Methods for Calculating the Optical Band Structure of Photonic Composites

Lately, there has been an increasing interest in studying the propagation of electromagnetic waves in periodic dielectric structures (photonic crystals). Like the electron propagation in semiconductors, these structures are represented by band diagrams in which gaps can be found where the electromagnetic propagation is forbidden. Much effort is dedicated to find structures that can prohibit the propagation of light in all directions. This effect could lead to light localization.Singapore-MIT Alliance (SMA

Photonic Crystals: Numerical Predictions of Manufacturable Dielectric Composite Architectures

Photonic properties depend on both dielectric contrast in a microscopic composite and the arrangement of the microstructural components. No theory exists for direct prediction of photonic properties, and so progress relies on numerical methods combined with insight into manufacturable composite architectures. We present a discussion of effective photonic crystal production techniques and several numerical methods to predict dispersion relations of hypothetical but fabricable structures.Singapore-MIT Alliance (SMA

Quasi-two-dimensional optomechanical crystals with a complete phononic bandgap

A fully planar two-dimensional optomechanical crystal formed in a silicon microchip is used to create a structure devoid of phonons in the GHz frequency range. A nanoscale photonic crystal cavity is placed inside the phononic bandgap crystal in order to probe the properties of the localized acoustic modes. By studying the trends in mechanical damping, mode density, and optomechanical coupling strength of the acoustic resonances over an array of structures with varying geometric properties, clear evidence of a complete phononic bandgap is shown.Comment: 9 pages, 7 figure

Optical Properties of Gyroid Structured Materials: From Photonic Crystals to Metamaterials

The gyroid is a continuous and triply periodic cubic morphology which possesses a constant mean curvature surface across a range of volumetric ll fractions. Found in a variety of natural and synthetic systems which form through self-assembly, from buttery wing scales to block copolymers, the gyroid also exhibits an inherent chirality not observed in any other similar morphologies. These unique geometrical properties impart to gyroid structured materials a host of interesting optical properties. Depending on the length scale on which the constituent materials are organised, these properties arise from starkly di erent physical mechanisms (such as a complete photonic band gap for photonic crystals and a greatly depressed plasma frequency for optical metamaterials). This article reviews the theoretical predictions and experimental observations of the optical properties of two fundamental classes of gyroid structured materials: photonic crystals (wavelength scale) and metamaterials (subwavelength scale).This work was supported by the EPSRC through the Cambridge NanoDTC EP/G037221/1, EP/G060649/1, EP/L027151/1, and ERC LINASS 320503.This is the accepted manuscript version of the article. The final version is available from Wiley via http://dx.doi.org/10.1002/adom.20140033

Numerical simulations of the effects of microstructure on photonic crystals

Thesis (S.M.)--Massachusetts Institute of Technology, Dept. of Materials Science and Engineering, 2001.Includes bibliographical references (p. 46).The optical properties of photonic crystals were studied to gain an understanding of the response of these structures to electromagnetic waves. In particular, multilayer films and two-dimensional systems were studied. It is known that multilayer films present omnidirectional reflectivity for a certain range of frequencies. A numerical technique for solving the Maxwell's equations within infinite periodic structures is presented. The media are assumed to be isotropic, linear, and non-conducting. This technique is based on the Fourier Transform of the dielectric constant of the structure. The numerical formulation allows the determination of the dispersion curves for photonic crystals. From them, omnidirectional reflectivity can be studied. The formulation was used to find the range of forbidden frequencies within an infinite multilayer film composed of two alternating materials. The energy gap for several constitutive parameters is shown. A two-dimensional system composed of infinite rods in a square lattice was also treated with this method. The Matrix Translation Method for studying omnidirectional reflectivity in one-dimensional systems was also used. This method considers homogeneous media and the electromagnetic fields are matched at the interfaces between the media. The formulation is used to obtain the reflectivity of the multilayer film as a function of the direction and frequency of the incident wave. By taking into account all directions and frequencies, omnidirectionality can be found. The method was applied to a finite multilayer film and the results are compared to those obtained with the Fast Fourier Transform Method. Similar results were obtained. An understanding of the properties of photonic crystals was achieved and guidelines for the determination of energy gaps are presented.by Martin Maldovan.S.M

Exploring for new photonic band gap structures

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Materials Science and Engineering, 2004.Includes bibliographical references (leaves 103-104).In the infinite set of possible photonic band gap structures there are no simple rules to serve as a guide in the search for optimal designs. The existence and characteristics of photonic band gaps depend on such factors as dielectric contrast, volume fraction, symmetry and connectivity of the dielectric structure. In this thesis a large set of photonic structures are developed to help understand the nature of the dependencies and provide a platform for easy fabrication of three-dimensional structures with large complete photonic band gaps. Two approaches for accessing new structures are examined. A systematic method based on crystallography to search for photonic band gap structures is established in this thesis. A search within the FCC space groups is undertaken resulting in the discovery of two new photonic band gap structures. Specific structures found in self-organizing systems, the single P, the single G, and single D structures, are shown to possess large photonic band gaps. Design guidelines to fabricate these structures by interference lithography are given. A layer-by-layer approximation of the single D structure amenable to fabrication by conventional semiconductor fabrication techniques is proposed. A second technique for obtaining photonic band gap structures with different topologies is based on the splitting of nodes in the diamond network. The realization of these structures using block copolymer self assembly and layer-by-layer lithographic technique are briefly examined.by Martin Maldovan.Ph.D

Narrow Low-Frequency Spectrum and Heat Management by Thermocrystals

By transforming heat flux from particle to wave phonon transport, we introduce a new class of engineered material to control thermal conduction. We show that rationally designed nanostructured alloys can lead to a fundamental new approach for thermal management, guiding heat as photonic and phononic crystals guide light and sound, respectively. Novel applications for these materials include heat waveguides, thermal lattices, heat imaging, thermo-optics, thermal diodes, and thermal cloaking

Enhancing Thermal Transport in Layered Nanomaterials

Abstract A comprehensive rational thermal material design paradigm requires the ability to reduce and enhance the thermal conductivities of nanomaterials. In contrast to the existing ability to reduce the thermal conductivity, methods that allow to enhance heat conduction are currently limited. Enhancing the nanoscale thermal conductivity could bring radical improvements in the performance of electronics, optoelectronics, and photovoltaic systems. Here, we show that enhanced thermal conductivities can be achieved in semiconductor nanostructures by rationally engineering phonon spectral coupling between materials. By embedding a germanium film between silicon layers, we show that its thermal conductivity can be increased by more than 100% at room temperature in contrast to a free standing thin-film. The injection of phonons from the cladding silicon layers creates the observed enhancement in thermal conductivity. We study the key factors underlying the phonon injection mechanism and find that the surface conditions and layer thicknesses play a determining role. The findings presented here will allow for the creation of nanomaterials with an increased thermal conductivity