TY - JOUR

T1 - Roadmap on Transformation Optics

AU - McCall, Martin

AU - Pendy, John

AU - Galdi, Vincenzo

AU - Lai, Yun

AU - Horsely, Simon

AU - Zhu, Jain

AU - Mitchell-Thomas, Rhiannon

AU - Quevedo-Teruel, Oscar

AU - Tassin, Philippe

AU - Ginis, Vincent

AU - Martini, Enrica

AU - Minatti, Gabriele

AU - Maci, Stefano

AU - Hao, Yang

AU - Kinsler, Paul

AU - Gratus, Jonathan

AU - Lukens, Joseph

AU - Weiner, Andrew

AU - Leonhardt, Ulf

AU - Smolyaninov, Igor

AU - Smolyaninova, Vera

AU - Thompson, Robert

AU - Wegener, Martin

AU - Kadic, Muamer

AU - Cummer, Steven

N1 - This is an author-created, un-copyedited version of an article accepted for publication/published in Journal of Optics. IOP Publishing Ltd is not responsible for any errors or omissions in this version of the manuscript or any version derived from it. The Version of Record is available online at doi: 10.1088/2040-8986/aab976

PY - 2018/5/22

Y1 - 2018/5/22

N2 - 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 veryprecisely 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 ofscience fiction writers, but realised in the laboratory a few months after the papers proposing thepossibility 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 restricted domain do not appear to have been exhausted yet.Beyond cloaking, the enhanced electromagnetic landscape provided by Transformation Optics hasshown how fully analytic solutions can be found to a number of physical scenarios such as plasmonic systems used in electron energy loss spectroscopy (EELS) and cathodoluminescence (CL). Are there further fields to be enriched?A new twist to Transformation Optics was the extension to the space-time domain. By applyingtransformations to space-time, rather than just space, it was shown that events rather than objectscould be hidden from view; Transformation Optics had provided a means of effectively redactingevents from history. The hype quickly settled into serious nonlinear optical experiments thatdemonstrated the soundness of the idea, and it is now possible to consider the practical implications, particularly in optical signal processing, of having an `interrupt-without-interrupt' facility that the so-called temporal cloak provides. Inevitable issues of dispersion in actual systems have only begun to be addressed.Now that time is included in the programme of Transformation Optics, it is natural to ask what roleideas from General Relativity can play in shaping the future of Transformation Optics. Indeed, one of the earliest papers on Transformation Optics was provocatively titled `General Relativity in Electrical Engineering'. The answer that curvature does not enter directly into transformation optics merely encourages us to speculate on the role of Transformation Optics in defining laboratory analogues.Quite why Maxwell's theory defines a `perfect' transformation theory, while other areas of physicssuch as acoustics are not apparently quite so amenable, is a deep question whose precise,mathematical answer will help inform us of the extent to which similar ideas can be extended to other fields.The contributors to this roadmap review, who are all renowned practitioners or inventors ofTransformation Optics, will give their perspectives into the field's status and future development.

AB - 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 veryprecisely 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 ofscience fiction writers, but realised in the laboratory a few months after the papers proposing thepossibility 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 restricted domain do not appear to have been exhausted yet.Beyond cloaking, the enhanced electromagnetic landscape provided by Transformation Optics hasshown how fully analytic solutions can be found to a number of physical scenarios such as plasmonic systems used in electron energy loss spectroscopy (EELS) and cathodoluminescence (CL). Are there further fields to be enriched?A new twist to Transformation Optics was the extension to the space-time domain. By applyingtransformations to space-time, rather than just space, it was shown that events rather than objectscould be hidden from view; Transformation Optics had provided a means of effectively redactingevents from history. The hype quickly settled into serious nonlinear optical experiments thatdemonstrated the soundness of the idea, and it is now possible to consider the practical implications, particularly in optical signal processing, of having an `interrupt-without-interrupt' facility that the so-called temporal cloak provides. Inevitable issues of dispersion in actual systems have only begun to be addressed.Now that time is included in the programme of Transformation Optics, it is natural to ask what roleideas from General Relativity can play in shaping the future of Transformation Optics. Indeed, one of the earliest papers on Transformation Optics was provocatively titled `General Relativity in Electrical Engineering'. The answer that curvature does not enter directly into transformation optics merely encourages us to speculate on the role of Transformation Optics in defining laboratory analogues.Quite why Maxwell's theory defines a `perfect' transformation theory, while other areas of physicssuch as acoustics are not apparently quite so amenable, is a deep question whose precise,mathematical answer will help inform us of the extent to which similar ideas can be extended to other fields.The contributors to this roadmap review, who are all renowned practitioners or inventors ofTransformation Optics, will give their perspectives into the field's status and future development.

U2 - 10.1088/2040-8986/aab976

DO - 10.1088/2040-8986/aab976

M3 - Journal article

VL - 20

JO - Journal of Optics

JF - Journal of Optics

SN - 2040-8978

IS - 6

M1 - 063001

ER -