Modelling of plasmonic systems:advanced numerical methods and applications

Metallic nanostructures interact in complex ways with light, forming the subject of plasmonics and bringing novel physical phenomena and practical applications. The fundamental and practical importance of plasmonics necessitates the development of a multitude of simulation techniques. Surface integral equation (SIE) is a numerical method which is particularly suited for simulating many plasmonic systems. In this thesis, we develop SIE-based numerical methods for plasmonics and use them to study plasmonic systems of interest. Electric and magnetic surface currents are the basic quantities calculated in SIE, and it is appealing to directly compute various physical quantities directly using them. We develop a formalism to compute optical forces and torques, polarisation charges and multipole moments using the surface currents for better accuracy and efficiency. Numerical simulation is all about finding the right balance between accuracy and computational cost. SIE allows to choose this tradeoff in computing the integrals for the simulation matrix. We study the effect of the integration routine on the accuracy of the matrix and propose an optimised recipe for evaluating the integrals. Although this recipe incurs an overhead, we show how it becomes necessary in computing some physical quantities and simulating some systems, and how it allows simulations using a coarser discretisation. One drawback of SIE is that it can only simulate domains for which the response of each domain can be expressed in terms of the Green's function for the domain. Only homogeneous and periodic domains could be dealt with till now, limiting its applicability. We extend SIE to simulate nanostructures embedded in the layers of a stratified medium to partly overcome this restriction, paving the way for further improvements. SIE has the ability to model complex and realistic geometries. We exploit this feature to study the effect of fabrication-induced rounding on nanorods and gap antennae. We show how rounding results in blue shift of resonances, migration of charges from corners to edges to faces, and reduced coupling between nanostructures. The surface current-based formalism to calculate optical forces and torques permits their computation for particles in close proximity. We use this to study the internal forces in compound plasmonic systems, and show the presence of strong internal forces between their components. We also demonstrate surprising features such as force and torque reversal, and circular polarisation-dependent behaviour in achiral systems. We then numerically investigate the possibility of using optical torques to orient and rotate plasmonic nanostructures, relying on surface plasmon resonance, retardation effects and circular polarisation. Polarisation charges contain useful information about the behaviour of plasmonic systems, but there are difficulties in understanding and visualising them. We discuss the complex nature of polarisation charges and suggest various techniques to visualise them in complicated systems in a manner which is easy to understand without loss of information. Finally, we utilise the ability of SIE to compute accurate near fields to study the Raman enhancement in multi-walled carbon nanotubes on coating with metal, and the analogous quenching of Raman signal from silicon substrates

Review—Origin and Promotional Effects of Plasmonics in Photocatalysis

Plasmonic effects including near-field coupling, light scattering, guided mode through surface plasmon polaritons (SPPs), Förster resonant energy transfer (FRET), and thermoplasmonics are extensively used for harnessing inexhaustible solar energy for photovoltaics and photocatalysis. Recently, plasmonic hot carrier-driven photocatalysis has received additional attention thanks to its specific selectivity in the catalytic conversion of gas molecules and organic compounds, resulting from the direct injection of hot carriers into the lowest unoccupied molecular orbital of the adsorbate molecule. The excellent light trapping property and high efficiency of hot charge-carrier generation through electromagnetic surface plasmon decay have been identified as the dominant mechanisms that promote energy-intensive chemical reactions at room temperature and atmospheric pressure. However, understanding the electromagnetic effects of plasmonics and distinguishing them from chemical effects in photocatalysis is challenging. While there exist several reviews underlining the experimental observations of plasmonic effects, this critical review addresses the physical origin of the various plasmon-related phenomena and how they can promote photocatalysis. The conditions under which each plasmonic effect dominates and how to distinguish one from another is also discussed. Finally, future research directions are proposed with the aim to accelerate progress in this field at the interface between chemistry and physics

WTHD minimisation in hybrid multilevel inverter using biogeographical based optimisation

Harmonic minimisation in hybrid cascaded multilevel inverter involves complex nonlinear transcendental equation with multiple solutions. Hybrid cascaded multilevel can be implemented using reduced switch count when compared to traditional cascaded multilevel inverter topology. In this paper Biogeographical Based Optimisation (BBO) technique is applied to Hybrid multilevel inverter to determine the optimum switching angles with weighted total harmonic distortion (WTHD) as the objective function. Optimisation based on WTHD combines the advantage of both OMTHD (Optimal Minimisation of Total Harmonic Distortion) and SHE (Selective Harmonic Elimination) PWM. WTHD optimisation has the benefit of eliminating the specific lower order harmonics as in SHEPWM and minimisation of THD as in OMTHD. The simulation and experimental results for a 7 level multilevel inverter were presented. The results indicate that WTHD optimization provides both elimination of lower order harmonics and minimisation of Total Harmonic Distortion when compared to conventional OMTHD and SHE PWM. Experimental prototype of a seven level hybrid cascaded multilevel inverter is implemented to verify the simulation results

Decoding Apatite in Volcanic Carbonatitic breccia from Mongra, Northwest of Amba Dongar Carbonatite Complex, Gujarat, India: Insights to Genesis & Rare Earth Element Budgets

Chemical variations in apatite in context to carbonatites and trace element partitioning between apatites and carbonatite-rich liquids are important in assessing the petrogenesis and evolution of the carbonatites as well as the associated carbonatitic breccia due to apatite's sensitivity to surrounding magma composition. Volcanic carbonatitic breccia is one of the constituent rock types found outside the ring structure of the Amba Dongar Carbonatite Complex (ADC) situated in western India. In the present work, we report the mineral chemistry of apatites from the carbonatitic breccias of the Mongra region (ADC outer core) and compare them to apatites from ADC carbonatites.Apatite chemistry from Mongra displays a larger concentration of rare earth elements, manganese, and chlorine when compared to those of ADC carbonatites. Morphology and distinct zoning of these apatites represent late-stage magmatic processes with high heavy rare earth element concentrations (high Lutetium), followed by interaction of fluids from the surrounding alkaline rocks. Variation in sulphur concentration in the apatites of this study indicates crystallisation under mildly reducing conditions. Integrated field observations, petrography, and apatite mineral chemistry from the Mongra region allow for an understanding of the genesis of apatites in the ADC outer core, with possible implications for late-stage mineral-melt interactions

Outcome of Patients with Cholinergic Insecticide Poisoning Treated with Gastric Lavage: A Prospective Observational Cohort Study

Background: Gastric lavage (GL) is one of the most commonly used decontamination method for cholinergic insecticide ingestion in developing countries despite lack of supporting evidence. This study was designed to evaluate the outcome of patients with cholinergic insecticide poisoning treated with GL in regards to timing and frequency of the procedure. Methods: In this prospective observational cohort study, GL was planned to be administered to patients with cholinergic insecticide poisoning after initial stabilization irrespective of lavage given in peripheral hospitals. Therefore, some patients received one procedure (single GL) and some received more than one procedure (multiple GL). Early GL was defined as GL given within one hour of poison exposure and late GL was referred to performing the procedure after one hour. Results: During the study period, 238 patients with cholinergic insecticide poisoning received GL comprising of 93 who received early, 145 who received late, 127 who received single and 111 who received multiple GL. Seventy-six GL treated patients (31.9%) died. Mortality, early RF and duration of assisted ventilation were not significantly different between patients receiving early and late, or single and multiple GL. Patients receiving multiple GL compared to those who received single GL developed late RF (9.0% vs. 20.5%, P = 0.01) and IMS (9.9% vs. 23.6%, P = 0.005) in significantly lesser extents. In multiple logistic regression analysis, effect of multiple GL on IMS and late RF remained significant (P = 0.04). Conclusion: Number or timing of GL does not show any association with mortality while multiple GL had protective effect against development of late RF and IMS. Hence, GL might be beneficial in cholinergic insecticide poisoning