Chapter 7 Conclusions
The controllable and tunable surface plasmon excitation of metallic nanoparticles was studied analytically and numerically by Mie scattering theory and FDTD simulation method, respec- tively. For a single particle system, optical properties of nanoshells and nanocylinders were investigated analytically by the theory for scattering of a plane wave by a coated nanosphere or a coated nanocylinder. According to our calculated results, the polarization charge distri- bution on the surface of a coated silver nanocylinder is changed from a dipole-like distribution to a quadrupole-like distribution by changing the permittivity of the coated material. It is meant that the quadrupole contribution was enhanced and led in the local-field enhancement of the coated particle. In other words, the local-field enhancement around the coated silver nanocylinder can be manipulated and the higher-mode contribution to the enhanced local field became significant by controlling the permittivity of the coated material.
The higher mode or called the whispering-gallery mode with n = 9 of a high-permittivity sphere of rcore = 300 nm was excited by an incident plane wave at λ = 400 nm. The TE- like and TM-like-mode whispering-gallery mode were observed and exhibited different near- field distributions when the permittivities of the sphere are 7.18 and 8.14, respectively. The whispering-gallery mode of a coated metallic sphere was presented and the field intensity was influenced strongly by the radius and permittivity of the core material. The field intensity of the whispering-gallery mode of a coated silver sphere decreased dramatically with increasing radius of the silver core and the whispering-gallery mode pattern vanished when the radius of the core is larger than 130 nm. For future study, the whispering-gallery mode could be treated as a high spatial frequency (high k vector) light source and used to excite the controllable
higher mode surface plasmon resonance directly on the surface of metallic nanoparticles.
In chapter 4, the surface plasmon behaviors of an asymmetric nanocylinder pair and pair- array structure of silver nanocylinder were studied using a two-dimensional FDTD simulation.
For an asymmetric pair, TM-mode near-field distributions showed the high local-field enhance- ment appeared in the gap between nanocylinder pairs as well as on the other side of the smaller nanocylinder. With changing radius ratio of the two nanocylinders, the nano-focusing effect was observed and that associates with the asymmetric polarization charge distribution on the surface of the asymmetric nanocylinder pair. Besides, by controlling the illumination direction of incident light and the radius ratios of nanocylinder pairs, the confined polarization charges will lead to the local-field enhancement in the gap and became concentrated when the asym- metric polarization charges were induced abundantly on the surface around the gap by the mutual interaction between nanocylinders. Poynting vector showed that energy flows can be effectively concentrated on a nanoscale gap between the asymmetric nanocylinder pair and the direction of the energy flows in the vicinity of nanocylinders can be controlled by changing the illumination direction. The far-field signals indicated that the polarization charge distribution were similar to a dipole-like distribution and the far-field optical responses were similar to those from dipole-dipole interaction.
In section 4.2, we presented the numerical results of controlling surface plasmon excitation and local-field confinement of silver pair arrays. An array structure which is based on silver nanocylinder pairs was proposed to study extensively the localized surface plasmon excitation in the gaps of the adjacent silver nanocylinders. TM-mode near-field distributions demonstrated that high local-field enhancement appears in the vicinity of silver nanocylinders and light can be confined effectively in the gap of each pair due to surface plasmon excitation. The local- field enhancements due to pair-pair interaction were observed in our simulations. For a two- pair array, local-field enhancement due to asymmetric polarization charge distribution were observed by changing different illumination directions of incident light. The enhanced local fields became more concentrated and more stronger as compared with the single pair structure in section 4.2.2 since the multi-particle interaction could strengthen the contribution of higher multipole to the enhanced local fields in the gaps. Besides, the interpair distance of four-pair arrays could be tuned to control the intensities of enhanced local field in the gap of each pair.
The controllable and predictable surface plasmon resonance behaviors were observed in three- pair array structures. By analyzing the phase distribution of the electric field, that resonant behaviors can be understood by a simplified open cavity model. For future study, a pair-array structure can be used as a waveguide structure in nanoscale region. Compared with the linear metallic chain structure [53], electromagnetic energy in near-field region can be transported along the gaps of pairs of pair-array structures, thus the energy loss on the metallic surface can be reduced effectively during the transport process and a nanoscale waveguide with high transmission rate can be obtained.
Localized surface plasmon excitation of metallic nanoparticles have been applied to high- density near-field optical disks and biosensor. In chapter 5, the mechanism of AgOx-type near- field optical disks was studied systematically by FDTD method. The collective effect of local- field enhancement due to localized surface plasmon of randomly distributed Ag nanoparticles in AgOx layer became main element to the superresolution of an AgOx-type near-field optical disk. Fourier optics approach combined with angular spectrum representation was proposed to provide a theoretical background to understand the superresolution of a near-field optical disk. Randomly distributed Ag nanoparticles in AgOx layer could be represented by a random optical function with random and broader spatial spectrum and the evanescent components from subwavelength recording marks could be convoluted by the random optical function into propagating components and became the detectable far-field signals. The intensity of far-field signals of an AgOx-type near-field optical disk can be enhanced by the local-field enhancement around the Ag nanoparticles and the subwavelength recording marks smaller than λ/10 were distinguished i.e the resolution beyond the optical diffraction limit could be obtained. Actually, the optical function of the AgOx layer with random Ag nanoparticles was complicated. The near-field coupling between the Ag nanoparticles would influence the optical function of the near-field active layer of a near-field optical disk.
Influences of distributions of metallic nanoparticles in near-field active layer on the resolu- tion capability of near-field optical disks were studied to obtain a high resolution performance near-field optical disk. The far-field signals of specific mark size can be selectively controlled and enhanced by periodic Ag nanoparticles or nanoclusters. When the period of the periodic nanoparticles or nanoclusters matched with the period of the subwavelength recording marks,
the evanescent components corresponding to the period of recording marks can be convoluted into propagating components and the far-field detectable signals would be enhanced also. Be- sides, the distributions of metallic nanoparticles in near-field active layer can be optimized artificially by doping or sputtering process and the near-field active layer can be formed by a metal-dielectric nanocomposite thin layer. The strongest enhanced local fields appeared in the center of the thin layer due to the collective effect of localized surface plasmon of Au nanopar- ticles or nanoclusters. The variations of the far-field signals of the subwavelength recording marks became smooth relatively because of the distributions of the enhanced local field of the nanocomposite layer were uniform as compared with the AgOx-type near-field optical disks.
Therefore, a higher intensity and stable far-field signal of a near-field optical disk could be obtained with using a metal-dielectric nanocomposite near-field active layer.
Next, the influences of the collective effect of localized surface plasmon of random-distributed Ag nanoparticles on the near-field and far-field optical responses of near-field optical disks were studied at different illuminating frequencies. When an AgOx-type near-field optical disk was illuminated by incident light with different frequencies, different near-field distributions of en- hanced local field appear around Ag nanoparticles due to the excitation of localized surface plasmon at different frequencies. The simulation results showed that the resolution of subwave- length recording marks was influenced directly by the local-field enhancement of Ag nanoparti- cles and the far-field signals from randomly distributed Ag nanoparticles were closely related to the scattering efficiency of single Ag nanoparticle. However, the peaks of random nanoparticles were shifted to the longer wavelength than those of single Ag nanoparticle due to the complex particle-particle interaction.
Recently, several studies indicate that the readout mechanism of near-field optical disks associates with laser-induced local material change (phase change or chemical reaction) of the near-field active layer of near-field optical disks. Therefore the temperature distribution of near-field optical disks become an important information in a readout process as well as the electric-field distribution. For future study, a numerical method consisting of a heat transport simulation model and a FDTD method will be used to simulate the optical and thermal be- haviors of near-field optical disks simultaneously. If the physical or chemical properties of a near-field optical disk are changed by laser induced heating, the material property change of
the optical disk will feedback into an electromagnetic simulation method like a FDTD method.
Thus, not only the resolution capability but also the readout thermal stability of a near-field optical disk can be considered in our simulation study.
In chapter 6, the fluorescence signal enhancement of the LSPCF fiber-optic biosensor with Au nanoparticles was studied by the scattering theory of evanescent waves by a single Au nanoparticle when the distance between nanoparticles and interface of fiber d was larger than 2r. The fluorescence signal enhancement associated with the local-field enhancement of Au nanoparticles due to localized surface plasmon excitation. According to our calculations, averaged-field intensity around an Au nanoparticle, which was illuminated by evanescent waves from an uncladded fiber’s surface can be enhanced about 8 times of the field intensity without Au nanoparticle. The calculated results were consistent with the experimental measurements in reference [72]. Although we did not choose the resonance frequency of Au nanoparticles as the frequency of incident light since the fluorescence signals of the LSPCF fiber-optic biosensor would be absorbed by Au nanoparticles when the emission frequency of fluorescence signals were near to the resonance frequency of Au nanoparticles, the local field around Au nanoparti- cles still can be enhanced few times of the field intensity without nanoparticles due to surface plasmon excitation. The first-order approximation provides a qualitative theoretical under- standing of the mechanism of the fluorescence signal enhancement of the LSPCF fiber-optic biosensor.
