Near-field imaging of spin-locked edge states in all-dielectric topological metasurfaces

Abstract

A new class of phenomena stemming from topological states of quantum matter has recently found a variety of analogies in classical systems 1,2,3,4,5,6,7,8,9,10 . Spin-locking and one-way propagation have been shown to drastically alter our view on scattering of electromagnetic waves, thus offering an unprecedented robustness to defects and disorder 11,12,13,14,15,16,17,18,19,20 . Despite these successes, bringing these new ideas to practical grounds meets a number of serious limitations. In photonics, when it is crucial to implement topological photonic devices on a chip 21 , two major challenges are associated with electromagnetic dissipation into heat and out-of-plane radiation into free space. Both these mechanisms may destroy the topological state and seriously affect the device performance. Here we experimentally demonstrate that the topological order for light can be implemented in all-dielectric on-chip prototype metasurfaces, which mitigate the effect of Ohmic losses by using exclusively dielectric materials, and reveal that coupling of the system to the radiative continuum does not affect the topological properties. Spin-Hall effect of light for spin-polarized topological edge states is revealed through near-field spectroscopy measurements.The experimental realization of photonic topological systems have demonstrated fascinating properties for their electromagnetic modes and opened avenues for their use in practical systems and devices 11,15,14,22,23,24,25,26,27 . Unfortunately, many of these original proposals have been based on metal containing metamaterials, 22,28,29,30,26 whose functionality cannot be extended into IR and visible domains. In this respect, all-dielectric topological structures based on arrays of ring resonators 14,31,32 or chiral waveguides 15,27,33 have demonstrated desirable properties at optical frequencies. Nonetheless, this comes at the cost of a very large footprint, with arrays of resonators that can be hundreds to thousands of wavelengths in size, rendering such systems rather unpractical. It is of practical relevance to realize topological systems not only made of photonic compatible materials, but also compact enough for integration with modern photonics circuitry. In this case one has to consider structures with characteristic length scales that are smaller 34 or comparable in size 35 with the wavelength of light. In this regard all-dielectric metamaterials and metasurfaces 36,37,38

Recent advances in 2D, 3D and higher-order topological photonics

“…The nontrivial 3D topology can be understood as a weak 3D topological insulator, which can be produced by layering arrays of 2D topological systems. Indeed, a bianisotropic metasurface, an array of dielectric bianisotropic unit structures as a 2D version of 3D bianisotropic metamaterials, has been reported to support a 2D topological phase 151 . In contrast to the linear surface Dirac cones in the triangular lattice, quadratic surface dispersions have been demonstrated in the tetragonal lattice of a similar unit structure 100 .…”

Manipulating topological valley modes in plasmonic metasurfaces

“…Exploiting the radiative coupling regime between the NPs [7], whose interactions are mediated by diffractive modes in the plane of the array (surface lattice resonances), has enabled the experimental demonstration of lasing [8,9] and Bose-Einstein condensation [10] in plasmonic lattices, due to their dramatic quality factor enhancement. On the other hand, the near field coupling regime, where the distance between the NPs is very subwavelength, is currently regaining interest due to the possibility of realising topological phases of light confined at nanoscale dimensions using metal NPs as well as other nanoresonators such as dielectric NPs [11][12][13][14][15][16][17][18][19]. This has been sparked by the potential of topological protection to provide robust light propagation immune to certain kinds of disorder and imperfections in samples; in analogy to the effects present * matthew.proctor12@imperial.ac.uk in topological insulators, materials which are insulating in the bulk and possess protected conduction states along their edge [20].…”

Near‐Field Characterization of Higher‐Order Topological Photonic States at Optical Frequencies

“…Near-field techniques have been shown to be very useful in studies of topological photonic systems: for instance, they have allowed the accurate extraction of subwavelength-detailed field structures evidencing the topology of the boundary modes at microwaves. [29][30][31] The use of scattering scanning near-field optical microscopy (s-SNOM) to directly map near-fields in the optical domain, which was successfully used to study the properties of complex optical media, [32,33] was recently applied to characterize topologically robust transport in valley photonic crystal waveguides. [34] Higher-order topological insulators (HOTIs) represent a new type of topological system, supporting boundary states localized over boundaries, two or more dimensions lower than the dimensionality of the system itself.…”