Probing ultra-subwavelength inhomogeneities embedded within dielectric targets using photonic nanojets

The use of optics to detect ultra-subwavelength features embedded within structures is a hot topic for a broad diversity of applications like spectroscopy, nanotechnology, microscopy, and optical data storage discs. Conventional objective lens based optical systems have a fundamental limit on the best possible resolution of about 200 \u03b7m due to the diffraction of light as it propagates into the far-field. There already exist several near-field techniques with the capability to overcome this limitation, but each of these systems has certain drawbacks related to the complexity of the system or to limitations imposed by the system. A photonic nanojet is a very particular beam of light that can provide a practical way to overcome the diffraction limit inherent to far-field techniques. A nanojet is an electromagnetic field envelope formed on the shadow-side surface of a plane-wave-illuminated dielectric microsphere of diameter larger than the wavelength and with refractive index contrast relative to the background medium of less than 2:1. It can maintain a subwavelength transversal beamwidth for distances greater than 2 wavelengths away from the surface of the generating microsphere. This Dissertation provides a computational test of the hypothesis that the backscattered spectrum resulting from photonic nanojet illumination of a three-dimensional (3-D) dielectric structure can reveal the presence and location of ultra-subwavelength, nanoscale-thin weakly contrasting dielectric inhomogeneities within dielectric targets. The effect of surface roughness on the illuminated side of the target is analyzed, and targets ranging from simple dielectric slabs to complex biological cells are studied. The present work is performed through computational electrodynamics modeling based upon the rigorous, large-scale solution of Maxwells equations. Specifically, the 3-D finite-difference time-domain (FDTD) method is employed to test the above hypothesis.\u2

Selected developments in computational electromagnetics for radio engineering

This thesis deals with the development and application of two simulation methods commonly used in radio engineering, namely the Finite-Difference Time-Domain method (FDTD) and the Finite Element Method (FEM). The main emphasis of this thesis is in FDTD. FDTD has become probably the most popular computational technique in radio engineering. It is a well established, fairly accurate and easy-to-implement method. Being a time-domain method, it can provide wide-band information in a single simulation. It simulates physical wave propagation in the computational volume, and is thus especially useful for educational purposes and for gaining engineering insight into complicated wave interaction and coupling phenomena. In this thesis, numerical dispersion taking place in the FDTD algorithm is analyzed, and a novel dispersion reduction procedure is described, based on artificial anisotropy. As a result, larger cells can be used to obtain the same accuracy in terms of dispersion error. Simulation experiments suggest that typically the dispersion reduction allows roughly doubling the cell size in each coordinate direction, without sacrificing the accuracy. The obtainable advantage is, however, dependent on the problem. In the open literature, a few other procedures are also presented to reduce the dispersion error. However, the rather dominating effect of unequal grid resolution along different coordinate directions has been neglected in previous studies. The so-called Perfectly Matched Layer (PML) has proven to be a very useful absorbing boundary condition (ABC) in FDTD simulations. It is reliable, works well in wide frequency band and is easy to implement. The most notable deficiency of PML is that it enlarges the computational volume - in open 3-D structures easily by a factor of two. However, due to its advantages, PML has become a standard ABC. In this thesis, the operation of PML in FDTD has been studied theoretically, and some interesting properties of it not known before are uncovered. For example, it is shown that, surprisingly, PML can absorb perfectly (i.e. with zero reflection) plane waves propagating towards almost arbitrary given direction at given frequency. Optimizing the conductivity profile allows reduction of the PML thickness. A typical application of the FDTD method is the design of a mobile handset antenna. An improved coaxial probe model has been developed for antenna simulations. The well-known resistive voltage source (RVS) model has also been discussed. A reference plane transformation is proposed to correct the simulated input impedance. A popular thin-wire model in 2-D FDTD is discussed, and it is shown to be based on erroneous reasoning. The error has been corrected by a simple procedure, and the corrected model has been demonstrated to simulate infinite long thin wires much better than the commonly used model. A novel way to implement singular basis functions in FEM is discussed. It is shown theoretically and demonstrated by examples that if a waveguide propagation mode contains field singularities, then explicit inclusion of singularities in finite element analysis is crucial in order to obtain accurate cut-off wavenumbers.reviewe

Orders of Magnitude Enhancement of Optical Nonlinear Phenomena in Subwavelength Metal-Dielectric Gratings

Nonlinear optical materials give rise to a multitude of phenomena that have important applications in technology and science. Due to small nonlinearities in naturally occurring materials, large optical fields are necessary to realize measurable nonlinear phenomena. The necessity of high intensity sources severely limits its use in practical applications, especially in low-powered devices. Several methods for enhancement of nonlinearity have been proposed, including use of conjugate polymers, resonators, and metallic nanoparticles. In this thesis, the nonlinear enhancement properties of subwavelength metal-dielectric gratings are explored. Enhancement in nonlinearity by several orders of magnitude is achieved, with the enhancement entirely controlled by the geometry of the structure, and independent of the wavelength of incident light. Ultimately, the nonlinear enhancement properties of metal-dielectric gratings allows for the reduction of input light intensity in producing nonlinear optical phenomena, and is an important step in the design low-powered nonlinear optical applications

A Single-Field Finite-Difference Time-Domain Formulations for Electromagnetic Simulations

In this dissertation, a set of general purpose single-field finite-difference time-domain updating equations for solving electromagnetic problems is derived. The formulation uses a single-field expression for full-wave solution. This formulation can provide numerical results similar to those obtained using the traditional formulation with less required computer resources. Traditional finite-difference time-domain updating equations are based on Maxwell\u27s curl equations whereas the single-field updating equations used here are based on the vector wave equation. General formulations are derived for normal and oblique incidence plane wave cases for linear, isotropic, homogeneous and non-dispersive as well as dispersive media. To compare the single-field updating equations with the traditional ones, two-dimensional transverse magnetic, two-dimensional transverse electric and one-dimensional electromagnetic problems are solved. Fields generated by a current sheet and a filament electric current are calculated for one and two-dimensional formulations, respectively. Performance analyses of the single-field formulation in terms of CPU time, memory requirement, stability, dispersion, and accuracy are presented. Based on the simulations of several two-dimensional problems excited by a filament of electric current, it was observed that the single-field method is more efficient than the traditional one in terms of speed and memory requirements. One scattering problem consisting of three infinitely long dielectric cylinders excited by an obliquely incident plane wave and another scattering problem consisting of a point source exciting a dispersive sphere, utilizing Lorentz-Drude model, are also formulated and analyzed. The numerical results obtained confirmed the validity and efficiency of the single-field formulations

Computation of electromagnetic fields in assemblages of biological cells using a modified finite difference time domain scheme. Computational electromagnetic methods using quasi-static approximate version of FDTD, modified Berenger absorbing boundary and Floquet periodic boundary conditions to investigate the phenomena in the interaction between EM fields and biological systems.

yesThere is an increasing need for accurate models describing the electrical behaviour of individual biological cells exposed to electromagnetic fields. In this area of solving linear problem, the most frequently used technique for computing the EM field is the Finite-Difference Time-Domain (FDTD) method. When modelling objects that are small compared with the wavelength, for example biological cells at radio frequencies, the standard Finite-Difference Time-Domain (FDTD) method requires extremely small time-step sizes, which may lead to excessive computation times. The problem can be overcome by implementing a quasi-static approximate version of FDTD, based on transferring the working frequency to a higher frequency and scaling back to the frequency of interest after the field has been computed. An approach to modeling and analysis of biological cells, incorporating the Hodgkin and Huxley membrane model, is presented here. Since the external medium of the biological cell is lossy material, a modified Berenger absorbing boundary condition is used to truncate the computation grid. Linear assemblages of cells are investigated and then Floquet periodic boundary conditions are imposed to imitate the effect of periodic replication of the assemblages. Thus, the analysis of a large structure of cells is made more computationally efficient than the modeling of the entire structure. The total fields of the simulated structures are shown to give reasonable and stable results at 900MHz, 1800MHz and 2450MHz. This method will facilitate deeper investigation of the phenomena in the interaction between EM fields and biological systems. Moreover, the nonlinear response of biological cell exposed to a 0.9GHz signal was discussed on observing the second harmonic at 1.8GHz. In this, an electrical circuit model has been proposed to calibrate the performance of nonlinear RF energy conversion inside a high quality factor resonant cavity with known nonlinear device. Meanwhile, the first and second harmonic responses of the cavity due to the loading of the cavity with the lossy material will also be demonstrated. The results from proposed mathematical model, give good indication of the input power required to detect the weakly effects of the second harmonic signal prior to perform the measurement. Hence, this proposed mathematical model will assist to determine how sensitivity of the second harmonic signal can be detected by placing the required specific input power

Design and simulation for the fabrication of integrated semiconductor optical logic gates

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Electrical Engineering and Computer Science, February 2006.This electronic version was submitted by the student author. The certified thesis is available in the Institute Archives and Special Collections.Includes bibliographical references.Development of ultrafast all-optical logic requires accurate and efficient modeling of optical components and interfaces. In this research, we present an all-optical logic unit cell with complete Boolean functionality as a representative circuit for modeling and optimization of monolithically integrated components. Proposed optical logic unit cell is based on an integrated balanced Mach-Zehnder interferometer (MZI) with semiconductor optical amplifiers (SOAs) in each arm and includes straight ridge waveguides, ridge waveguide bends, and multimode interference (MMI) devices. We use beam propagation method (BPM) to model, design and optimize dilute ridge waveguides, MMIs, and asymmetric twin waveguide (ATG) adiabatic taper couplers. We assess device robustness with respect to variations in fabrication, including lateral pattern transfer and etching. Bending losses in curved waveguides are evaluated using complex-frequency leaky mode computations with perfectly matched layer (PML) boundary conditions. Finite difference time domain (FDTD) method with PML is utilized in calculating reflections produced by abrupt interfaces, including a tip of an adiabatic taper coupler.(cont.) We demonstrate that evaluating reflections based on local effective indices on two sides of the junction offers a simple, accurate, and time-efficient alternative to FDTD. We show a strategy for development of SOAs for linear amplification and phase shifting using the same layered semiconductor structure. Our model of optical pulse propagation in SOA is based on rate equations for carrier density and photon density and using a wavelength-dependent parametric model for gain. We demonstrate a tradeoff between injection current density and device length for both linear and non-linear SOAs.by Aleksandra Markina.Ph.D

Sub-wavelength optical phenomena and their applications in nano-fabrication

textThis dissertation presents the numerical study of sub-wavelength optical phenomena and experimental demonstration of their applications in nanoscale patterning. The optical near-field enhancement associated with sub-wavelength scale nanostructures, such as nano-ridges and nano-tips, were utilized to produce nanoscale patterns. Numerical simulation using finite difference time domain (FDTD) method were employed as a modeling tool to predict and optimize a scheme to achieve parallel patterning by near-field enhanced direct nano-molding. In order to pattern the whole area of the substrate, the laser light was chosen to be transparent to the substrate and was shine from the back with an incident angle. The near-field enhancement facilitates the local ablation to form line or dot patterns on the thin film coated on the substrate. Fabrication process to manufacture the nanostructure used as the mold was also developed. Further investigation into the near-field enhancement phenomena leads to the study of underlying physics: surface plasmons (SPs) and SP-light coupling. Previous research efforts have shown that transmission through sub-wavelength apertures can be orders of magnitude higher than predicted by aperture theory due to SP-light coupling. In this dissertation study, the SPs excited on the nano apertures were combined with the SPs on the substrate to overcome the two fundamental restraints of light at sub-wavelength scale, transmission and diffraction. The discovery was exploited for nanolithography, coined Surface Plasmon Assisted Nanolithography (SPAN), and one-to-one nanoscale pattern transfer has been achieved using both laser light and UV lamp without losing convenience and simplicity of traditional photolithography technique. High contrast optical near-field interference generated by SP-light coupling was also studied and combined with the photolithography technique to produce threedimensional (3D) nanostructures. Different multi-layer 2D/3D periodic polymeric nanostructures have been directly fabricated using such a mechanism in a typical photolithography setup. The nanostructures fabricated can be easily controlled in terms of size, layout, and defects by designing the mask. This so-called Surface Plasmon Assisted 3D Nanolithography (3D-SPAN) was demonstrated in experiments to offer flexible and convenient 3D nanofabrication capabilities.Mechanical Engineerin

Numerical simulation of wave-plasma interactions in the ionosphere

Ionospheric modification by means of high-power electromagnetic (EM) waves can result in the excitation of a diverse range of plasma waves and instabilities. This thesis presents the development and application of a GPU-accelerated finite-difference time-domain (FDTD) code designed to simulate the time-explicit response of an ionospheric plasma to incident EM waves. Validation tests are presented in which the code achieved good agreement with the predictions of plasma theory and the computations of benchmark software. The code was used to investigate the mechanisms behind several recent experimental observations which have not been fully understood, including the effect of 2D density inhomogeneity on the O-mode to Z-mode conversion process and thus the shape of the conversion window, and the influence of EM wave polarisation and frequency on the growth of density irregularities. The O-to-Z-mode conversion process was shown to be responsible for a strong dependence of artificially-induced plasma perturbation on both the EM wave inclination angle and the 2D characteristics of the background plasma. Allowing excited Z-mode waves to reflect back towards the interaction region was found to cause enhancement of the electric field and a substantial increase in electron temperature. Simulations of O-mode and X-mode polarised waves demonstrated that both are capable of exciting geomagnetic field-aligned density irregularities, particularly at altitudes where the background plasma frequency corresponds to an electron gyroharmonic. Inclusion of estimated electrostatic fields associated with irregularities in the simulation algorithm resulted in an enhanced electron temperature. Excitation of these density features could address an observed asymmetry in anomalous absorption and recent unexplained X-mode heating results reported at EISCAT. Comparing simulations with ion motion allowed or suppressed indicated that a parametric instability was responsible for irregularity production. Simulation of EM wave fields confirmed that X-mode waves are capable of exceeding the threshold for parametric instability excitation under certain conditions