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Design of Plasmonic Nanostructures for Improving the Detection in Raman Spectroscopy

Khajeahsani, Mohammad Sadegh | 2016

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  1. Type of Document: Ph.D. Dissertation
  2. Language: Farsi
  3. Document No: 49189 (05)
  4. University: Sharif University of Technology
  5. Department: Electrical Engineering
  6. Advisor(s): Rashidian, Bizhan; Shahmansouri, Afsaneh
  7. Abstract:
  8. Excitation of Localized Surface Plasmons, which is one of the features of the metal nanostructures, results in the confinement of light in the dimensions smaller than the incidence wavelength, and even lower than the fundamental diffraction limit. This property causes electric field enhancement at surface, helping the occurrence of Surface Enhanced Raman Scattering (SERS). The amount of electric field enhancement and its resonance frequency depend on the type of material, its geometrical properties and its shape, and the arrangement of the scattering particles. Also, properties of surrounding environment material and the type of the excited mode of the structure, and the incidence wave affect this behavior. In this thesis, the occurrence of Lorentzian and FANO resonances in plasmonic structures is investigated phenomenologically. For this purpose in the first step, by exploiting semi-analytical T-matrix method, a new analytical method for calculation of eigenmodes of a scattering structure is introduced. In this method each eigenmode is characterized by two quantities. An eigenvalue, which may represent the reflection coefficient or system transfer function, and an eigenvector which contains the coefficients of the electric field expansion of the eigenmode. In the second step, abased on the calculated eigenmodes, a criterion is established for identification of the type of resonance points, either being Lorentzian, or and FANO type. Similarities with the well-known resonances in simple physical systems such as RLC circuits are addressed. Next, the field expansion of a mode (a linear combination of eigenmodes) is obtained using the fundamental orthogonality relation of the eigenmodes. This expansion leads to deriving analytical modal scattering and absorption power formula. An important outcome of these modal power formula, is the capability of analytical modeling of interference interaction between eigenmodes, which is reported for the first time here. In addition, it is shown that the presented model is in accord to the common accurate definitions of FANO resonance phenomena in the plasmonic nanostructures. Also, it has been shown that due to the analytic form of the presented model, engineering of the incidence wave to achieve the optimized modal behavior is feasible. By using the techniques developed for finding T-matrix, a software has been developed in MATLAB, for analysis of electromagnetic wave scattering problem from structures containing arbitrary number of spherical boundaries, By implementing the eigenmode extraction method and the presented model for analysis of wave and matter interaction in this software, a powerful tool has been provided for analysis and design of the above mentioned plasmonic nanostructures. As an example, a three layers nonconcentric nanoshell structure is designed to achieve a mode with FANO resonance characteristic, and the necessary condition for excitation of this mode has been obtained, by using the developed software. The scattering power spectra have been calculated, under two different incidence waves, which are superposition of two different plane waves, and it has been shown that the optimized mode can be excited by tailoring the incidence wave
  9. Keywords:
  10. Fano Resonances ; Surface Enhanced Raman Scattering (SERS) ; PLasmonic Structure ; Localized Surface Plasmons ; T-Matrix Method ; Eigen Modes

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