Morphology and plasmonic properties of self-organized arrays of ...

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92 CHAPTER 5. MODELLING AND ANALYSIS OF THE OPT. PROP.55Re[]432103xxε effyyε effzzε effRe[]43213Im[]2Im[]211020040060080010001200020040060080010001200 [nm] [nm]Figure 5.13: Real and imaginary parts of the computed principal components of theeffective dielectric tensor for the square (left panel) and rectangular (right panel) samples.of the imaginary part. It should therefore be expected for the LSP peaks observed inreflection to be red-shifted with respect to the corresponding minima of transmissivity.In order to better clarify the effect of the NPs shape dispersion, the R S and R P spectracalculated in absence of L spread are also reported in fig. 5.11 and fig. 5.12 as the thindashed lines. Comparing these curves to the ones calculated for σ L > 0, we can see that,introducing a dispersion of depolarization factors, both the L and T LSP modes redshift,and the resonances become weaker and broader. These effects are especially pronouncedfor the L mode of the square sample, i.e. for elongated particles along the major axis, whilethe corresponding T LSP peak is only slightly modified; for the rectangular sample thefinite dispersion effects are instead equally observed for all the involved plasmon modes.DiscussionIn order to fully exploit the possibility of engineering the plasmonic response of the selforganizedarrays, the intrinsic single particles properties and the effects of the dipolarcoupling on the LSP resonances must be clearly highlighted. In particular, we are interestedin understanding how the parameters of fabrication of our samples (substratetemperatures, thickness of deposited gold) affect the LSP, and the relative weights ofthe intrinsic and collective effects on the optical anisotropy. Calculating the evolution ofthe LSP peak wavelength as a function of increasing array dimensionality, i.e. graduallyintroducing mutual interactions in the system along and across the ripples, provides aparticularly simple way to achieve this understanding.We therefore computed the optical response of the square and rectangular samples forthree steps of increasing dimensionality: no EM coupling between NPs (0D, equivalent toisolated NPs), EM coupling between NPs allowed only within the single chains (1D, wherea “chain” is defined as being oriented along the LiF ridges), and EM coupling between allNPs in the array (2D). The variation in the (theoretical) position of the LSP will provideinteresting clues about the mechanism of EM coupling in the system.
5.2. 2-DIMENSIONAL ARRAYS OF GOLD NANOPARTICLES 93LSP L mode LSP T modeR Sa.R S0.180.140.104000.300.250.200.155000D1D2D600 [nm]7000D1D2D800LSPR position [nm]L/T gap [nm]LSPR width [nm]6005755505257550250150100LTLTb.400500600 [nm]700800c.0D 1D 2DFigure 5.14: Panels a, b: R S reflectivity spectra computed as a function of the systemdimensionality for the “square” sample, in parallel (a) and perpendicular (b) optical configurations.Panelc: wavelengthpositionandlinewidthoflongitudinal(solidmarkers)andtransverse (open markers) LSP resonances and their splitting (diamonds) as a function ofthe system dimensionality, deduced from the R S spectra in panels (a), (b).In fig. 5.14 and fig. 5.15 we report the calculated R S , in parallel (panels (a)) andperpendicular (panels (b)) optical configurations (cfr. fig. 4.5), as a function of the systemdimensionalityforthesquareandtherectangularcase, respectively; theNPsarrangementssketched in fig. 5.11(e) and fig. 5.12(e) were employed for the calculations. In panels (c)of fig. 5.14 and fig. 5.15 we also summarize the values of the LSP position (λ S T,P , solid andopen squares) and width (Γ S T,P , solid and open circles) as a function of the dimensionality,along with the L/T modes splitting (solid diamonds).From a quick inspection of the graphs, it can be clearly seen that the LSPs wavelengthand their corresponding width evolve as a function of the system’s dimensionality. As ageneral trend, the variations of both λ S and Γ S are not monotonous, and different for theT and L modes.Let us first discuss in depth the square case: there, the particles are equally spacedand ellipsoidal, with the longest axis parallel to the LiF ridges.0D Due to the anisotropic shape of the particles, the 0D R S spectra (red curves infig. 5.14(a, b)) already exhibit intrinsically non-degenerate peaks, located at λ 0 L =567 nm and λ 0 T = 527 nm, for exciting field parallel and transverse to the LiF ridges,
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Università degli studi di GenovaFa
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4 CONTENTS5.2.2 Optical anisotropy
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6 Introductionconfining the EM ener
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8 Introduction
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10 CHAPTER 1. THEORYEλBkFigure 1.1
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12 CHAPTER 1. THEORYfrom which, sep
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14 CHAPTER 1. THEORY≈ ω 0 like
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16 CHAPTER 1. THEORYε 1 (λ) = N 2
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18 CHAPTER 1. THEORYThe correspondi
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20 CHAPTER 1. THEORYso the induced
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22 CHAPTER 1. THEORYε MG −ε aε
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24 CHAPTER 1. THEORYUV range, this
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26 CHAPTER 1. THEORYwhich is called
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28 CHAPTER 1. THEORYwhere Γ 0 = v
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30 CHAPTER 1. THEORYpoints of the c
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32 CHAPTER 1. THEORYand then monoto
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34 CHAPTER 1. THEORYposition, has t
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36 CHAPTER 1. THEORYfieldorequivale
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38 CHAPTER 1. THEORY
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40 CHAPTER 2. EXPERIMENTAL METHODSt
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Page 44 and 45: 44 CHAPTER 2. EXPERIMENTAL METHODS2
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Page 48 and 49: 48 CHAPTER 2. EXPERIMENTAL METHODSt
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Page 104 and 105: 104 Conclusionsparticular cases of
Page 106 and 107: 106 Acknowledgements
Page 108 and 109: 108 BIBLIOGRAPHY[14] Shouheng Sun,
Page 110 and 111: 110 BIBLIOGRAPHY[44] Q A Pankhurst,
Page 112 and 113: 112 BIBLIOGRAPHY[74] Prashant K. Ja
Page 114 and 115: 114 BIBLIOGRAPHY[103] Tobias Kraus,
Page 116 and 117: 116 BIBLIOGRAPHY[137] F. Brouers, J
Page 118 and 119: 118 BIBLIOGRAPHY[169] Christy L. Ha
Page 120 and 121: 120 BIBLIOGRAPHY[201] G. Baldacchin
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