Imperial College London

DrMatthewForeman

Faculty of Natural SciencesDepartment of Physics

Research Associate(Royal Society University Research Fellow)
 
 
 
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Contact

 

+44 (0)20 7594 7721matthew.foreman Website

 
 
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Location

 

642Blackett LaboratorySouth Kensington Campus

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Summary

 

Publications

Publication Type
Year
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70 results found

Byrnes N, Foreman MR, 2021, Polarisation statistics of vector scattering matrices from the circular orthogonal ensemble, Optics Communications, Pages: 127462-127462, ISSN: 0030-4018

Journal article

Berk J, Foreman MR, 2021, Role of multiple scattering in single particle perturbations in absorbing random media, Physical Review Research, Vol: 3, Pages: 1-14, ISSN: 2643-1564

Speckle patterns produced by disordered scattering systems exhibit asensitivity to addition of individual particles which can be used for sensingapplications. Using a coupled dipole model we investigate how multiplescattering can enhance field perturbations arising in such random scatteringbased sensors. Three distinct families of multiple scattering paths are shownto contribute and the corresponding complex enhancement factors derived.Probability distributions of individual enhancement factors over the complexplane are characterised numerically within the context of surface plasmonpolariton scattering in which absorption is shown to play an important role. Weshow that enhancements become more strongly dependent on individual scattererproperties when absorption losses are larger, however, amplitude enhancements$\sim 10^2$, comparable to low loss surface plasmons, are achievable throughsensor optimisation. Approximate analytic expressions for the complex meanenhancements are also found, which agree well with simulations when loopcontributions are negligible.

Journal article

Berk J, Foreman MR, 2021, Theory of multiple scattering enhanced single particle plasmonic sensing, ACS Photonics, Vol: 8, Pages: 2227-2233, ISSN: 2330-4022

Methods to increase the light scattered from small particles can help improve the sensitivity of many sensing techniques. Here, we investigate the role multiple scattering plays in perturbing the scattered signal when a particle is added to a random scattering environment. Three enhancement factors, parametrizing the effect of different classes of multiple scattering trajectories on the field perturbation, are introduced and their mean amplitudes explored numerically in the context of surface plasmon polariton scattering. We demonstrate that there exists an optimum scatterer density at which the sensitivity enhancement is maximized, with factors on the order of 102 achievable. Dependence of the enhancement factors on scatterer properties are also studied.

Journal article

Berk J, Paterson C, Foreman MR, 2021, Tracking single particles using surface plasmon leakage radiation speckle, Journal of Lightwave Technology, Vol: 39, Pages: 3950-3960, ISSN: 0733-8724

Label free tracking of small bio-particles such as proteins or viruses is of great utility in the study of biological processes, however such experiments are frequently hindered by weak signal strengths and a susceptibility to scattering impurities. To overcome these problems we here propose a novel technique leveraging the enhanced sensitivity of both interferometric detection and the strong field confinement of surface plasmons. Specifically, we show that interference between the field scattered by an analyte particle and a speckle reference field, derived from random scattering of surface plasmons propagating on a rough metal film, enables particle tracking with sub-wavelength accuracy. We present the analytic framework of our technique and verify its robustness to noise through Monte Carlo simulations.

Journal article

Azeem F, Trainor LS, Devane PA, Norman DS, Rueda A, Lambert NJ, Kumari M, Foreman MR, Schwefel HGLet al., 2021, Dielectric perturbations: anomalous resonance frequency shifts in optical resonators, Optics Letters, Vol: 46, Pages: 2477-2480, ISSN: 0146-9592

Small perturbations in the dielectric environment around resonant dielectric structures usually lead to a frequency shift of the resonator modes directly proportional to the polarizability of the perturbation. Here, we report experimental observations of strong frequency shifts that can oppose and even exceed the contribution of the perturbations’ polarizability. We show in particular how the mode frequencies of a lithium niobate whispering-gallery-mode resonator are shifted by planar substrates—of refractive indices ranging from 1.50 to 4.22—contacting the resonator rim. Both blue- and redshifts are observed, as well as an increase in mode linewidth, when substrates are moved into the evanescent field of the whispering gallery mode. We compare the experimental results to a theoretical model by Foreman et al. [J. Opt. Soc. Am. B 33, 2177 (2016) [CrossRef] ] and provide an additional intuitive explanation based on the Goos–Hänchen shift for the optical domain, with applications to dielectric structures ranging from meta-surfaces to photonic crystal cavities.

Journal article

Byrnes N, Foreman MR, 2021, Symmetry constraints for vector scattering and transfer matrices containing evanescent components: energy conservation, reciprocity and time reversal, Physical Review Research, Vol: 3, Pages: 013129 – 1-013129 – 13, ISSN: 2643-1564

In this work we study the scattering and transfer matrices for electricfields defined with respect to an angular spectrum of plane waves. For thesematrices, we derive the constraints that are enforced by conservation ofenergy, reciprocity and time reversal symmetry. Notably, we examine the generalcase of vector fields in three dimensions and allow for evanescent fieldcomponents. Moreover, we consider fields described by both continuous anddiscrete angular spectra, the latter being more relevant to practicalapplications, such as optical scattering experiments. We compare our results tobetter-known constraints, such as the unitarity of the scattering matrix forfar-field modes, and show that previous results follow from our framework asspecial cases. Finally, we demonstrate our results numerically with a simpleexample of wave propagation at a planar glass-air interface, including theeffects of total internal reflection. Our formalism makes minimal assumptionsabout the nature of the scattering medium and is thus applicable to a widerange of scattering problems.

Journal article

Seow KLC, Török P, Foreman MR, 2020, Single pixel polarimetric imaging through scattering media, Optics Letters, Vol: 45, Pages: 5740-5743, ISSN: 0146-9592

Polarimetric imaging can provide valuable information about biological samples in a wide range of applications. Detrimental tissue scattering and depolarization however currently hamper in vivo polarization imaging. In this work, single pixel imaging is investigated as a means of reconstructing polarimetric images through scattering media. A theoretical imaging model is presented, and the recovery of the spatially resolved Mueller matrix of a test object behind a scattering phantom is demonstrated experimentally.

Journal article

Foreman M, Byrnes N, 2020, Universal bounds for imaging in scattering media, New Journal of Physics, Vol: 22, Pages: 1-15, ISSN: 1367-2630

In this work we establish universal ensemble independent bounds on the mean and variance of the mutual information and channel capacity for imaging through a complex medium. Both upper and lower bounds are derived and are solely dependent on the mean transmittance of the medium and the number of degrees of freedom N. In the asymptotic limit of large N, upper bounds on the channel capacity are shown to be well approximated by that of a bimodal channel with independent identically Bernoulli distributed transmission eigenvalues. Reflection based imaging modalities are also considered and permitted regions in the transmission-reflection information plane defined. Numerical examples drawn from the circular and DMPK random matrix ensembles are used to illustrate the validity of the derived bounds. Finally, although the mutual information and channel capacity are shown to be non-linear statistics of the transmission eigenvalues, the existence of central limit theorems is demonstrated and discussed.

Journal article

Xiang Y, Foreman MR, Török P, 2020, SNR Enhancement in Brillouin Microspectroscopy using Spectrum Reconstruction, Biomedical Optics Express, Vol: 11

Brillouin imaging suffers from intrinsically low signal-to-noise ratios(SNR). Such low SNRs can render common data analysis protocols unreliable,especially for SNRs below $\sim10$. In this work we exploit two denoisingalgorithms, namely maximum entropy reconstruction (MER) and wavelet analysis(WA), to improve the accuracy and precision in determination of Brillouinshifts and linewidth. Algorithm performance is quantified using Monte-Carlosimulations and benchmarked against the Cram\'er-Rao lower bound. Superiorestimation results are demonstrated even at low SNRS ($\geq 1$). Denoising wasfurthermore applied to experimental Brillouin spectra of distilled water atroom temperature, allowing the speed of sound in water to be extracted.Experimental and theoretical values were found to be consistent to within$\pm1\%$ at unity SNR.

Journal article

Foreman MR, Favaro A, Aiello A, 2019, Erratum: optimal frames for polarization state reconstruction [Phys. Rev. Lett. 115, 263901 (2015)], Physical Review Letters, Vol: 123, ISSN: 0031-9007

Journal article

Foreman MR, 2019, Field correlations in surface plasmon speckle, Publisher: arXiv

In this work fluctuations in the electric field of surface plasmon polaritons undergoing random scattering on a rough metallic surface are considered. A rigorous closed form analytic expression is derived describing second order correlations in the resulting plasmon speckle pattern assuming statistically stationary and isotropic roughness. Partially coherent planar Schell-model source fields can also be described within the developed framework. Behaviour of the three-dimensional degree of cross polarisation and spectral degree of coherence is also discussed. Expressions derived take full account of dissipation in the metal with non-universal behaviour exhibited within the correlation length of the surface and source fields.

Working paper

Foreman M, 2019, Field correlations in surface plasmon speckle, Scientific Reports, Vol: 9, ISSN: 2045-2322

In this work fluctuations in the electric field of surface plasmon polaritons undergoing random scattering on a rough metallic surface are considered. A rigorous closed form analytic expression is derived describing second order correlations in the resulting plasmon speckle pattern assuming statistically stationary and isotropic roughness. Partially coherent planar Schell-model source fields can also be described within the developed framework. Behaviour of the three-dimensional degree of cross polarisation and spectral degree of coherence is also discussed. Expressions derived take full account of dissipation in the metal with non-universal behaviour exhibited within the correlation length of the surface and source fields.

Journal article

Torok P, Foreman MR, 2019, Precision and informational limits in inelastic optical spectroscopy, Scientific Reports, Vol: 9, Pages: 1-16, ISSN: 2045-2322

Using Fisher information and the Cramér-Rao lower bound, we analyse fundamental precision limits in the determination of spectral parameters in inelastic optical scattering. General analytic formulae are derived which account for the instrument response functions of the dispersive element and relay optics found in practical Raman and Brillouin spectrometers. Limiting cases of dispersion and diffraction limited spectrometers, corresponding to measurement of Lorentzian and Voigt lineshapes respectively, are discussed in detail allowing optimal configurations to be identified. Effects of defocus, spherical aberration, detector pixelation and a finite detector size are also considered.

Journal article

Foreman MR, Goudail F, 2019, On the equivalence of optimization metrics in Stokes polarimetry, Optical Engineering, Vol: 58, ISSN: 0091-3286

Optimization of polarimeters has historically been achieved using an assortment of performance metrics. Selection of an optimization parameter is, however, frequently made on an ad hoc basis. We rigorouslydemonstrate that optimization strategies in Stokes polarimetry based on three common metrics, namely theFrobenius condition number of the instrument matrix, the determinant of the associated Gram matrix, or theequally weighted variance, are frequently formally equivalent. In particular, using each metric, we derive the sameset of constraints on the measurement states, correcting a previously reported proof, and show that these can besatisfied using spherical 2 designs. Discussion of scenarios in which equivalence between the metrics breaksdown is also given. Our conclusions are equally applicable to optimization of the illumination states in Muellermatrix polarimetry.

Journal article

, 2019, Detecting single molecule interactions with plasmon-enhanced optical microcavities

Conference paper

Urban NT, Foreman MR, Hell SW, Sivan Yet al., 2018, Nanoparticle-assisted STED nanoscopy with gold nanospheres, ACS Photonics, Vol: 5, Pages: 2574-2583, ISSN: 2330-4022

We demonstrate stimulated emission depletion (STED) microscopy with 20 nmgold nanospheres coated by fluorescent silica. Compared with previousdemonstrations of STED with a hybrid plasmonic fluorescent label, the currentimplementation offers a substantially smaller label and a better resolutionimprovement of up to 2.5-fold beyond the diffraction limit of confocalmicroscopy. This is achieved at approximately 2 times lower intensity thanconventional STED based on dyes alone, and in an aqueous environment,demonstrating the relevance to bio-imaging. Finally, we also show, for thefirst time in this context, a 3-fold reduction in the rate of photobleachingcompared to standard dye-based STED, thus, enabling brighter images.

Journal article

Sedlmeir F, Foreman MR, Vogl U, Zeltner R, Schunk G, Strekalov DV, Marquardt C, Leuchs G, Schwefel HGLet al., 2017, Polarization-selective out-coupling of whispering gallery modes, Physical Review Applied, Vol: 7, ISSN: 2331-7019

Whispering gallery mode (WGM) resonators are an important platform for linear, nonlinear and quantum optical experiments. In such experiments, independent control of in- and out-coupling rates to different modes can lead to higher conversion efficiencies and greater flexibility in the generation of non-classical states based on parametric down conversion. In this work, we introduce a scheme that enables selective out-coupling of WGMs belonging to a specific polarization family, while the orthogonally polarized modes remain largely unperturbed. Our technique utilizes material birefringence in both the resonator and the coupler such that a negative (positive) birefringence allows for polarization-selective coupling to TE (TM) WGMs. We formulate a new coupling condition suitable for describing the case where the refractive indices of the resonator and the coupler are almost the same, from which we derive a criterion for polarization-selective coupling. Finally, we experimentally demonstrate our proposed method using a lithium niobate disk resonator coupled to a lithium niobate prism, where we show a \SI{22}{dB} suppression of coupling to TM modes relative to TE modes.

Journal article

Foreman MR, Keng D, Treasurer E, Lopez JR, Arnold Set al., 2017, Whispering gallery mode single nanoparticle detection and sizing: the validity of the dipole approximation, Optics Letters, Vol: 42, Pages: 963-966, ISSN: 1539-4794

Interactions between whispering gallery modes (WGMs) and small nanoparticles are commonly modeled by treating the particle as a point dipole scatterer. This approach is assumed to be accurate as long as the nanoparticle radius, , is small compared to the WGM wavelength . In this Letter, however, we show that the large field gradients associated with the evanescent decay of a WGM causes the dipole theory to significantly underestimate the interaction strength and, hence, the induced WGM resonance shift, even for particles as small as ∼ /10. To mitigate this issue, we employ a renormalized Born approximation to more accurately determine nanoparticle-induced resonance shifts and, hence, enable improved particle sizing. The domain of validity of this approximation is investigated, and supporting experimental results are presented.

Journal article

Arnold S, Keng D, Treasurer E, Foreman MRet al., 2017, How latitude location on a micro-world enables real-time nanoparticle sizing, Nano-Optics: Principles Enabling Basic Research and Applications, Editors: DiBartolo, Collins, Silvestri, Publisher: SPRINGER, Pages: 235-245, ISBN: 978-94-024-0848-5

We have devised a method for using the nanoparticle induced frequency shift of whispering gallery modes (WGMs) in a microspheroid for the accurate determination of the nanoparticle size in real time. Before the introduction of this technique, size determination from the mode shift could only be obtained statistically based on the assumption that the largest perturbation occurs for binding at the equator. Determining the latitude of the binding event using two polar WGMs results in an analytic method for size determination using a single binding event. The analysis proceeds by incorporating the binding latitude into the Reactive Sensing Principle (RSP), itself containing a shape dependent form factor found using the Born approximation. By comparing this theory with experiments we find that our theoretical approach is more accurate than point dipole theory even though the optical size (circumference/wavelength) is considerably less than one.

Book chapter

Foreman MR, 2016, Single-particle spectroscopy: whispers of absorption, Nature Photonics, Vol: 10, Pages: 755-757, ISSN: 1749-4893

The combination of whispering-gallery-mode sensing with photothermal absorption spectroscopy promises significant advances in single-molecule identification.

Journal article

Foreman MR, Sedlmeir F, Schwefel HGL, Leuchs Get al., 2016, Dielectric tuning and coupling of whispering gallery modes using an anisotropic prism, Journal of the Optical Society of America B - Optical Physics, Vol: 33, Pages: 2177-2195, ISSN: 0740-3224

Optical whispering gallery mode (WGM) resonators are a powerful and versatile tool in many branches of science. Fine-tuning of the central frequency and linewidth of individual resonances is, however, desirable in a number of applications, including frequency conversion, optical communications, and efficient light–matter coupling. To this end we present a detailed theoretical analysis of dielectric tuning of WGMs supported in axisymmetric resonators. Using the Bethe–Schwinger equation and adopting an angular spectrum field representation, we study the resonance shift and mode broadening of high-Q WGMs when a planar dielectric substrate is brought close to the resonator. Particular focus is given to use of a uniaxial substrate with an arbitrarily aligned optic axis. Competing red and blue resonance shifts (~30  MHz), deriving from generation of a near-field material polarization and back action from the radiation continuum, respectively, are found. Anomalous resonance shifts can hence be observed depending on the substrate material, whereas mode broadening on the order of ∼50  MHz can also be realized. Furthermore, polarization-selective coupling with extinction ratios of >10^4 can be achieved when the resonator and substrate are of the same composition and their optic axes are chosen correctly. Double refraction and properties of outcoupled beams are also discussed.

Journal article

Chen WT, Torok P, Foreman MR, Liao CY, Tsai W-Y, Wu PR, Tsai DPet al., 2016, Integrated plasmonic metasurfaces for spectropolarimetry, Nanotechnology, Vol: 27, ISSN: 1361-6528

Plasmonic metasurfaces enable simultaneous control of the phase, momentum, amplitude and polarization of light and hence promise great utility in realization of compact photonic devices. In this paper, we demonstrate a novel chip-scale device suitable for simultaneous polarization and spectral measurements through use of six integrated plasmonic metasurfaces (IPMs), which diffract light with a given polarization state and spectral component into well-defined spatial domains. Full calibration and characterization of our device is presented, whereby good spectral resolution and polarization accuracy over a wavelength range of 500–700 nm is shown. Functionality of our device in a Müller matrix modality is demonstrated through determination of the polarization properties of a commercially available variable waveplate. Our proposed IPM is robust, compact and can be fabricated with a single photolithography step, promising many applications in polarization imaging, quantum communication and quantitative sensing.

Journal article

Foreman MR, Favaro A, Aiello A, 2015, Optimal frames for polarization state reconstruction, Physical Review Letters, Vol: 115, ISSN: 1079-7114

Complete determination of the polarization state of light requires at least four distinct projective measurements of the associated Stokes vector. Stability of state reconstruction, however, hinges on the condition number κ of the corresponding instrument matrix. Optimization of redundant measurement frames with an arbitrary number of analysis states, m, is considered in this Letter in the sense of minimization of κ. The minimum achievable κ is analytically found and shown to be independent of m, except for m=5 where this minimum is unachievable. Distribution of the optimal analysis states over the Poincaré sphere is found to be described by spherical 2 designs, including the Platonic solids as special cases. Higher order polarization properties also play a key role in nonlinear, stochastic, and quantum processes. Optimal measurement schemes for nonlinear measurands of degree t are hence also considered and found to correspond to spherical 2t designs, thereby constituting a generalization of the concept of mutually unbiased bases.

Journal article

Foreman MR, Swaim JD, Vollmer F, 2015, Whispering gallery mode sensors (vol 7, pg 168, 2015), ADVANCES IN OPTICS AND PHOTONICS, Vol: 7, Pages: 632-634, ISSN: 1943-8206

Journal article

Foreman MR, Swaim JD, Vollmer F, 2015, Whispering gallery mode sensors, ADVANCES IN OPTICS AND PHOTONICS, Vol: 7, Pages: 168-240, ISSN: 1943-8206

Journal article

Kim E, Foreman MR, Baaske MD, Vollmer Fet al., 2015, Thermal characterisation of (bio)polymers with a temperature-stabilised whispering gallery mode microsensor, APPLIED PHYSICS LETTERS, Vol: 106, ISSN: 0003-6951

Journal article

Foreman MR, Vollmer F, 2015, Optical Tracking of Anomalous Diffusion Kinetics in Polymer Microspheres, Physical Review Letters, Vol: 114, ISSN: 0031-9007

In this Letter we propose the use of whispering gallery mode resonance tracking as a label-free optical means to monitor diffusion kinetics in glassy polymer microspheres. Approximate solutions to the governing diffusion equations are derived for the case of slow relaxation and small Stefan number. Transduction of physical changes in the polymer, including formation of a rubbery layer, swelling, and dissolution, into detectable resonance shifts are described using a perturbative approach. Concrete examples of poly(methyl methacrylate) and polystyrene spheres in water are considered.

Journal article

Vollmer F, Baaske M, Foreman M, 2015, Detecting single molecule interactions with plasmon-enhanced optical microcavities

Conference paper

Baaske MD, Foreman MR, Vollmer F, 2014, Single-molecule nucleic acid interactions monitored on a label-free microcavity biosensor platform, Nature Nanotechnology, Vol: 9, Pages: 933-939, ISSN: 1748-3395

Biosensing relies on the detection of molecules and their specific interactions. It is therefore highly desirable to develop transducers exhibiting ultimate detection limits. Microcavities are an exemplary candidate technology for demonstrating such a capability in the optical domain and in a label-free fashion. Additional sensitivity gains, achievable by exploiting plasmon resonances, promise biosensing down to the single-molecule level. Here, we introduce a biosensing platform using optical microcavity-based sensors that exhibits single-molecule sensitivity and is selective to specific single binding events. Whispering gallery modes in glass microspheres are used to leverage plasmonic enhancements in gold nanorods for the specific detection of nucleic acid hybridization, down to single 8-mer oligonucleotides. Detection of single intercalating small molecules confirms the observation of single-molecule hybridization. Matched and mismatched strands are discriminated by their interaction kinetics. Our platform allows us to monitor specific molecular interactions transiently, hence mitigating the need for high binding affinity and avoiding permanent binding of target molecules to the receptors. Sensor lifetime is therefore increased, allowing interaction kinetics to be statistically analysed.

Journal article

Foreman MR, Avino S, Zullo R, Loock H-P, Vollmer F, Gagliardi Get al., 2014, Enhanced nanoparticle detection with liquid droplet resonators, EUROPEAN PHYSICAL JOURNAL-SPECIAL TOPICS, Vol: 223, Pages: 1971-1988, ISSN: 1951-6355

Journal article

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