of plasmonic wave plates, but also include important considerations in relation to plasmonic super absorbers and thermal emitters.

Acknowledgments

We acknowledge financial support for this work from the Danish Council for Independent Research (the FTP project ANAP, contract no. 09-072949, and the FNU project, contract no. 12-124690).

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S.-Y. Hsu, K.-L. Lee, E.-H. Lin, M.-C. Lee, and P.-K. Wei, “Giant birefringence induced by plasmonic nanoslit arrays,” Appl. Phys. Lett. 95, 013105 (2009). [CrossRef]

7.

A. Drezet, C. Genet, and T. W. Ebbesen, “Miniature plasmonic wave plates,” Phys. Rev. Lett. 101, 043902 (2008). [CrossRef] [PubMed]

8.

J. Hao, Q. Ren, Z. An, X. Huang, Z. Chen, M. Qiu, and L. Zhou, “Optical metamaterial for polarization control,” Phys. Rev. A 80, 023807 (2009). [CrossRef]

9.

Y. Zhao and A. Alú, “Manipulating light polarization with ultrathin plasmonic metasurfaces,” Phys. Rev. B 84, 205428 (2011). [CrossRef]

10.

E. H. Khoo, E. P. Li, and K. B. Crozier, “Plasmonic wave plate based on subwavelength nanoslits,” Opt. Lett. 36, 2498–2500 (2011). [CrossRef] [PubMed]

11.

P. F. Chimento, N. V. Kuzmin, J. Bosman, P. F. A. Alkemade, G. W. Hooft, and E. R. Eliel, “A subwavelength slit as a quarter-wave retarder,” Opt. Express 19, 24219–24227 (2011). [CrossRef] [PubMed]

12.

A. Roberts and L. Lin, “Plasmonic quarter-wave plate,” Opt. Lett. 37, 1820–1822 (2012). [CrossRef] [PubMed]

13.

F. Wang, A. Chakrabarty, F. Minkowski, K. Sun, and Q.-H. Wei, “Polarization conversion with elliptical patch nanoantennas,” Appl. Phys. Lett. 101, 023101 (2012). [CrossRef]

14.

F. I. Baida, M. Boutria, R. Oussaid, and D. Van Labeke, “Enhanced-transmission metamaterials as anisotropic plates,” Phys. Rev. B 84, 035107 (2011). [CrossRef]

15.

J. Hao, Y. Yuan, L. Ran, T. Jiang, J. A. Kong, C. T. Chan, and L. Zhou, “Manipulating electromagnetic wave polarization by anisotropic metamaterials,” Phys. Rev. Lett. 99, 063908 (2007). [CrossRef] [PubMed]

16.

A. B. Evlyukhin, S. I. Bozhevolnyi, A. Pors, M. G. Nielsen, I. P. Radko, M. Willatzen, and O. Albrektsen, “Detuned electrical dipoles for plasmonic sensing,” Nano Lett. 10, 4571–4577 (2010). [CrossRef] [PubMed]

17.

S. I. Bozhevolnyi, A. B. Evlyukhin, A. Pors, M. G. Nielsen, M. Willatzen, and O. Albrektsen, “Optical transparency by detuned electrical dipoles,” New J. Phys. 13, 023034 (2011). [CrossRef]

18.

A. Pors, M. G. Nielsen, G. Della Valle, M. Willatzen, O. Albrektsen, and S. I. Bozhevolnyi, “Plasmonic metamaterial wave retarders in reflection by orthogonally oriented detuned electrical dipoles,” Opt. Lett. 36, 1626–1628 (2011). [CrossRef] [PubMed]

19.

J. Jung, T. Søndergaard, and S. I. Bozhevolnyi, “Greens function surface integral equation method for theoretical analysis of scatterers close to a metal interface,” Phys. Rev. B 77, 245310 (2008). [CrossRef]

20.

M. G. Nielsen, D. K. Gramotnev, A. Pors, O. Albrektsen, and S. I. Bozhevolnyi, “Continuous layer gap plasmon resonators,” Opt. Express 19, 19310–19322 (2011). [CrossRef] [PubMed]

21.

D. K. Gramotnev, A. Pors, M. Willatzen, and S. I. Bozhevolnyi, “Gap-plasmon nanoantennas and bowtie resonators,” Phys. Rev. B 85, 045434 (2012). [CrossRef]

22.

D. L. Markovich, A. Andryieuski, M. Zalkovskij, R. Malureanu, and A. V. Lavrinenko, “Metamaterial polarization converter analysis: limits of performance,” (2012). ArXiv:1209.0095v1.

23.

N. Liu, M. Mesch, T. Weiss, M. Hentschel, and H. Giessen, “Infrared perfect absorber and its application as plasmonic sensor,” Nano Lett. 10, 2342–2348 (2010). [CrossRef] [PubMed]

24.

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

25.

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, 1–7 (2011). [CrossRef]

26.

J. Le Perchec, Y. Desieres, N. Rochat, and R. Espiau de Lamaestreb, “Subwavelength optical absorber with an integrated photon sorter,” Appl. Phys Lett. 100, 113305 (2012). [CrossRef]

27.

P. Bouchon, C. Koechlin, F. Pardo, R. Haider, and J.-L. Pelouard, “Wideband omnidirectional infrared absorber with a patchwork of plasmonic nanoantennas,” Opt. Lett. 37, 1038–1040 (2012). [CrossRef] [PubMed]

28.

M. G. Nielsen, A. Pors, O. Albrektsen, and S. I. Bozhevolnyi, “Efficient absorption of visible radiation by gap plasmon resonators,” Opt. Express 20, 13311–13319 (2012). [CrossRef] [PubMed]

29.

I. Puscasu and W. L. Schaich, “Narrow-band, tunable infrared emission from arrays of microstrip patches,” Appl. Phys. Lett. 92, 233102 (2008). [CrossRef]

30.

W. Sun, Q. He, J. Hao, and L. Zhou, “A transparent metamaterial to manipulate electromagnetic wave polarizations,” Opt. Lett. 36, 927–929 (2011). [CrossRef] [PubMed]

31.

E. J. Zeman and G. C. Schatz, “An accurate electromagnetic theory study of surface enhancement factors for Ag, Au, Cu, Li, Na, Al, Ga, In, Zn, and Cd,” J. Phys. Chem. 91, 634–643 (1987). [CrossRef]

32.

C. F. Bohren and D. R. Huffman, Absorption and scattering of light by small particles (Wiley, Weinheim, 1983).

33.

P. B. Johnson and R. W. Christy, “Optical constants of the noble metals,” Phys. Rev. B 6, 4370–4379 (1972). [CrossRef]

34.

Y. Zhao, N. Engheta, and A. Alú, “Homogenization of plasmonic metasurfaces modeled as transmission-line loads,” Metamaterials 5, 90–96 (2011). [CrossRef]

35.

T. Søndergaard and S. I. Bozhevolnyi, “Vectorial model for multiple scattering by surface nanoparticles via surface polariton-to-polariton interactions,” Phys. Rev. B 67, 165405 (2003). [CrossRef]

36.

G. Della Valle, T. Søndergaard, and S. I. Bozhevolnyi, “Plasmon-polariton nano-strip resonators: from visible to infra-red,” Opt. Express 16, 6867–6876 (2008). [CrossRef] [PubMed]

37.

Y. Chu and K. B. Crozier, “Experimental study of the interaction between localized and propagating surface plasmons,” Opt. Lett. 34, 244–246 (2009). [CrossRef] [PubMed]

38.

S. I. Bozhevolnyi and T. Søndergaard, “General properties of slow-plasmon resonant nanostructures: nano-antennas and resonators,” Opt. Express 15, 10869–10877 (2007). [CrossRef] [PubMed]

39.

G. Dolling, C. Enkrich, M. Wegener, J. F. Zhou, C. M. Soukoulis, and S. Linden, “Cut-wire pairs and plate pairs as magnetic atoms for optical metamaterials,” Opt. Lett. 30, 3198–3200 (2005). [CrossRef] [PubMed]

40.

W. Cai, U. K. Chettiar, H.-K. Yuan, V. C. de Silva, A. V. Kildishev, V. P. Drachev, and V. M. Shalaev, “Metamagnetics with rainbow colors,” Opt. Express 15, 3333–3341 (2007). [CrossRef] [PubMed]

41.

D. Sievenpiper, L. Zhang, R. F. Jimenez Broas, N. G. Alexópolous, and E. Yablonovitch, “High-impedance electromagnetic surfaces with a forbidden frequency band,” IEEE Trans. Microwave Theory Tech. 47, 2059–2074 (1999). [CrossRef]

42.

J. M. Hao, L. Zhou, and C. T. Chan, “An effective-medium model for high-impedance surfaces,” Appl. Phys. A 87, 281–284 (2007). [CrossRef]

OCIS Codes
(260.3910) Physical optics : Metal optics
(260.5430) Physical optics : Polarization
(250.5403) Optoelectronics : Plasmonics
(310.6628) Thin films : Subwavelength structures, nanostructures

ToC Category:
Optics at Surfaces

History
Original Manuscript: October 10, 2012
Revised Manuscript: January 9, 2013
Manuscript Accepted: January 23, 2013
Published: January 31, 2013

Citation
Anders Pors and Sergey I. Bozhevolnyi, "Efficient and broadband quarter-wave plates by gap-plasmon resonators," Opt. Express 21, 2942-2952 (2013)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-21-3-2942


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References

  1. F. L. Pedrotti and L. S. Pedrotti, Introduction to Optics (Prentice Hall, 1993), 2nd ed.
  2. L. H. Cescato, E. Gluch, and N. Streibl, “Holographic quarterwave plates,” Appl. Opt.29, 3286–3290 (1990). [CrossRef] [PubMed]
  3. F. Xu, R.-C. Tyan, P.-C. Sun, Y. Fainman, C.-C. Cheng, and A. Scherer, “Fabrication, modeling, and characterization of form-birefringent nanostructures,” Opt. Lett.20, 2457–2459 (1995). [CrossRef] [PubMed]
  4. G. P. Nordin and P. C. Deguzman, “Broadband form birefringent quarter-wave plate for the mid-infrared wavelength region,” Opt. Express5, 163–168 (1999). [CrossRef] [PubMed]
  5. W. Yu, A. Mizutani, H. Kikuta, and T. Konishi, “Reduced wavelength-dependent quarter-wave plate fabricated by a multilayered subwavelength structure,” Appl. Opt.45, 2601–2606 (2006). [CrossRef] [PubMed]
  6. S.-Y. Hsu, K.-L. Lee, E.-H. Lin, M.-C. Lee, and P.-K. Wei, “Giant birefringence induced by plasmonic nanoslit arrays,” Appl. Phys. Lett.95, 013105 (2009). [CrossRef]
  7. A. Drezet, C. Genet, and T. W. Ebbesen, “Miniature plasmonic wave plates,” Phys. Rev. Lett.101, 043902 (2008). [CrossRef] [PubMed]
  8. J. Hao, Q. Ren, Z. An, X. Huang, Z. Chen, M. Qiu, and L. Zhou, “Optical metamaterial for polarization control,” Phys. Rev. A80, 023807 (2009). [CrossRef]
  9. Y. Zhao and A. Alú, “Manipulating light polarization with ultrathin plasmonic metasurfaces,” Phys. Rev. B84, 205428 (2011). [CrossRef]
  10. E. H. Khoo, E. P. Li, and K. B. Crozier, “Plasmonic wave plate based on subwavelength nanoslits,” Opt. Lett.36, 2498–2500 (2011). [CrossRef] [PubMed]
  11. P. F. Chimento, N. V. Kuzmin, J. Bosman, P. F. A. Alkemade, G. W. Hooft, and E. R. Eliel, “A subwavelength slit as a quarter-wave retarder,” Opt. Express19, 24219–24227 (2011). [CrossRef] [PubMed]
  12. A. Roberts and L. Lin, “Plasmonic quarter-wave plate,” Opt. Lett.37, 1820–1822 (2012). [CrossRef] [PubMed]
  13. F. Wang, A. Chakrabarty, F. Minkowski, K. Sun, and Q.-H. Wei, “Polarization conversion with elliptical patch nanoantennas,” Appl. Phys. Lett.101, 023101 (2012). [CrossRef]
  14. F. I. Baida, M. Boutria, R. Oussaid, and D. Van Labeke, “Enhanced-transmission metamaterials as anisotropic plates,” Phys. Rev. B84, 035107 (2011). [CrossRef]
  15. J. Hao, Y. Yuan, L. Ran, T. Jiang, J. A. Kong, C. T. Chan, and L. Zhou, “Manipulating electromagnetic wave polarization by anisotropic metamaterials,” Phys. Rev. Lett.99, 063908 (2007). [CrossRef] [PubMed]
  16. A. B. Evlyukhin, S. I. Bozhevolnyi, A. Pors, M. G. Nielsen, I. P. Radko, M. Willatzen, and O. Albrektsen, “Detuned electrical dipoles for plasmonic sensing,” Nano Lett.10, 4571–4577 (2010). [CrossRef] [PubMed]
  17. S. I. Bozhevolnyi, A. B. Evlyukhin, A. Pors, M. G. Nielsen, M. Willatzen, and O. Albrektsen, “Optical transparency by detuned electrical dipoles,” New J. Phys.13, 023034 (2011). [CrossRef]
  18. A. Pors, M. G. Nielsen, G. Della Valle, M. Willatzen, O. Albrektsen, and S. I. Bozhevolnyi, “Plasmonic metamaterial wave retarders in reflection by orthogonally oriented detuned electrical dipoles,” Opt. Lett.36, 1626–1628 (2011). [CrossRef] [PubMed]
  19. J. Jung, T. Søndergaard, and S. I. Bozhevolnyi, “Greens function surface integral equation method for theoretical analysis of scatterers close to a metal interface,” Phys. Rev. B77, 245310 (2008). [CrossRef]
  20. M. G. Nielsen, D. K. Gramotnev, A. Pors, O. Albrektsen, and S. I. Bozhevolnyi, “Continuous layer gap plasmon resonators,” Opt. Express19, 19310–19322 (2011). [CrossRef] [PubMed]
  21. D. K. Gramotnev, A. Pors, M. Willatzen, and S. I. Bozhevolnyi, “Gap-plasmon nanoantennas and bowtie resonators,” Phys. Rev. B85, 045434 (2012). [CrossRef]
  22. D. L. Markovich, A. Andryieuski, M. Zalkovskij, R. Malureanu, and A. V. Lavrinenko, “Metamaterial polarization converter analysis: limits of performance,” (2012). ArXiv:1209.0095v1.
  23. N. Liu, M. Mesch, T. Weiss, M. Hentschel, and H. Giessen, “Infrared perfect absorber and its application as plasmonic sensor,” Nano Lett.10, 2342–2348 (2010). [CrossRef] [PubMed]
  24. J. Hao, J. Wang, X. Liu, W. J. Padilla, L. Zhou, and M. Qiu, “High performance optical absorber based on a plasmonic matematerial,” Appl. Phys. Lett.96, 251104 (2010). [CrossRef]
  25. 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, 1–7 (2011). [CrossRef]
  26. J. Le Perchec, Y. Desieres, N. Rochat, and R. Espiau de Lamaestreb, “Subwavelength optical absorber with an integrated photon sorter,” Appl. Phys Lett.100, 113305 (2012). [CrossRef]
  27. P. Bouchon, C. Koechlin, F. Pardo, R. Haider, and J.-L. Pelouard, “Wideband omnidirectional infrared absorber with a patchwork of plasmonic nanoantennas,” Opt. Lett.37, 1038–1040 (2012). [CrossRef] [PubMed]
  28. M. G. Nielsen, A. Pors, O. Albrektsen, and S. I. Bozhevolnyi, “Efficient absorption of visible radiation by gap plasmon resonators,” Opt. Express20, 13311–13319 (2012). [CrossRef] [PubMed]
  29. I. Puscasu and W. L. Schaich, “Narrow-band, tunable infrared emission from arrays of microstrip patches,” Appl. Phys. Lett.92, 233102 (2008). [CrossRef]
  30. W. Sun, Q. He, J. Hao, and L. Zhou, “A transparent metamaterial to manipulate electromagnetic wave polarizations,” Opt. Lett.36, 927–929 (2011). [CrossRef] [PubMed]
  31. E. J. Zeman and G. C. Schatz, “An accurate electromagnetic theory study of surface enhancement factors for Ag, Au, Cu, Li, Na, Al, Ga, In, Zn, and Cd,” J. Phys. Chem.91, 634–643 (1987). [CrossRef]
  32. C. F. Bohren and D. R. Huffman, Absorption and scattering of light by small particles (Wiley, Weinheim, 1983).
  33. P. B. Johnson and R. W. Christy, “Optical constants of the noble metals,” Phys. Rev. B6, 4370–4379 (1972). [CrossRef]
  34. Y. Zhao, N. Engheta, and A. Alú, “Homogenization of plasmonic metasurfaces modeled as transmission-line loads,” Metamaterials5, 90–96 (2011). [CrossRef]
  35. T. Søndergaard and S. I. Bozhevolnyi, “Vectorial model for multiple scattering by surface nanoparticles via surface polariton-to-polariton interactions,” Phys. Rev. B67, 165405 (2003). [CrossRef]
  36. G. Della Valle, T. Søndergaard, and S. I. Bozhevolnyi, “Plasmon-polariton nano-strip resonators: from visible to infra-red,” Opt. Express16, 6867–6876 (2008). [CrossRef] [PubMed]
  37. Y. Chu and K. B. Crozier, “Experimental study of the interaction between localized and propagating surface plasmons,” Opt. Lett.34, 244–246 (2009). [CrossRef] [PubMed]
  38. S. I. Bozhevolnyi and T. Søndergaard, “General properties of slow-plasmon resonant nanostructures: nano-antennas and resonators,” Opt. Express15, 10869–10877 (2007). [CrossRef] [PubMed]
  39. G. Dolling, C. Enkrich, M. Wegener, J. F. Zhou, C. M. Soukoulis, and S. Linden, “Cut-wire pairs and plate pairs as magnetic atoms for optical metamaterials,” Opt. Lett.30, 3198–3200 (2005). [CrossRef] [PubMed]
  40. W. Cai, U. K. Chettiar, H.-K. Yuan, V. C. de Silva, A. V. Kildishev, V. P. Drachev, and V. M. Shalaev, “Metamagnetics with rainbow colors,” Opt. Express15, 3333–3341 (2007). [CrossRef] [PubMed]
  41. D. Sievenpiper, L. Zhang, R. F. Jimenez Broas, N. G. Alexópolous, and E. Yablonovitch, “High-impedance electromagnetic surfaces with a forbidden frequency band,” IEEE Trans. Microwave Theory Tech.47, 2059–2074 (1999). [CrossRef]
  42. J. M. Hao, L. Zhou, and C. T. Chan, “An effective-medium model for high-impedance surfaces,” Appl. Phys. A87, 281–284 (2007). [CrossRef]

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