arXiv subjects

Martin W. McCall

Publications and source records attributed to Martin W. McCall.

17 recordsLinked to original sources

arXiv2025

Chirality-driven all-optical image differentiation

Optical analog computing enables powerful functionalities, including spatial differentiation, image processing, and ultrafast linear operations. Yet, most existing approaches rely on resonant or periodic structures, whose performance is strongly wavelength-dependent, imposing bandwidth limitations and demanding stringent fabrication tolerances. Here, to address some of these challenges, we introduce a highly tunable platform for optical processing, composed of two cascaded uniform slabs exhibiting both circular and linear birefringence, whose response exhibits features relevant to optical processing without relying on resonances. Specifically, using a coupled-wave theory framework we show that sharp reflection minima, referred to as spectral holes, emerge from destructive interference between counter-propagating circularly polarized waves in uniform birefringent slabs, and can be engineered solely through parameter tuning without requiring any spatial periodicity. Unlike traditional Bragg scattering, this mechanism operates without a resonance condition and enables a comparatively broader spectral response through material parameter tuning in spatially uniform media. When operated in the negative refraction regime enabled by giant chirality, the proposed system acts as a polarization-selective Laplacian-like operator, whose functionality is evidenced by an edge-detection proof of concept. The required material parameters align closely with recent experimental demonstrations of giant, tunable chirality via meta-optics, presenting a promising pathway towards compact and reconfigurable platforms for all-optical pattern recognition and image restoration.

physics.optics↗

arXiv2025

Electromagnetic Boundary Conditions for Space--time Interfaces

We give a general family of electromagnetic boundary conditions applicable to arbitrary space--time interfaces between electromagnetic media, which include the known space--only and time--only boundary conditions as special cases. These boundary conditions describe a broad class of electromagnetic interfaces, including surfaces in arbitrary motion, ultra-thin (metasurface) media, and cases where the media on one or both sides of the boundary can be both spatially and temporally dispersive. Our approach utilizes 4-dimensional spacetime and addresses the question of how, and in what ways, an electromagnetic field may be connected across a 3-dimensional hypersurface. We show that our proposed boundary conditions are the most general conditions consistent with causality and linearity.

physics.optics↗

arXiv2025

Coupling Light Waves to Gravitational Waves

We demonstrate analytically that gravitational waves, upon interacting with co-propagating electromagnetic radiation in a plasma, induce distinctive sidebands on the modulated light, thereby providing a detectable signature of their presence. Employing a fully covariant coupled-wave framework, we envision gravitational waves as phase-insensitive ``luminal moving gratings'' and derive explicit phase-matching conditions that articulate such an interaction whilst conserving both energy and momentum. Beyond preserving the directional signature of gravitational waves, the coupling mechanism imposes no coherence requirements on the photon-by-graviton scattering, hence enabling possibilities for exploiting cosmic microwave background radiation. Although detection at low frequencies is constrained by the requirement of long interaction lengths, advances in laser technology are poised to enable high-frequency gravitational wave detection, potentially unveiling insights into the primordial spacetime ripples that have been traversing the cosmos since the inflationary epoch.

physics.gen-ph↗

arXiv2024

Light propagation in time-periodic bi-isotropic media

Photonic structures and time-crystals, wherein time is incorporated as an additional degree of freedom for light manipulation, have necessitated the development of analytical and semi-analytical tools. However, such tools are currently limited to specific configurations, leaving several unexplored physical phenomena akin to photonic time-crystals elusive. In this communication, using a coupled-wave theory approach, we unveil the occurring light propagation phenomena in a time-periodic bi-isotropic medium whose permittivity, permeability, and chirality parameter are periodic functions of time. Contrary to their static counterparts, we demonstrate that the considered dynamic medium couples only co-handed counter-propagating waves. In cases of non-constant impedance, we prove that two first-order momentum gaps are formed in the Brillouin diagram, resulting in parametric amplification with different amplification factors and corresponding momenta for the right- and left-handed modes, respectively. The presence of chirality plays a major role in manipulating lightwave signals by controlling the center of resonance, the corresponding bandwidth, and the amplification factor in a distinct fashion for each mode. For a finite ``time-slab'' of the medium, we analytically derive the scattering coefficients as functions of time and momentum, discussing how extreme values of optical rotation grant access to the temporal analog of the chirality-induced negative refraction regime. Finally, we demonstrate the mechanism under which elliptical polarizations may change field orientation whilst the electric field propagates in a momentum gap, thus simultaneously showcasing parametric amplification.

physics.optics↗

arXiv2022

Reverse Circular Bragg Phenomenon

The axial propagation of circularly polarized light in an optically active structurally chiral medium is exactly solved via full electromagnetic analysis. Some symmetries of the system's characteristic matrix reveal new insights, which are confirmed by coupled wave theory. For extreme values of chirality, now accessible via metamaterials, a reverse circular Bragg resonance arises in the negative refraction regime where handedness reversal of counterpart modes occurs. A condition is identified under which optical activity offsets structural chirality, rendering the medium simply birefringent.

physics.optics↗

arXiv2022

Broadband Bragg phenomenon in a uniform medium

A new mechanism of Bragg reflection is identified, one that, remarkably, occurs in a uniform medium and relies on resonant tuning of the medium's parameters. Due to uniformity, reflection ensues over a broad wavelength range, much like a metal, but it is polarization dependent: one circular state is reflected, whilst the other is transmitted. Such a medium can thus provide a broadband, low-loss polarization divider/combiner. Assessment of the required parameters suggests that manufacturing is within range of current metamaterials technology. By externally regulating the medium's parameters, a highly efficient optical modulator is possible with potential applications across optics, optoelectronics, and photonics.

physics.optics↗

arXiv2022

Electromagnetism, Axions, and Topology: a first-order operator approach to constitutive responses provides greater freedom

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-Ampere equations with the constitutive relations, eliminating (D,H). The result is that we can admit more types of electromagnetic media -- notably, the new relations can allow coupling in the bulk to a homogeneous axionic material; in contrast to standard EM where any homogeneous axion-like field is completely decoupled in the bulk, and only accessible at boundaries. We also consider a wider context, including the role of topology, extended non-axionic constitutive parameters, and treatment of Ohmic currents. A range of examples including an axonic 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).

physics.optics↗

arXiv2022

Temporary Singularities and Axions: an analytic solution that challenges charge conservation

We construct an analytic solution to Maxwell's equations that violates global charge conservation, by building on the possibilities demonstrated in [Foundations of Physics 49, 330 (2019)]. The construction is valid for a spacetime containing a temporary singularity and a Maxwellian electrodynamics containing a proposed "topological" axion field. As such, it demonstrates that although "physics breaks down" at a singularity, the physical laws on the spacetime still impose constraints on what can happen. Further, the concepts of transformation optics can be applied to show that our specific mathematical solution has a much wider applicability.

gr-qc↗

arXiv2022

Evaporating black-holes, wormholes, and vacuum polarisation: must they always conserve charge?

A careful examination of the fundamentals of electromagnetic theory shows that due to the underlying mathematical assumptions required for Stokes' Theorem, global charge conservation cannot be guaranteed in topologically non-trivial spacetimes. However, in order to break the charge conservation mechanism we must also allow the electromagnetic excitation fields D, H to possess a gauge freedom, just as the electromagnetic scalar and vector potentials phi and A do. This has implications for the treatment of electromagnetism in spacetimes where black holes both form and then evaporate, as well as extending the possibilities for treating vacuum polarisation. Using this gauge freedom of D, H we also propose an alternative to the accepted notion that a charge passing through a wormhole necessarily leads to an additional (effective) charge on the wormhole's mouth.

gr-qc↗

arXiv2022

The surprising persistence of time-dependent quantum entanglement

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 a "positive-P" approach ideally suited to such problems, we show how the time-domain entanglement crucial for QNI can be -- and is -- recoverable despite the obscuring effects of real-world complications.

quant-ph↗

arXiv2019

Maxwell's (D, H ) excitation fields: lessons from permanent magnets

Macroscopic Maxwellian electrodynamics consists of four field quantities along with electric charges and electric currents. The fields occur in pairs, the primary ones being the electric and magnetic fields (E,B), and the other the excitation fields (D,H). The link between the two pairs of field is provided by constitutive relations, which specify (D,H) in terms of (E,B); this last connection enabling Maxwell's (differential) equations to be combined in a way that supports waves. In this paper we examine the role played by the excitation fields (D,H), showing that they can be regarded as not having a physical existence, and are merely playing a mathematically convenient role. This point of view is made particularly relevant when we consider competing constitutive models of permanent magnets, which although having the same measurable magnetic properties, have startlingly different behaviours for the magnetic excitation field H.

physics.class-ph↗

arXiv2017

Generalized Transformation Design: metrics, speeds, and diffusion

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

physics.class-ph↗

arXiv2016

On Spacetime Transformation Optics: Temporal and Spatial Dispersion

The electromagnetic implementation of cloaking, the hiding of objects from sight by diverting and reassembling illuminating electromagnetic fields has now been with us ten years, while the notion of hiding events is now five. Both schemes as initially presented neglected the inevitable dispersion that arises when a designed medium replaces vacuum under transformation. Here we define a transformation design protocol that incorporates both spacetime transformations and dispersive material responses in a natural and rigorous way. We show how this methodology is applied to an event cloak designed to appear as a homogeneous and isotropic but dispersive medium. The consequences for spacetime transformation design in dispersive materials are discussed, and some parameter and bandwidth constraints identified.

physics.optics↗

arXiv2016

Four Poynting Theorems

The Poynting vector is an invaluable tool for analysing electromagnetic problems. However, even a rigorous stress-energy tensor approach can still leave us with the question: is it best defined as $\Vec{E} \cross \Vec{H}$ or as $\Vec{D} \cross \Vec{B}$? Typical electromagnetic treatments provide yet another perspective: they regard $\Vec{E} \cross \Vec{B}$ as the appropriate definition, because $\Vec{E}$ and $\Vec{B}$ are taken to be the fundamental electromagnetic fields. The astute reader will even notice the fourth possible combination of fields: i.e. $\Vec{D} \cross \Vec{H}$. Faced with this diverse selection, we have decided to treat each possible flux vector on its merits, deriving its associated energy continuity equation but applying minimal restrictions to the allowed host media. We then discuss each form, and how it represents the response of the medium. Finally, we derive a propagation equation for each flux vector using a directional fields approach; a useful result which enables further interpretation of each flux and its interaction with the medium.

physics.optics↗

arXiv2015

Cloaks, editors, and bubbles: applications of spacetime transformation theory

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, Favaro, Kinsler, Boardman 2011]. Here we describe how event cloaks can be designed for simple wave systems, using either an approximate `speed cloak' method, or an exact full-wave one. Further, we discuss in detail many of the implications of spacetime transformation devices, covering their (usually) directional nature, spacetime distortions (as opposed to cloaks), and how leaky cloaks manifest themselves. We also address more exotic concepts that follow naturally on from considerations of simple spacetime transformation devices, such as spacetime modeling and causality editors; and describe a proposal for implementing the interrupt-without-interrupt concept suggested by McCall et al. We also describe how we might make a simple time-dependent `bubbleverse' by embedding a modulated Maxwell's Fisheye transformation device (T-device) in a flat background spacetime.

physics.optics↗

arXiv2015

Transformation devices: carpets in space and space-time

Here we extend the theory of space-time or event cloaking into that based on the carpet or ground-plane reflective surface. Further, by recasting and generalizing a scalar acoustic wave model into a new mathematically covariant form, we also show how transformation theories for optics and acoustics can be combined into a single prescription. The single prescription, however, still respects the fundamental differences between electromagnetic and acoustic waves, which then provide us with an existence test for any desired transformation device -- are the required material properties (the required constitutive parameters) physically permitted by the wave theory being designed for? Whilst electromagnetism is a flexible theory permitting almost any transformation device (T-device) design, we show that the acoustic model used here is more restricted.

physics.optics↗

arXiv2002

On mediums with negative phase velocity: a brief overview

Several issues relating to oppositely directed phase velocity and power flow are reviewed. A necessary condition for the occurrence of this phenomenon in isotropic dielectric-magnetic mediums is presented. Ramifications for aberration-free lenses, homogenization approaches, and complex mediums are discussed.

physics.class-ph↗