## Publications

158 results found

Koufidis SF, McCall MW, 2023, Wavelength-independent Bragg-like reflection in uniaxial bi-anisotropic media, *Journal of the Optical Society of America B*, Vol: 40, Pages: 2829-2829, ISSN: 0740-3224

<jats:p>We have recently shown that a uniform birefringent medium exhibits a circular Bragg phenomenon that relies solely on resonant tuning of the medium’s parameters, rather than on a particular wavelength resonance, thus rendering its electromagnetic response arbitrarily broadband. The resonant condition, however, necessitated a chirality parameter equal to the average refractive index. Here, we demonstrate that non-axial wave propagation in an axially bi-anisotropic uniaxial medium also enacts such a response and, moreover, relaxes the severity of the tuning condition, offering a convenient platform for controlling both the location of the resonance and the corresponding bandwidth. Anomalous wave propagation at a singular point is additionally identified, in the vicinity of which a remarkably high and intrinsically broadband refractive index can be realized. Recent demonstrations of meta-media with giant and controllable chirality pave the path towards the realistic embodiment of a highly efficient optical modulator.</jats:p>

Koufidis SF, Koutserimpas TT, McCall MW, 2023, Temporal analog of Bragg gratings, *Optics Letters*, Vol: 48, Pages: 4500-4500, ISSN: 0146-9592

<jats:p>Recently, scalar coupled-wave theory has been employed to analyze a medium with periodic time-varying permittivity, providing simple expressions and, consequently, straightforward insights into the parametric amplification mechanism. Here, we combine such an approach with the Möbius transformation method to investigate the dispersion and optical response of a finite “time-slab” of the aforementioned medium. We demonstrate the temporal analog of a Bragg grating, discuss the differences with its spatial counterpart, and examine nontrivial scenarios of the permittivity’s time-modulation, such as chirping and apodization. Furthermore, we propose a highly selective and, moreover, single-spatial-interface optical sensor, based on phase delineation.</jats:p>

McCall MW, Koufidis SF, 2023, Broadband Bragg phenomenon in a uniform birefringent medium, *OPTICS LETTERS*, Vol: 48, Pages: 1096-1099, ISSN: 0146-9592

Koufidis SF, McCall MW, 2022, M?bius transformation and coupled-wave theory: Complete identification of the transfer matrix, *PHYSICAL REVIEW A*, Vol: 106, ISSN: 2469-9926

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- Citations: 1

McCall M, Koufidis S, 2022, Reverse circular Bragg phenomenon, *Physical Review Research*, Vol: 4, ISSN: 2643-1564

he problem of axial propagation of circularly polarized light in a circularly birefringent structurally chiral medium is exactly solved via full electromagnetic analysis. Underlying symmetries of the system's characteristic matrix reveal interesting insights, which are confirmed by coupled wave theory. For extreme values of chirality, a reverse circular Bragg resonance arises in the negative refraction regime where handedness reversal of counterpart modes occurs. A condition is identified under which circular birefringence precisely offsets structural chirality, rendering the medium simply linearly birefringent. Manufacturing such a medium is feasible via current metamedia and inorganic materials technology and has applications in optics, optoelectronics, and sensing.

Kinsler P, McCall MW, Oulton RF,
et al., 2022, The surprising persistence of time-dependent quantum entanglement, *New Journal of Physics*, Vol: 24, Pages: 1-14, ISSN: 1367-2630

The mismatch between elegant theoretical models and the detailed experimental reality is particularly pronounced in quantum nonlinear interferometry (QNI). In stark contrast to theory, experiments contain pump beams that start in impure states and that are depleted, quantum noise that affects—and drives—any otherwise gradual build up of the signal and idler fields, and nonlinear materials that are far from ideal and have a complicated time-dependent dispersive response. Notably, we would normally expect group velocity mismatches to destroy any possibility of measurable or visible entanglement, even though it remains intact—the mismatches change the relative timings of induced signal–idler entanglements, thus generating 'which path' information. Using an approach based on the positive-P representation, which is ideally suited to such problems, we are able to keep detailed track of the time-domain entanglement crucial for QNI. This allows us to show that entanglement can be—and is—recoverable despite the obscuring effects of real-world complications; and that recovery is attributable to an implicit time-averaging present in the detection process.

Gratus J, Kinsler P, McCall MW, 2021, Temporary Singularities and Axions: An Analytic Solution that Challenges Charge Conservation, *ANNALEN DER PHYSIK*, Vol: 533, ISSN: 0003-3804

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Gratus J, McCall MW, Kinsler P, 2020, Electromagnetism, axions, and topology: A first-order operator approach to constitutive responses provides greater freedom, *Physical Review A: Atomic, Molecular and Optical Physics*, Vol: 101, Pages: 043804-1-043804-18, ISSN: 1050-2947

We show how the standard constitutive assumptions for the macroscopic Maxwell equations can be relaxed. This is done by arguing that the Maxwellian excitation fields (D, H) should be dispensed with, on the grounds that they (a) cannot be measured, and (b) act solely as gauge potentials for the charge and current. In the resulting theory, it is only the links between the fields (E, B) and the charge and current (ρ, J) that matter; and so we introduce appropriate linear operator equations that combine the Gauss and Maxwell-Ampère equations with the constitutive relations, eliminating (D, H). The result is that we can admit more types of electromagnetic media, notably, these relations can allow coupling in the bulk to a homogeneous axionic material; in contrast to standard electromagnetism where any homogeneous axionlike field is completely decoupled in the bulk, and only accessible at boundaries. We also consider a wider context, including the role of topology, extended nonaxionic constitutive parameters, and treatment of Ohmic currents. A range of examples including an axionic response material is presented, including static electromagnetic scenarios, a possible metamaterial implementation, and how the transformation optics paradigm would be modified. Notably, these examples include one where topological considerations make it impossible to model using (D, H).

Gratus J, Kinsler P, McCall M, 2019, Evaporating black-holes, wormholes, and vacuum polarisation: must they always conserve charge?, *Foundations of Physics*, Vol: 49, Pages: 330-350, ISSN: 0015-9018

A careful examination of the fundamentals of electromagnetic theory showsthat due to the underlying mathematical assumptions required for Stokes’ Theorem,global charge conservation cannot be guaranteed in topologically non-trivial space-times. However, in order to break the charge conservation mechanism we must alsoallow the electromagnetic excitation fieldsD,Hto possess a gauge freedom, just asthe electromagnetic scalar and vector potentialsφandAdo. This has implicationsfor the treatment of electromagnetism in spacetimes where black holes both form andthen evaporate, as well as extending the possibilities for treating vacuum polarisation.Using this gauge freedom ofD,Hwe also propose an alternative to the acceptednotion that a charge passing through a wormhole necessarily leads to an additional(effective) charge on the wormhole’s mouth.

Gratus J, Kinsler P, McCall MW, 2019, Maxwell's (D, H) excitation fields: lessons from permanent magnets, *EUROPEAN JOURNAL OF PHYSICS*, Vol: 40, ISSN: 0143-0807

McCall M, Pendry J, Galdi V,
et al., 2018, Roadmap on transformation optics, *Journal of Optics A: Pure and Applied Optics*, Vol: 20, ISSN: 1464-4258

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

Kinsler P, McCall MW, 2017, Generalized transformation design: metrics, speeds, and diffusion, *Wave Motion*, Vol: 77, Pages: 91-106, ISSN: 0165-2125

We show that a unified and maximally generalized approach to spatialtransformation design is possible, one that encompasses all second order waves,rays, and diffusion processes in anisotropic media. Until the final step, it isunnecessary to specify the physical process for which a specific transformationdesign is to be implemented. The principal approximation is the neglect of waveimpedance, an attribute that plays no role in ray propagation, and is thereforeirrelevant for pure ray devices; another constraint is that for waves thespatial variation in material parameters needs to be sufficiently smallcompared with the wavelength. The key link between our general formulation anda specific implementation is how the spatial metric relates to the speed ofdisturbance in a given medium, whether it is electromagnetic, acoustic, ordiffusive. Notably, we show that our generalised ray theory, in allowing foranisotropic indexes (speeds), generates the same predictions as does a wavetheory, and the results are closely related to those for diffusion processes.

McCall MW, Kinsler P, 2017, Space-time Cloaking, World Scientific Handbook of Metamaterials and Plasmonics: In 4 Volumes, Editors: Craster, Guenneau, Publisher: World Scientific Series in Nan, Pages: 173-203, ISBN: 9789813227613

McCall MW, Carter IE, Weir K, 2017, Investigation of the variation of near-circular polarization in Scarabaeoidea beetles, *Materials Today: Proceedings*, Vol: 4, Pages: 4942-4951, ISSN: 2214-7853

Variation in the reflection of circularly polarized light (CP) of a substantial number of beetles, of both the Hybosoridae and Scarabaeidae families, is discussed. Classifications of the spectral shapes were made for Cetonia aurata aurata beetles, which were related to variations within the chiral chitin structure and have been computationally modelled. It was seen that single peaked spectra were not the predominant spectral shape and that more complex structures are responsible for the spectra observed. Two structural perturbations methods to the single pitched structure are proposed to be responsible for the more complex spectral shapes. Further CP analysis of the genus rutelinae:Chrysina was undertaken with variations in broadband reflection observed within the optima species.

McCall MW, Gratus J, Kinsler P,
et al., 2016, On spacetime transformation optics: temporal and spatial dispersion, *New Journal of Physics*, Vol: 18, ISSN: 1367-2630

The electromagnetic implementation of cloaking, the hiding ofobjectsfrom sight by di-verting and reassembling illuminating electromagnetic fields has now been with us ten years,while the notion of hidingeventsis now five. Both schemes as initially presented neglectedthe inevitable dispersion that arises when a designed medium replaces vacuum under trans-formation. Here we define a transformation design protocol that incorporates both spacetimetransformations and dispersive material responses in a natural and rigorous way. We showhow this methodology is applied to an event cloak designed to appear as a homogeneousand isotropic butdispersivemedium. The consequences for spacetime transformation de-sign in dispersive materials are discussed, and some parameter and bandwidth constraintsidentified.

Kinsler P, Gratus J, McCall MW, et al., 2016, Dispersion in space-time transformation optics, URSI International Symposium on Electromagnetic Theory (EMTS), Publisher: IEEE, ISSN: 2163-405X

The use of spacetime cloaking to hide events is an intriguing trick, but the unavoidable presence of dispersion limits the performance of any implementation, and needs to be accounted for. We show how the dispersion changes under transformation.

Carter IE, Weir K, McCall MW,
et al., 2016, Variation in the circularly polarized light reflection of Lomaptera (Scarabaeidae) beetles, *Journal of the Royal Society Interface*, Vol: 13, ISSN: 1742-5689

An extended spectroscopic study on the left-through-left circularly polarised reflection spectra of a large number of beetles from the Australasian Scrabaeidae:Cetoniinae of the Lomaptera genus was undertaken. We have obtained a five-category spectral classification.The principal spectral features, which even within the genus range from blue to infra-red, are related to structural chirality in the beetle shells. The detailed features of each spectral classification are related to different structural perturbations of the helix including various pitch values and abrupt twist defects. These spectral characteristics and associated shell structures are confirmed on the basis of simple modelling. An important conclusion from our study is that the simple helical structure resulting in a single symmetric Bragg peak is not thedominant spectral type. Rather the reality is a rich tapestry of spectral types. One intriguing specimen is identified via an SEM micrograph to consist of a double interstitial helix leadingto a particular double-peak spectrum.

McCall MW, 2016, Transformation optics, curvature and beyond (Conference Presentation), Conference on Metamaterials X, Publisher: Society of Photo-optical Instrumentation Engineers (SPIE), ISSN: 1996-756X

Although the transformation algorithm is very well established and implemented, some intriguing questions remain unanswered. 1) In what precise mathematical sense is the transformation optics algorithm ‘exact’? The invariance of Maxwell’s equations is well understood, but in what sense does the same principle not apply to acoustics (say)? 2) Even if the fields are transformed in a way that apparently mimic vacuum perfectly, it is easy to construct very simple examples where the impedance of the transformed medium is no longer isotropic and homogeneous. This would seem to imply a fundamental shortcoming in any claim that electromagnetic cloaking has been reduced to technology. 3) Transformations are known to exist that introduce a discrepancy between the Poynting vector and the wave-vector. Does this distinction carry any physical significance? We have worked extensively on understanding a commonality between transformation theories that operates at the level of rays – being interpreted as geodesics of an appropriate manifold. At this level we now understand that the *key* problem underlying all attempts to unify the transformational approach to disparate areas of physics is how to relate the transformation of the base metric (be it Euclidean for spatial transformation optics, or Minkowskian for spacetime transformation optics) to the medium parameters of a given physical domain (e.g. constitutive parameters for electromagnetism, bulk modulus and mass density for acoustics, diffusion constant and number density for diffusion physics). Another misconception we will seek to address is the notion of the relationship between transformation optics and curvature. Many have indicated that transformation optics evinces similarities with Einstein’s curvature of spacetime. Here we will show emphatically that transformation optics cannot induce curvature. Inducing curvature in an electromagnetic medium requires the equivalent of a gravitational sou

McCall MW, Kinsler P, Topf RDM, 2016, The refractive index of reciprocal electromagnetic media, *Journal of Optics*, Vol: 18, ISSN: 2040-8978

We study the electromagnetics of media described by identical inhomogeneous relative dielectric and magnetic tensors, ${\boldsymbol{\epsilon }}={\boldsymbol{\mu }}.$ Such media occur generically as spatial transformation media, i.e. electromagnetic media that are defined by a deformation of space. We show that such media are completely described by a refractive index $n({\bf{r}},\hat{{\bf{s}}})$ that depends on position ${\bf{r}}$ and direction $\hat{{\bf{s}}},$ but is independent of polarization. The phase surface is always ellipsoidal, and $n({\bf{r}},\hat{{\bf{s}}})$ is therefore represented by the radius vector to the surface of the ellipsoid. We apply our method to calculate the angular dependence of the refractive index in the well-studied cylindrical cloak and to a new kind of structurally chiral medium induced by a twist deformation. By way of a simple example we also show that media for which ${\boldsymbol{\epsilon }}={\boldsymbol{\mu }}$ do not in general preserve the impedance properties of vacuum. The implications of this somewhat surprising conclusion for the field of transformation optics are discussed.

McCall MW, 2015, The Refractive Index of Electromagnetically Reciprocal Media, *Journal of Optics*, ISSN: 2040-8978

We study the electromagnetics of media described by identical inhomogeneous relative dielectric and magnetic tensors, € = µ. Such media occur generically as spatial transformation media , i.e. electromagnetic media that are defined by a deformation of space. We show that such media are completely described by a refractive index n (r,ŝ) that depends on position r and directionŝ, but is independent of polarization. The phase surface is alwaysellipsoidal, and n (r,ŝ) is therefore represented by the radius vector to the surface of the ellipsoid. We apply our method to calculate the angular dependence of the refractive index in the well-studied cylindrical cloak and to a new kind of structurally chiral medium induced by a twist deformation. By way of a simple example we also show that media for which € = µ do notin general preserve the impedance properties of vacuum. The implications ofthis somewhat surprising conclusion for the field of transformation optics are discussed.

Kinsler P, McCall MW, 2015, The futures of transformations and metamaterials, *Photonics and Nanostructures-Fundamentals and Applications*, Vol: 15, Pages: 10-23, ISSN: 1569-4429

McCall MW, 2015, Cloaking and Transformation Optics, Photonics, Volume 3 Photonics Technology and Instrumentation, Editors: Andrews, Publisher: John Wiley & Sons, Pages: 215-252, ISBN: 9781118225547

The underlying principles of Transformation Optics are reviewed. The covariant representation of Maxwell’s equations used enables us to describe two canonical examples of cloaking – spatial (object) and spacetime (event) cloaking – within a single unified formalism. Experimental implementations of both types of cloak are described that exploit reduced parameter schemes. Other methods that reduce the technological challenges for object cloaking include so-called carpet cloaking, a form of which can be achieved with naturally occurring birefringent media, and conformal (‘refractive index only’) cloaking in 2-D.

McCall MW, Hodgkinson IJ, Wu Q, 2014, Birefringent Thin Films and Polarizing Elements, Publisher: World Scientific, ISBN: 9781783265374

Birefringent Thin Films and Polarizing Elements (2nd Edition) includes the significant advances that have been made since the first book on tilted-columnar films was published.

Topf RDM, McCall MW, 2014, Modes of structurally chiral lasers, *PHYSICAL REVIEW A*, Vol: 90, ISSN: 1050-2947

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Kinsler P, McCall MW, 2014, Transformation devices: Event carpets in space and space-time, *PHYSICAL REVIEW A*, Vol: 89, ISSN: 1050-2947

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- Citations: 16

Kinsler P, McCall MW, 2014, Cloaks, editors, and bubbles: applications of spacetime transformation theory, *Annalen der Physik*, Vol: 526, Pages: 51-62, ISSN: 0003-3804

Spacetime or ‘event’ cloaking was recently introduced as a concept, and the theoretical design for such a cloak was presented for illumination by electromagnetic waves [McCall et al., J. Opt. 2011]. Here it is described how event cloaks can be designed for simple wave systems, using either an approximate ‘speed cloak’ method, or an exact full‐wave one. Further, details of many of the implications of spacetime transformation devices are discussed, including their (usually) directional nature, spacetime distortions (as opposed to cloaks), and how leaky cloaks manifest themselves. More exotic concepts are also addressed, in particular concepts that follow naturally on from considerations of simple spacetime transformation devices, such as spacetime modeling and causality editors. A proposal for implementing an interrupt‐without‐interrupt concept is described. Finally, the design for a time‐dependent ‘bubbleverse’ is presented, based on temporally modulated Maxwell's Fisheye transformation device (T‐device) in a flat background spacetime.

McCall M, 2013, Transformation optics and cloaking, *Contemporary Physics*, Vol: 54, Pages: 273-286, ISSN: 0010-7514

McCall MW, Favaro A, Kinsler P,
et al., 2011, A spacetime cloak, or a history editor, *JOURNAL OF OPTICS*, Vol: 13, ISSN: 2040-8978

We introduce a new type of electromagnetic cloak, the spacetime cloak (STC), which conceals events rather than objects. Non-emitting events occurring during a restricted period are never suspected by a distant observer. The cloak works by locally manipulating the speed of light of an initially uniform light distribution, whilst the light rays themselves always follow straight paths. Any 'perfect' spacetime cloak would necessarily rely upon the technology of electromagnetic metamaterials, which has already been shown to be capable of deforming light in ways hitherto unforeseen-to produce, for example, an electromagnetic object cloak. Nevertheless, we show how it is possible to use intensity-dependent refractive indices to construct an approximate STC, an implementation that would enable the distinct signature of successful event cloaking to be observed. Potential demonstrations include systems that apparently violate quantum statistics, 'interrupt-without-interrupt' computation on convergent data channels and the illusion of a Star Trek transporter.

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- Citations: 147

McCall MW, Kinsler P, Favaro A, 2011, All kinds of cloaks, all kinds of transformations, Conference on Metamaterials - Fundamentals and Applications IV, Publisher: SPIE-INT SOC OPTICAL ENGINEERING, ISSN: 0277-786X

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- Citations: 2

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