Partially-disordered photonic-crystal thin films for enhanced and robust photovoltaics

We present a general framework for the design of thin-film photovoltaics based on a partially-disordered photonic crystal that has both enhanced absorption for light trapping and reduced sensitivity to the angle and polarization of incident radiation. The absorption characteristics of different lattice structures are investigated as an initial periodic structure is gradually perturbed. We find that an optimal amount of disorder controllably introduced into a multi-lattice photonic crystal causes the characteristic narrow-band, resonant peaks to be broadened resulting in a device with enhanced and robust performance ideal for typical operating conditions of photovoltaic applications.Comment: 5 pages, 4 figure

A novel boundary element method using surface conductive absorbers for full-wave analysis of 3-D nanophotonics

Fast surface integral equation (SIE) solvers seem to be ideal approaches for simulating 3-D nanophotonic devices, as these devices generate fields both in an interior channel and in the infinite exterior domain. However, many devices of interest, such as optical couplers, have channels that can not be terminated without generating reflections. Generating absorbers for these channels is a new problem for SIE methods, as the methods were initially developed for problems with finite surfaces. In this paper we show that the obvious approach for eliminating reflections, making the channel mildly conductive outside the domain of interest, is inaccurate. We describe a new method, in which the absorber has a gradually increasing surface conductivity; such an absorber can be easily incorporated in fast integral equation solvers. Numerical experiments from a surface-conductivity modified FFT-accelerated PMCHW-based solver are correlated with analytic results, demonstrating that this new method is orders of magnitude more effective than a volume absorber, and that the smoothness of the surface conductivity function determines the performance of the absorber. In particular, we show that the magnitude of the transition reflection is proportional to 1/L^(2d+2), where L is the absorber length and d is the order of the differentiability of the surface conductivity function.Comment: 10 page

A fundamental relation between phase and group velocity, and application to the failure of perfectly matched layers in backward-wave structures

http://link.aps.org/doi/10.1103/PhysRevE.79.065601We demonstrate that the ratio of group to phase velocity has a simple relationship to the orientation of the electromagnetic field. In nondispersive materials, opposite group and phase velocity corresponds to fields that are mostly oriented in the propagation direction. More generally, this relationship (including the case of dispersive and negative-index materials) offers a perspective on the phenomena of backward waves and left-handed media. As an application of this relationship, we demonstrate and explain an irrecoverable failure of perfectly matched layer absorbing boundaries in computer simulations for constant cross-section waveguides with backward-wave modes and suggest an alternative in the form of adiabatic isotropic absorbers

Computation and design for nanophotonics

Thesis (Sc. D.)--Massachusetts Institute of Technology, Dept. of Materials Science and Engineering, 2010.This electronic version was submitted by the student author. The certified thesis is available in the Institute Archives and Special Collections.Cataloged from student submitted PDF version of thesis.Includes bibliographical references (p. 191-209).The versatility of computational design as an alternative to design by nanofabrication has made computers a reliable design tool in nanophotonics. Given that almost any 2d pattern can be fabricated at infrared length scales, there exists a large number of degrees of freedom in nanophotonic device design. However current designs are adhoc and could potentially benefit from optimization but there are several outstanding issues regarding PDE-based optimization for electromagnetism that must first be addressed: continuously and accurately deforming geometric objects represented on a discrete uniform grid while avoiding staircasing effects, reducing the computational expense of large simulations while improving accuracy, resolving the breakdown of standard absorbing boundary layers for important problems, finding robust designs that are impervious to small perturbations, and finally distinguishing global from local minima. We address each of these issues in turn by developing novel subpixel smoothing methods that markedly improve the accuracy of simulations, demonstrate the failure of perfectly matched layers (PML) in several important cases and propose a workaround, develop a simple procedure to determine the validity of any PML implementation and incorporate these and other enhancements into a flexible, free software package for electromagnetic simulations based on the finite-difference time-domain (FDTD) method. Next we investigate two classes of design problems in nanophotonics. The first involves finding cladding structures for holey photoniccrystal fibers at low-index contrasts that permit a larger class of materials to be used in the fabrication process. The second is the development of adiabatic tapers for coupling to slow-light modes of photonic-crystal waveguides that are insensitive to manufacturing and operational variability.by Ardavan Oskooi.Sc.D

Investigation of the PML for inhomogeneous media

Thesis (S.M.)--Massachusetts Institute of Technology, Computation for Design and Optimization Program, 2008.This electronic version was submitted by the student author. The certified thesis is available in the Institute Archives and Special Collections.Includes bibliographical references (p. 41-45).Although perfectly matched layers (PMLs) have been widely used to truncate numerical simulations of electromagnetism and other wave equations, we point out important cases in which a PML fails to be reflectionless even in the limit of infinite resolution. In particular, the underlying coordinate-stretching idea behind PML breaks down in photonic crystals and in other structures where the material is not an analytic function in the direction perpendicular to the boundary, leading to substantial reflections. The alternative is an adiabatic absorber, in which reflections are made negligible by gradually increasing the material absorption at the boundaries, similar to a common strategy to combat discretization reflections in PMLs. We demonstrate the fundamental connection between such reflections and the smoothness of the absorption profile via coupled-mode theory, and show how to obtain higher-order and even exponential vanishing of the reflection with absorber thickness.by Ardavan F. Oskooi.S.M