Probing the Photonic Spin–Orbit Interactions in the Near Field of Nanostructures

Abstract

Photonic spin-orbit interactions (SOI) provide a new design paradigm of functional nanomaterials and nanostructures, and have especially accelerated advances in spin-orbit photonics. The berry phase or the geometric phase, a salient property of SOI, plays a vital role in this process. Thus, the char acterization of photonic SOI processes together with the Berry phase is highly demanded for studies such as the optical spinHall effect, spintovortex conversion, and Rashba effect. Here, a spinselective and phaseresolved near field microscopic method is proposed and experimentally demonstrated for realtime probing and direct visualization of photonic SOI at mesoscale, and a 3D tomographic technique for imaging the spatial evolutions of the optical phases is also properly realized. By analyzing a metallic metasurface as a spin tovortex conversion platform, the abrupt geometric phase and the spatially evolutional dynamic phases are directly measured and intuitively illustrated. This work provides a powerful tool for the study of spin-orbit phenomena in nearfield optics, and can hold the promise for directly exploring the spin dependent surface states in plasmonics and photonic topological insulators.

Topology‐Induced Phase Transitions in Spin‐Orbit Photonics

“…Our findings provide a unified picture to understand two previously discovered effects on the same foot, and can be extended to study other SOI-induced effects in light (e.g., spin-controlled vortex generation [1,[9][10][11][12] and spin-Hall momentum shift [35][36][37][38][39] in inhomogeneous anisotropic media) and other waves (e.g., vortexbearing electron beams). [40,41] More importantly, as the SOI of light plays an increasingly important role in nanophotonics, [42][43][44][45][46][47][48] plasmonics, [49][50][51] and topological photonics, [52][53][54][55] our results…”

“…Our findings provide a unified picture to understand two previously discovered effects on the same foot, and can be extended to study other SOI‐induced effects in light (e.g., spin‐controlled vortex generation [ 1,9–12 ] and spin‐Hall momentum shift [ 35–39 ] in inhomogeneous anisotropic media) and other waves (e.g., vortex‐bearing electron beams). [ 40,41 ] More importantly, as the SOI of light plays an increasingly important role in nanophotonics, [ 42–48 ] plasmonics, [ 49–51 ] and topological photonics, [ 52–55 ] our results may pave the way for a variety of applications, such as precision metrology, [ 56,57 ] edge detection, [ 58,59 ] particle manipulation, [ 60,61 ] and various spin‐photonic components. [ 42–45,62,63 ]…”

Spin‐Symmetry‐Selective Generation of Ultracompact Optical Vortices in Nanoapertures without Chirality

“…Moreover, the generated OV has a very small size, whose diameter is only 1.06 μm at a distance of 2 μm above the PAN structure (see Section 3, Supporting Information). Therefore, compared with the metasurfaces composed of discrete meta‐atoms, [ 25,29,33 ] a much smaller footprint of the OV beam and a much larger polarization conversion ratio of ≈80% are obtained here. It means that a large amount of the transmitted energy is carried by the cross‐polarized OV component and most of the copolarized incident field is screened out by the opaque gold film.…”

“…Different from the resonance-type metasurfaces, the metasurfaces based on the Pancharatnam-Berry phase (PB phase) or the so-called geometric phase can generate OVs through the photonic SOI, which shows robust and dispersion-free features. [17,[24][25][26][27][28][29][30][31][32][33] Despite the versatile functionalities of both types of metasurfaces, they have some evident challenges and drawbacks. On one hand, metasurfaces consist of discrete meta-atoms interacting with light, leading to discontinuous wavefront modulation and relatively low efficiency.…”

“…To achieve this goal, we established a near-field heterodyne interferometric setup combined with an aperture-type SNOM (NT-MDT, SNLG112NTF) to characterize the OV. [33] The heterodyne scheme allows us to realize phase measurements while a novel cross-polarization detection mechanism is implemented to distinguish different spin states. The spatial resolution of the near-field measurements depends on the aperture size of individual probes, which is typically 100 nm in our experiments.…”