X-ray computed tomography

X-ray computed tomography (CT) can reveal the internal details of objects in three dimensions non-destructively. In this Primer, we outline the basic principles of CT and describe the ways in which a CT scan can be acquired using X-ray tubes and synchrotron sources, including the different possible contrast modes that can be exploited. We explain the process of computationally reconstructing three-dimensional (3D) images from 2D radiographs and how to segment the 3D images for subsequent visualization and quantification. Whereas CT is widely used in medical and heavy industrial contexts at relatively low resolutions, here we focus on the application of higher resolution X-ray CT across science and engineering. We consider the application of X-ray CT to study subjects across the materials, metrology and manufacturing, engineering, food, biological, geological and palaeontological sciences. We examine how CT can be used to follow the structural evolution of materials in three dimensions in real time or in a time-lapse manner, for example to follow materials manufacturing or the in-service behaviour and degradation of manufactured components. Finally, we consider the potential for radiation damage and common sources of imaging artefacts, discuss reproducibility issues and consider future advances and opportunities

Roadmap on Label-Free Super-resolution Imaging

Label-free super-resolution (LFSR) imaging relies on light-scattering processes in nanoscale objects without a need for fluorescent (FL) staining required in super-resolved FL microscopy. The objectives of this Roadmap are to present a comprehensive vision of the developments, the state-of-the-art in this field, and to discuss the resolution boundaries and hurdles that need to be overcome to break the classical diffraction limit of the label-free imaging. The scope of this Roadmap spans from the advanced interference detection techniques, where the diffraction-limited lateral resolution is combined with unsurpassed axial and temporal resolution, to techniques with true lateral super-resolution capability that are based on understanding resolution as an information science problem, on using novel structured illumination, near-field scanning, and nonlinear optics approaches, and on designing superlenses based on nanoplasmonics, metamaterials, transformation optics, and microsphere-assisted approaches. To this end, this Roadmap brings under the same umbrella researchers from the physics and biomedical optics communities in which such studies have often been developing separately. The ultimate intent of this paper is to create a vision for the current and future developments of LFSR imaging based on its physical mechanisms and to create a great opening for the series of articles in this field.Peer reviewe

Modern Applications in Optics and Photonics: From Sensing and Analytics to Communication

Optics and photonics are among the key technologies of the 21st century, and offer potential for novel applications in areas such as sensing and spectroscopy, analytics, monitoring, biomedical imaging/diagnostics, and optical communication technology. The high degree of control over light fields, together with the capabilities of modern processing and integration technology, enables new optical measurement systems with enhanced functionality and sensitivity. They are attractive for a range of applications that were previously inaccessible. This Special Issue aims to provide an overview of some of the most advanced application areas in optics and photonics and indicate the broad potential for the future

Single Molecule Fluorescence Spectroscopy and Imaging: Advanced Methods and Applications in Life Sciences

The visualization of biological structures down to the molecular length scale has been recently made possible by the development of super-resolution fluorescence microscopy. These techniques now routinely resolve biological structures down to a few nanometers. Various super-resolution techniques have been developed, the most successful being Stimulated Depletion Emission (STED) microscopy and Single Molecule Localization Microscopy (SMLM). In what follows, I will focus on the latter class of techniques which is based on the fact that a single molecule image allows for localizing the molecule with a much higher accuracy than the diffraction limit of resolution of the used microscope. However, a big challenge of SMLM is to achieve a similar super-resolution along the optical axis of a microscope. For this purpose, metal-induced energy transfer (MIET) imaging was recently introduced as an elegant method for axially localizing fluorophores with nanometer precision. The underlying principle of MIET is based on an electromagnetic near-field-mediated energy transfer from an excited fluorescent emitter (donor) to a thin planar metal film (acceptor). This energy transfer leads to a distance-dependent modulation of an emitter’s fluorescence lifetime (quenching), that can be easily measured with conventional fluorescence lifetime measurement techniques. The power of MIET is that it works with any fluorophore, and it only requires a conventional fluorescence lifetime imaging (FLIM) microscope. In this thesis, I present a powerful modification and further development of MIET, that is called graphene-induced energy transfer (GIET). GIET replaces the metal film of MIET with a single sheet of graphene which reduces the quenching range by one order of magnitude, leading to a tenfold improvement in axial resolution. This enables the localization of fluorophores with sub-nanometer accuracy. We demonstrate the potential of GIET by quantifying inter-leaflet distances in supported lipid bilayers (SLBs) and discuss the potential of the technique particularly in membrane biophysics applications. The second line of this thesis is devoted to the complementary topic of fast molecular dynamics. While super-resolution microscopy succeeds in resolving structural details with nanometer resolution, it is too slow for temporally resolving the fast dynamics of the observed molecules. For this purpose, spectroscopic techniques such as single molecule fluorescence spectroscopy (SMFS) have become an important tool that can resolve molecular dynamics down to timescales of nanoseconds. In my thesis, I focus on fluorescence lifetime correlation spectroscopy (FLCS), an advanced variant of fluorescence correlation spectroscopy (FCS). Using FLCS, I could disentangle two emission states in an autofluorescent protein that have otherwise highly overlapping spectra, and I could quantify the microsecond switching rates between these two states. As compared to other existing methods, FLCS offers the unique advantage of probing such fast switching kinetics with nanosecond temporal resolution under equilibrium conditions at room temperature, making it the method of choice for similar studies of complex luminescent emitters. Finally, I will also present another study where I utilized advanced FCS for studying protein self-assembly. In summary, my thesis presents several advanced methods in SMLM and SMFS which significantly enhance the spatial and temporal resolution at the single molecule level. I believe that the presented methods will find a wide range of applications in the life sciences.2021-06-0

Light in correlated disordered media

The optics of correlated disordered media is a fascinating research topic emerging at the interface between the physics of waves in complex media and nanophotonics. Inspired by photonic structures in nature and enabled by advances in nanofabrication processes, recent investigations have unveiled how the design of structural correlations down to the subwavelength scale could be exploited to control the scattering, transport and localization of light in matter. From optical transparency to superdiffusive light transport to photonic gaps, the optics of correlated disordered media challenges our physical intuition and offers new perspectives for applications. This article reviews the theoretical foundations, state-of-the-art experimental techniques and major achievements in the study of light interaction with correlated disorder, covering a wide range of systems -- from short-range correlated photonic liquids, to L\'evy glasses containing fractal heterogeneities, to hyperuniform disordered photonic materials. The mechanisms underlying light scattering and transport phenomena are elucidated on the basis of rigorous theoretical arguments. We overview the exciting ongoing research on mesoscopic phenomena, such as transport phase transitions and speckle statistics, and the current development of disorder engineering for applications such as light-energy management and visual appearance design. Special efforts are finally made to identify the main theoretical and experimental challenges to address in the near future.Comment: Submitted to Reviews of Modern Physics. Feedbacks are welcom

Development of X-ray Tomography Tools for Characterisation of Lithium-Sulfur Batteries

Electrochemical energy storage devices are becoming increasingly ubiquitous in both consumer and industrial applications, driven by a pressing need to reduce carbon emissions for the mitigation of global warming. The electrification of the transport and mobility sector and growth in portable electronic devices demand portable power sources with high energy densities, and lithium-ion (Li-ion) batteries have been adopted extensively in these applications. However, conventional transition metal oxide-based intercalation materials used at the positive electrode are reaching their theoretical limitations, and only relatively minor improvements in theoretical specific capacity can be achieved. // Lithium-sulfur (Li-S) batteries offer higher gravimetric theoretical specific capacity and energy density and are billed as a potential successor to Li-ion technology but suffer from limited cycle life and self-discharge due to complex multi-phase chemistry and parasitic side reactions. // To better understand the fundamental mechanisms behind these processes, advanced characterisation methods involving the use of penetrating radiation (such as X-rays and neutrons) have become invaluable tools to capture the operation and degradation of the Li-S battery. Three-dimensional techniques such as X-ray micro-tomography (micro-CT) are particularly suited to probe the heterogeneous nature of battery electrode microstructures. // In this thesis, main areas of focus will include the application of ex situ and in situ X-ray micro-CT on Li-S batteries and the broader development of in situ tomography cells. The overall scientific aims of this thesis include: measuring the three-dimensional microstructural characteristics of sulfur electrodes; elucidating the three-dimensional nature of both sulfur dissolution and redeposition as a function of state of charge; and developing a better understanding of the transport processes occurring within the Li-S battery and the influence of porosity and tortuosity on electrochemical performance. In parallel, the development of in situ tomography cells capable of electrochemical cycling is an extensive component of this thesis, with applications not solely limited to Li-S batteries or X-ray micro-CT