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Optics Express

Optics Express

  • Editor: C. Martijn de Sterke
  • Vol. 19, Iss. 21 — Oct. 10, 2011
  • pp: 21081–21090
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Plasmon-enhanced depolarization of reflected light from arrays of nanoparticle dimers

Gary F. Walsh, Carlo Forestiere, and Luca Dal Negro  »View Author Affiliations


Optics Express, Vol. 19, Issue 21, pp. 21081-21090 (2011)
http://dx.doi.org/10.1364/OE.19.021081


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Abstract

Using spectroscopic ellipsometry and analytical multiple scattering theory, we demonstrate significant depolarization of far-field reflected light due to plasmonic near-field concentration in dimer arrays of metallic nanoparticles fabricated by electron beam lithography. By systematically investigating dimer arrays with varying sub-wavelength interparticle separations, we show that the measured depolarization presents a sharp peak at the Rayleigh cutoff condition for efficient in-plane diffraction. Moreover, by investigating the depolarization of reflected light as a function of the excitation angle, we demonstrate that maximum depolarization occurs in the spectral regions of plasmon-enhanced near-fields. Our results demonstrate that far-field reflection measurements encode information on the near-field spectra of complex nanoparticle arrays, and can be utilized to experimentally determine the optimal conditions for the excitation of sub-wavelength plasmonic resonances. The proposed approach opens novel opportunities for the engineering of nanoparticle arrays with optimized enhancement of optical cross sections for spectroscopic and sensing applications.

© 2011 OSA

1. Introduction to diffractive coupling of LSP resonances

Localized surface plasmon (LSP) resonances have received increasing attention during the past decade due to their ability to dramatically enhance near-field optical intensities and boost nanoscale light-matter interactions [1

1. M. L. Brongersma and P. G. Kik, Surface Plasmon Nanophotonics (Springer, 2007).

,2

2. S. A. Maier, Plasmonics: Fundamentals and Applications (Springer, 2007).

]. However, in order to fully take advantage of plasmonic effects in metal nanostructures, their excitation conditions must be carefully selected in order to produce maximum enhancement of the electromagnetic near-fields within targeted wavelength and angular spectra. Much work has been performed on tailoring LSP resonances through the engineering of nanoparticle’s size, shape, and near-field coupling on length scales much shorter than the wavelength of light [3

3. P. Nordlander, C. Oubre, E. Prodan, K. Li, and M. I. Stockman, “Plasmon hybridization in nanoparticle dimers,” Nano Lett. 4(5), 899–903 (2004). [CrossRef]

]. Recently, studies of periodic [4

4. S. Zou, N. Janel, and G. C. Schatz, “Silver nanoparticle array structures that produce remarkably narrow plasmon lineshapes,” J. Chem. Phys. 120(23), 10871–10875 (2004). [CrossRef] [PubMed]

8

8. Y. Chu, E. Schonbrun, T. Yang, and K. B. Crozier, “Experimental observation of narrow surface plasmon resonances in gold nanoparticle arrays,” Appl. Phys. Lett. 93(18), 181108 (2008). [CrossRef]

] and aperiodic arrays with complex geometries [9

9. L. Dal Negro, N.-N. Feng, and A. Gopinath, “Electromagnetic coupling and plasmon localization in deterministic aperiodic arrays,” J. Opt. A, Pure Appl. Opt. 10(6), 064013 (2008). [CrossRef]

13

13. J. Trevino, H. Cao, and L. Dal Negro, “Circularly symmetric light scattering from nanoplasmonic spirals,” Nano Lett. 11(5), 2008–2016 (2011). [CrossRef] [PubMed]

] have demonstrated that diffractive light coupling of particle clusters plays a significant role in shaping their near and far-field properties by enhancing the coupling efficiency to LSP resonances.

In this work, using depolarization ellipsometry, we experimentally investigate the complex interplay of short-range quasi-static and long-range diffractive coupling in two-dimensional (2D) periodic diffraction gratings made of closely spaced nanoparticle dimers with varying sub-wavelength interparticle separations. By investigating a large number of structures under different excitation angles, we show that optimized photonic-plasmonic coupling results in strong depolarization of far-field scattered light. Moreover, by comparing our experimental results with rigorous analytical multiple scattering calculations based on the Generalized Mie Theory (GMT) [12

12. A. Gopinath, S. V. Boriskina, N.-N. Feng, B. M. Reinhard, and L. Dal Negro, “Photonic-plasmonic scattering resonances in deterministic aperiodic structures,” Nano Lett. 8(8), 2423–2431 (2008). [CrossRef] [PubMed]

,14

14. C. F. Bohren and D. R. Huffman, Absorption and Scattering of Light by Small Particles (Wiley-VCH, 1986).

19

19. Y. L. Xu, “Electromagnetic scattering by an aggregate of spheres,” Appl. Opt. 34(21), 4573–4588 (1995). [CrossRef] [PubMed]

], we demonstrate that the observed far-field depolarization peaks originate from the enhanced spatial localization of plasmonic excitations and that depolarization measurements can be utilized to extract information on the wavelength spectra of near-fields in plasmonic nanostructures.

Anomalous diffraction from metallic gratings has been known and studied for more than a century [20

20. U. Fano, “The theory of anomalous diffraction gratings and of quasi-stationary waves on metallic surfaces (Sommerfeld's waves),” J. Opt. Soc. Am. 31(3), 213–222 (1941). [CrossRef]

,21

21. A. Hessel and A. A. Oliner, “A new theory of Wood's anomalies on optical gratings,” Appl. Opt. 4(10), 1275–1297 (1965). [CrossRef]

]. Recently, Zou et al. [4

4. S. Zou, N. Janel, and G. C. Schatz, “Silver nanoparticle array structures that produce remarkably narrow plasmon lineshapes,” J. Chem. Phys. 120(23), 10871–10875 (2004). [CrossRef] [PubMed]

] predicted sharp resonances in the scattering and extinction spectra of periodic chains of metal nanoparticles and later theoretically showed that these give rise to large values of near-field enhancement [22

22. S. Zou and G. C. Schatz, “Silver nanoparticle array structures that produce giant enhancements in electromagnetic fields,” Chem. Phys. Lett. 403(1-3), 62–67 (2005). [CrossRef]

]. Hicks et al. [5

5. E. M. Hicks, S. Zou, G. C. Schatz, K. G. Spears, R. P. Van Duyne, L. Gunnarsson, T. Rindzevicius, B. Kasemo, and M. Käll, “Controlling plasmon line shapes through diffractive coupling in linear arrays of cylindrical nanoparticles fabricated by electron beam lithography,” Nano Lett. 5(6), 1065–1070 (2005). [CrossRef] [PubMed]

] first reported enhanced dark field scattering intensity form chains of silver nanoparticles with lattice constants approximately equal to the isolated particle’s resonant wavelength. Later using ellipsometry, Kravets et al. [6

6. V. G. Kravets, F. Schedin, and A. N. Grigorenko, “Extremely narrow plasmon resonances based on diffraction coupling of localized plasmons in arrays of metallic nanoparticles,” Phys. Rev. Lett. 101(8), 087403 (2008). [CrossRef] [PubMed]

] reported the first experimental observation, both in reflection and extinction, of collective photonic-plasmonic resonances in periodic arrays of gold nano-particles and demonstrated that the spectral location of these resonances could be tailored by varying the incident angle and grating period. They recently utilized these effects to engineer an ultrasensitive refractive index sensor [23

23. V. G. Kravets, F. Schedin, A. V. Kabashin, and A. N. Grigorenko, “Sensitivity of collective plasmon modes of gold nanoresonators to local environment,” Opt. Lett. 35(7), 956–958 (2010). [CrossRef] [PubMed]

]. These findings where later confirmed by Auguié et al. [7

7. B. Auguié and W. L. Barnes, “Collective resonances in gold nanoparticle arrays,” Phys. Rev. Lett. 101(14), 143902 (2008). [CrossRef] [PubMed]

] and Chu et al. [8

8. Y. Chu, E. Schonbrun, T. Yang, and K. B. Crozier, “Experimental observation of narrow surface plasmon resonances in gold nanoparticle arrays,” Appl. Phys. Lett. 93(18), 181108 (2008). [CrossRef]

] and can be explained by the Fano-type coupling of narrow band photonic grating modes with the broad plasmonic response of metallic nanoparticles [24

24. A. Christ, S. G. Tikhodeev, N. A. Gippius, J. Kuhl, and H. Giessen, “Waveguide-plasmon polaritons: strong coupling of photonic and electronic resonances in a metallic photonic crystal slab,” Phys. Rev. Lett. 91(18), 183901 (2003). [CrossRef] [PubMed]

,25

25. B. Luk’yanchuk, N. I. Zheludev, S. A. Maier, N. J. Halas, P. Nordlander, H. Giessen, and C. T. Chong, “The Fano resonance in plasmonic nanostructures and metamaterials,” Nat. Mater. 9(9), 707–715 (2010). [CrossRef] [PubMed]

]. Recently, numerous studies of deterministic aperiodic structures have shown broadband coupling of critically localized photonic modes to LSP resonances leading to large and reproducible Surface Enhanced Roman Scattering (SERS) enhancement factors [26

26. A. Gopinath, S. V. Boriskina, W. R. Premasiri, L. Ziegler, B. M. Reinhard, and L. Dal Negro, “Plasmonic nanogalaxies: multiscale aperiodic arrays for surface-enhanced Raman sensing,” Nano Lett. 9(11), 3922–3929 (2009). [CrossRef] [PubMed]

,27

27. A. Gopinath, S. V. Boriskina, B. M. Reinhard, and L. Dal Negro, “Deterministic aperiodic arrays of metal nanoparticles for surface-enhanced Raman scattering (SERS),” Opt. Express 17(5), 3741–3753 (2009). [CrossRef] [PubMed]

].

Most studies on lithographically-defined plamonic arrays have focused on measurements of extinction and scattering or on effects such as SERS, but there has been little work on the depolarization properties of light reflected by plasmonic arrays. On the other hand, numerous experimental and theoretical studies of the depolarization of scattered light from suspensions of randomly oriented non-spherical metal nanoparticles provided detailed information on LSP resonances [28

28. B. N. Khlebtsov, V. A. Khanadeev, and N. G. Khlebtsov, “Observation of extra-high depolarized light scattering spectra from gold nanorods,” J. Phys. Chem. C 112(33), 12760–12768 (2008). [CrossRef]

,29

29. N. G. Khlebtsov, A. G. Melnikov, V. A. Bogatyrev, L. A. Dykman, A. V. Alekseeva, L. A. Trachuk, and B. N. Khlebtsov, “Can the light scattering depolarization ratio of small particles be greater than 1/3?” J. Phys. Chem. B 109(28), 13578–13584 (2005). [CrossRef] [PubMed]

].

Here we investigate depolarization effects in nanoscale plasmonic particles diffractively coupled via tuneable long-range grating modes, and we show that the localization of LSP resonances causes sharp depolarization peaks in the far-field reflection bearing information on the near-field spectra of plasmonic excitations. Additionally, we show that depolarization reflection techniques can be used to unambiguously determine the condition for optimal excitation of LSP resonances in photonic-plasmonic arrays.

The relation between spatial frequencies and diffracted waves by particle arrays can easily be understood within the scalar Fourier optics of thin diffractive elements. By matching the wavefront of the incoming wave with the complex amplitude transmittance of the grating it follows that the diffracted field is spread angularly into a sum of plane waves propagating away from the grating, each characterized by a unique wavevector k. The components of the diffracted wavevectors travelling away from the grating are
kz=2πλ2νx2νy2
(1)
where νx and νy are the spatial frequencies of the complex amplitude at the grating surface and λ is the wavelength. From this simplified picture we can already appreciate that the interaction of the diffracted field with the plasmonic particles is greatest when the wave is diffracted into the array plane and propagates at grazing incidence, namely when kz = 0. In the context of grating optics, this condition is known as the Rayleigh cutoff condition [30

30. J. W. Goodman, Introduction to Fourier Optics (McGraw-Hill, 1968).

] and it determines the transition between a propagating diffractive order carrying energy away from the grating and an evanescent one with spatial frequency above the propagation cutoff. For a plane wave at oblique angle of incidence θ0 on a regular 1D periodic grating with lattice constant a, it is readily shown that the Rayleigh cutoff wavelength is given by [30

30. J. W. Goodman, Introduction to Fourier Optics (McGraw-Hill, 1968).

]
λ=am(n±sinθ0),
(2)
where n is the refractive index of the substrate and m in an integer.

In this study, we will focus on 1D periodic gratings of fixed lattice constant and systematically investigate the effects of tuning the coupling strength to LSP resonances by adjusting the angle of incidence and the sub-wavelength gaps between periodically arranged plasmonic dimers.

2. Samples fabrication and experimental apparatus

Using a Woollams V-VASE Spectroscopic Ellipsometer we investigated the depolarization properties of lithographically defined periodic arrays of identical gold nano-particles on fused silica substrates. The array geometry is shown schematically in Fig. 1(a)
Fig. 1 (a) Schematic of nanoplasmonic dimer array geometry. Particles are cylinders with diameter D, dimer gap separation dmin, edge-to-edge separation between dimers in the Y direction d2 and the center-to-center separation in the X direction a. (b-e) Scanning electron micrographs of representative dimer arrays fabricated by EBL on fused silica substrates with D = 120nm, d2 = 130nm and dmin = 20nm (a), 30nm (b), 50nm (c), and 130nm (d).
. Gold nano-cylinders with diameter D are arranged into dimers with minimum edge-to-edge gap separation dmin. The edge-to-edge distance between adjacent dimers in the Y-direction is held constant at d2 so that as dmin is increased the spacing between the dimers does not decrease. When dmin = d2 the lattice becomes a rectangular array of monomers. The lattice is spaced in the X-direction in a regular grid with period a.

Figures 1(b-e) show scanning electron micrographs of representative arrays of dimers and monomers fabricated by electron beam lithography (EBL). Patterns were defined by EBL in a 180nm thick layer of PMMA (Microchem PMMA 950 A3) spin cast on fused silica substrates then sputter coated with less than 10nm of gold. After exposure, the gold layer is removed with a gold etchant, the resist is developed for 70 seconds in a 1:3 solution of methyl isobutyl ketone to isopropanol and the samples are cleaned in an oxygen plasma. 2nm of Cr and 28nm of Au are deposited by electron beam evaporation then lift off is performed in heated acetone. The samples are rinsed in isopropanol and dried with nitrogen before ellipsometric measurements are performed. The arrays are 200μm x 200μm which is equal to the spot size of the ellipsometer beam using its focusing probes.

3. Ellipsometry of plasmonic arrays

Spectroscopic ellipsometry is a powerful phase sensitive technique that measures the change in the polarization state of light upon reflection from a surface. This technique is well suited to provide accurate phase information at the Rayleigh cutoff of periodic gratings where it has previously been shown that the p component of the specular reflection is nearly completely suppressed [6

6. V. G. Kravets, F. Schedin, and A. N. Grigorenko, “Extremely narrow plasmon resonances based on diffraction coupling of localized plasmons in arrays of metallic nanoparticles,” Phys. Rev. Lett. 101(8), 087403 (2008). [CrossRef] [PubMed]

]. As is illustrated in Fig. 2(a)
Fig. 2 (a) Schematic of ellipsometry experiment. The array is excited at an angle of incidence θ0 and the polarization state of the scattered light is measured at θs = θ0. (b) Squares of ellipsometric coefficients for an array of dimers (αd2, βd2, γd2) in blue with a = 320nm, D = 100nm, d2 = 80nm and dmin = 40nm measured at θ0 = 50° and a comparable array of monomers (αd2, βd2, γd2) in red with dmin = d2 = 80nm. Also depolarization for the two arrays (Δdp_d, Δdp_m). (c1-c3) Scaling of the squares of the ellipsometric parameters α2, β2 and γ2 respectively, with dimer gap separation (dmin = 20, 30, 50 and 130nm) at θ0 = 60° for an array with D = 120nm, a = 320nm and d2 = 130nm. (d) Scaling of the depolarization with dimer gap separation. (inset) Maximum depolarization vs. gap separation.
, our samples are illuminated at an angle of incidence θ0 and the specular reflection is measured at the angle θs = θ0. The plasmonic structures are oriented such that the s component of the incident field is parallel to the dimer axis and the p component is aligned with the grating direction. The three ellipsometric coefficients α, β, and γ are determined from a series of intensity measurements performed with various incident polarization states and analyzer positions [31

31. R. Joerger, K. Forcht, A. Gombert, M. Köhl, and W. Graf, “Influence of incoherent superposition of light on ellipsometric coefficients,” Appl. Opt. 36(1), 319–327 (1997). [CrossRef] [PubMed]

,32

32. H. Fujiwara, Spectroscopic Ellipsometry (John Wiley & Sons, Ltd, 2007).

]. The degree of polarization of the reflected field is than given by:

P=α2+γ2+β2.
(3)

In this paper we report this value as the percentage of depolarization, Δdp = 100%(1-P). When Δdp = 0, the reflected light is totally polarized, meaning that it can be represented by one specific state of polarization. When 0 < Δdp < 100, the reflected light is only partially polarized, meaning it consists of an incoherent mixture of multiple waves with no phase correlation between them [31

31. R. Joerger, K. Forcht, A. Gombert, M. Köhl, and W. Graf, “Influence of incoherent superposition of light on ellipsometric coefficients,” Appl. Opt. 36(1), 319–327 (1997). [CrossRef] [PubMed]

]. Shown in Fig. 2(b) are the squares of the three normalized ellipsometric parameters and Δdp obtained at specular reflection from an array of dimers (denoted by subscript d and blue lines) with a = 320nm, D = 100nm, dmin = 40nm, d2 = 80nm and a comparable array of monomers (denoted by subscript m and red lines) with dmin = d2 = 80nm measured at θ0 = 50°. The grating resonance around 575nm produces sharp features (one peak and a two polarization dip) in the three ellipsometric parameters. Note that γ2 is very small compared to the other two parameters therefore we have increased its magnitude by 20 times in Fig. 2(b). Notice additionally that in Fig. 2(b) the magnitude of the dip in the parameter α2 is greater than the peak intensity in β22 being negligible) and therefore it follows from Eq. (3) that a net depolarization of the reflected light is induced by the plasmonic resonance of the array.

It can be observed from the solid curves in Fig. 2(b) that this effect is five times stronger for the array of dimers than for monomers, due to the increased field concentration in the nanoscale dimer gap regions. In order to investigate the role of near-field coupling in the dimers on the reflected depolarization spectrum we systematically varied the sub-wavelength interparticle separations in the dimer gap regions dmin, and found a high dependence of the strength of depolarization on this parameter, as shown in Fig. 2(d). Figures 2(c1–c3) show the spectra of the three ellipsometric parameters measured at θ0 = 60° for arrays of 120nm diameter particles with a = 320nm and d2 = 130nm for dmin = 20, 30, 50, and 130nm. It can be noticed that the magnitude of the sharp variations (especially in α2) increases as the dimer gap is reduced. The corresponding depolarization spectrum for each of these arrays is shown in Fig. 2(d) and the maximum of each is plotted versus the dimer gap separation in the inset. Also notice that a second peak, red shifted from the main one, appears in Figs. 2(c–d). This corresponds to the Rayleigh cutoff wavelength for diffracted waves propagating in the substrate [6

6. V. G. Kravets, F. Schedin, and A. N. Grigorenko, “Extremely narrow plasmon resonances based on diffraction coupling of localized plasmons in arrays of metallic nanoparticles,” Phys. Rev. Lett. 101(8), 087403 (2008). [CrossRef] [PubMed]

,21

21. A. Hessel and A. A. Oliner, “A new theory of Wood's anomalies on optical gratings,” Appl. Opt. 4(10), 1275–1297 (1965). [CrossRef]

], which can be easily predicted by inserting the refractive index of fused silica (n = 1.45) into Eq. (2).

While it is well established that plasmonic quasi-static coupling of closely spaced dimers dramatically increases the LSP field localization compared to monomers [33

33. C. E. Talley, J. B. Jackson, C. Oubre, N. K. Grady, C. W. Hollars, S. M. Lane, T. R. Huser, P. Nordlander, and N. J. Halas, “Surface-enhanced Raman scattering from individual au nanoparticles and nanoparticle dimer substrates,” Nano Lett. 5(8), 1569–1574 (2005). [CrossRef] [PubMed]

], it is not directly obvious that this enhanced nanoscale field localization can lead to an increase in depolarization of reflected radiation in the far-field. In fact, the field components that are localized within the nanoscale dimer gaps have spatial frequencies well in excess of the propagation cutoff. However, for the structures considered in this work, our experimental results clearly demonstrate that by decreasing the dimer gap separation it is possible to increase the depolarization of the far-field reflected (i.e., radiative) components. In order to rule out the contribution to our ellipsometric measurements of additional depolarization effects due to the incoherent addition of backside reflections from the substrate [31

31. R. Joerger, K. Forcht, A. Gombert, M. Köhl, and W. Graf, “Influence of incoherent superposition of light on ellipsometric coefficients,” Appl. Opt. 36(1), 319–327 (1997). [CrossRef] [PubMed]

,32

32. H. Fujiwara, Spectroscopic Ellipsometry (John Wiley & Sons, Ltd, 2007).

], we repeated the experiments on arrays fabricated on single and double side polished silicon substrates. In both cases, we observed similar depolarization peaks at wavelengths above the silicon bandgap where the samples are opaque. For the double side polished silicon sample, a background depolarization of around 3% was observed for wavelengths below the bandgap where it is transparent. Moreover, no depolarization was observed in regions outside the arrays for either the single side polished silicon or fused silica substrate at any wavelength.

In order to obtain a better understanding of our experimental results we will now discuss more closely the physical origin of plasmon-enhanced field depolarization as measured by ellipsometry on nanostructured surfaces. It is well known that multiple light scattering by rough inhomogeneous surfaces produces diffuse reflection of light with rapidly varying spatial distribution of polarization states. Standard ellipsometric instruments collect a finite angular range of k vectors (as opposed to a single direction) and therefore detect an inhomogeneous mixture of wave fields, each characterized by a well-defined polarization state. This type of inhomogeneous angular spread of polarization states, known as quasi-depolarization [32

32. H. Fujiwara, Spectroscopic Ellipsometry (John Wiley & Sons, Ltd, 2007).

], is the primary cause for the polarization loss mechanism (i.e. depolarization) of scattered light by rough surfaces or irregular objects. However, in our experimental work we deal with homogeneous samples consisting of identical nanoparticles deposited on optically smooth surfaces, and the significant depolarization shown in Fig. 2 occurs only when the Rayleigh cutoff condition is met. This condition maximizes the coupling to localized plasmonic excitations and the spread of scattered k vectors within the finite collection angle, contributing to plasmon-enhanced quasi-depolarization in the specular reflection [34

34. L. Novotny and B. Hecht, Principles of Nano-Optics (Cambridge University Press, 2006).

]. It is important to note that angularly diffuse scattering occurs over a narrower frequency range determined by the Rayleigh cutoff condition. The increased spatial localization of the LSP resonances results in angularly diffuse scattered field components with spatially varying polarization states, leading to net depolarization. Interestingly, this effect can already be understood by investigating the interplay between plasmon resonances and the degree of linear depolarization of scattered fields.

4. Theoretical results

The polarization state of the scattered beam along a given direction can be fully characterized by its Stokes vectors [14

14. C. F. Bohren and D. R. Huffman, Absorption and Scattering of Light by Small Particles (Wiley-VCH, 1986).

]. GMT is used to calculate the Mueller matrix of the scattered field in the specular direction, which provides the relation between the incident [I0Q0U0V0]T and scattered [IQUV]T Stokes vector, that is,

[IQUV]=M[I0Q0U0V0].
(4)

If a linearly polarized wave is incident on the nanoparticle array the scattered light will be in general elliptically polarized, featuring a degree of linear polarization given by

DoLP=Q2+U2I.
(5)

Therefore, during the scattering process with the nanoparticles array, light undergoes a loss of linear polarization with respect to the original polarization state (100% linearly polarized), which is described by the degree of linear depolarization (1-DoLP).

In order to gain a rigorous, yet qualitative understanding of our results, we analytically calculated the degree of linear depolarization from linear arrays of nanospheres dimers. As we will show below, the degree of linear depolarization, which can be conveniently calculated within the GMT analytical framework, provides qualitative information on the behavior of the measured depolarization spectra without the need of complex angular averages for the scattered field. In Fig. 3
Fig. 3 GMT calculations of gold sphere of diameter D = 120nm. (a) The degree of linear depolarization at the specular direction for isolated dimers with gap separations dmin = 10, 20, 30 and 40nm. (b) Maximum field enhancement spectra calculated in the plane of the array for a isolated dimers with varying gap separations. (c) Degree of linear depolarization at the specular direction for a chain of 51 gold dimers with lattice spacing a = 350nm and varying gap separations. (d) Maximum field enhancement spectra for chains of dimers with varying gap separations. Structures are illuminated at an incident angle θ0 = 60° with a plane wave linearly polarization 45° with respect to the plane of incidence.
, we investigate the scattering from isolated dimers of gold [35

35. P. B. Johnson and R. W. Christy, “Optical constants of the noble metals,” Phys. Rev. B 6(12), 4370–4379 (1972). [CrossRef]

] sphere with diameter D = 120nm and varying inter-particle gap separation, illuminated by a plane wave launched at an angle θ0 = 60°, with the electric field linearly polarized at 45° with respect to the plane of incidence. Figures 3(a) and 3(b) show the quantity (1-DoLP) at the specular direction with respect to the incident beam and the maximum field enhancement calculated in the plane of the array, respectively. Consistently with our experimental observations, we observe that by decreasing the dimer gap separation, both the degree of linear depolarization and the maximum field enhancement increase. Additionally, we note that the peak in the degree of linear depolarization is significantly blue-shifted from the calculated peak of maximum field enhancement by approximately 70nm. Figures 3(c) and 3(d) show these same parameters for linear chains of dimers with varying gap separation and chains of monomers. The chains are 51 clusters (dimers or monomers) long with a lattice constant a = 350nm. The spheres are 120nm in diameter with a minimum edge-to-edge separation dmin = 10, 20, 30 and 40nm. An incident plane wave linearly polarized at 45° is launched at an angle of θ0 = 60°. Both the linear depolarization and maximum field enhancement feature sharp peaks at the grating resonance wavelength which increase in magnitude as the dimer gap separation decreases, as observed in our experiments. Unlike the case of isolated dimers, the spectral location of the maxima in linear depolarization and near-field enhancement only differ by 10nm. Despite a direct calculation of depolarization spectra cannot be easily achieved, these GMT results on the behavior of the DoLP support our experimental demonstration of far-field depolarization occurring in the frequency range of plasmon-enhanced near-field intensity.

4. The effect of incident angle

Next, we numerically modelled the effects of the incident angle on both the near-field and far-field polarization properties by using GMT. Figures 5(a)
Fig. 5 GMT calculations of the maximum field enhancement (blue) and degree of linear depolarization (1-DoLP) (red) for periodic chains of 51 dimers of gold spheres with diameter D = 120nm, gap dmin = 20nm, and lattice constant a = 350nm, illuminated at an angle of incidence θ0 = 60° (a), 70° (b), 80°(c) with the electric field linearly polarized at 45° with respect to the plane of incidence. (d) Peak position of the maximum field enhancement (blue) and degree of linear depolarization (red) as a function of angle of incidence compared with the wavelength of the first order grating mode predicted by the Rayleigh condition.
5(c) show the linear depolarization and maximum field enhancement calculated for a chain of 51 dimers with 350nm lattice spacing and minimum edge-to-edge gap separation of 20nm illuminated at various angles of incidence, namely θ0 = 60° (a), θ0 = 70° (b), θ0 = 80° (c), with an electric field linearly polarized at 45° with respect to the plane of incidence. Both the peaks in the linear depolarization and the maximum field enhancement follow the grating condition. This can be seen in Fig. 5(d), which shows both of the peak positions as a function of angle of incidence together with the wavelength of the first order grating mode predicted by the Rayleigh condition shown in Eq. (2) with n = m = 1.

From this study, we conclude that the maximum near-field intensity occurs at wavelengths slightly red-shifted from the maximum of linear depolarization. However, these results demonstrate that the knowledge of far-field depolarization spectra bears information on the spectra of near-field plasmonic excitations. The interplay between depolarization and plasmonic-enhanced near-field spectra can be used to determine the optimal conditions to engineer near-field intensity enhancement in photonic-plasmonic arrays.

5. Conclusions

We have demonstrated for the first time strong depolarization of specular reflection from arrays of plasmonic nanoparticles diffractively coupled in a periodic grating geometry. We have shown experimentally that at the Rayleigh cutoff condition the depolarization features a sharp maximum corresponding to the efficient excitation of LSP localized modes. Additionally, we demonstrated that there is an optimal angle of incidence that gives rise to the strongest depolarization effects and that the simultaneous engineering of diffractive coupling and LSP resonance bandwidths of individual nanoparticle is required to maximize photonic-plasmonic coupling in nanoparticle arrays.

The origin of the observed far-field depolarization has been explained by the increased spatial localization of the LSP resonances, that results in angularly diffuse scattered field components with spatially varying degrees of linear polarization, leading to a net quasi-depolarization effect. Analytical scattering calculations of the degree of linear polarization performed on single nanospheres and periodic chains confirm that the observed depolarization spectra overlap closely the plasmonic-enhanced near-field spectra. Our results demonstrate that far-field depolarization spectra carry optical information on the near-field frequency spectra of plasmonic arrays, and can be utilized to determine the optimal excitation conditions for the engineering of plasmonic resonances in complex arrays.

Acknowledgments

This work was partially supported by the U.S. Army through the Natick Soldier Center (W911NF-07-D-001), the SMART Scholarship Program, the Air Force program “Deterministic Aperiodic Structures for On-chip Nanophotonic and Nanoplasmonic Device Applications” under Award FA9550-10-1-0019, and from the NSF Career Award ECCS-0846651. This document has been approved for public release. NSRDEC PAO # U11-475.

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2.

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3.

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4.

S. Zou, N. Janel, and G. C. Schatz, “Silver nanoparticle array structures that produce remarkably narrow plasmon lineshapes,” J. Chem. Phys. 120(23), 10871–10875 (2004). [CrossRef] [PubMed]

5.

E. M. Hicks, S. Zou, G. C. Schatz, K. G. Spears, R. P. Van Duyne, L. Gunnarsson, T. Rindzevicius, B. Kasemo, and M. Käll, “Controlling plasmon line shapes through diffractive coupling in linear arrays of cylindrical nanoparticles fabricated by electron beam lithography,” Nano Lett. 5(6), 1065–1070 (2005). [CrossRef] [PubMed]

6.

V. G. Kravets, F. Schedin, and A. N. Grigorenko, “Extremely narrow plasmon resonances based on diffraction coupling of localized plasmons in arrays of metallic nanoparticles,” Phys. Rev. Lett. 101(8), 087403 (2008). [CrossRef] [PubMed]

7.

B. Auguié and W. L. Barnes, “Collective resonances in gold nanoparticle arrays,” Phys. Rev. Lett. 101(14), 143902 (2008). [CrossRef] [PubMed]

8.

Y. Chu, E. Schonbrun, T. Yang, and K. B. Crozier, “Experimental observation of narrow surface plasmon resonances in gold nanoparticle arrays,” Appl. Phys. Lett. 93(18), 181108 (2008). [CrossRef]

9.

L. Dal Negro, N.-N. Feng, and A. Gopinath, “Electromagnetic coupling and plasmon localization in deterministic aperiodic arrays,” J. Opt. A, Pure Appl. Opt. 10(6), 064013 (2008). [CrossRef]

10.

C. Forestiere, M. Donelli, G. F. Walsh, E. Zeni, G. Miano, and L. Dal Negro, “Particle-swarm optimization of broadband nanoplasmonic arrays,” Opt. Lett. 35(2), 133–135 (2010). [CrossRef] [PubMed]

11.

C. Forestiere, G. F. Walsh, G. Miano, and L. Dal Negro, “Nanoplasmonics of prime number arrays,” Opt. Express 17(26), 24288–24303 (2009). [CrossRef] [PubMed]

12.

A. Gopinath, S. V. Boriskina, N.-N. Feng, B. M. Reinhard, and L. Dal Negro, “Photonic-plasmonic scattering resonances in deterministic aperiodic structures,” Nano Lett. 8(8), 2423–2431 (2008). [CrossRef] [PubMed]

13.

J. Trevino, H. Cao, and L. Dal Negro, “Circularly symmetric light scattering from nanoplasmonic spirals,” Nano Lett. 11(5), 2008–2016 (2011). [CrossRef] [PubMed]

14.

C. F. Bohren and D. R. Huffman, Absorption and Scattering of Light by Small Particles (Wiley-VCH, 1986).

15.

J. H. Bruning and Y. T. Lo, “Multiple scattering of EM waves by spheres Part I: Multiple expansion and ray optical solutions,” IEEE Trans. Antennas Propag. 19(3), 378–390 (1971). [CrossRef]

16.

U. Kreibigv and M. Vollme, eds., Optical Properties of Metal Clusters (Springer-Verlag, 1995).

17.

D. W. Mackowski, “Calculation of total cross-sections of multiple sphere clusters,” J. Opt. Soc. Am. A 11(11), 2851–2861 (1994). [CrossRef]

18.

M. Quinten and U. Kreibig, “Absorption and elastic scattering of light by particle aggregates,” Appl. Opt. 32(30), 6173–6182 (1993). [CrossRef] [PubMed]

19.

Y. L. Xu, “Electromagnetic scattering by an aggregate of spheres,” Appl. Opt. 34(21), 4573–4588 (1995). [CrossRef] [PubMed]

20.

U. Fano, “The theory of anomalous diffraction gratings and of quasi-stationary waves on metallic surfaces (Sommerfeld's waves),” J. Opt. Soc. Am. 31(3), 213–222 (1941). [CrossRef]

21.

A. Hessel and A. A. Oliner, “A new theory of Wood's anomalies on optical gratings,” Appl. Opt. 4(10), 1275–1297 (1965). [CrossRef]

22.

S. Zou and G. C. Schatz, “Silver nanoparticle array structures that produce giant enhancements in electromagnetic fields,” Chem. Phys. Lett. 403(1-3), 62–67 (2005). [CrossRef]

23.

V. G. Kravets, F. Schedin, A. V. Kabashin, and A. N. Grigorenko, “Sensitivity of collective plasmon modes of gold nanoresonators to local environment,” Opt. Lett. 35(7), 956–958 (2010). [CrossRef] [PubMed]

24.

A. Christ, S. G. Tikhodeev, N. A. Gippius, J. Kuhl, and H. Giessen, “Waveguide-plasmon polaritons: strong coupling of photonic and electronic resonances in a metallic photonic crystal slab,” Phys. Rev. Lett. 91(18), 183901 (2003). [CrossRef] [PubMed]

25.

B. Luk’yanchuk, N. I. Zheludev, S. A. Maier, N. J. Halas, P. Nordlander, H. Giessen, and C. T. Chong, “The Fano resonance in plasmonic nanostructures and metamaterials,” Nat. Mater. 9(9), 707–715 (2010). [CrossRef] [PubMed]

26.

A. Gopinath, S. V. Boriskina, W. R. Premasiri, L. Ziegler, B. M. Reinhard, and L. Dal Negro, “Plasmonic nanogalaxies: multiscale aperiodic arrays for surface-enhanced Raman sensing,” Nano Lett. 9(11), 3922–3929 (2009). [CrossRef] [PubMed]

27.

A. Gopinath, S. V. Boriskina, B. M. Reinhard, and L. Dal Negro, “Deterministic aperiodic arrays of metal nanoparticles for surface-enhanced Raman scattering (SERS),” Opt. Express 17(5), 3741–3753 (2009). [CrossRef] [PubMed]

28.

B. N. Khlebtsov, V. A. Khanadeev, and N. G. Khlebtsov, “Observation of extra-high depolarized light scattering spectra from gold nanorods,” J. Phys. Chem. C 112(33), 12760–12768 (2008). [CrossRef]

29.

N. G. Khlebtsov, A. G. Melnikov, V. A. Bogatyrev, L. A. Dykman, A. V. Alekseeva, L. A. Trachuk, and B. N. Khlebtsov, “Can the light scattering depolarization ratio of small particles be greater than 1/3?” J. Phys. Chem. B 109(28), 13578–13584 (2005). [CrossRef] [PubMed]

30.

J. W. Goodman, Introduction to Fourier Optics (McGraw-Hill, 1968).

31.

R. Joerger, K. Forcht, A. Gombert, M. Köhl, and W. Graf, “Influence of incoherent superposition of light on ellipsometric coefficients,” Appl. Opt. 36(1), 319–327 (1997). [CrossRef] [PubMed]

32.

H. Fujiwara, Spectroscopic Ellipsometry (John Wiley & Sons, Ltd, 2007).

33.

C. E. Talley, J. B. Jackson, C. Oubre, N. K. Grady, C. W. Hollars, S. M. Lane, T. R. Huser, P. Nordlander, and N. J. Halas, “Surface-enhanced Raman scattering from individual au nanoparticles and nanoparticle dimer substrates,” Nano Lett. 5(8), 1569–1574 (2005). [CrossRef] [PubMed]

34.

L. Novotny and B. Hecht, Principles of Nano-Optics (Cambridge University Press, 2006).

35.

P. B. Johnson and R. W. Christy, “Optical constants of the noble metals,” Phys. Rev. B 6(12), 4370–4379 (1972). [CrossRef]

36.

J. D. Jackson, Classical Electrodynamics (Wiley, 1998).

OCIS Codes
(240.5770) Optics at surfaces : Roughness
(240.6680) Optics at surfaces : Surface plasmons
(240.6690) Optics at surfaces : Surface waves
(290.5855) Scattering : Scattering, polarization
(050.6624) Diffraction and gratings : Subwavelength structures
(240.2130) Optics at surfaces : Ellipsometry and polarimetry

ToC Category:
Optics at Surfaces

History
Original Manuscript: August 16, 2011
Revised Manuscript: September 20, 2011
Manuscript Accepted: September 20, 2011
Published: October 7, 2011

Virtual Issues
Vol. 6, Iss. 11 Virtual Journal for Biomedical Optics

Citation
Gary F. Walsh, Carlo Forestiere, and Luca Dal Negro, "Plasmon-enhanced depolarization of reflected light from arrays of nanoparticle dimers," Opt. Express 19, 21081-21090 (2011)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-19-21-21081


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References

  1. M. L. Brongersma and P. G. Kik, Surface Plasmon Nanophotonics (Springer, 2007).
  2. S. A. Maier, Plasmonics: Fundamentals and Applications (Springer, 2007).
  3. P. Nordlander, C. Oubre, E. Prodan, K. Li, and M. I. Stockman, “Plasmon hybridization in nanoparticle dimers,” Nano Lett.4(5), 899–903 (2004). [CrossRef]
  4. S. Zou, N. Janel, and G. C. Schatz, “Silver nanoparticle array structures that produce remarkably narrow plasmon lineshapes,” J. Chem. Phys.120(23), 10871–10875 (2004). [CrossRef] [PubMed]
  5. E. M. Hicks, S. Zou, G. C. Schatz, K. G. Spears, R. P. Van Duyne, L. Gunnarsson, T. Rindzevicius, B. Kasemo, and M. Käll, “Controlling plasmon line shapes through diffractive coupling in linear arrays of cylindrical nanoparticles fabricated by electron beam lithography,” Nano Lett.5(6), 1065–1070 (2005). [CrossRef] [PubMed]
  6. V. G. Kravets, F. Schedin, and A. N. Grigorenko, “Extremely narrow plasmon resonances based on diffraction coupling of localized plasmons in arrays of metallic nanoparticles,” Phys. Rev. Lett.101(8), 087403 (2008). [CrossRef] [PubMed]
  7. B. Auguié and W. L. Barnes, “Collective resonances in gold nanoparticle arrays,” Phys. Rev. Lett.101(14), 143902 (2008). [CrossRef] [PubMed]
  8. Y. Chu, E. Schonbrun, T. Yang, and K. B. Crozier, “Experimental observation of narrow surface plasmon resonances in gold nanoparticle arrays,” Appl. Phys. Lett.93(18), 181108 (2008). [CrossRef]
  9. L. Dal Negro, N.-N. Feng, and A. Gopinath, “Electromagnetic coupling and plasmon localization in deterministic aperiodic arrays,” J. Opt. A, Pure Appl. Opt.10(6), 064013 (2008). [CrossRef]
  10. C. Forestiere, M. Donelli, G. F. Walsh, E. Zeni, G. Miano, and L. Dal Negro, “Particle-swarm optimization of broadband nanoplasmonic arrays,” Opt. Lett.35(2), 133–135 (2010). [CrossRef] [PubMed]
  11. C. Forestiere, G. F. Walsh, G. Miano, and L. Dal Negro, “Nanoplasmonics of prime number arrays,” Opt. Express17(26), 24288–24303 (2009). [CrossRef] [PubMed]
  12. A. Gopinath, S. V. Boriskina, N.-N. Feng, B. M. Reinhard, and L. Dal Negro, “Photonic-plasmonic scattering resonances in deterministic aperiodic structures,” Nano Lett.8(8), 2423–2431 (2008). [CrossRef] [PubMed]
  13. J. Trevino, H. Cao, and L. Dal Negro, “Circularly symmetric light scattering from nanoplasmonic spirals,” Nano Lett.11(5), 2008–2016 (2011). [CrossRef] [PubMed]
  14. C. F. Bohren and D. R. Huffman, Absorption and Scattering of Light by Small Particles (Wiley-VCH, 1986).
  15. J. H. Bruning and Y. T. Lo, “Multiple scattering of EM waves by spheres Part I: Multiple expansion and ray optical solutions,” IEEE Trans. Antennas Propag.19(3), 378–390 (1971). [CrossRef]
  16. U. Kreibigv and M. Vollme, eds., Optical Properties of Metal Clusters (Springer-Verlag, 1995).
  17. D. W. Mackowski, “Calculation of total cross-sections of multiple sphere clusters,” J. Opt. Soc. Am. A11(11), 2851–2861 (1994). [CrossRef]
  18. M. Quinten and U. Kreibig, “Absorption and elastic scattering of light by particle aggregates,” Appl. Opt.32(30), 6173–6182 (1993). [CrossRef] [PubMed]
  19. Y. L. Xu, “Electromagnetic scattering by an aggregate of spheres,” Appl. Opt.34(21), 4573–4588 (1995). [CrossRef] [PubMed]
  20. U. Fano, “The theory of anomalous diffraction gratings and of quasi-stationary waves on metallic surfaces (Sommerfeld's waves),” J. Opt. Soc. Am.31(3), 213–222 (1941). [CrossRef]
  21. A. Hessel and A. A. Oliner, “A new theory of Wood's anomalies on optical gratings,” Appl. Opt.4(10), 1275–1297 (1965). [CrossRef]
  22. S. Zou and G. C. Schatz, “Silver nanoparticle array structures that produce giant enhancements in electromagnetic fields,” Chem. Phys. Lett.403(1-3), 62–67 (2005). [CrossRef]
  23. V. G. Kravets, F. Schedin, A. V. Kabashin, and A. N. Grigorenko, “Sensitivity of collective plasmon modes of gold nanoresonators to local environment,” Opt. Lett.35(7), 956–958 (2010). [CrossRef] [PubMed]
  24. A. Christ, S. G. Tikhodeev, N. A. Gippius, J. Kuhl, and H. Giessen, “Waveguide-plasmon polaritons: strong coupling of photonic and electronic resonances in a metallic photonic crystal slab,” Phys. Rev. Lett.91(18), 183901 (2003). [CrossRef] [PubMed]
  25. B. Luk’yanchuk, N. I. Zheludev, S. A. Maier, N. J. Halas, P. Nordlander, H. Giessen, and C. T. Chong, “The Fano resonance in plasmonic nanostructures and metamaterials,” Nat. Mater.9(9), 707–715 (2010). [CrossRef] [PubMed]
  26. A. Gopinath, S. V. Boriskina, W. R. Premasiri, L. Ziegler, B. M. Reinhard, and L. Dal Negro, “Plasmonic nanogalaxies: multiscale aperiodic arrays for surface-enhanced Raman sensing,” Nano Lett.9(11), 3922–3929 (2009). [CrossRef] [PubMed]
  27. A. Gopinath, S. V. Boriskina, B. M. Reinhard, and L. Dal Negro, “Deterministic aperiodic arrays of metal nanoparticles for surface-enhanced Raman scattering (SERS),” Opt. Express17(5), 3741–3753 (2009). [CrossRef] [PubMed]
  28. B. N. Khlebtsov, V. A. Khanadeev, and N. G. Khlebtsov, “Observation of extra-high depolarized light scattering spectra from gold nanorods,” J. Phys. Chem. C112(33), 12760–12768 (2008). [CrossRef]
  29. N. G. Khlebtsov, A. G. Melnikov, V. A. Bogatyrev, L. A. Dykman, A. V. Alekseeva, L. A. Trachuk, and B. N. Khlebtsov, “Can the light scattering depolarization ratio of small particles be greater than 1/3?” J. Phys. Chem. B109(28), 13578–13584 (2005). [CrossRef] [PubMed]
  30. J. W. Goodman, Introduction to Fourier Optics (McGraw-Hill, 1968).
  31. R. Joerger, K. Forcht, A. Gombert, M. Köhl, and W. Graf, “Influence of incoherent superposition of light on ellipsometric coefficients,” Appl. Opt.36(1), 319–327 (1997). [CrossRef] [PubMed]
  32. H. Fujiwara, Spectroscopic Ellipsometry (John Wiley & Sons, Ltd, 2007).
  33. C. E. Talley, J. B. Jackson, C. Oubre, N. K. Grady, C. W. Hollars, S. M. Lane, T. R. Huser, P. Nordlander, and N. J. Halas, “Surface-enhanced Raman scattering from individual au nanoparticles and nanoparticle dimer substrates,” Nano Lett.5(8), 1569–1574 (2005). [CrossRef] [PubMed]
  34. L. Novotny and B. Hecht, Principles of Nano-Optics (Cambridge University Press, 2006).
  35. P. B. Johnson and R. W. Christy, “Optical constants of the noble metals,” Phys. Rev. B6(12), 4370–4379 (1972). [CrossRef]
  36. J. D. Jackson, Classical Electrodynamics (Wiley, 1998).

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