arXiv subjects

David B. Phillips

Publications and source records attributed to David B. Phillips.

At least 19 recordsLinked to original sources

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↗

arXiv2026

Miniaturised transmissive multi-plane light converters via laser-written geometric phase holograms cascaded in glass

Multi-plane light converters (MPLCs) are an emerging beam shaping technology capable of deterministically mapping a basis of input spatial light modes to a different basis of output modes. The ability to perform such multi-modal spatial reformatting operations has many future applications in both classical and quantum photonics, spanning from optical communications to photonic computing and advanced imaging. In this work we fabricate miniaturised transmissive MPLCs fully-encapsulated within a fused silica glass chip using single-step 3D direct laser writing. Our approach relies on the formation of femto-second laser induced birefringent nanogratings with a spatially controllable slow-axis orientation. Multiple layers of these nanogratings are laser-written throughout the volume of the glass to create a sequence of axially separated geometric phase holograms which imprint controllable phase patterns onto circularly polarised read-out light propagating through them. We construct and test a range of proof-of-concept laser-written MPLCs operating in the visible (lambda = 633nm). These miniature beam multiplexers are formed from up to 5 separate phase masks of width ~260um, cascaded along a total length of ~2.7mm, thus occupying a compact volume of ~0.15 mm3. We first demonstrate Hermite-Gaussian (HG) mode sorters capable of diverting the energy carried by up to 28 overlapping HG modes into spatially separated output channels. We next create a 7-mode orthogonal speckle sorter, highlighting the universal nature of the spatial transformations it is possible to encode. Finally, we show analogue optical matrix multiplications achieved by passively scattering light through these 3D structured glass elements. Our work begins to merge the concepts of free-space optics with 3D integrated photonics in glass and plots a path towards the rapid prototyping of robust monolithic MPLC technology.

physics.optics↗

arXiv2025

The radial memory effect in multimode optical fibres

We present a study of a novel memory effect in multimode optical fibres, which manifests itself as an output ring of excess energy at the same radius as an input focussed spot. This effect is robust against fibre perturbations, and we discuss its possible use for spatial multiplexing.

physics.optics↗

arXiv2025

Self-configuring high-speed multi-plane light conversion

Multi-plane light converters (MPLCs) - also known as linear diffractive neural networks - are an emerging optical technology, capable of converting an orthogonal set of optical fields into any other orthogonal set via a unitary transformation. MPLC design is a non-linear problem typically solved by optimising a digital model of the optical system. However, inherently high levels of design complexity mean that even a minor mismatch between this digital model and the physically realised MPLC leads to a severe reduction in real-world performance. Here we address this challenge by creating a self-configuring free-space MPLC. Despite the large number of parameters to be optimised (typically tens of thousands or more), our proof-of-principle device converges in minutes using a method in which light only needs to be transmitted in one direction through the MPLC. Two innovations make this possible. Firstly, we devise an in-situ optimisation algorithm combining wavefront shaping with the principles of wavefront matching that would conventionally be used to inverse-design MPLCs offline in simulation. Secondly, we introduce a new MPLC platform incorporating a microelectromechanical system (MEMS) phase-only light modulator - allowing rapid MPLC switching at up to kiloHertz rates. Our scheme automatically accounts for the physical characteristics of all system components and absorbs any unknown misalignments and aberrations into the final design. We demonstrate self-configured MPLCs capable of mapping random orthogonal speckle input fields to well-defined Laguerre-Gaussian and Hermite-Gaussian output modes, as well as universal mode sorters. Our work paves the way towards large-scale ultra-high-fidelity fast-switching MPLCs and diffractive neural networks, which promises to unlock new applications in areas ranging from optical communications to optical computing and imaging.

physics.optics↗

arXiv2024

Fast and light-efficient wavefront shaping with a MEMS phase-only light modulator

Over the last two decades, spatial light modulators (SLMs) have revolutionised our ability to shape optical fields. They grant independent dynamic control over thousands of degrees-of-freedom within a single light beam. In this work we test a new type of SLM, known as a phase-only light modulator (PLM), that blends the high efficiency of liquid crystal SLMs with the fast switching rates of binary digital micro-mirror devices (DMDs). A PLM has a 2D mega-pixel array of micro-mirrors. The vertical height of each micro-mirror can be independently adjusted with 4-bit precision. Here we provide a concise tutorial on the operation and calibration of a PLM. We demonstrate arbitrary pattern projection, aberration correction, and control of light transport through complex media. We show high-speed wavefront shaping through a multimode optical fibre -- scanning over 2000 points at 1.44 kHz. We make available our custom high-speed PLM control software library developed in C++. As PLMs are based upon micro-electromechanical system (MEMS) technology, they are polarisation agnostic, and possess fundamental switching rate limitations equivalent to that of DMDs -- with operation at up to 10 kHz anticipated in the near future. We expect PLMs will find high-speed light shaping applications across a range of fields including adaptive optics, microscopy, optogenetics and quantum optics.

physics.optics↗

arXiv2024

Self-attention-based non-linear basis transformations for compact latent space modelling of dynamic optical fibre transmission matrices

Multimode optical fibres are hair-thin strands of glass that efficiently transport light. They promise next-generation medical endoscopes that provide unprecedented sub-cellular image resolution deep inside the body. However, confining light to such fibres means that images are inherently scrambled in transit. Conventionally, this scrambling has been compensated by pre-calibrating how a specific fibre scrambles light and solving a stationary linear matrix equation that represents a physical model of the fibre. However, as the technology develops towards real-world deployment, the unscrambling process must account for dynamic changes in the matrix representing the fibre's effect on light, due to factors such as movement and temperature shifts, and non-linearities resulting from the inaccessibility of the fibre tip when inside the body. Such complex, dynamic and nonlinear behaviour is well-suited to approximation by neural networks, but most leading image reconstruction networks rely on convolutional layers, which assume strong correlations between adjacent pixels, a strong inductive bias that is inappropriate for fibre matrices which may be expressed in a range of arbitrary coordinate representations with long-range correlations. We introduce a new concept that uses self-attention layers to dynamically transform the coordinate representations of varying fibre matrices to a basis that admits compact, low-dimensional representations suitable for further processing. We demonstrate the effectiveness of this approach on diverse fibre matrix datasets. We show our models significantly improve the sparsity of fibre bases in their transformed bases with a participation ratio, p, as a measure of sparsity, of between 0.01 and 0.11. Further, we show that these transformed representations admit reconstruction of the original matrices with < 10% reconstruction error, demonstrating the invertibility.

cs.LG↗

arXiv2024

Shrinking a gradient index lens antenna system with a spaceplate

The miniaturisation of optical systems is an ongoing challenge across the electromagnetic spectrum. While the thickness of optical elements themselves can be reduced using advances in metamaterials, it is the voids between these elements -- which are necessary parts of an optical system -- that occupy most of the volume. Recently, a novel optical element coined a `spaceplate' was proposed, that replaces a region of free space with a thinner optical element that emulates the free-space optical response function -- thus having the potential to substantially shrink the volume of optical systems. While there have been a few proof-of-principle demonstrations of spaceplates, they have not yet been deployed in a real-world optical system. In this work, we use a bespoke-designed spaceplate to reduce the length of a gradient index (GRIN) lens microwave antenna. Our antenna is designed to operate at 23.5 GHz, and the incorporation of a nonlocal metamaterial spaceplate enables the distance between the antenna feed and the GRIN lens to be reduced by almost a factor of two. We find the radiation patterns from a conventional and space-squeezed antenna are very similar, with a very low cross-polarisation, and only a minor increase in the side-lobe levels when introducing the spaceplate. Our work represents a first example of a spaceplate integrated into a functional optical system, highlighting the potential for this concept to reduce the physical size of optical systems in real-world applications.

physics.app-ph↗

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↗

arXiv2023

All-optically untangling light propagation through multimode fibres

When light propagates through a complex medium, such as a multimode optical fibre (MMF), the spatial information it carries is scrambled. In this work we experimentally demonstrate an all-optical strategy to unscramble this light again. We first create a digital model capturing the way light has been scattered, and then use this model to inverse-design and build a complementary optical system - which we call an optical inverter - that reverses this scattering process. Our implementation of this concept is based on multi-plane light conversion, and can also be understood as a diffractive artificial neural network or a physical matrix pre-conditioner. We present three design strategies allowing different aspects of device performance to be prioritised. We experimentally demonstrate a prototype optical inverter capable of simultaneously unscrambling up to 30 spatial modes that have propagated through a 1m long MMF, and show how this enables near instantaneous incoherent imaging, without the need for any beam scanning or computational processing. We also demonstrate the reconfigurable nature of this prototype, allowing it to adapt and deliver a new optical transformation if the MMF it is matched to changes configuration. Our work represents a first step towards a new way to see through scattering media. Beyond imaging, this concept may also have applications to the fields of optical communications, optical computing and quantum photonics.

physics.optics↗

arXiv2023

Photon-efficient optical tweezers via wavefront shaping

Optical tweezers enable non-contact trapping of micro-scale objects using light. Despite their widespread use, it is currently not known how tightly it is possible to three-dimensionally trap micro-particles with a given photon budget. Reaching this elusive limit would enable maximally-stiff particle trapping for precision measurements on the nanoscale, and photon-efficient tweezing of light-sensitive objects. Here we solve this problem by customising a trapping light field to suit a specific particle, with the aim of simultaneously optimising trap stiffness in all three dimensions. Initially taking a theoretical approach, we develop an efficient multi-parameter optimisation routine to design bespoke optical traps for a wide range of micro-particles. We show that the confinement volume of micro-spheres held in these sculpted traps can be reduced by one-to-two orders-of-magnitude in comparison to a conventional optical tweezer of the same power. We go on to conduct proof-of-principle experiments, and use a wavefront shaping inspired strategy to suppress the Brownian fluctuations of optically trapped micro-spheres in every direction concurrently, thus demonstrating order-of-magnitude reductions in their confinement volumes. Our work paves the way towards the fundamental limits of optical control over the mesoscopic realm.

physics.optics↗

arXiv2023

Threading light through dynamic complex media

The scattering of light impacts sensing and communication technologies throughout the electromagnetic spectrum. Overcoming the effects of time-varying scattering media is particularly challenging. In this article we introduce a new way to control the propagation of light through dynamic complex media. Our strategy is based on the observation that many dynamic scattering systems exhibit a range of decorrelation times -- meaning that over a given timescale, some parts of the medium may essentially remain static. We experimentally demonstrate a suite of new techniques to identify and guide light through these networks of static channels -- threading optical fields around multiple dynamic pockets hidden at unknown locations inside opaque media. We first show how a single stable light field propagating through a partially dynamic medium can be found by optimising the wavefront of the incident field. Next, we demonstrate how this procedure can be accelerated by 2 orders of magnitude using a physically realised form of adjoint gradient descent optimisation. Finally, we describe how the search for stable light modes can be posed as an eigenvalue problem: we introduce a new optical matrix operator, the time-averaged transmission matrix, and show how it reveals a basis of fluctuation-eigenchannels that can be used for stable beam shaping through time-varying media. These methods rely only on external camera measurements recording scattered light, require no prior knowledge about the medium, and are independent of the rate at which dynamic regions move. Our work has potential future applications to a wide variety of technologies reliant on general wave phenomena subject to dynamic conditions, from optics to acoustics.

physics.optics↗

arXiv2022

High-dimensional spatial mode sorting and optical circuit design using multi-plane light conversion

Multi-plane light converters (MPLCs) are an emerging class of optical device capable of converting a set of input spatial light modes to a new target set of output modes. This operation represents a linear optical transformation - a much sought after capability in photonics. MPLCs have potential applications in both the classical and quantum optics domains, in fields ranging from optical communications, to optical computing and imaging. They consist of a series of diffractive optical elements (the 'planes'), typically separated by free-space. The phase delays imparted by each plane are determined by the process of inverse-design, most often using an adjoint algorithm known as the wavefront matching method (WMM), which optimises the correlation between the target and actual MPLC outputs. In this work we investigate high mode capacity MPLCs to create arbitrary spatial mode sorters and linear optical circuits. We focus on designs possessing low numbers of phase planes to render these MPLCs experimentally feasible. To best control light in this scenario, we develop a new inverse-design algorithm, based on gradient ascent with a specifically tailored objective function, and show how in the low-plane limit it converges to MPLC designs with substantially lower modal cross-talk and higher fidelity than achievable using the WMM. We experimentally demonstrate several prototype few-plane high-dimensional spatial mode sorters, operating on up to 55 modes, capable of sorting photons based on their Zernike mode, orbital angular momentum state, or an arbitrarily randomized spatial mode basis. We discuss the advantages and drawbacks of these proof-of-principle prototypes, and describe future improvements. Our work points to a bright future for high-dimensional MPLC-based technologies.

physics.optics↗

arXiv2022

How to build the optical inverse of a multimode fibre

When light propagates through a multimode optical fibre (MMF), the spatial information it carries is scrambled. Wavefront shaping can undo this scrambling, typically one spatial mode at a time - enabling deployment of MMFs as ultra-thin micro-endoscopes. In this work we go beyond serial wavefront shaping by showing how to simultaneously unscramble all spatial modes emerging from an MMF in parallel. We introduce a passive multiple-scattering element - crafted through the process of inverse design - that is complementary to an MMF and undoes its optical effects. This optical inverter makes possible both single-shot wide-field imaging and super-resolution imaging through MMFs. Our design consists of a cascade of diffractive elements, and can be understood from the perspective of both multi-plane light conversion, and as a physically inspired deep diffractive neural network. This physical architecture can outperform state-of-the-art electronic neural networks tasked with unscrambling light, as it preserves the phase and coherence information of the optical signals flowing through it. Here we demonstrate our MMF inversion concept through numerical simulations, and efficiently sort and unscramble up to ~400 step-index fibre modes, reforming incoherent images of scenes at arbitrary distances from the distal fibre facet. We also describe how our optical inverter can dynamically adapt to see through flexible fibres with a range of experimentally realistic TMs - made possible by moulding optical memory effects into the structure of our design. Although complex, our inversion scheme is based on current fabrication technology so could be realised in the near future. Beyond imaging through scattering media, these concepts open up a range of new avenues for optical multiplexing, communications and computation in the realms of classical and quantum photonics.

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↗

arXiv2021

Roadmap on Wavefront Shaping and deep imaging in complex media

The last decade has seen the development of a wide set of tools, such as wavefront shaping, computational or fundamental methods, that allow to understand and control light propagation in a complex medium, such as biological tissues or multimode fibers. A vibrant and diverse community is now working on this field, that has revolutionized the prospect of diffraction-limited imaging at depth in tissues. This roadmap highlights several key aspects of this fast developing field, and some of the challenges and opportunities ahead.

physics.optics↗

arXiv2021

Memory effect assisted imaging through multimode optical fibres

When light propagates through opaque material, the spatial information it holds becomes scrambled, but not necessarily lost. Two classes of techniques have emerged to recover this information: methods relying on optical memory effects, and transmission matrix (TM) approaches. Here we develop a general framework describing the nature of memory effects in structures of arbitrary geometry. We show how this framework, when combined with wavefront shaping driven by feedback from a guide-star, enables estimation of the TM of any such system. This highlights that guide-star assisted imaging is possible regardless of the type of memory effect a scatterer exhibits. We apply this concept to multimode fibres (MMFs) and identify a `quasi-radial' memory effect. This allows the TM of an MMF to be approximated from only one end - an important step for micro-endoscopy. Our work broadens the applications of memory effects to a range of novel imaging and optical communication scenarios.

physics.optics↗

arXiv2021

Time of flight 3D imaging through multimode optical fibres

Time-of-flight (ToF) 3D imaging has a wealth of applications, from industrial inspection to movement tracking and gesture recognition. Depth information is recovered by measuring the round-trip flight time of laser pulses, which usually requires projection and collection optics with diameters of several centimetres. In this work we shrink this requirement by two orders of magnitude, and demonstrate near video-rate 3D imaging through multimode optical fibres (MMFs) - the width of a strand of human hair. Unlike conventional imaging systems, MMFs exhibit exceptionally complex light transport resembling that of a highly scattering medium. To overcome this complication, we implement high-speed aberration correction using wavefront shaping synchronised with a pulsed laser source, enabling random-access scanning of the scene at a rate of $\sim$23,000 points per second. Using non-ballistic light we image moving objects several metres beyond the end of a $\sim$40 cm long MMF of 50$μ$m core diameter, with millimetric depth resolution, at frame-rates of $\sim$5Hz. Our work extends far-field depth resolving capabilities to ultra-thin micro-endoscopes, and will have a broad range of applications to clinical and remote inspection scenarios.

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↗