Ibrahim Kursat Sendur

Light Delivery Techniques for Heat-Assisted Magnetic Recording.

Heat-assisted magnetic recording (HAMR), also known as hybrid recording, has been proposed to enable storage densities greater than 1 Tb/in2 in hard disc drives while circumventing the superparamagnetic limit. Light is delivered in the near field to the recording medium to heat just the spot which is to be recorded. Techniques based on apertures, antennas, waveguides, and solid immersion lenses have been suggested for delivering substantial amounts of optical power into subwavelength spots in the near field. A practical transducer for HAMR may require a combination of techniques.

Scattered field formulation of finite difference time domain for a focused light beam in dense media with lossy materials.

Using the scattered field finite difference time domain (FDTD) formalism, equations for a plane wave incident from a dense medium onto lossy media are derived. The Richards-Wolf vector field equations are introduced into the scattered field FDTD formalism to model an incident focused beam. The results are compared to Mie theory scattering from spherical lossy dielectric and metallic spheres.

Numerical simulation of thermal signatures of buried mines over a diurnal cycle

3D thermal and radiometric models have been developed to study the passive IR signature of a land mine buried under a rough soil surface. A finite element model is used to describe the thermal phenomena, including temporal variations, the spatial structure of the signature, and environmental effects. The Crank-Nicholson algorithm is used for time-stepping the simulation. The mine and the surroundings are approximated by pentahedral elements having linear interpolation functions. The FEM grid for the soil includes a random rough surface having a normal probability density and specified covariance function. The mine is modeled as a homogeneous body of deterministic shape having the thermal properties of TNT. Natural solar insolation and the effects of convective heat transfer are represented by linearized boundary conditions. The behavior over a periodic diurnal cycle is studied by running the simulation to steady state. Finite element solutions for the thermal emissions are combined with reflected radiometric components to predict the signatures seen by an IR camera. Numerical simulations are presented for a representative target, a 25 cm anti-tank mine simulant developed by the US Army. The temporal evolution of the temperature distribution and IR signature are presented for both smooth and rough surfaces.

Role of environmental factors and mine geometry in thermal IR mine signatures

Thermal IR signatures of buried land mines are affected by various environmental conditions as well as the mines composition, size and burial geometry. In this work we present quantitative relations for the effect of those factors on the signatures peak contrast and apparent diameter. We begin with a review of the relevant phenomena and the underlying physics. A three-dimensional simulation tool developed by the authors is used to simulate signatures for the case of a static water distribution. We discuss efforts to validate the model using experimental data collected at Fort A.P. Hill, VA. Using this simulation tool a variety of factors are considered, including soil water content, soil sand content, wind speed, mine diameter and mine burial depth.

Analysis of polarimetric IR phenomena for detection of surface mines

It has long been recognized that surface-laid land mines and other man-made objects tend to have different polarization characteristics than natural materials. This fact has been used to advantage in a number of mine detecting sensors developed over the last two decades. In this work we present the theoretical basis for this polarization dependence. The theory of scattering from randomly rough surfaces is employed to develop a model for scattering and emission from mines and natural surfaces. The emissivity seen by both polarized and unpolarized sensors is studied for smooth and rough surfaces. The polarized and unpolarized emissivities of rough surfaces are modeled using the solution of the reciprocal active scattering problem via the second order small perturbation method/small slope approximation(SPM/SSA). The theory is used to determine the most suitable angle for passive polarimetric IR detection of surface mines.

Finite difference frequency domain scattered field formulation for near-field optical data storage

We present a novel technique for the modeling of near field optical devices. The key features of this technique are the accuracy of the finite difference time domain method, the advantages of a scattered field formulation, and the direct use the complx permittivity of metals at the frequency of interest.

Using physical models to improve thermal IR detection of buried mines

Many aspects of a buried mines thermal IR signature can be predicted through physical models, and insight provided by such models can lead to better detection. Several techniques for exploiting this information are described. The first approach involves ML estimation of model parameters and followed by classification of those parameters. We show that this approach is related to an approximate evaluation of an integral over the parameters that arises in a Bayesian formulation. This technique is compared with a generalized likelihood ratio test (GLRT) and with computationally efficient, model-free approaches, in which soil temperature data are classified directly. The benefit of using the temporal information is also investigated. Algorithm performance is illustrated using broadband IR imagery of buried mines acquired over a 24 hour period. It is found that the detection performance at a suitably selected time is comparable to the performance achieved by processing all times. The performance of the GLRT, for which detection is based only on the residual error, is inferior to a classifier using the parameters.

Solution of radiation problems using the fast multipole method

EM radiation problems involving electrically large radiators and reflectors are to be solved using the fast multipole method (FMM). Structures that are intentional radiators, such as antennas, have to be designed to optimize the required radiation characteristics. On the other hand, unintentional radiators, such as electronic systems, have to be designed to suppress the radiation mechanisms. Computational simulation of both types of radiators can be used to improve the design in a shorter time spending less resources. However, some interesting real-life radiation problems are electrically very large and thus cannot be solved using traditional solution algorithms due to the limitations on the computational resources. The FMM enables the solution of larger problems with existing computational resources by reducing the computational complexity and the memory requirement of the solution without sacrificing the accuracy. This is achieved by replacing the matrix-vector multiplications of O(N/sup 2/) complexity by a faster equivalent of O(N/sup 1.5/) complexity in each iteration of an iterative scheme. Some versions of the FMM have complexities that are even lower than O(N/sup 1.5/) per iteration. In contrast, a direct solution would require O(N/sup 3/) operations. 3D radiation problems involving complicated geometries are modelled using arbitrary surface triangulations. Piecewise linear basis functions defined on triangular domains due to Rao, Wilton, and Glisson (RWG) (1982) and referred to as RWG basis functions are used to approximate the induced currents.

Optimization of plasmonic nano-antennas

The interaction of light with plasmonic nano-antennas is investigated. First, an extensive parametric study is performed on the material and geometrical effects on dipole and bow-tie nano-antennas. The transmission efficiency is studied for various parameters including length, thickness, width, and composition of the antenna as well as the wavelength of incident light. The modeling and simulation of these structures is done using 3-D finite element method based full-wave solutions of Maxwell’s equations. Next, a modeling-based automated design optimization framework is developed to optimize nano-antennas. The electromagnetic model is integrated with optimization solvers such as gradient-based optimization tools and genetic algorithms.

Interaction of Spherical Nanoparticles with a Highly Focused Beam of Light

Interaction of a highly focused beam of light with spherical nanoparticles is investigated for various incident field polarizations. First, an analytical solution is obtained to calculate the interaction of a highly focused beam of light with a spherical particle. To accurately express the incident electric field near the focus of an aplanatic lens, the technique established by Richards and Wolf is used. Using this analytical solution, electric field distributions of various spherical nanoparticles made of silver are investigated for highly focused linearly and radially polarized beams. The effect of the half angle of the focused beam is investigated. In addition a three-dimensional finite element method based solution is obtained and compared with the analytical solution for problems involving a highly focused beam of light interacting with a spherical particle.