Morphology-Dependent Resonances in Two Concentric Spheres with Variable Refractive Index in the Outer Layer: Analytic Solutions

In many applications constant or piecewise constant refractive index profiles are used to study the scattering of plane electromagnetic waves by a spherical object. When the structured media has variable refractive indices, this is more of a challenge. In this paper, we investigate the morphology dependent resonances for the scattering of electromagnetic waves from two concentric spheres when the outer shell has a variable refractive index. The resonance analysis is applied to the general solutions of the radial Debye potential for both transverse magnetic and transverse electric modes. Finally, the analytic conditions to determine the resonance locations for this system are derived in the closed form of both modes. Our numerical results are provided with discussion

Gradient Optics of subwavelength nanofilms

Propagation and tunneling of light through subwavelength photonic barriers, formed by dielectric layers with continuous spatial variations of dielectric susceptibility across the film are considered. Effects of giant heterogeneity-induced non-local dispersion, both normal and anomalous, are examined by means of a series of exact analytical solutions of Maxwell equations for gradient media. Generalized Fresnel formulae, visualizing a profound influence of gradient and curvature of dielectric susceptibility profiles on reflectance/transmittance of periodical photonic heterostructures are presented. Depending on the cutoff frequency of the barrier, governed by technologically managed spatial profile of its refractive index, propagation or tunneling of light through these barriers are examined. Nonattenuative transfer of EM energy by evanescent waves, tunneling through dielectric gradient barriers, characterized by real values of refractive index, decreasing in the depth of medium, is shown. Scaling of the obtained results for different spectral ranges of visible, IR and THz waves is illustrated. Potential of gradient optical structures for design of miniaturized filters, polarizers and frequency-selective interfaces of subwavelength thickness is considered

Whispering-gallery-mode dye laser emission from liquid in a capillary fiber

Bibliography: p. 153-170.The nature of optical whispering-gallery-mode resonances in a layered microcylinder is investigated numerically by studying the scattering characteristics and the internal electromagnetic fields of a normally-illuminated cladded dielectric fiber calculated using the boundary-value method. Computed resonant mode configurations are compared to the better-known results for homogeneous spheres and cylinders and coated spheres. It is shown that high-Q whispering-gallery-mode resonances can be supported by the curved interface between the core and cladding regions of a layered fiber if the core refractive index is sufficiently greater than that of the outer layer, and that these modes can be directly related to the so-called morphology-dependent resonances of a homogeneous cylinder of the same size and relative refractive index as the fiber core. The implications of these resonant modes for inelastic optical processes are made clear by developing a model for optical emissions from a molecule in the core of a capillary fiber. The results of the model show that the transition rates of molecules in the fiber core and near to the core/cladding interface are enhanced at frequencies corresponding to cavity resonances. It is shown experimentally that these high-Q cavity modes can be excited to above the threshold for laser emission by providing gain in the fiber core material. We have used a refractive dye-doped solvent as a gain medium and a fused-silica capillary to form the resonant cavity. Upon optical excitation of the dye by illuminating the fiber normally with the green beam from a frequency-doubled Nd:YAG laser, laser emission is emitted from the fiber core in the plane perpendicular to the fiber axis. We explain the novel spatial and spectral dependences of the laser emission in terms of the calculated frequencies and Q-values of the resonant cavity modes and the bulk properties of the cavity medium. We show that the thresholds observed in the laser system can be explained using a simplified rate-equation approach, and that this also explains some of the other observed features of the emissions. The heating of the dye solvent during a laser pulse has an observable effect on the resonance mode locations due to the temperature dependence of the refractive index. We demonstrate the use of observed laser spectra to determine the size and taper of the capillary fiber core

Relation between raman backscattering from droplets and bulk water: Effect of refractive index dispersion

A theoretical framework is presented that permits investigations of the relation between inelastic backscattering from microparticles and bulk samples of Raman-active materials. It is based on the Lorentz reciprocity theorem and no fundamental restrictions concerning the microparticle shape apply. The approach provides a simple and intuitive explanation for the enhancement of the differential backscattering cross-section in particles in comparison to bulk. The enhancement factor for scattering of water droplets in the diameter range from 0 to 60 mu m (vitally important for the a priori measurement of liquid water content of warm clouds with spectroscopic Raman lidars) is about a factor of 1.2-1.6 larger (depending on the size of the sphere) than an earlier study has shown. The numerical calculations are extended to 1000 mu m and demonstrate that dispersion of the refractive index of water becomes an important factor for spheres larger than 100 mu m. The physics of the oscillatory phenomena predicted by the simulations is explained. (C) 2018 Elsevier Ltd. All rights reserved

Optical Nanostructures For Controllable And Tunable Optical Properties

Optical nanostructures are heterogeneous media containing subwavelength inclusions in periodic or aperiodic fashion. The optical properties of optical nanostructure can be controlled and tuned using their constituent material properties and spatial arrangement of the inclusions. While optical nanostructures have been widely studied, controllable and tunable nanostructures using low loss transparent materials has not been studied in detail in the literature. The objective of this research is to perform efficient design and analyses of controllable and tunable optical nanostructures using low loss transparent materials. To that end, versatile and highly accurate numerical methods like finite different tie domain and plane wave expansion methods are reviewed first. These methods and compared in terms of their speed, accuracy, and memory requirement. Different kind of optical nanostructures, consisting of low index transparent materials, are analyzed to study their controllability. For example, single scatterers are optimized to obtain highly direction forward scattering using low index materials. Then, the minimum refractive index required for establishing optical bandgap in a planar periodic nanostructure was established. Using the bandgap, highly sensitive transparent sensors are designed using low index materials. It is found that the numerical methods can analyze small or periodic nanostructure, while requiring significant computational resources. As an alternative to numerical modelling, analytical effective medium approximations are considered. The available approximations are reviewed, and their limitations are pointed out. Using the Mie scattering theory, the Maxwell-Garnett approximation is extended so that it can account for arbitrary size, as well as different physical structures, of the inclusions. The derived effective medium approximation is tested on a wide variety of optical nanostructure, both periodic and aperiodic. Good agreement between analytical and experimental results are established. The utility of the approximation in designing controllable and tunable optical nanostructure is demonstrated by modelling the dynamic optical properties of magnetic colloids and verifying them experimentally. The effective medium approximation can be a very fast, and efficient method of modelling the controllable and tunable properties of optical nanostructure, when applied judiciously. The applicability, limits of validity, and limitation of the approximation is also discussed. Using the analytical framework, controllable optical nanostructure that can mimic optical elements, e.g., focusing lenses, are designed. The relationship between physical structure of the inclusions and the imparted phase by the nanostructure is studied using effective medium approximation and numerical methods. The effective medium approximation can predict the imparted phase with high accuracy, while requiring a fraction of the computation resources compared to numerical methods. Based on the relationship between imparted phase and physical structure of the inclusions, it is possible to design optical nanostructure with controllable spatial phase profile. Using this property, nanostructured optical elements are designed. Their far-field properties are calculated using analytical scalar theory. The analytical results matched well with numerical and experimental results. In conclusion, an analytical method for designing and analyzing tunable and controllable optical nanostructure is derived and verified with experimental results. The analytical method is significantly more efficient compared to numerical methods, while being similarly accurate compared to experimental results. The research in this work can lead to efficient design of optical nanostructure for many different fields

Colloquium: Graphene spectroscopy

Spectroscopic studies of electronic phenomena in graphene are reviewed. A variety of methods and techniques are surveyed, from quasiparticle spectroscopies (tunneling, photoemission) to methods probing density and current response (infrared optics, Raman) to scanning probe nanoscopy and ultrafast pump-probe experiments. Vast complimentary information derived from these investigations is shown to highlight unusual properties of Dirac quasiparticles and many-body interaction effects in the physics of graphene.Comment: 36 pages, 16 figure

Control and characterization of nano-structures with the symmetries of light

Light beams can be symmetric under different transformations: translations, rotations, mirror symmetries, duality transformations, etc. In this thesis, a systematic way of characterizing these symmetries is presented. Then, it is shown that light beams symmetric under different transformations can be used to control light-matter interactions at the nano-scale. Particular applications are developed, both theoretically and experimentally. Inducing a dual behaviour on a non-dual sample, the excitation of high multipolar order resonances and the measurement of circular dichroism using vortex beams are among them.Comment: PhD Thesis, Department of Physics and Astronomy, Macquarie University. PhD Supervisor: Gabriel Molina-Terriz