arXiv subjects

Ian R. Hooper

Publications and source records attributed to Ian R. Hooper.

11 recordsLinked to original sources

arXiv2026

Real-time THz ptychography using an optically modulated aperture

Ptychography allows ordinary cameras to perform both quantitative phase and intensity imaging. However, the requirement for relative object to probe motion and high measurement count significantly limits imaging speed and application scope. In this work, we introduce optically modulated aperture ptychography, using a silicon photomodulator as a rapidly reconfigurable aperture, deployable across the microwave, millimeter wave and terahertz bands. We demonstrate experimentally at ~ 0.1 THz real-time ptychographic image capture, without moving parts. Furthermore, a reconfigurable aperture facilitates diverse and spatially multiplexed probes. We use a multiplexed known-probe reconstruction strategy that improves signal-to-noise ratio and increases the information content per measurement, enabling the capture of fluid dynamics at 16 fps. Our optical modulation approach enables high-throughput ptychographic imaging of dynamic samples, for applications such as non-destructive evaluation and in-vivo biomedical imaging.

physics.optics↗

arXiv2026

A microwave super-resolution imaging approach towards breast cancer margin mapping

Accurate characterisation of margins in excised breast cancer tumours is critical to the success of surgical interventions. Yet margin status is typically confirmed post-operatively using histopathology. Here we present a microwave single pixel imaging technique designed for use in intraoperative margin assessment. By leveraging the photo-induced change in microwave transparency of a silicon modulator placed under the sample, we map the microwave reflectivity of tissue-mimicking phantoms with deeply sub-wavelength resolution, allowing hydration mapping across large areas (10 x 10 cm) at ~1 mm resolution. We evaluate the discriminatory capability of our method using gelatine-based tumour phantoms with water-content variations designed to mimic the contrast between malignant tissue and tumour margins in resected breast specimens. We demonstrate the capability to identify, locate and quantify inadequate margins up to the typically targeted minimum thickness of 2 mm. Furthermore, using numerical modelling, we show that our approach is expected to be resilient to patient-specific tissue differences. These results establish microwave single-pixel imaging as a promising route towards real-time intraoperative assessment of margins in excised breast tumours.

physics.optics↗

arXiv2025

Scattering in Time-Varying Drude-Lorentz Models

Motivated by recent experiments, the theoretical study of wave propagation in time varying materials is of current interest. Although significant in nearly all such experiments, material dispersion is commonly neglected in theoretical studies. Yet, as we show here, understanding the precise microscopic model for the material dispersion is crucial for predicting experimental outcomes. Here we study the temporal scattering coefficients of four different time-varying Drude-Lorentz models, exploring how an incident continuous wave splits into forward and backward waves due to an abrupt change in plasma frequency. The differences in the predicted scattering are unique to time-varying media, and arise from the exact way in which the time variation appears in the various model parameters. We verify our results using a custom finite difference time domain algorithm, concluding with a discussion of the limitations that arise from using these models with an abrupt change in plasma frequency.

physics.optics↗

arXiv2025

Quasi Normal Modes in Dispersive Photonic Time-Crystals

Quasinormal modes characterise the transient response of static optical cavities. Here, we introduce the notion of a Floquet quasinormal mode to describe transient responses in photonic time crystals. Contrasting their static counterparts, exceptional points associated with symmetry transitions are an inherent feature, as modes spontaneously and non-perturbatively lock their phase to the oscillations of the material. We further investigate the limiting behaviour of the Floquet quasinormal modes in large cavities. Distinct non-perturbative behaviour arises in time-modulated systems as increasingly large time-crystal cavities come closer to achieving the maximum gain predicted from a bulk wavenumber bandgap.

physics.optics↗

arXiv2024

The 2024 Active Metamaterials Roadmap

Active metamaterials are engineered structures that possess novel properties that can be changed after the point of manufacture. Their novel properties arise predominantly from their physical structure, as opposed to their chemical composition and can be changed through means such as direct energy addition into wave paths, or physically changing/morphing the structure in response to both a user or environmental input. Active metamaterials are currently of wide interest to the physics community and encompass a range of sub-domains in applied physics (e.g. photonic, microwave, acoustic, mechanical, etc.). They possess the potential to provide solutions that are more suitable to specific applications, or which allow novel properties to be produced which cannot be achieved with passive metamaterials, such as time-varying or gain enhancement effects. They have the potential to help solve some of the important current and future problems faced by the advancement of modern society, such as achieving net-zero, sustainability, healthcare and equality goals. Despite their huge potential, the added complexity of their design and operation, compared to passive metamaterials creates challenges to the advancement of the field, particularly beyond theoretical and lab-based experiments. This roadmap brings together experts in all types of active metamaterials and across a wide range of areas of applied physics. The objective is to provide an overview of the current state of the art and the associated current/future challenges, with the hope that the required advances identified create a roadmap for the future advancement and application of this field.

physics.app-ph↗

arXiv2024

Symmetry-Protected Lossless Modes in Dispersive Time-Varying Media

We give an exact application of a recently developed, operator-based theory of wave propagation in dispersive, time-varying media. Using this theory we find that the usual symmetry of complex conjugation plus changing the sign of the frequency, required for real valued fields, implies that the allowed propagation constants in the medium are either real valued or come in conjugate pairs. The real valued wave numbers are only present in time-varying media, implying that time variation leads to modes that are free from dissipation, even in a lossy medium. Moreover, these symmetry-unbroken waves lack a defined propagation direction. This can lead to a divergent transmission coefficient when waves are incident onto a finite, time-varying slab. The techniques used in this work present a route towards further analytic applications of this operator formalism.

physics.optics↗

arXiv2023

Terahertz imaging through emissivity control

Adoption of terahertz technologies is hindered by the lack of cost-effective THz sources. Here we demonstrate a fundamentally new way to generate and control THz radiation, via spatio-temporal emissivity modulation. By patterning the optical photoexcitation of a surface-passivated silicon wafer, we locally control the free-electron density, and thereby pattern the wafer's emissivity in the THz part of the electromagnetic spectrum. We show how this unconventional source of controllable THz radiation enables a new form of incoherent computational THz imaging. We use it to image various concealed objects, demonstrating this scheme has the penetrating capability of state-of-the-art THz imaging approaches, without the requirement of femto-second pulsed laser sources. Furthermore, the incoherent nature of thermal radiation also ensures the obtained images are free of interference artifacts. Our spatio-temporal emissivity control paves the way towards a new family of long-wavelength structured illumination, imaging and spectroscopy systems.

physics.optics↗

arXiv2022

Space-squeezing optics in the microwave spectral region

Optical systems often consist largely of empty space, as diffraction effects that occur through free-space propagation can be crucial to their function. Contracting these voids offers a path to the miniaturisation of a wide range of optical devices. Recently, a new optical element - coined a 'spaceplate' - has been proposed, that is capable of emulating the effects of diffraction over a specified propagation distance using a thinner non-local metamaterial [Nat. Commun. 12, 3512 (2021)]. The compression factor of such an element is given by the ratio of the length of free-space that is replaced to the thickness of the spaceplate itself. In this work we test a prototype spaceplate in the microwave spectral region (20-23\,GHz) - the first such demonstration designed to operate in ambient air. Our device consists of a Fabry-Pérot cavity formed from two reflective metasurfaces, with a compression factor that can be tuned by varying the size of perforations within each layer. Using a pair of directive horn antennas, we measure a space compression factor of up to approx. 6 over an NA of 0.34 and fractional bandwidth of 6%. We also investigate the fundamental trade-offs that exist between the compression factor, transmission efficiency, numerical aperture (NA) and bandwidth of this single resonator spaceplate design, and highlight that it can reach arbitrarily high compression factors by restricting its NA and bandwidth.

physics.optics↗

arXiv2020

Total internal reflection based super-resolution imaging for sub-IR frequencies

For measurements designed to accurately determine layer thickness, there is a natural trade-off between sensitivity to optical thickness and lateral resolution due to the angular ray distribution required for a focused beam. We demonstrate a near-field imaging approach that enables both sub-wavelength lateral resolution and optical thickness sensitivity. We illuminate a sample in a total internal reflection geometry, with a photo-activated spatial modulator in the near-field, which allows optical thickness images to be computationally reconstructed in a few seconds. We demonstrate our approach at 140 GHz (wavelength 2.15 mm), where images are normally severely limited in spatial resolution, and demonstrate mapping of optical thickness variation in inhomogeneous biological tissues.

physics.app-ph↗

arXiv2017

Absence of Anderson localization in certain random lattices

We report on the transition between an Anderson localized regime and a conductive regime in a 1D scattering system with correlated disorder. We show experimentally that when long-range correlations, in the form of a power-law spectral density with power larger than 2, are introduced the localization length becomes much bigger than the sample size and the transmission peaks typical of an Anderson localized system merge into a pass band. As other forms of long-range correlations are known to have the opposite effect, i.e. to enhance localization, our results show that care is needed when discussing the effects of correlations, as different kinds of long-range correlations can give rise to very different behavior.

physics.optics↗

arXiv2014

Topological Modes in One Dimensional Solids and Photonic Crystals

It is shown theoretically that a one-dimensional crystal with time reversal symmetry is characterized by a Z_{2} topological invariant that predicts the existence or otherwise of edge states. This is confirmed experimentally through the construction and simulation of a photonic crystal analogue in the microwave regime.

cond-mat.mes-hall↗