419 results found
Huidobro PA, Galiffi E, Guenneau S, et al., 2019, Fresnel drag in space-time-modulated metamaterials.
A moving medium drags light along with it as measured by Fizeau and explained by Einstein's theory of special relativity. Here we show that the same effect can be obtained in a situation where there is no physical motion of the medium. Modulations of both the permittivity and permeability, phased in space and time in the form of traveling waves, are the basis of our model. Space-time metamaterials are represented by effective bianisotropic parameters, which can in turn be mapped to a moving homogeneous medium. Hence these metamaterials mimic a relativistic effect without the need for any actual material motion. We discuss how both the permittivity and permeability need to be modulated to achieve these effects, and we present an equivalent transmission line model.
Galiffi E, Arroyo Huidobro P, Pendry J, et al., Probing Graphene's nonlocality with singular metasurfaces, Nanophotonics, ISSN: 2192-8606
: Singular graphene metasurfaces, conductivity gratings realized by periodically suppressing the local dopinglevel of a graphene sheet, have recently been proposed to efficiently harvest THz light and couple it to surface plasmonsover broad absorption bands, achieving remarkably highfield enhancement. However, the large momentum wavevectors thus attained are sensitive to the nonlocal behaviourof the underlying electron liquid. Here, we extend the theory of singular graphene metasurfaces to account for thefull nonlocal optical response of graphene and discuss theresulting impact on the plasmon resonance spectrum. Finally, we propose a simple local analogue model that isable to reproduce the effect of nonlocality in local-responsecalculations by introducing a constant conductivity offset,which could prove a valuable tool in the modelling of morecomplex experimental graphene-based platforms.
Galiffi E, Huidobro PA, Pendry JB, 2019, Broadband nonreciprocal THz amplification in luminal graphene metasurfaces, Publisher: arXiv
Time has emerged as a new degree of freedom for metamaterials, promising newpathways in wave control. However, electromagnetism suffers from limitations inthe modulation speed of material parameters. Here we argue that theselimitations can be circumvented by introducing a traveling-wave refractiveindex modulation, with the same phase velocity of the waves. We show how theconcept of "luminal grating" can yield giant nonreciprocity, achieve efficientone-way amplification, pulse compression and frequency up-conversion, proposinga realistic implementation in double-layer graphene.
Yang F, Wang Y-T, Huidobro PA, et al., 2019, Nonlocal effects in singular plasmonic metasurfaces, Physical Review B, Vol: 99, ISSN: 2469-9950
A local model of the dielectric response of a metal predicts that singular surfaces, such as sharp-edged structures, have a continuous absorption spectrum and extreme concentration of energy at the singularity. Here, we show that nonlocality drastically alters this picture: The spectrum is now discrete and the energy concentration, though still substantial, is greatly reduced.
Galiffi E, Pendry J, Arroyo Huidobro P, 2019, Singular Graphene Metasurfaces, EPJ Applied Metamaterials, Vol: 6, ISSN: 2272-2394
The spatial tunability of the electron density in graphene enables the dynamic engineering of metasurfaces in the form of conductivity gratings, which can bridge the momentum gap between incident radiation and surface plasmons. Here, we discuss singular graphene metasurfaces, whose conductivity is strongly suppressed at the grating valleys. By analytically characterising their plasmonic response via transformation optics, we first review the physical principles underlying these structures, which were recently found to exhibit broadband, tunable THz absorption. We characterise the spectrum with different common substrates and then move to study in further detail how conductivity gratings may be finely tuned by placing an array of charged gold nanowires at sub-micron distance from the graphene.
Pendry JB, Huidobro PA, Ding K, 2019, Computing one-dimensional metasurfaces, Physical Review B, Vol: 99, ISSN: 2469-9950
We show that complex periodic metasurfaces can be simply represented by conformal transformations from the flat surface of a slab of material to a periodic grating leading to a methodology for computing their properties. Matrix equations are solved to give accurate solutions of Maxwell's equations with detailed derivations given in the Supplemental Material.
Yang F, Huidobro PA, Pendry JB, 2018, Transformation optics approach to singular metasurfaces, Physical Review B, Vol: 98, ISSN: 2469-9950
Surface plasmons dominate the optical response of metal surfaces, and their nature is controlled by surface geometry. Here we study metasurfaces containing singularities in the form of sharp edges and characterized by three quantum numbers despite the two-dimensional nature of the surface. We explore the nature of the plasmonic excitations, their ability to generate large concentrations of optical energy, and the transition from the discrete excitation spectrum of a nonsingular surface to the continuous spectrum of a singular metasurface.
Transformation Optics asks Maxwell’s equations what kind of electromagnetic medium recreate some smooth deformation of space. The guiding principle is Einstein’s principle of covariance: that any physical theory must take the same form in any coordinate system. This requirement fixes very precisely the required electromagnetic medium.The impact of this insight cannot be overestimated. Many practitioners were used to thinking that only a few analytic solutions to Maxwell’s equations existed, such as the monochromatic plane wave in a homogeneous, isotropic medium. At a stroke, Transformation Optics increases that landscape from ‘few’ to ‘infinity’, and to each of the infinitude of analytic solutions dreamt up by the researcher, corresponds an electromagnetic medium capable of reproducing that solution precisely. The most striking example is the electromagnetic cloak, thought to be an unreachable dream of science fiction writers, but realised in the laboratory a few months after the papers proposing the possibility were published. But the practical challenges are considerable, requiring meta-media that are at once electrically and magnetically inhomogeneous and anisotropic. How far have we come since the first demonstrations over a decade ago? And what does the future hold? If the wizardry of perfect macroscopic optical invisibility still eludes us in practice, then what compromises still enable us to create interesting, useful, devices? While 3D cloaking remains a significant technical challenge, much progress has been made in 2-dimensions. Carpet cloaking, wherein an object is hidden under a surface that appears optically flat, relaxes the constraints of extreme electromagnetic parameters. Surface wave cloaking guides sub-wavelength surface waves, making uneven surfaces appear flat. Two dimensions is also the setting in which conformal and complex coordinate transformations are realisable, and the possibilities in this restr
Chen W-J, Hou B, Zhang Z-Q, et al., 2018, Metamaterials with index ellipsoids at arbitrary k-points, NATURE COMMUNICATIONS, Vol: 9, ISSN: 2041-1723
Galiffi E, Pendry JB, Huidobro PA, 2018, Broadband tunable THz absorption with singular graphene metasurfaces, ACS Nano, Vol: 12, Pages: 1006-1013, ISSN: 1936-0851
By exploiting singular spatial modulations of the graphene conductivity, we design a broadband, tunable THz absorber whose efficiency approaches the theoretical upper bound for a wide absorption band with a fractional bandwidth of 185%. Strong field enhancement is exhibited by the modes of this extended structure, which is able to excite a wealth of high-order surface plasmons, enabling deeply subwavelength focusing of incident THz radiation. Previous studies have shown that the conductivity can be modulated at GHz frequencies, which might lead to the development of efficient high-speed broadband switching by an atomically thin layer.
Pendry JB, Huidobro PA, Luo Y, et al., 2017, Compacted dimensions and singular plasmonic surfaces, Science, Vol: 358, Pages: 915-917, ISSN: 0036-8075
In advanced field theories, there can be more than four dimensions to space, the excess dimensions described as compacted and unobservable on everyday length scales. We report a simple model, unconnected to field theory, for a compacted dimension realized in a metallic metasurface periodically structured in the form of a grating comprising a series of singularities. An extra dimension of the grating is hidden, and the surface plasmon excitations, though localized at the surface, are characterized by three wave vectors rather than the two of typical two-dimensional metal grating. We propose an experimental realization in a doped graphene layer.
Demetriadou A, Hamm J, Luo Y, et al., 2017, Spatio-temporal dynamics and control of strong coupling in plasmonic nano-cavities, ACS Photonics, Vol: 4, Pages: 2410-2418, ISSN: 2330-4022
In the light-matter strong coupling regime, the excited state of quantum emitters is inextricably linked to a photonic mode, leading to hybrid states that are part-light and part-matter. Recently, there has been huge effort to realize strong coupling with nanoplasmonics, since it provides a versatile environment to study and control molecules in ambient conditions. Amongst the most promising designs are plasmonic nano-cavities that confine light to unprecedentedly small volumes. Such nano-cavities though support multiple types of modes, with different field profiles and radiative decay rates (bright and dark modes). Here, we show theoretically that the different nature of these modes leads to mode beating within the nano-cavity and the Rabi-oscillations, which alters the spatio-temporal dynamics of the hybrid system. By specifically designing the illumination set-up, we decompose and control the dark and bright plasmon mode excitation and therefore their coupling with quantum emitters. Hence, this work opens new routes for dynamically dressing emitters, to tailor their hybrid states with external radiation.
Zhang Y-L, Pendry JB, Lei DY, 2017, Radial anisotropy from a geometric viewpoint: Topological singularity and effective medium realization, PHYSICAL REVIEW B, Vol: 96, ISSN: 2469-9950
Arroyo-Huidobro P, Maier SA, Pendry, 2017, Tunable plasmonic metasurface for perfect absorption, EPJ Applied Metamaterials, Vol: 4, ISSN: 2272-2394
Tunable metasurfaces, whose functionality can be dynamically modified, open up the possibil-ity of ultra-compact photonic components with reconfigurable applications. Here we consider agraphene monolayer subject to a spatially periodic gate bias, which, thank to surface plasmons inthe graphene, acts as a tunable and extremely compact metasurface for terahertz radiation. Aftercharacterizing its functionality, we show that it serves as the basic building block of an ultrathincomplete absorber. In this subwavelength-thickness device, transmission and reflection channels areblocked and electromagnetic energy is completely absorbed by the metasurface building blocks. Theproposed structure can be used as a modulator, and its frequency of operation can be changed byscaling its size or adjusting the doping level.
Sasihithlu K, Pendry JB, Craster RV, 2017, Van der Waals Force Assisted Heat Transfer, Zeitschrift für Naturforschung - Section A Journal of Physical Sciences, Vol: 72, Pages: 181-188, ISSN: 0932-0784
Phonons (collective atomic vibrations in solids) are more effective in transporting heat than photons. This is the reason why the conduction mode of heat transport in nonmetals (mediated by phonons) is dominant compared to the radiation mode of heat transport (mediated by photons). However, since phonons are unable to traverse a vacuum gap (unlike photons), it is commonly believed that two bodies separated by a gap cannot exchange heat via phonons. Recently, a mechanism was proposed [J. B. Pendry, K. Sasihithlu, and R. V. Craster, Phys. Rev. B 94, 075414 (2016)] by which phonons can transport heat across a vacuum gap – through the Van der Waals interaction between two bodies with gap less than the wavelength of light. Such heat transfer mechanisms are highly relevant for heating (and cooling) of nanostructures; the heating of the flying heads in magnetic storage disks is a case in point. Here, the theoretical derivation for modelling phonon transmission is revisited and extended to the case of two bodies made of different materials separated by a vacuum gap. Magnitudes of phonon transmission, and hence the heat transfer, for commonly used materials in the micro- and nano-electromechanical industry are calculated and compared with the calculation of conduction heat transfer through air for small gaps as well as the heat transfer calculation due to photon exchange.
Pendry JB, 2016, Low frequency plasmons in thin-wire structures: a commentary., Journal of Physics: Condensed Matter, Vol: 28, ISSN: 0953-8984
Pendry JB, Sasihithlu K, Craster RV, 2016, Phonon-assisted heat transfer between vacuum-separated surfaces, Physical Review B - Condensed Matter and Materials Physics, Vol: 94, ISSN: 1098-0121
With increasing interest in nanotechnology, the question arises of how heat is exchanged between materials separated by only a few nanometers of vacuum. Here, we present calculations of the contribution of phonons to heat transfer mediated by van der Waals forces and compare the results to other mechanisms such as coupling through near field fluctuations. Our results show a more dramatic decay with separation than previous work.
Pendry JB, 2016, Controlling nanoscale light (Conference Presentation), Conference on Metamaterials X, Publisher: SPIE-INT SOC OPTICAL ENGINEERING, ISSN: 0277-786X
Our intuitive understanding of light has its foundation in the ray approximation and is intimately connected with our vision: as far as our eyes are concerned light behaves like a stream of particles. Here we look inside the wavelength and study the properties of plasmonic structures with dimensions of just a few nanometres: a tenth or even a hundredth of the wavelength of visible light, where the ray picture fails utterly. In this talk we show how the new concept of transformation optics that manipulates electric and magnetic field lines rather than rays can provide an equally intuitive understanding of sub wavelength phenomena and at the same time be an exact description at the level of Maxwell’s equations. The concepts are applied to a number of plasmonic structures
Kraft M, Luo Y, Pendry JB, 2016, Transformation Optics: A Time- and Frequency-Domain Analysis of Electron-Energy Loss Spectroscopy., Nano Letters, Vol: 16, Pages: 5156-5162, ISSN: 1530-6992
Electron energy loss spectroscopy (EELS) and cathodoluminescence (CL) play a pivotal role in many of the cutting edge experiments in plasmonics. EELS and CL experiments are usually supported by numerical simulations, which-though accurate-may not provide as much physical insight as analytical calculations do. Fully analytical solutions to EELS and CL systems in plasmonics are rare and difficult to obtain. This paper aims to narrow this gap by introducing a new method based on transformation optics that allows to calculate the quasistatic frequency- and time-domain response of plasmonic particles under electron beam excitation. We study a nonconcentric annulus (and ellipse in the Supporting Information ) as an example.
We present a simple design to achieve bianisotropy at visible wavelengths: an ultrathinplasmonic grating made of a gold grating covered by a thin flat layer of gold.We show experimentally and through simulations that the grating exhibits magnetoelectriccoupling and features asymmetric reflection and absorption, all that with adevice thickness of a tenth of the operating wavelength. We compared the spectralresults and retrieved the effective material parameters of different polarizations anddevices. We show that both asymmetry and strong coupling between the incominglight and the optically interacting surfaces are required for obtaining asymmetric opticalbehavior in metasurfaces
Huidobro PA, Kraft M, Maier SA, et al., 2016, Graphene as a Tunable Anisotropic or IsotropicPlasmonic Metasurface, ACS Nano, Vol: 10, Pages: 5499-5506, ISSN: 1936-086X
We demonstrate a tunable plasmonic metasurface by considering a graphene sheetsubject to a periodically patterned doping level. The unique optical properties ofgraphene result in electrically tunable plasmons that allow for extreme confinementof electromagnetic energy in the technologically significant regime of THz frequencies.Here we add an extra degree of freedom by using graphene as a metasurface, proposingto dope it with an electrical gate patterned in the micron or sub-micron scale. Byextracting the effective conductivity of the sheet we characterize metasurfaces periodicallymodulated along one or two directions. In the first case, and making use of theanalytical insight provided by transformation optics, we show an efficient control ofTHz radiation for one polarization. In the second case, we demonstrate a metasurfacewith an isotropic response that is independent of wave polarization and orientation.
Chen H, Pendry JB, Smith DR, 2016, Special issue on transformation optics, Journal of Optics, Vol: 18, ISSN: 2040-8986
Here we study subwavelength gratings for coupling into graphene plasmons by means of an an-alytical model based on transformation optics that is not limited to very shallow gratings. Weconsider gratings that consist of a periodic modulation of the charge density in the graphene sheet,and gratings formed by this conductivity modulation together with a dielectric grating placed inclose vicinity of the graphene. Explicit expressions for the dispersion relation of the plasmon po-laritons supported by the system, and reectance and transmittance under plane wave illuminationare given. We discuss the conditions for maximising the coupling between incident radiation andplasmons in the graphene, finding the optimal modulation strength for a conductivity grating.
Bak AO, Yoxall EO, Sarriugarte P, et al., 2016, Harnessing a Quantum Design Approach for Making Low-Loss Superlenses, NANO LETTERS, Vol: 16, Pages: 1609-1613, ISSN: 1530-6984
Kraft M, Luo Y, Pendry JB, 2016, Transformation Optics and EELS, a Frequency- and Time-domain Analysis, Progress in Electromagnetic Research Symposium (PIERS), Publisher: IEEE, Pages: 1702-1706
Huidobro PA, Kraft M, Maier SA, et al., 2016, Graphene as a Tunable Plasmonic Metasurface with Transformation Optics, Progress in Electromagnetic Research Symposium (PIERS), Publisher: IEEE, Pages: 14-14
Zhao R, Luo Y, Pendry JB, 2016, Transformation optics applied to van der Waals interactions, SCIENCE Bulletin, Vol: 61, Pages: 59-67, ISSN: 2095-9273
The van der Waals force originates from the electromagnetic interaction between quantum fluctuation-induced charges. It is a ubiquitous but subtle force which plays an important role and has a wide range of applications in surface related phenomena like adhesion, friction, and colloidal stability. Calculating the van der Waals force between closely spaced metallic nanoparticles is very challenging due to the strong concentration of electromagnetic fields at the nanometric gap. Especially, at such a small length scale, the macroscopic description of the dielectric properties no longer suffices. The diffuse nonlocal nature of the induced surface electrons which are smeared out near the boundary has to be considered. Here, we review the recent progress on using three-dimensional transformation optics to study the van der Waals forces between closely spaced nanostructures. Through mapping a seemingly asymmetric system to a more symmetric counterpart, transformation optics enables us to look into the behavior of van der Waals forces at extreme length scales, where the effect of nonlocality is found to dramatically weaken the van der Waals interactions.
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