Electromagnetic doughnuts: localised and propagating toroidal excitations
Professor Nikolay Zheludev, University of Southampton, UK
Recent progress in toroidal electrodynamics that has been possible with artificial metamaterials will be reviewed. The toroidal dipole is a localised electromagnetic excitation independent from the familiar magnetic and electric dipoles. While the electric dipole can be understood as separated opposite charges and the magnetic dipole as a current loop, the toroidal dipole introduced by Y. B. Zaldovich in 1958 it corresponds to currents ﬂowing on the surface of a torus. Resonant interactions of induced toroidal dipoles with electromagnetic waves have recently been observed in metamaterial structures at microwave, terahertz and optical frequencies. They provide distinct and physically significant contributions to the basic characteristics of matter including absorption, dispersion, and optical activity, the origin of which cannot be comprehensively interpreted in the context of standard multipoles alone. Interference of radiating induced toroidal and electric dipoles leads to transparency windows in artificial materials as a manifestation of the dynamic anapole. Toroidal excitations also exist in free-space as spatially and temporally localised electromagnetic pulses propagating at the speed of light and interacting with matter.
Near-field terahertz spectroscopy: studying terahertz resonators on a micro-scale
Dr Irinia Khromova, King's College London, UK
Terahertz radiation allows for non-destructive detection of objects and processes ’invisible’ at optical and microwave frequencies. Modern terahertz science promises break-through security, medical, and quality control techniques, as well as access to crucial astronomical observation and environmental monitoring. However, the emerging terahertz technology is held back by the scarcity of functional materials and devices required for manipulation of terahertz radiation.
This talk demonstrates opportunities and advantages of the near-field terahertz time-domain spectroscopy for direct studies of terahertz electromagnetic resonances occurring on a micrometre scale. As examples of micro-resonators, it considers conductive micro-fibres and dielectric micro-spheres. Micro-resonators are at the heart of numerous promising terahertz solutions, including the metamaterial approach – creating functional materials from artificial pre-designed resonant micrometre-sized ‘meta-atoms’. Experimental studies of micrometre-scale terahertz resonances are essential, yet inaccessible to common far-field spectroscopic techniques due to extreme sensitivity requirements.
This non-contact technique maps the field patterns of terahertz resonant modes excited in individual conductive or insulating micro-objects, and gives access to essential parameters of micro-resonators, including their resonance frequency, local field enhancement and quality factors. Depending on the underlying physics of observed terahertz resonances, it allows for material and structural characterisation of micro-objects.
This work uses the examples of carbon micro-fibres and titanium dioxide micro-spheres to show the advantages of near-field terahertz time-domain spectroscopy for non-contact terahertz conductivity probing and anisotropic material characterisation; and direct observation of versatile resonant modes, including surface-plasmon resonances in conductive dipoles, and magnetic dipole resonances in dielectric subwavelength terahertz resonators.
Transformation optics applied to electron-loss problems in plasmonics
Mr Matthias Kraft, Imperial College London, UK
Transformation optics is a relatively new subfield in electromagnetic research. Yet, it has been at the heart of many of the most promising advancements in electromagnetism in recent years. Originally developed to aid numerical simulations in cylindrical geometries, it has since been applied as a design tool for such exotic devices as invisibility cloaks or negative refractive index lenses. Within the last 5 years, transformation optics has entered the field of plasmonics. It has proved valuable as a design tool for devices such as beam shifters, surface cloaks or light harvesters. Moreover, it has also proven itself as an analytical tool in the study of interacting plasmonic nano-particles or van der Waals forces. This talk would like to add an entry to the already long list of fields where transformation optics can make a difference. The study of electron energy loss spectroscopy of plasmonic nano-particles.
From spinning fields to chiral optical forces
Dr Cyriaque Genet, ISIS - University of Strasbourg and CNRS, France
Direct manipulation of particles through light-induced forces has led to formidable progress, which has been impacting research in many areas ranging from ultracold-matter physics to biology. The rise of nano-optics has offered the experimentalists new types of optical excitations associated with inhomogeneous fields and complex beam topologies that lead to a great variety of dynamical effects. It was pointed out recently that light radiation pressure is determined by the sole orbital part of the Poynting vector, with no contribution from its spin part. This has important consequences that can be clearly illustrated in the context of surface plasmon optics. The intrinsic spinning character of a plasmonic field brings indeed a clear dynamical distinction between orbital and spin energy flows that can be readily discussed in terms of induced optical forces and torques. Simultaneously, the interest focuses on exploiting the connections between spin-orbit interaction and the concept of chirality at the level of the plasmonic near field. These connections have direct fundamental implications with new possibilities opened in the context of chirality. In particular, new types of optical forces have been recently unveiled when a chiral object is illuminated by a chiral light field that can lead to new chiral separation and discrimination schemes. Such new effects and schemes will be presented and discussed when aiming at manipulating chiral nano-objects using tailored chiral optical fields.
Spin Hall effects in photonics
Dr Francisco Rodríguez-Fortuño, King's College London, UK
Scalar waves can be uniquely described by their amplitude and phase distributions through space; while the amplitude determines the position, the phase distribution determines the propagation direction (wave-vector) of the waves. In addition to those spatial degrees of freedom, electromagnetic waves, being described by vector fields, also posses polarization degrees of freedom, determined by the directions in which the electric and magnetic fields oscillate with time. Spin-orbit interactions of light describe how, under certain circumstances, the polarization of light can affect its propagation direction. This coupling between the spatial and polarization degrees of freedom is completely described by Maxwell's equations applied to the appropriate geometry, and can be analogous to spin-orbit interactions of relativistic quantum particles and electrons in solids. After mentioning some general examples, this talk will focus on a novel kind of spin-orbit interaction that arises on near fields, which are associated with an elliptical or circular polarization in the electric and/or magnetic fields, and can be exploited for an extremely robust polarization-controlled nano-routing of electromagnetic waves in a wide range of scenarios. Experimental examples in different platforms will be described, as well as the reciprocal scenario, in which light propagation can be used to synthesize polarizations.
Multipoles, spherical t-designs and polarization state reconstruction
Dr Matthew Foreman, Max Planck Institute for the Science of Light, Germany
Measurement of the polarisation state of light is a common problem in many branches of fundamental and applied science. Accurate and robust measurements are essential in all such applications. Recently, interest has grown in determining higher order polarisation properties, as these can play a key role in nonlinear and quantum processes. Whilst polarisation dependent optical nonlinear processes can provide important insights into crystal and molecular structure, higher order properties described through a multipolar expansion of the polarisation matrix can contain “hidden” polarisation correlations, which are of interest both in a quantum and in a classical context.
Optimisation of linear polarisation measurements is well studied, however, the problem of optimally reconstructing higher order polarisation properties has to date remained unsolved. The authors present their recent work in which they derive an analytic solution to this problem using an arbitrary number of measurements. Their analysis hence generalises existing results in the linear domain which have been predominately confined to minimal measurement sets, however, critically the authors present optimal measurement strategies for higher order problems. The presented method employs the elegant mathematical framework of spherical t-designs, thereby the derived optimal measurement sets constitute a powerful generalisation of the concept of mutually unbiased bases.