Browsing by Subject "Metasurfaces"

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Bio-inspired nanophotonics : manipulating light at the nanoscale with plasmonic metamaterials
(2013-05) Zhao, Yang, active 21st century; Alù, Andrea
Metals interact very differently with light than with radio waves and finite conductivities and losses often limit the way that RF concepts can be directly transferred to higher frequencies. Plasmonic materials are investigated here for various optical applications, since they can interact, confine and focus light at the nanoscale; however, regular plasmonic devices are severely limited by frequency dispersion and absorption, and confined signals cannot travel along plasmonic lines over few wavelengths. For these reasons, novel concepts and materials should be introduced to successfully manipulate and radiate light in the same flexible way we operate at lower frequencies. In line with these efforts, optical metamaterials exploit the resonant wave interaction of collections of plasmonic nanoparticles to produce anomalous light effects, beyond what naturally available in optical materials and in their basic constituents. Still, these concepts are currently limited by a variety of factors, such as: (a) technological challenges in realizing 3-D bulk composites with specific nano-structured patterns; (b) inherent sensitivity to disorder and losses in their realization; (c) not straightforward modeling of their interaction with nearby optical sources. In this study, we develop a novel paradigm to use single-element nanoantennas, and composite nanoantenna arrays forming two-dimensional metasurfaces and three-dimensional metamaterials, to control and manipulate light and its polarization at the nanoscale, which can possibly bypass the abovementioned limitations in terms of design procedure and experimental realization. The final design of some of the metamaterial concepts proposed in this work was inspired by biological species, whose complex structure can exhibit superior functionalities to detect, control and manipulate the polarization state of light for their orientation, signaling and defense. Inspired by these concepts, we theoretically investigate and design metasurfaces and metamaterial models with the help of fully vectorial numerical simulation tools, and we are able to outline the limitations and ultimate conditions under which the average optical surface impedance concept may accurately describe the complex wave interaction with planar plasmonic metasurfaces. We also experimentally explore various technological approaches compatible with these goals, such as the realization of lithographic single-element nanoantenna and nanoantenna arrays with complex circuit loads, periodic arrays of plasmonic nanoparticles or nanoapertures, and stacks of rotated plasmonic metasurfaces. At the conclusion of this effort, we have theoretically analyzed, designed and experimentally realized and characterized the feasibility of using discrete metasurfaces to realize phenomena and performance that are not available in natural materials, oftentimes inspired by the biological world.
Breaking temporal symmetries in metamaterials and metasurfaces
(2015-08) Fleury, Romain; Alù, Andrea; Becker, Michael F; Haberman, Michael R; Belkin, Mikhail; Wang, Zheng
Metamaterials are artificially structured materials that are engineered to interact with waves in extraordinary ways, leading to unconventional physical phenomena not found in natural materials, such as negative refraction and cloaking. So far, they have been for the most part based on structures that are inherently symmetric upon time reversal. In this work, I will explore the largely uncharted properties of electromagnetic and acoustic metamaterials that are designed to purposely break time-reversal symmetry. I will show how time-reversal symmetry breaking can be exploited to build a novel class of non-reciprocal acoustic and electromagnetic devices, such as isolators and circulators. These devices will then be used as building blocks to construct the acoustic equivalent of topological insulators, a metamaterial that supports one-way phononic transport on its edges with strong topological protection against defects and disorder. I will study the exceptional properties of time-asymmetric systems that fulfill a special kind of space-time symmetry, consisting in taking their mirror image and running time backwards. Known as Parity-Time (PT) symmetry, this property leads to anomalous scattering behaviors, such as unidirectional invisibility and phase compensation. I will demonstrate theoretically and experimentally how PT-symmetric metasurface pairs can replicate electromagnetic phenomena usually associated with bulk metamaterials, including negative refraction, planar focusing and invisibility effects, with the clear advantage of being completely loss-immune and potentially overcoming the bandwidth limitations of passive metamaterials.
Diffusion of airborne sound using acoustic metamaterials
(2018-08-16) Cheung, Fiona S.; Haberman, Michael R. (Michael Richard), 1977-; Wilson, Preston S.
Acoustic surface treatments, such as absorbers and diffusers, are used to control unwanted reflections in rooms. These reflections, when excessive, can create an unpleasant experience for both audiences and performers in any space, not just performance spaces. Existing acoustic absorbers, including Helmholtz resonators, quarter-wave resonators, and panel absorbers, are discussed in this thesis. The most common diffuser, the Quadratic Residue Diffuser (QRD), is also explored in detail. While QRDs are well known for their predictability and ease of design, they suffer from two main drawbacks: size and aesthetics. This thesis explores the use of acoustic metamaterials, specifically coiled space metamaterials, to replace the QRD. These metamaterials seek to address these specific problems with QRD designs while replicating its ability to scatter acoustic waves in a predictable fashion. Two specific coiled space metamaterial designs are discussed in detail, and their responses are compared to that of the QRD to determine whether they can be viable replacements. The results of the comparisons, while unable to replicate the response of the standard QRD exactly, did show modest improvements. More validation must be done before a definitive answer can be given as to whether either of these designs are able to be successful replacements for the QRD
Intersubband polaritonic metasurfaces for flat nonlinear optics
(2019-12) Nookala, Nishant; Belkin, Mikhail A.; Alu, Andrea; Bank, Seth R; Brener, Igal; Wasserman, Daniel
Much attention has been drawn in recent years towards the creation of two-dimensional equivalents of traditionally three-dimensional optical elements. The reduction in dimensionality offers advantages such as a smaller spatial footprint, reduced manufacturing and operating complexity, and access to more exotic optical functionalities than can be achieved in a traditional bulk optics approach. Metasurface-based flat optical components, comprised of arrays of subwavelength-spaced scatterers, have emerged as a promising candidate for the realization of this vision, but have been largely constrained to the domain of linear interactions of light with matter. Nonlinear effects are intrinsically weak in bulk materials, and even more so across subwavelength volumes, meaning realization of practical and efficient nonlinear optical metasurfaces has generally been out of reach. In this thesis, a series of metasurface designs are proposed and experimentally verified to exhibit record setting nonlinearities across deeply subwavelength volumes. The metasurfaces are comprised of metallic nanoantennas patterned onto multiple-quantum well structures possessing tailored intersubband electronic resonances. We term the ensemble structure an Intersubband Polaritonic Metasurface (IPM), owing to the nature of the coupling of the electromagnetic antenna mode with the electronic intersubband transition. Suitable design of the antenna geometry allows one to access and enhance the giant intrinsic nonlinearity of intersubband transitions, which are subject to polarization-selection rules that typically inhibit their operation configuration. Further, due to their subwavelength thickness, IPMs are not constrained to the typical phase-matching considerations of bulk nonlinear optics. In the first part of this study, we investigate IPMs for efficient second-harmonic generation. A novel architecture consisting of etched nanoantenna volumes is proposed and experimentally demonstrated to exhibit a record setting second-order nonlinearity in the mid-infrared spectral range. Using this design, we experimentally demonstrate a means to control the phase of the generated nonlinear signal via the Pancharatnam-Berry geometric phase approach. We then present and demonstrate a simplified fabrication procedure for the creation of efficient IPMs, forgoing the rather cumbersome wafer-bonding and substrate removal process inherent to earlier designs. Finally, we propose and verify a metasurface design exhibiting an ultrafast third-order nonlinearity
Metasurfaces based on nanoresonator modes coupling to intersubband transitions
(2023-09-10) Xu, Jiaming, Ph. D.; Belkin, Mikhail A.; Wasserman, Daniel, 1976-; Belyanin, Alexey; Bank, Seth R; Alu, Andrea
Metasurfaces, which are planar structures made of massive numbers of nanoresonators, have attracted significant attention due to their ability to control the frequency, intensity, and wavefronts of transmitted or reflected waves while providing a small spatial footprint. These advantages, together with the ability to efficiently couple light into the material in their nanocavities, make them promising candidate structures to harness the huge nonlinearities associated with intersubband transitions multi-quantum-well semiconductor. Optical transitions between electron subbands in semiconductor heterostructures are polarized along the material growth direction. This property makes it difficult to harness intersubband optical response using light incident or emitted normal to the surface of a semiconductor material. Our group have addressed this problem by coupling intersubband transitions to optical modes in metal antennas fabricated on top of multi-quantum-well heterostructures. This approach has led to the development of nonlinear metasurfaces with record-high nonlinear optical response, demonstrated by our group, as well as to the demonstrations of other photonic structures, e.g., quantum cascade laser metasurfaces that operate as vertical external cavity surface emitting lasers. However, there are also limitations which hinder their further developments. These include: (1) all successful intersubband metasurfaces demonstrated to date use metal nanoantennas, and thus are very lossy and vulnerable to the thermal damages; (2) most of the intersubband metasurfaces demonstrated to date are based on InP or GaAs materials systems and their operating wavelengths are limited to the mid-infrared and terahertz parts of the spectrum by the limited conduction band offset of InP- and GaAs-based semiconductor heterostructures; The focus of my PhD work was to investigate and overcome the limitations mentioned above using the following approaches: (1) the development of an all-dielectric nanoresonator metasurface based on Mie resonators for second harmonic generation; (2) investigation of intersubband transitions in GaN/AlGaN heterostructures grown on various non-polar substrates for extending to extend the intersubband metasurface material pool into higher energy range; (3) besides, I have also theoretically investigated the use of metasurfaces to create Purcell-enhanced mid-infrared light emitting devices using quantum cascade laser material that naturally has very low efficiency of spontaneous light emission.
Metasurfaces for cell capturing and infrared spectroscopy
(2018-08) Kelp, Glen; Fitzpatrick, Richard, 1963-; Shvets, G.; Tsoi, Maxim; Sreenivasan, S V; Gordon, Vernita
Metasurfaces, a 2-dimensional subclass of metamaterials, have attracted attention due to the ability to fabricate thin devices with extraordinary properties. High localized field enhancement of mid-infrared (IR) Fano resonant metasurfaces makes them an appealing platform for sensing of biological materials. Metasurface-enhanced infrared spectroscopy (MEIRS) has been used in this work to study mid-IR spectra of human cells, a methodology known as spectral cytopathology. Various characteristic vibrational modes of cells are recognized using MEIRS of fixed and live cells. Fixed normal and cancerous colon cells are distinguished by their mid-IR spectra using principal component analysis (PCA). A microfluidic device is proposed and constructed for capturing and spectral analysis of live cells. Dielectrophoresis (DEP) is used to capture cells in aqueous solution directly onto metasurfaces for their interrogation with MEIRS. It is shown that live cell spectra can be collected within a few minutes. DEP captured epithelial cancer cells and lymphocytes are distinguished by their spectra using PCA. Furthermore, it is demonstrated that only epithelial cells can be captured on the metasurfaces using DEP, while preventing blood cells from being collected. This opens the possibility to use such a device for selective capturing of circulating tumor cells from blood samples for immediate spectral characterization.
Multi-physics modeling of electromagnetically driven surface plasma discharges
(2019-12-06) Kim, Yunho, 1989-; Raja, Laxminarayan L.; Varghese, Philip L.; Goldstein, David B.; Bisetti, Fabrizio; Hallock, Gary A.
This dissertation presents the computational modeling of non-equilibrium plasma discharges on an electromagnetically driven surface and its application to plasma assisted combustion. We address challenges often encountered in high pressure plasma discharges such as the non-uniform formation of plasmas due to filamentations and show how they could be handled by using a particular type of metamaterial. A metamaterial in the present context is an artificial composite assembled with periodic elements smaller than an incident wavelength. Metamaterials have drawn significant interest in engineering communities during the past few decades due to their extraordinary electromagnetic (EM) characteristics, e.g., a negative refractive index, that cannot be naturally excited using conventional methods or materials. An interesting electrodynamic phenomenon associated with metamaterials is the possible surface wave excitation on the artificially engineered surfaces. In particular, by carefully designing the assembly of periodic elements consisting of conductors and dielectrics, a strongly localized surface wave mode known as a spoof surface plasmon polariton (SSPP) can be efficiently excited. The extraordinary electromagnetic property of SSPP is its ability to imitate the behaviors of a surface plasmon polariton (SPP) in a wide range of frequencies (GHz -THz) while SPP can exist only in the optical regime (100’s THz). In this study, our goal is to provide the in-depth analysis of the electrodynamics of SSPP, transients of surface plasma generation due to SSPP resonances, and to demonstrate the feasibility of using it for plasma assisted combustion. We have used multiple computational models that have been developed by our group and added necessary features to simulate the phenomena more accurately. In the first part of this work, we describe the numerical schemes employed for simulations. The computational tool consists of solvers for three different sets of equations: Maxwell’s equations for high frequency (HF) electromagnetics, plasma governing equations for discharge physics, and reactive Navier Stokes equations for combustion. Coupling of these equations must be done carefully due to the multi-scale nature of the high frequency plasma discharges and combustion. The length and time scales range from micrometers to centimeters and nanoseconds to milliseconds, respectively. We provide the details of the coupling of the equations as well as the discretization methods for each set of equations. In this work, one of chief contributions to improving the models is the implementation of an enhanced version of absorbing boundary condition (CFS-PML) for second order Maxwell’s equations. CFS-PML is especially suited for electromagnetic wave simulations that involve conductors which we demonstrate by solving a model problem for the verification of the code. In the second part, we present the computational study of argon surface plasma discharges generated by SSPP. The EM surface wave excitation is first analyzed in depth because the electromagnetic power absorption by electrons determines the transients of plasma breakdown. Electrodynamics of the SSPP excitation is investigated using broadband and monochromatic wave simulations. Instead of the infinite array of periodic elements, we have studied the metamaterial with a finite length for practical engineering applications. It is found that over a wide range of length scales from millimeters to centimeters, the EM waves always have a single node structure at resonance frequencies. The surface wave excited on the metasurface is characteristic of coupling between the cavity mode and surface wave mode. We refer to the resonance pertinent to such coupling as hybrid resonance. The shift of the hybrid resonance frequency is investigated in terms of varying dielectric permittivities, distances between perforations, and the whole lengths of the metasurfaces. Using an optimal configuration of the metasurface, the transients of the surface plasma generation due to the field intensification is studied. Interactions among the surface plasma, SSPP and the incident wave are presented. Multiple simulations show that even if the metasurfaces have different lengths, the transients of surface plasma formation are qualitatively identical at the hybrid resonance frequencies. Such scalability is one of the primary features of metamaterials that can be extended to the plasma discharge. In the third part, plasma assisted combustion induced by microwave sources is studied. Previous research in combustion engineering communities have addressed the importance of volumetric formation of flame kernel for successful combustion. Another key point in plasma assisted combustion is the volumetric generation of radical species in nanosecond timescale, which can significantly reduce the ignition delay for lean fuel-air mixtures. Motivated by the need for mechanisms that can generate combustion enhancing radicals over a large area, we have investigated the feasibility of using the SSPP generated surface plasmas for plasma assisted combustion. A kinetic mechanism of H₂ - air mixture that was previously established by our group is used for this study. A mixture with the equivalence ratio of 0.3 at the initial pressure and temperature of 1 atm and 1000 K is assumed, respectively. Fully coupled simulations show that the cm-scale plasma kernel can be efficiently transitioned into successful ignition and flame propagation with shortened ignition delay. In the last part, we discuss strategies to parallelize the simulation tools for high performance computing. The governing equations solved in this study are spatially discretized using either finite edge element method or cell-centered finite volume method. They require different approaches to achieve parallel scalability, and in particular, the Maxwell’s equations needs a special preconditioning technique to reduce computational time. The technique is known as nodal auxiliary space preconditioning whose theoretical background and performance on a supercomputer are presented. Additionally, the module which solves reactive Navier-Stokes equations is also parallelized to study large scale (centimeters) ignition phenomena. For both plasma-wave coupled solver and combustion solver, we discuss the details of MPI(Message Passing Interface)-based parallelization processes.
Nonlinear and wavelength-tunable plasmonic metasurfaces and devices
(2014-12) Lee, Jongwon; Belkin, Mikhail A.
Wavelength-tunable optical response from solid-state optoelectronic devices is a desired feature for a variety of applications such as spectroscopy, laser emission tuning, and telecommunications. Nonlinear optical response, on the other hand, has an important role in modern photonic functionalities, including efficient frequency conversions, all-optical signal processing, and ultrafast switching. This study presents the development of optical devices with wavelength tunable or nonlinear optical functionality based on plasmonic effects. For the first part of this study, widely wavelength tunable optical bandpass filters based on the unique properties of long-range surface plasmon polaritons (LR SPP) are presented. Planar metal stripe waveguides surrounded by two different cladding layers that have dissimilar refractive index dispersions were used to develop a wide wavelength tuning. The concept was demonstrated using a set of index-matching fluids and over 200nm of wavelength tuning was achieved with only 0.004 of index variation. For practical application of the proposed concept, a thermo-optic polymer was used to develop a widely tunable thermo-optic bandpass filter and over 220 nm of wavelength tuning was achieved with only 8 ºC of temperature variation. Another novel approach to produce a widely wavelength tunable optical response for free-space optical applications involves integrating plasmonic metasurfaces with quantum-electronic engineered semiconductor layers for giant electro-optic effect, which is proposed and experimentally demonstrated in the second part of this study. Coupling of surface plasmon modes formed by plasmonic nanoresonators with Stark tunable intersubband transitions in multi-quantum well structures induced by applying bias voltages through the semiconductor layer was used to develop tunable spectral responses in the mid-infrared range. Experimentally, over 310 nm of spectral peak tuning around 7 μm of wavelength with 10 ns response time was achieved. As the final part of this study, highly nonlinear metasurfaces based on coupling of electromagnetically engineered plasmonic nanoresonators with quantum-engineered intersubband nonlinearities are proposed and experimentally demonstrated. In the proof-of-concept demonstration, an effective nonlinear susceptibility over 50 nm/V was measured and, after further optimization, over 480 nm/V was measured for second harmonic generation under normal incidence. The proposed concept shows that it is possible to engineer virtually any element of the nonlinear susceptibility tensor of the nonlinear metasurface.
Plasmonic moiré metamaterials and metasurfaces : tunable optical properties and nanophotonic applications
(2018-05) Wu, Zilong, Ph. D.; Zheng, Yuebing; Fan, Donglei; Akinwande, Deji; Belkin, Mikhail
Optical metamaterials and metasurfaces, which are properly designed assembly of man-made building blocks with strong interactions with electromagnetic waves, have emerged as promising candidates to replace natural materials due to extraordinary capabilities in light manipulation. In particular, plasmonic metamaterials and metasurfaces have shown their potentials in surpassing diffraction limit, manipulating light beams, and enhancing energy conversion by substantially enhancing light-matter interactions through effects of surface plasmon polaritons (SPPs) or localized surface plasmons (LSPs). Although enormous breakthroughs in plasmonic metamaterials and metasurfaces have been made in the recent decades, the further development of this field towards practical applications has been limited by the lack of high throughput fabrication and the poor tunability in conventional designs. This dissertation presents the cost-effective nanofabrication, rationale design, numerical modelling, experimental demonstration, and application prototyping of new classes of plasmonic metamaterials and metasurfaces featured by moiré patterns. Firstly, we developed novel techniques based on conventional nanosphere lithography to achieve high-throughput nanofabrication of metamaterials and metasurfaces with three-dimensional (3D) and two-dimensional (2D) moiré configurations. Secondly, we demonstrated multiband moiré metasurfaces with flexible tunability based on plasmonic materials including Au and graphene, which have shown potentials as multifunctional biomedical platforms. Thirdly, we developed moiré chiral metamaterials with ultrathin thickness and precisely tunable chiroptical responses, which have been applied as ultrasensitive sensors to achieve label-free enantiodiscrimination of chiral molecules. Finally, we introduced dynamic tunability in moiré chiral metamaterials through stimuli-responsive manipulation of optical coupling in the metamaterials. The results presented in this dissertation could provide guidance to the development of tunable moiré metamaterials and metasurfaces from design and fabrication to characterization and device implementation, benefiting a range of applications from light manipulation to molecular sensing
Radio-frequency metamaterials for cloaking, absorption and wave control
(2015-08) Soric, Jason Christopher; Alù, Andrea; Kerkhoff, Aaron; Ling, Hao; Pearce, John; Yilmaz, Ali
Metamaterial technology has provided us with new tools to explore fascinating applications and physical phenomena. Metamaterials have also inspired many scientists and engineers to think about concepts and ideas under a new light. In this work, we look at ways to use metamaterial technologies to solve modern-day challenges. In particular, we have realized covers enabled by metamaterials and metasurfaces that strongly suppress scattering at all angles, still allowing for field penetration inside the “cloaked” region. This important property opens great opportunities across a multitude of sensor applications. There are many current and forward-looking applications where a sensor with control over its presence to a given environment is of great interest. Advantages of our approach are simplicity of fabrication, resiliency to loss and manufacturing imperfections, extraordinarily low and conformal profile, and natural integration with electronics. In this work, we explore practical applications of our cloaking techniques including: low observability, interference reduction from closely spaced co-located antennas, and reduction or modulation of the scattering of receiving dipole antennas, while maintaining any desired level of received power. With such designs, high-performance sensing and measurement techniques, as well as commercial antenna applications, such as co-site antenna platforms and side-lobe level reduction can be envisioned.