Tuning optical properties in random arrays of plasmon resonant nanoparticles

University of Technology, Sydney. Dept. of Applied Physics.Small conductive particles show a resonant behaviour at wavelengths where bulk or thin film samples have no features. This resonance is caused by the collective oscillation of the free electrons in the particle and is called localised surface plasmon resonance. It is influenced by the shape of the particle, the surrounding medium and particle interaction. I studied shape, matrix and interaction effects of metallic and metal-like particles in various systems with the aim to rationally tune the resonance to specific wavelengths for different applications. Dilute samples of small LaB6 particles were studied with regard to their NIR blocking performance. My analysis showed that they are more efficient than the alternative materials ITO and ATO. This is mainly due to the position of the LaBg particle resonance, which lies precisely in the spectral region which needs to be blocked (around 1 /mi). I was able to model the optical properties of the window samples, using a dilute quasi-static approach for anisotropic particles. Different embedding matrices and particle interaction have also an influence on the localised surface plasmon resonance. An example for a combination of matrix and interaction effects is a self-assembled gold particle him with organic linkers. Structural effects were especially important in these films, as was verified by electron microscopy. The optical properties were successfully modeled, using a two level effective medium approximation. A different way to tune the resonance is to change the shell thickness to core size ratio in metallic nanoshells. The resulting spectral shift, though, is limited by experimental realities for the metal coating and the onset of scattering for larger particles. The shell has two resonances, of which the low energy one can be tuned by the ratio mentioned above. This resonance also shows a different electric field profile to the normal dipole (and high energy shell) resonance. The field pattern also highlights a strong field gradient across the external shell interface and along the incident polarisation direction. The properties were calculated using Mie theory and the quasi-static approximation. Finally, the far and near-field optical properties of thin silver films with randomly distributed holes were studied. They showed an enhanced absorption, due to coupling of the incident light into surface plasmon polaritons by the holes. Whereas the films did not show the enhanced transmission, which occurs in regular hole arrays, they still might provide some insight in the processes involved by helping to exclude some possible explanations

Field profiles for spherical conductive nanoparticles and metallic-shell/dielectric-core nano-composites

Profiles of the electric field strength |E|2/|E 0|2 for spherical metallic shells on a dielectric core are presented both inside the particle and outside. The dependence of the near-field strength and extent on shell thickness and total particle size is discussed qualitatively. Although the internal fields inside the shell and in the core are larger than for homogeneous particles, for not too thick shells, this does not translate into a stronger near-field away from the surface of the shell. The fields inside the shell, at the low energy resonance and close to it, are rotated by π/2 with respect to fields inside homogeneous particles, which means that the maximum field strengths in the shell are perpendicular to the incident polarisation. This follows from the fact that the low energy resonance for a shell is for the largest dipole moment of the whole system, which compensates the incident field. The largest moment is created when the same charges are collected at both interfaces (shell/medium and core/shell) along the incident polarisation. This creates regions of low field densities at the poles along the incident polarisation, because same charge fields repel each other. Following from that, the field lines are bunched up at the perpendicular poles, creating large field line densities and hence large fields at these points. The case for opposite charges across the interfaces creates the high energy, antisymmetric resonance

Evaluation of the limits of resonance tunability in metallic nanoshells with a spectral averaging method

Spectral selectivity based on tuning the surface plasmon resonance in metallic nanoshells by variation of the relative shell thickness is shown to be limited by the interplay between scattering and absorption. To achieve resonance energies in the near infrared and infrared, relatively large cores are needed, which lead to strong and broad scattering bands and multipolar contributions in the visible. The scattering contribution to extinction is described with a new parameter SΛ, which is defined for a wavelength range of interest Λ. This parameter can help in designing materials for specific applications where scattering is either hindering, as in near-infrared absorbers for visually clear windows, or actually desired, such as in particle-array-based sensors. © 2005 Optical Society of America

Optical response of nanostructured metal/dielectric composites and multilayers

The homogeneous optical response in conducting nanostructured layers, and in insulating layers containing dense arrays of self assembled conducting nanoparticles separated by organic linkers, is examined experimentally through their effective complex indices (n*, k*). Classical effective medium models, modified to account for the 3-phase nanostructure, are shown to explain (n*, k*) in dense particulate systems but not inhomogeneous layers with macroscopic conductance for which a different approach to homogenisation is discussed, (n*, k*) data on thin granular metal films, thin mesoporous gold, and on thin metal layers containing ordered arrays of voids, is linked to properties of the surface plasmon states which span the nanostructured film. Coupling between evanescent waves at either surface counterbalanced by electron scattering losses must be considered. Virtual bound states for resonant photons result, with the associated transit delay leading to a large rise in n* in many nanostructures. Overcoating n-Ag with alumina is shown to alter (n*, k*) through its impact on the SP coupling. In contrast to classical optical homogenisation, effective indices depend on film thickness. Supporting high resolution SEM images are presented

Optical properties of dense self-assembled gold nanoparticle layers with organic linker molecules

Films consisting of self-assembled gold nanoparticles cross-linked with alkane-dithiols were prepared by a filtration method and studied with scanning electron microscopy to determine the structure of the films and spectrophotometry and ellipsometry to ascertain their optical properties. The structural characterization showed the existence of nanometer-sized voids within the films. This previously unmentioned feature is responsible for the previous difficulties in modelling the optical properties with effective medium models. This can be remedied, using a two-tiered hierarchical effective medium model, which takes into account the existence of the voids. Using this model we were able to fit the experimental data, with only the void volume fraction to be determined by the overall fit, while the gold volume fraction in the linker medium is fixed by the wavelength of the resonance peak. Our model should be applicable to all such films, when the deposition method, which determines the microstructure, is properly taken into account

Plasmonically Enhanced Reflectance of Heat Radiation from Low-Bandgap Semiconductor Microinclusions

Increased reflectance from the inclusion of highly scattering particles at low volume fractions in an insulating dielectric offers a promising way to reduce radiative thermal losses at high temperatures. Here, we investigate plasmonic resonance driven enhanced scattering from microinclusions of low-bandgap semiconductors (InP, Si, Ge, PbS, InAs and Te) in an insulating composite to tailor its infrared reflectance for minimizing thermal losses from radiative transfer. To this end, we compute the spectral properties of the microcomposites using Monte Carlo modeling and compare them with results from Fresnel equations. The role of particle size-dependent Mie scattering and absorption efficiencies, and, scattering anisotropy are studied to identify the optimal microinclusion size and material parameters for maximizing the reflectance of the thermal radiation. For composites with Si and Ge microinclusions we obtain reflectance efficiencies of 57 - 65% for the incident blackbody radiation from sources at temperatures in the range 400 - 1600 {\deg}C. Furthermore, we observe a broadbanding of the reflectance spectra from the plasmonic resonances due to charge carriers generated from defect states within the semiconductor bandgap. Our results thus open up the possibility of developing efficient high-temperature thermal insulators through use of the low-bandgap semiconductor microinclusions in insulating dielectrics.Comment: Main article (8 Figures and 2 Tables) + Supporting Information (8 Figures

Dilute LaB<inf>6</inf> nanoparticles in polymer as optimized clear solar control glazing

The performance of window samples with a LaB6 nanoparticle-doped polymer laminate in the reduction of solar heat gain was studied. The reason behind the near-infrared absorption was the excitation of surface plasmons. Windows, considered suitable for solar control glazing, transmitted visible radiation but blocked the near infrared (NIR) between 750 and 2500 nm