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

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66 CHAPTER 4. SELF-ORGANIZED NPS ARRAYS: OPTICAL PROP.0.16R P0.040.02R P||R S0.12R S┴0.080.0040080012001600a. [nm]b.400800 [nm]120016000.15R S||0.030.02R p┴R S0.10R P0.010.0540080012001600c. [nm]d.0.00400800 [nm]12001600p1.0psR0.5R SR Pse.400800 1200 [nm]1600Figure 4.6: Sets of reflectivities at an angle of incidence θ = 50 ◦ for s- and p-polarized incidentlight, measured on various gold “nanowires” samples (t Au ≈ 5 nm) on nanopatternedLiF(110). Black curves: Λ = 45 nm; red curves: Λ = 40 nm; blue curves: Λ = 25 nm.Plane of incidence either parallel (left panels) or perpendicular (right panels) to the LiFridges. Exciting electric field along (panels a, b) and transverse (panels c, d) to the goldwires. Panel e: s- and p-polarized reflectivities of bulk gold.curve) and perpendicular (black curve) to the ripples.In full analogy with ellipsometric data, a strong optical anisotropy is observed also inthe reflectivity/transmission spectra, when the electric field oscillates either perpendicularor parallel the ripple direction: in the first case (R ||S and R⊥ P ) the reflectivities are peakedat λ ≈ 600 nm and then decrease at longer wavelengths; at variance with this, RS ⊥ andR || Phave an higher intensity, with a very broad maximum around 1000 nm. Both of thesetrends are quite different from the bulk reference, where the reflectivity approaches unityfor λ 600 nm. Considering the transmission, the same features observed as enhancedreflectivity are observed as minima of the transmitted intensity: for light polarized alongthe ripples, the transmittance is as low as 0.4 over a large region from 800 nm to thelargest wavelength (cfr. the transmittance of the bare substrate in fig. 4.1(a)); when theelectric field is instead perpendicular to the ripples, the overall transmitted intensity ishigher, but shows a sharp minimum at λ ≈ 650 nm.With the support of the AFM data, we can safely assign the peaks of reflectivityand the minima of transmittance to the occurrence of plasmonic resonances inside the
4.2. OPTICAL PROPERTIES OF AU NANOPARTICLES ARRAYS 670.80.7T0.60.5||0.4a.0.34006008001000 [nm]120014001600T ||1.0T ┴0.90.8T0.70.6||0.5b.0.44006008001000 [nm]120014001600Figure 4.7: Transmissivity spectra measured at normal incidence on a nanopatternedLiF(110) (Λ ≈ 25 nm) after the deposition of t Au ≈ 5 nm gold at T = 100 ◦ C (panel (a))and subsequent annealing at T = 400 ◦ C (panel (b)). The transmissivities were acquiredwith linearly polarized light, parallel (red curves) and perpendicular (black curves) to theLiF ridges.gold grains. In the direction perpendicular the ripples, the grains are well separatedby the ripples ridges, so the electrons are effectively confined and the LSP peaks quitesharp. On the contrary, inside each groove the Au grains can accommodate as close aspossible to one another, and there are no constraints on their dimensions; the broadeningof the resonances, when the electric field is parallel the ripples, can then be attributedin part to the dispersion of the grains lengths, so that the observed peak is actually thesuperimposition of several peaks slightly shifted in wavelength, in part to the coalescenceof several contiguous grains, which reduces the electronic confinement restoring a metallicbehaviour somewhat analogous to a bulk sample. Thus, we will refer to this class ofsamples as gold “nanowires”.Having measured the reflectivities, we can now give a qualitative interpretation of theΨ spectra, which are roughly equivalent to the ratio R P /R S between the reflectivities;we consider only the region of interests λ 500 nm. When the plane of incidence isperpendicular to the ripples, Ψ (fig. 4.4(b)) corresponds to the ratio between RP ⊥, peakedat λ ≈ 600 nm, and RS ⊥, slowly varying, so it has a sharp peak similar to R⊥ P but shiftedat a different wavelength, because RS ⊥ is not a constant. On the contrary, Ψ as measuredin || geometry (fig. 4.4(a)) is roughly the ratio between a nearly constant curve (R || P )
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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
Page 16 and 17: 16 CHAPTER 1. THEORYε 1 (λ) = N 2
Page 18 and 19: 18 CHAPTER 1. THEORYThe correspondi
Page 20 and 21: 20 CHAPTER 1. THEORYso the induced
Page 22 and 23: 22 CHAPTER 1. THEORYε MG −ε aε
Page 24 and 25: 24 CHAPTER 1. THEORYUV range, this
Page 26 and 27: 26 CHAPTER 1. THEORYwhich is called
Page 28 and 29: 28 CHAPTER 1. THEORYwhere Γ 0 = v
Page 30 and 31: 30 CHAPTER 1. THEORYpoints of the c
Page 32 and 33: 32 CHAPTER 1. THEORYand then monoto
Page 34 and 35: 34 CHAPTER 1. THEORYposition, has t
Page 36 and 37: 36 CHAPTER 1. THEORYfieldorequivale
Page 38 and 39: 38 CHAPTER 1. THEORY
Page 40 and 41: 40 CHAPTER 2. EXPERIMENTAL METHODSt
Page 44 and 45: 44 CHAPTER 2. EXPERIMENTAL METHODS2
Page 46 and 47: 46 CHAPTER 2. EXPERIMENTAL METHODSs
Page 50 and 51: [001][100]50 CHAPTER 3. SELF-ORGANI
Page 52 and 53: 52 CHAPTER 3. SELF-ORGANIZED NPS AR
Page 76 and 77: 76 CHAPTER 5. MODELLING AND ANALYSI
Page 98 and 99: 98 CHAPTER 6. COMPOSITE MEDIA BASED
Page 100 and 101: 100 CHAPTER 6. COMPOSITE MEDIA BASE
Page 102 and 103: 102 CHAPTER 6. COMPOSITE MEDIA BASE
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,
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116 BIBLIOGRAPHY[137] F. Brouers, J
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118 BIBLIOGRAPHY[169] Christy L. Ha
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120 BIBLIOGRAPHY[201] G. Baldacchin
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