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Alzheimer's disease (AD), a debilitating neurodegenerative disorder, remains one of the foremost public health challenges of our time. Despite decades of research, its etiology largely remains enigmatic. Recently, attention has turned to the gut-brain axis, a complex network of communication between the gastrointestinal tract and the brain, as a potential player in the pathogenesis of AD. Here we exploited X-ray Phase Contrast Tomography to provide an in-depth analysis of the link between the gut condition and AD, exploring gut anatomy and structure in murine models. We conducted a comprehensive analysis by comparing the outcomes in various mouse models of cognitive impairment, including AD, frail mice, and frontotemporal dementia (FTD) affected mice. We discovered an association between substantial changes in the gut structure and the presence of amyloid-beta (A\b{eta}) in the brain. We found that the most important gut alterations are related to A\b{eta} occurrence in the brain. In particular, we investigated the gut morphology, the distribution of enteric micro-processes and neurons in the ileum. Understanding the intricate interplay between gut condition and dementia may open new avenues for early AD diagnosis and treatment offering hope for a future where these diseases may be more effectively addressed., Comment: 14 pages and 7 figures

Optical bound states in the continuum (BIC) allow to totally prevent a photonic mode from radiating into free space along a given spatial direction. Polariton excitations derived from the strong radiation-matter interaction of a BIC with an excitonic resonance inherit an ultralong radiative lifetime and significant nonlinearities due to their hybrid nature. However, maximizing the light-matter interaction in these structures remains challenging, especially with 2D semiconductors, thus preventing the observation of room temperature nonlinearities of BIC polaritons. Here we show a strong light-matter interaction enhancement at room temperature by coupling monolayer WS2 excitons to a BIC, while optimizing for the electric field strength at the monolayer position through Bloch surface wave confinement. By acting on the grating geometry, the coupling with the active material is maximized in an open and flexible architecture, allowing to achieve a 100 meV photonic bandgap with the BIC in a local energy minimum and a record 70 meV Rabi splitting. Our novel architecture provides large room temperature optical nonlinearities, thus paving the way to tunable BIC-based polariton devices with topologically-protected robustness to fabrication imperfections., Comment: 23 pages, 10 figure

Exciton-polaritons are hybrid light-matter states that arise from strong coupling between an exciton resonance and a photonic cavity mode. As bosonic excitations, they can undergo a phase transition to a condensed state that can emit coherent light without a population inversion. This aspect makes them good candidates for thresholdless lasers, yet short exciton-polariton lifetime has made it difficult to achieve condensation at very low power densities. In this sense, long-lived symmetry-protected states are excellent candidates to overcome the limitations that arise from the finite mirror reflectivity of monolithic microcavities. In this work we use a photonic symmetry protected bound state in the continuum coupled to an excitonic resonance to achieve state-of-the-art polariton condensation threshold in GaAs/AlGaAs waveguide. Most important, we show the influence of fabrication control and how surface passivation via atomic layer deposition provides a way to reduce exciton quenching at the grating sidewalls.

Turbulent phenomena are among the most striking effects that both classical and quantum fluids can exhibit. While classical turbulence is ubiquitous in nature, the observation of quantum turbulence requires the precise manipulation of quantum fluids such as superfluid helium or atomic Bose-Einstein condensates. In this work we demonstrate the turbulent dynamics of a 2D quantum fluid of exciton-polaritons, hybrid light-matter quasiparticles, both by measuring the kinetic energy spectrum and showing the onset of vortex clustering. We demonstrate that the formation of clusters of quantum vortices is triggered by the increase of the incompressible kinetic energy per vortex, showing the tendency of the vortex-gas towards highly excited configurations despite the dissipative nature of our system. These results lay the basis for the investigations of quantum turbulence in two-dimensional fluids of light.

We use X-ray phase-contrast tomography (XPCT) in a multi-organ approach to identify early imaging markers predictive of multiple sclerosis (MS) in EAE animal model. As the majority of neurodegenerative diseases, MS is characterized by a progressive accumulation of biological deficits across different organs and systems. A simultaneous imaging of different disease-relevant networks and a multiscale imaging, ranging from the single cell through to the organ as a whole, are required to provide complete reliable information. XPCT offers the unprecedented possibility to investigate structural and cellular alterations at brain, gut, and eye levels in a multiscale approach. The comparison between naive mice and EAE-affected mice sacrificed at different time points and the correlation of the data from different organs and different time points, unveils the identification of early changes in organs possible predictive of the disease.

The still unmet ability of following the fate of specific cells or molecules within the whole body would give an outstanding breakthrough in the comprehension of disease mechanisms and in the monitoring of therapeutic approaches. Our idea is to push forward the bio-nanotechnology to a level where it serves the most advanced 3D bio-medical imaging to provide a multi-scale imaging ranging from the whole organ down to the cellular level, enabling high-resolution visualization of disease-relevant cells within the whole disease-altered biological context. We present here the first proof-of-concept of a novel tomography procedure, Immuno-Histo-X-ray Phase Contrast Tomography (XPCT) that combines cutting-edge XPCT, which provides detailed image of the whole organ, with molecular imaging at the cellular level, identifying the relevant cells via an immunohistochemistry-based approach. We combine metal nanoparticles and a single-domain antibody that target relevant cells. Our results lay the foundation for a new generation of 3D-bio-medical X-ray imaging., Comment: 11 pages 5 figures

The requirement for optical-electrical-optical conversion of signals in optical technologies is often one of the majors bottleneck in terms of speed and energy consumption. The use of dressed photons (also called polaritons), that allows for intrinsic sizable interactions, could significantly improve the performances of optical integrated elements such as switches or optical gates. In this work we demonstrate the ultrafast switch of a laser coupled into a polaritonic waveguide triggered by an optical pulse resonant with the same dispersion but at a lower energy. Our experiments show two effects capable to interrupt the transmission of the laser in two different time ranges: a sub-picosecond time range due to the optical Stark effect, and a picosecond range governed by the creation of a charge reservoir. In the latter regime we found that at certain power of excitation the activation of dark states allows for a long persistence of the switching much beyond the bright exciton lifetime., Comment: Main text: 6 pages, 4 figures. Supplementary Information: 4 pages, 4 figures

Optical bound states in the continuum (BIC) are peculiar topological states that, when realized in a planar photonic crystal lattice, are symmetry-protected from radiating in the far field despite lying within the light cone, i.e., in the energy-momentum dispersion region for which radiation can propagate out of the lattice plane. These BICs possess an invariant topological charge given by the winding number of the polarization vectors, similarly to vortices in quantum fluids, such as superfluid helium and atomic Bose-Einstein condensates. In spite of several reports of optical BICs in patterned dielectric slabs with evidence of lasing, their potential as topologically protected states with theoretically infinite lifetime has not been fully exploited, yet. Here we show Bose-Einstein condensation of polaritons, hybrid light-matter excitations, occuring in a BIC thanks to its peculiar non-radiative nature. The combination of the ultra-long BIC lifetime and the tight confinement of the waveguide geometry allow to achieve an extremely low threshold density for condensation, which is not reached in the dispersion minimum but at a saddle point in reciprocal space. By bridging bosonic condensation and symmetry-protected radiation eigenmodes, we unveil new ways of imparting topological properties onto macroscopic quantum states with unexplored dispersion features. Such an observation may open a route towards energy-efficient polariton condensation in cost-effective integrated devices, ultimately suited for the development of hybrid light-matter optical circuits

The tumour microenvironment (TME) strongly influences tumorigenesis and metastasis. Two of the most characterized properties of the TME are acidosis and hypoxia, both of which are considered hallmarks of tumours as well as critical factors in response to anticancer treatments. Currently, various imaging approaches exist to measure acidosis and hypoxia in the TME, including magnetic resonance imaging (MRI), positron emission tomography and optical imaging. In this review, we will focus on the latest fluorescent-based methods for optical sensing of cell metabolism and MRI as diagnostic imaging tools applied both in vitro and in vivo. The primary emphasis will be on describing the current and future uses of systems that can measure intra- and extra-cellular pH and oxygen changes at high spatial and temporal resolution. In addition, the suitability of these approaches for mapping tumour heterogeneity, and assessing response or failure to therapeutics will also be covered., Comment: 36 pages, 13 figures

Exciton-polaritons are mixed light-matter particles offering a versatile solid state platform to study many-body physical effects. In this work we demonstrate an electrically controlled polariton laser, in a compact, easy-to-fabricate and integrable configuration, based on a semiconductor waveguide. Interestingly, we show that polariton lasing can be achieved in a system without a global minimum in the polariton energy-momentum dispersion. The surface cavity modes for the laser emission are obtained by adding couples of specifically designed diffraction gratings on top of the planar waveguide, forming an in-plane Fabry-Perot cavity. It is thanks to the waveguide geometry, that we can apply a transverse electric field in order to finely tune the laser energy and quality factor of the cavity modes. Remarkably, we exploit the system sensitivity to the applied electric field to achieve an electrically controlled population of coherent polaritons. The precise control that can be reached with the manipulation of the grating properties and of the electric field provides strong advantages to this device in terms of miniaturization and integrability, two main features for the future development of coherent sources from polaritonic technologies., Comment: 11 pages, 5 figures. Supplementary: 6 pages, 7 figures

Artificial gauge fields, simulating real phenomenologies that unfold in a vast variety of systems, offer extraordinary possibilities to study extreme physical effects in many different environments, from high energy physics to quantum mechanics and cosmology. They are also at the heart of topological physics. Here, exploiting a strongly anisotropic material under strong coupling regime, we experimentally synthesize a Yang-Mills non-Abelian gauge field acting on an exciton-polariton quantum flow like a magnetic field. We observe experimentally the corresponding curved trajectories and spin precession. This motion follows chromodynamics equations which normally describe the quarks strong interactions and their color. Our work therefore opens exciting perspectives of simulating quark-gluon dynamics using highly flexible photonic simulators. It makes of sub-atomic physics a potential new playground to apply topological physics concepts.

Machine learning software applications are nowadays ubiquitous in many fields of science and society for their outstanding capability of solving computationally vast problems like the recognition of patterns and regularities in big datasets. One of the main goals of research is the realization of a physical neural network able to perform data processing in a much faster and energy-efficient way than the state-of-the-art technology. Here we show that lattices of exciton-polariton condensates accomplish neuromorphic computing using fast optical nonlinearities and with lower error rate than any previous hardware implementation. We demonstrate that our neural network significantly increases the recognition efficiency compared to the linear classification algorithms on one of the most widely used benchmarks, the MNIST problem, showing a concrete advantage from the integration of optical systems in reservoir computing architectures.

Semiconductor devices are strong competitors in the race for the development of quantum com-putational systems. In this work, we interface two semiconductor building blocks of different di-mensionality and with complementary properties: (1) a quantum dot hosting a single exciton andacting as a nearly ideal single-photon emitter and (2) a quantum well in a 2D microcavity sustain-ing polaritons, which are known for their strong interactions and unique hydrodynamics propertiesincluding ultrafast real-time monitoring of their propagation and phase-mapping. In the presentexperiment we can thus observe how the injected single particles propagate and evolve inside themicrocavity, giving rise to hydrodynamics features typical of macroscopic systems despite their in-trinsic genuine quantum nature. In the presence of a structural defect, we observe the celebratedquantum interference of a single particle that produces fringes reminiscent of a wave propagation.While this behaviour could be theoretically expected, our imaging of such an interference pattern,together with a measurement of antibunching, constitutes the first demonstration of spatial mappingof the self-interference of a single quantum particle hitting an obstacle., Comment: 9 pages, 4 figures, supplementary 4 pages, 6 figures

Polaritonic devices exploit the coherent coupling between excitonic and photonic degrees of freedom to perform highly nonlinear operations with low input powers. Most of the current results exploit excitons in epitaxially grown quantum wells and require low temperature operation, while viable alternatives have yet to be found at room temperature. Here we show that large single-crystal flakes of two-dimensional layered perovskite are able to sustain strong polariton nonlinearities at room temperature with no need to be embedded in an optical cavity. In particular, exciton-exciton interaction energies are measured to be remarkably similar to the ones known for inorganic quantum wells at cryogenic temperatures, and more than one order of magnitude larger than alternative room temperature polariton devices reported so far. Thanks to their easy fabrication, large dipolar oscillator strengths and strong nonlinearities, these materials hold great promises to realize actual polariton devices at room temperature., Comment: 16 pages, 4 figures, plus Supporting file

Hybrid organic-inorganic perovskites have emerged as very promising materials for photonic applications, thanks to the great synthetic versatility that allows to tune their optical properties. In the two-dimensional (2D) crystalline form, these materials behave as multiple quantum-well heterostructures with stable excitonic resonances up to room temperature. In this work strong light-matter coupling in 2D perovskite single-crystal flakes is observed, and the polarization-dependent exciton-polariton response is used to disclose new excitonic features. For the first time, an out-of-plane component of the excitons is observed, unexpected for such 2D systems and completely absent in other layered materials, such as transition-metal dichalcogenides. By comparing different hybrid perovskites with the same inorganic layer but different organic interlayers, it is shown how the nature of the organic ligands controllably affects the out-of-plane exciton-photon coupling. Such vertical dipole coupling is particularly sought in those systems, e.g. plasmonic nanocavities, in which the direction of the field is usually orthogonal to the material sheet. Organic interlayers are shown to affect also the strong birefringence associated to the layered structure, which is exploited in this work to completely rotate the linear polarization degree in only few microns of propagation, akin to what happens in metamaterials., Comment: 11 pages, 14 figures (included supporting info)

In this work we experimentally demonstrate for the first time spontaneous generation of two-dimensional exciton-polariton X-waves. X-waves belong to the family of localized packets, which are capable of sustaining their shape with no spreading even in the linear regime. This allows to keep the packet shape and size for very low densities and very long times compared, for instance, to soliton waves, which always necessitate a nonlinearity to compensate the diffusion. Here we exploit the polariton nonlinearity and unique structured dispersion, comprising both positive- and negative-mass curvatures, to trigger an asymmetric four wave mixing in the momentum space. This ultimately enables self-formation of a spatial X-wave front. By means of ultrafast imaging experiments we observe the early reshaping of the initial Gaussian packet into the X-pulse and its propagation even for vanishing small densities. This allows us to outline the crucial effects and parameters driving the phenomena and to tune the degree of peak superluminal propagation, which we found to be in a good agreement with numerical simulations., Comment: 10 pages, 6 figures, 2 ancillary movie files

A collection of more than 1800 carbonized Greek and Latin papyri, discovered in the Roman Villa dei Papiri at Herculaneum in the middle of 18th century, is the unique classical library survived from antiquity. These ancient-Herculaneum-papyri were charred during 79 A.D. Vesuvius eruption, a circumstance which providentially preserved them until now. This magnificent collection contains valuable work by Greek philosophers, such as Epicurus, Chrysippus and Philodemus, in particular an impressive amount of extensive treatises by Philodemus of Gadara, an Epicurean philosopher of the 1st century BC. The aim of the present study is to read extended and hitherto unknown portions of text hidden inside carbonized-Herculaneum-papyri using enhanced X-ray-phase-contrast-tomography (XPCT) non-destructive technique and a new set of numerical algorithms for virtual-unrolling. This paper documents our success in revealing the largest portion of Greek text ever detected so far inside unopened scrolls, with unprecedented spatial resolution and contrast, all without damaging these precious historical manuscripts. Parts of texts have been decoded and the 'voice' of Epicurean philosopher Philodemus is brought back again after 2000 years from rolled-up carbonized Herculaneum-papyri.

Superfluidity, the ability of a liquid or gas to flow with zero viscosity, is one of the most remarkable implications of collective quantum coherence. In equilibrium systems like liquid 4He and ultracold atomic gases, superfluid behaviour conjugates diverse yet related phenomena, such as persistency of metastable flow in multiply connected geometries and the existence of a critical velocity for frictionless flow when hitting a static defect. The link between these different aspects of superfluid behaviour is far less clear in driven-dissipative systems displaying collective coherence, such as microcavity polaritons, which raises important questions about their concurrency. With a joint theoretical and experimental study, we show that the scenario is particularly rich for polaritons driven in a three-fluid collective coherent regime so-called optical parametric oscillator. On the one hand, the spontaneous macroscopic coherence following the phase locking of the signal and idler fluids has been shown to be responsible for their simultaneous quantized flow metastability. On the other hand, we show here that pump, signal and idler have distinct responses when hitting a static defect; while the signal displays hardly appreciable modulations, the ones appearing in pump and idler are determined by their mutual coupling due to nonlinear and parametric processes., Comment: paper (6 pages, 4 figures), Supplemental Material (6 pages, 7 figures)

We propose theoretically and demonstrate experimentally a generation of light pulses whose polarization varies temporally to cover selected areas of the Poincar\'e sphere with tunable swirling speed and total duration (1 ps and 10 ps respectively in our implementation). The effect relies on the Rabi oscillations of two polarized fields in the strong coupling regime, excited by two counter-polarized and delayed pulses. The interferences of the oscillating fields result in the precession of the Stokes vector of the emitted light while polariton lifetime imbalance results in its drift from a circle on the sphere of controllable radius to a single point at long times. The positioning of the initial and final states allows to engineer the type of polarization spanning, including a full sweeping of the Poincar\'e sphere. The universality and simplicity of the scheme should allow for the deployment of time varying polarization fields at a technologically exploitable level., Comment: 6 pages, 4 figures. Supplementary videos: https://drive.google.com/folderview?id=0B0QCllnLqdyBSHNtSjlyWjN1UFU

We report the experimental observation and control of space and time-resolved light-matter Rabi oscillations in a microcavity. Our setup precision and the system coherence are so high that coherent control can be implemented with amplification or switching off of the oscillations and even erasing of the polariton density by optical pulses. The data is reproduced by a fundamental quantum optical model with excellent accuracy, providing new insights on the key components that rule the polariton dynamics., Comment: 5 pages, 3 figures, supplementary 7 pages, 4 figures. Supplementary videos: https://drive.google.com/folderview?id=0B0QCllnLqdyBNjlMLTdjZlNhbTQ&usp=sharing

We report on a novel kind of transition in random lasers induced by the geometrical confinement of the emitting material. Different dye doped paper devices with controlled geometry are fabricated by soft-lithography and show two distinguished behaviors in the stimulated emission: in the absence of boundary constraints the energy threshold decreases for larger laser volumes showing the typical trend of diffusive {\it non-resonant} random lasers, while when the same material in lithographed into channels, the walls act as cavity and the {\it resonant} behavior typical of standard lasers is observed. The experimental results are consistent with the general theories of random and standard lasers and a clear phase diagram of the transition is reported., Comment: 4 pages, 3 figures

Spin-glass theory is one of the leading paradigms of complex physics and describes condensed matter, neural networks and biological systems, ultracold atoms, random photonics, and many other research fields. According to this theory, identical systems under identical conditions may reach different states and provide different values for observable quantities. This effect is known as Replica Symmetry Breaking and is revealed by the shape of the probability distribution function of an order parameter named the Parisi overlap. However, a direct experimental evidence in any field of research is still missing. Here we investigate pulse-to-pulse fluctuations in random lasers, we introduce and measure the analogue of the Parisi overlap in independent experimental realizations of the same disordered sample, and we find that the distribution function yields evidence of a transition to a glassy light phase compatible with a replica symmetry breaking., Comment: 10 pages, 5 figures

The Hanbury Brown--Twiss effect is one of the celebrated phenomenologies of modern physics that accommodates equally well classical (interferences of waves) and quantum (correlations between indistinguishable particles) interpretations. The effect was discovered in the late thirties with a basic observation of Hanbury Brown that radio-pulses from two distinct antennas generate signals on the oscilloscope that wiggle similarly to the naked eye. When Hanbury Brown and his mathematician colleague Twiss took the obvious step to propose bringing the effect in the optical range, they met with considerable opposition as single-photon interferences were deemed impossible. The Hanbury Brown--Twiss effect is nowadays universally accepted and, being so fundamental, embodies many subtleties of our understanding of the wave/particle dual nature of light. Thanks to a novel experimental technique, we report here a generalized version of the Hanbury Brown--Twiss effect to include the frequency of the detected light, or, from the particle point of view, the energy of the detected photons. In addition to the known tendencies of indistinguishable photons to arrive together on the detector, we find that photons of different colors present the opposite characteristic of avoiding each others. We postulate that fermions can be similarly brought to exhibit positive (boson-like) correlations by frequency filtering., Comment: 18 pages, includes supplementary material of the derivations

Spinorial or multi-component Bose-Einstein condensates may sustain fractional quanta of circulation, vorticant topological excitations with half integer windings of phase and polarization. Matter-light quantum fluids, such as microcavity polaritons, represent a unique test bed for realising strongly interacting and out-of-equilibrium condensates. The direct access to the phase of their wavefunction enables us to pursue the quest of whether half vortices ---rather than full integer vortices--- are the fundamental topological excitations of a spinor polariton fluid. Here, we are able to directly generate by resonant pulsed excitations, a polariton fluid carrying either the half or full vortex states as initial condition, and to follow their coherent evolution using ultrafast holography. Surprisingly we observe a rich phenomenology that shows a stable evolution of a phase singularity in a single component as well as in the full vortex state, spiraling, splitting and branching of the initial cores under different regimes and the proliferation of many vortex anti-vortex pairs in self generated circular ripples. This allows us to devise the interplay of nonlinearity and sample disorder in shaping the fluid and driving the phase singularities dynamics, Comment: New version complete with revised modelization, discussion and added material. 8 pages, 7 figures. Supplementary videos: https://drive.google.com/folderview?id=0B0QCllnLqdyBfmc2ai0yVF9fa2g2VnZodGUwemVkLThBb3BoOVRKRDJMS2dUdjlZdkRTQkU

Polaritons in microcavities are versatile quasi-2D bosonic particles with a high degree of coherence and strong nonlinearities, thanks to their hybrid light-matter character. In their condensed form, they display striking quantum hydrodynamic features analogous to atomic Bose-Einstein condensates, such as long-range order coherence, superfluidity and quantized vorticity. Their variegated dispersive and dissipative properties, however, set significant differences from their atomic counterpart. In this work, we report the unique phenomenology that is observed when a pulse of light impacts the polariton vacuum: the condensate that is instantaneously formed does not splash in real space but instead coheres into an enigmatic structure, featuring concentric rings and, most notably, a sharp and bright peak at the center. Using a state-of-the-art ultrafast imaging with 50 fs time steps, we are able to track the dynamics of the polariton mean-field wavefunction in both real and reciprocal space. The observation of the real-space collapse of the condensate into an extremely localized---resolution limited---peak is at odd with the repulsive interactions of polaritons and their positive effective mass. An unconventional mechanism is therefore at play to account for our observations. Our modeling suggests that self-trapping due to a local heating of the crystal lattice---that can be described as a collective polaron formed by a polariton condensate---could be involved. These observations hint at the fascinating fluid dynamics of polaritons in conditions of extreme intensities and ultrafast times., Comment: New version complete with revised theoretical interpretation and added material. 8 pages, 5 figures, supplementary 12 pages, 15 figures. Supplementary videos: https://drive.google.com/folderview?id=0B0QCllnLqdyBZUNXMTdTNUNhdUk

We investigate the cross-interactions in a two-component polariton quantum fluid coherently driven by two independent pumping lasers tuned at different energies and momenta. We show that both the hysteresis cycles and the ON/OFF threshold of one polariton signal can be entirely controlled by a second polariton fluid. Furthermore, we study the ultrafast switching dynamics of a driven polariton state, demonstrating the ability to control the polariton population with an external laser pulse, in less than a few picoseconds.

The random laser emission from the functionalized thienyl-S,S-dioxide quinquethiophene (T5OCx) in confined patterns with different shapes is demonstrated. Functional patterning of the light emitter organic material in well defined features is obtained by spontaneous molecular self-assembly guided by surface tension driven (STD) lithography. Such controlled supramolecular nano-aggregates act as scattering centers allowing the fabrication of one-component organic lasers with no external resonator and with desired shape and efficiency. Atomic force microscopy shows that different geometric pattern with different supramolecular organization obtained by the lithographic process tailors the coherent emission properties by controlling the distribution and the size of the random scatterers.

While photons in vacuum are massless particles that do not interact with each other, significant photon-photon interactions appear in suitable nonlinear media, leading to novel hydrodynamic behaviors typical of quantum fluids. Here we show the formation of vortex-antivortex pairs in a Bose-Einstein condensate of exciton-polaritons -a coherent gas of strongly dressed photons- flowing at supersonic speed against an artificial potential barrier created and controlled by a light beam in a planar semiconductor microcavity. The observed hydrodynamical phenomenology is in agreement with original theoretical predictions based on the Gross-Pitaevskii equation, recently generalized to the polariton context. However, in contrast to this theoretical work, we show how the initial position and the subsequent trajectory of the vortices crucially depend on the strength and size of the artificial barrier. Additionally, we demonstrate how a suitably tailored optical beam can be used to permanently trap and store the vortices that are hydrodynamically created in the wake of a natural defect. These observations are borne out by time-dependent theoretical simulations.