A field expansions method for scattering by periodic multilayered media

The interaction of acoustic and electromagnetic waves with periodic structures plays an important role in a wide range of problems of scientific and technological interest. This contribution focuses upon the robust and high-order numerical simulation of a model for the interaction of pressure waves generated within the earth incident upon layers of sediment near the surface. Herein is described a Boundary Perturbation Method for the numerical simulation of scattering returns from irregularly shaped periodic layered media. The method requires only the discretization of the layer interfaces (so that the number of unknowns is an order of magnitude smaller than Finite Difference and Finite Element simulations), while it avoids not only the need for specialized quadrature rules but also the dense linear systems characteristic of Boundary Integral/Element Methods. The approach is a generalization to multiple layers of Bruno & Reitich's "Method of Field Expansions" for dielectric structures with two layers. By simply considering the entire structure simultaneously, rather than solving in individual layers separately, the full field can be recovered in time proportional to the number of interfaces. As with the original Field Expansions method, this approach is extremely efficient and spectrally accurate.National Science Foundation (U.S.) (grant No. DMS–0810958)United States. Dept. of Energy (Award No. DE–SC0001549)Massachusetts Institute of Technology. Earth Resources Laborator

Two-photon fabrication of bio-inspired microstructures for optical topological applications

The control of the flow of light using photonic-band-gap materials has received considerable attention over the past decade, and the technological applications of artificially structured metamaterials, like photonic crystals, have demonstrated the potential of artificially engineered media in photonics. Photonic crystals are periodic structures with spatial period comparable in size to the wavelength of light. Unlike unstructured materials, their dispersive properties can be manipulated through the choice of materials and the geometrical design. The idea of periodic systems with a tailored modulation of the dielectric function was motivated by the well-known physics of electronic Bloch states, because the dielectric scattering of light in periodic media presents the same formal solutions as those for the scattering of electrons in periodic potentials. Analogous to the electronic bandgaps formed in semiconductors, photonic crystals possess photonic bandgaps, frequency bands where light is completely reflected due to interference. This unique control of light has inspired the development of many photonic crystal devices such as integrated optical waveguides, cavities, optical-switches and even super-prisms. Recently, however, in the study of electronic systems, it has become apparent that even in the absence of interaction effects, the dispersion relations of the energy bands do not fully characterize the dynamics of wave packets in all symmetry conditions. The additional information, which is not obtainable from the simple knowledge of the energy bands is the topological description of the photonic band structure. Topology, a property related to the global structure of the frequency dispersion of a photonic system, emerged as a new tool for the control of momentum space and an additional degree of freedom for the discovery of fundamentally new states of light. Topological ideas in photonics branch from exciting developments in solid-state physics, along with the discovery of new phases of matter called topological insulators, materials which are conventional insulators in the bulk but support dissipationless topologically protected edge states. Topological insulators have been of interest to physicists as much for their unique physics as for their plethora of potential applications, which include the whole range of possibilities from nano-scale electronic circuits to the realization of Majorana fermions and large-scale quantum computers. Recent works propose to transfer the key feature of topologically non trivial electronic models to the realm of photonics. There are many advantages to studying band topologies in photonic systems. First, in contrast to the topological insulator state in conventional materials, where we are limited to select atomic compositions and crystalline arrangements, in photonics we can literally build a topological system through the selection of materials and geometry and we can tune continuously the design to create any of the allowed bulk or edge dispersions. Second, photons have no Fermi levels, therefore the whole photonic band structure can be probed using photons with different energy. Moreover, there is no fundamental length scale in Maxwell’s equations, therefore experimentalists can work at any wavelength. Finally, the exploitation of topological effects could dramatically improve the robustness of photonic devices in the presence of imperfections. The field of topological photonics has grown exponentially in recent years. Non-trivial topological effects have been proposed across a variety of photonic systems. Much like the field of topological insulators in electronics, topological photonics promises an enormous variety of breakthroughs in both fundamental physics and technological outcomes. Despite these potentials, advancements in topological photonics research has been hindered by diffculties in fabricating 3D structures that fulfill the requirements to exhibit topologically non-trivial properties. Currently, the main challenge in this field is the realisation of topological optical structures for the development of on-chip optical systems that support states of light that are immune to back scatter, robust against perturbation and feature guaranteed unidirectional transmission. The aim of this thesis is to realise and investigate three-dimensional photonic microstructures with topologically non trivial properties and operative wavelength in the optical regime for application as optical signal processing devices. In particular, our goal is to realise photonic crystals that possess frequency isolated linear point degeneracies in their three-dimensional dispersion that define the illusive Weyl points. Weyl points act as monopoles or anti-monopoles Berry flux in momentum space, and carry chirality defined by quantised topological charges. Here we demonstrate a new technique for the experimental realisation of photonic type I Weyl points in a bio-inspired three-dimensional photonic crystal with operative wavelength in the middle-infrared. More importantly, we discover the chiral nature of the photonic Weyl points by coupling with spin-angular momentum carried by circularly polarised light. Our photonic structures are based on the biomimetic gyroid networks, structures that naturally occur in several biological nanostructures such as the wing scales of the \textit{Callophrys Rubi} butterfly. The gyroid network is a three-dimensional periodic network with both cubic symmetry and chirality and thus is an excellent platform for the development of chiral photonic crystals and a powerful platform for the study of novel photonic topological states. We demonstrate that by using a galvo-dithered direct laser writing technique it is possible to fabricate biomimetic gyroid structures with superior control over size, periodicity and filling fraction compared to the biological counterparts. This method is particularly suitable for fabricating achiral double gyroid micro-structures, which are the first step for the Weyl point realisation. Using this technique, we are able to incorporate defects into the double gyroid design and in this way break the parity symmetry, as required in the Weyl points systems. To obtain frequency isolated Weyl points in the band structure and detect them clearly, a high refractive index structure is required. We propose the idea of coating the polymer templates with high refractive index materials creating a core-cladding structure to increase the effective refractive index of the photonic crystals. The core-cladding structure is practically realised via atomic layer deposition of layered-composite nanometric antimony telluride on polymer templates created via three-dimensional direct laser writing. Finally, we characterise the Weyl point structures with angle-resolved transmission spectroscopy and investigate the chiral character of the opposite charged point degeneracies through the coupling with circularly polarised light. In this way we discover a Weyl point-induced mechanism that leads to reversed circular dichroism along the directions that intersect the oppositely charged topological photonic states. The operation of these topological structures at optical wavelengths and efficient fabrication via three-dimensional nano-lithography make them highly desirable in integrated photonic chips and nano-photonic devices. The discovery of the Weyl-point induced reversed circular dichroism provides an entirely new platform for developing topologically protected super-robust photonic devices in angular-momentum-based information processing, circular-dichroism-enabled protein sensing, spintronics and quantum optoelectronics

5th International Conference on Nanomaterials Science and Mechanical Engineering

No abstract available.publishe

Fluctuation-induced interactions and nonlinear nanophotonics

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Physics, 2010.Cataloged from PDF version of thesis.Includes bibliographical references (p. 293-329).We present theoretical and numerical methods for studying Casimir forces and nonlinear frequency conversion in nanophotonic media consisting of arbitrary geometries and materials. The first section of the thesis focuses on the study of various geometry-enabled resonant effects leading to strong nonlinear interactions. The starting point of this work is a coupled-mode theory framework for modeling a wide range of resonant nonlinear frequency-conversion processes in general geometries, ameliorating the need for repeated and expensive finite-difference time-domain simulations. We examine the predictions of the theory for two particular nonlinear processes: harmonic generation and difference-frequency generation. Our results demonstrate strong enhancement of nonlinear interactions at a "critical" input power leading to 100% frequency conversion, among many other interesting dynamical effects. Using a quantum-mechanical description of light, based on cavity quantum electrodynamics, similar enhancement effects are demonstrated at the single-photon level, leading to the possibility of achieving all-optical switching of a single signal photon by a single gating photon in a waveguide-cavity geometry consisting of pumped four-level atoms embedded in a cavity. Finally, we describe how one may tailor the geometry of certain materials to enhance their nonlinear susceptibilities by exploiting a consequence of the Purcell effect. The second section of the thesis, the main contribution of this work, presents a new formulation for studying Casimir forces in arbitrary geometries and materials that directly exploits efficient and well-developed techniques in computational electromagnetism. To begin with, we present the step-by-step conceptual development of our computational method, based on a well-known stress tensor formalism for computing Casimir forces. A proof-of- concept finite-difference frequency-domain implementation of the stress-tensor method is described and checked against known results in simple geometries. Building on this work, we then describe the basic theoretical ingredients of a new technique for determining Casimir forces via antenna measurements in tabletop experiments. This technique is based on a (derived) correspondence between the complex-frequency deformation of the Casimir frequency-integrand for any given geometry and the real-frequency classical electromagnetic response of the same geometry, but with dissipation added everywhere. This correspondence forms the starting point of a numerical Casimir solver based on the finite-difference time-domain method, which we describe and then implement via an off-the-shelf time-domain solver, requiring no modifications. These numerical methods are then used to explore a wide range of geometries and materials, of various levels of complexity: First, a four-body piston-like geometry consisting of two cylinders next to adjacent walls, which exhibits a non-monotonic lateral Casimir force (explained via ray optics and the method of images); Second, a zipper-like, glide-symmetric structure that leads to a net repulsive force arising from a competition between attractive interactions. Finally, we examine a number of geometries consisting of fluid-separated objects and find a number of interesting results. These include: stable levitation and suspension of compact objects, dispersion-induced orientation transitions, and strong non-zero temperature Casimir effects.by Alejandro Rodriguez-Wong.Ph.D

Specular and diffuse X-ray scattering studies of surfaces and interfaces

The behaviour of thin film semiconducting and magnetic devices depends upon the chemical and physical status of the as-grown structure. Since the dimensions of many devices can be in the Angstrom and nanometre region, characterisation techniques capable of measuring chemical and physical parameters in this regime are necessary if an understanding of the effect of specimen structure on observed properties is to be achieved. This thesis uses high resolution x-ray scattering techniques to characterise sub-micron layered structures of semiconducting and magnetic materials. Double crystal diffraction is routinely employed in the semiconductor industry for the on line inspection of sample quality. While material parameters such as sample perfection and layer composition may be rapidly deduced, the non-destructive measurement of layer thickness is more difficult (particularly for multilayered samples) and lengthy simulation procedures are often necessary to extract the thickness information from a double crystal diffraction profile. However, for semiconductor structures which act as Bragg case interferometers, oscillations (known as thickness fringes) appear in the diffracted profile. The period of these fringes can be directly related to layer thickness. Attempts to Fourier transform diffraction data, in order to automatically extract the frequency" of thickness fringes, have previously been only partially successful. It is shown that the relatively weak intensity of the thickness fringes and the presence of the substrate peak in the analysed diffraction data, drastically reduce the quality of the subsequent Fourier transform. A procedure for the manipulation of diffraction data is suggested, where an "average” envelope is fitted to the thickness fringes and used to normalise the data. The application of an auto-correlation is shown to further increase the quality of the Fourier transform of the normalised data. The application of Fourier transform techniques to the routine analysis of double crystal diffraction data is discussedA novel technique for the measurement of absolute lattice parameters of single crystals is presented, which is capable of determining lattice constants with an absolute accuracy of around 2 parts in 10(^5). The technique requires only the use of a conventional triple crystal diffractometer with motorised 20 circle movement and the provision for a fine, precise rocking motion of the analyser. To demonstrate the technique, exemplary measurements on GaAs and InAs crystals are presented. Triple crystal diffi-action analysis has been performed on three material systems of current technological interest; the Hg(_1-x)Mn(_x)Te on GaAs, the Cd(_1-x)Hg(_x)Te on CdTe/Cd(_1-x)Zn(_x)Te and the low temperature grown GaAs systems. Studies on the Hg(_1-x)Mn(_x)Te on GaAs system reveal that the principal contribution to the rocking curve widths of layers grown using the direct alloy growth (DAG) method, arise from the tilt (i.e., mosaicity) of layer sub-grains. This finding is confirmed by double crystal topography which shows that the layers are highly mosaic with a typical grain size of (130±5)µm. Topographic studies of Hg(_1-x)Mn(_x)Te on GaAs, grown using the interdiffused multilayer process (IMP), show that sample quality is significantly improved with single crystal material being produced using this growth method. Triple crystal diffraction studies of the Cd(_1-x)Hg(_x)Te on CdTe/Cd(_0.96)Zn(_0.04)Te systems reveal several findings. These are that the main contribution to rocking curve widths is from lattice tilts and that the tilt distribution increases as the layer thickness decreases. Further, the quality of the Cd(_0.96)Zn(_0.04)Te substrate analysed is superior to that of the CdTe and that Cd(_1-x)Hg(_x)Te layers grown on Cd(_0.96)Zn(_0.04)Te substrates are generally of a higher quality than those grown on CdTe. Triple crystal analysis of MBE and ALE grown GaAs films, deposited at low growth temperatures, show that, at equivalent temperatures, superior quality films are grown by the ALE technique. Narrow lattice dilation and tilt distributions are reported for GaAs films grown at temperatures as low as 300ºC by the ALE method. While diffraction techniques are highly suitable for the study of relatively perfect crystalline material, they are not appropriate to the analysis of heavily dislocated or even amorphous specimens. This is not the case for the Grazing Incidence X-Ray Reflectivity (GIXR) technique, whose sensitivity is not dependent upon sample structure. The GIXR technique is currently attracting increasing interest following the development of commercial instruments. In this thesis, GIXR has been used to probe the layer thickness and interfacial roughness of a series of magnetic multilayer samples and Si/Si(_x)Ge(_1-x) superlattices. The technique is shown to be capable of measuring layer thickness to an accuracy of one monolayer. Modelling of specular GIXR data for the Si/Si(_x)Ge(_1-x) superlattices has shown that the magnitude of interfacial roughness is different for the two types of interface within the high Ge content superlattice samples, the Si(_x)Ge(_1-x)→Si interface possessing a long range sinusoidal roughness of (0.9±0.3)nm, in addition to die short range roughness of (0.5±0.2)nm present at all interfaces. By collecting the diffuse scatter from a GIXR experiment, conformal, or correlated, roughness has been observed in both the multilayer and superlattice samples

Summaries of FY 1997 Research in the Chemical Sciences

The objective of this program is to expand, through support of basic research, knowledge of various areas of chemistry, physics and chemical engineering with a goal of contributing to new or improved processes for developing and using domestic energy resources in an efficient and environmentally sound manner. Each team of the Division of Chemical Sciences, Fundamental Interactions and Molecular Processes, is divided into programs that cover the various disciplines. Disciplinary areas where research is supported include atomic, molecular, and optical physics; physical, inorganic, and organic chemistry; chemical energy, chemical physics; photochemistry; radiation chemistry; analytical chemistry; separations science; heavy element chemistry; chemical engineering sciences; and advanced battery research. However, traditional disciplinary boundaries should not be considered barriers, and multi-disciplinary efforts are encouraged. In addition, the program supports several major scientific user facilities. The following summaries describe the programs

Entropie‐dominierte Selbstorganisationsprozesse birnenförmiger Teilchensysteme

The ambition to recreate highly complex and functional nanostructures found in living organisms marks one of the pillars of today‘s research in bio- and soft matter physics. Here, self-assembly has evolved into a prominent strategy in nanostructure formation and has proven to be a useful tool for many complex structures. However, it is still a challenge to design and realise particle properties such that they self-organise into a desired target configuration. One of the key design parameters is the shape of the constituent particles. This thesis focuses in particular on the shape sensitivity of liquid crystal phases by addressing the entropically driven colloidal self-assembly of tapered ellipsoids, reminiscent of „pear-shaped“ particles. Therefore, we analyse the formation of the gyroid and of the accompanying bilayer architecture, reported earlier in the so-called pear hard Gaussian overlap (PHGO) approximation, by applying various geometrical tools like Set-Voronoi tessellation and clustering algorithms. Using computational simulations, we also indicate a method to stabilise other bicontinuous structures like the diamond phase. Moreover, we investigate both computationally and theoretically(density functional theory) the influence of minor variations in shape on different pearshaped particle systems, including the stability of the PHGO gyroid phase. We show that the formation of the gyroid is due to small non-additive properties of the PHGO potential. This phase does not form in pears with a „true“ hard pear-shaped potential. Overall our results allow for a better general understanding of necessity and sufficiency of particle shape in regards to colloidal self-assembly processes. Furthermore, the pear-shaped particle system sheds light on a unique collective mechanism to generate bicontinuous phases. It suggests a new alternative pathway which might help us to solve still unknown characteristics and properties of naturally occurring gyroid-like nano- and microstructures.Ein wichtiger Bestandteil der heutigen Forschung in Bio- und Soft Matter Physik besteht daraus, Technologien zu entwickeln, um hoch komplexe und funktionelle Strukturen, die uns aus der Natur bekannt sind, nachzubilden. Hinsichtlich dessen ist vor allem die Methode der Selbstorganisation von Mikro- und Nanoteilchen hervorzuheben, durch die eine Vielzahl verschiedener Strukturen erzeugt werden konnten. Jedoch stehen wir bei diesem Verfahren noch immer vor der Herausforderung, Teilchen mit bestimmten Eigenschaften zu entwerfen, welche die spontane Anordnung der Teilchen in eine gewünschte Struktur bewirken. Einer der wichtigsten Designparameter ist dabei die Form der Bausteinteilchen. In dieser Dissertation konzentrieren wir uns besonders auf die Anfälligkeit von Flüssigkristallphasen bezüglich kleiner Änderungen der Teilchenform und nutzen dabei das Beispiel der Selbstorganisation von Entropie-dominierter Kolloide, die dem Umriss nach verjüngten Ellipsoiden oder "Birnen" ähneln. Mit Hilfe von geometrischen Werkzeugen wie z.B. Set-Voronoi Tessellation oder Cluster-Algorithmen analysieren wir insbesondere die Entstehung der Gyroidphase und der dazugehörigen Bilagenformation, welche bereits in Systemen von harten Birnen, die durch das pear hard Gaussian overlap (PHGO) Potential angenähert werden, entdeckt wurden. Des Weiteren zeigen wir durch Computersimulationen eine Strategie auf, um andere bikontinuierliche Strukturen, wie die Diamentenphase, zu stabilisieren. Schlussendlich betrachten wir sowohl rechnerisch (durch Simulationen) als auch theoretisch (durch Dichtefunktionaltheorie) die Auswirkungen kleiner Abweichungen der Teilchenform auf das Verhalten des kolloiden, birnenförmigen Teilchensystems, inklusive der Stabilität der PHGO Gyroidphase. Wir zeigen, dass die Entstehung des Gyroids auf kleinen nicht-additiven Eigenschaften des PHGO Birnenmodells beruhen. In ''echten'' harten Teilchensystemen entwickelt sich diese Struktur nicht. Insgesamt ermöglichen unsere Ergebnisse einen besseren Einblick auf das Konzept von notwendiger und hinreichender Teilchenform in Selbstorganistationsprozessen. Die birnenförmigen Teilchensysteme geben außerdem Aufschluss über einen ungewöhnlichen, kollektiven Mechanismus, um bikontinuierliche Phasen zu erzeugen. Dies deutet auf einen neuen, alternativen Konstruktionsweg hin, der uns möglicherweise hilft, noch unbekannte Eigenschaften natürlich vorkommender, gyroidähnlicher Nano- und Mikrostrukturen zu erklären

Nonlinear optics on the nanoscale: solid state and quantum applications

Nonlinear optical processes in solid-state systems are typically weak but the strength of the nonlinear interactions can be significantly increased in nanophotonic structures providing a local enhancement of the optical fields. This thesis deals with three experiments in this thematic area. The Kerr nonlinearity of 2D Ruddlesden-Popper-phase (2D RPP) lead halide perovskite flakes is investigated by means of the Z-scan method, after which the flakes are combined with an array of Al nanoantennae to form a nonlinear metasurface. Both nonlinear absorption and refraction are strongly enhanced near the exciton ground state of the 2D RPP with peak values of beta_eff = -256 cm/MW and n_2 = +- 10^-13 m^2/W comparable to top values in the literature. The combined metasurface leads to a further enhancement of the Kerr nonlinearity attributed to the strong near-fields of the nanoantennae, however, with a complicated saturation behaviour analysed separately. Moreover, a partial hybridisation of the exciton and plasmon modes is found. Wavelength-sized cones etched into a GaN layer are studied for their improvement of frequency mixing efficiencies in the visible-NIR range compared to an unstructured GaN layer. As a preliminary result, the second-harmonic generation (SHG) efficiency is found to be increased by a factor of 10. In future samples with smaller cones, pumped at the magnetic dipole resonance, a larger efficiency increase is expected. Frequency mixing in a metasurface of Au nanoantennae is investigated with the aim of working towards sources of visible-NIR photon pairs with tailored spectral properties. Frequency scans to map out the joint spectral density are conducted, which will allow for testing the spectral properties of improved future antenna designs. The expected spontaneous parametric down-conversion (SPDC) count rates in the GaN and Au nanoantenna samples are estimated via the stimulated emission tomography (SET) technique.Open Acces