Spin-orbit interactions (SOIs) of light are manifestations of coupling between components of light's angular momentum. They are at play in most basic optical processes, offering opportunities both to understand their fundamental origin and to control light in novel ways. Because SOIs become significant at subwavelength scale, they have largely been explored in the context of inhomogeneous materials exhibiting wavelength-scale structures, and notably metamaterials. Here we demonstrate that spin-orbit interactions can in fact analogously emerge in moving matter through the well-known light-dragging effects. SOIs in moving media are shown to manifest through a Berry phase induced by vorticity, which then leads to a rotation of the wave's polarization. In bringing together electrodynamics of moving media and SOIs of light, our work not only paves the way for the discovery of new fundamental effects but also uncovers novel means to harness SOIs to control light.
Spin-orbit interactions (SOIs) of light are manifestations of coupling between components of light's angular momentum. They are at play in most basic optical processes, offering opportunities both to understand their fundamental origin and to control light in novel ways. Because SOIs become significant at subwavelength scale, they have largely been explored in the context of inhomogeneous materials exhibiting wavelength-scale structures, and notably metamaterials. Here we demonstrate that spin-orbit interactions can in fact analogously emerge in moving matter through the well-known light-dragging effects. SOIs in moving media are shown to manifest through a Berry phase induced by vorticity, which then leads to a rotation of the wave's polarization. In bringing together electrodynamics of moving media and SOIs of light, our work not only paves the way for the discovery of new fundamental effects but also uncovers novel means to harness SOIs to control light.
Optical spin-orbit interactions (SOI), demonstrated experimentally in optical cavities, enable the polarization degrees of freedom of light to be described and controlled as an effective pseudospin, providing a foundation for spintronic-like control of light and the development of next-generation optical technologies. However, prior works describe this emergent effect using phenomenological models that fail to uncover its microscopic quantum origins, and as a result, a rigorous connection between material electronic structure and the optical SOI remains elusive. In this work, we address this fundamental challenge by developing a microscopic quantum theory starting from the non-relativistic quantum electrodynamical (QED) description of light-matter interactions. We show that this effect arises from virtual interactions between confined photons and material excitations. Crucially, our theoretical framework provides microscopic material-design principles for tuning the distinct components of optical SOI, which include the optical birefringence, photonic self-energies, TE-TM splitting, and optical Zeeman splitting, enabling precise engineering of effective magnetic fields, diabolical points, and pseudospin textures. We anticipate that the predictive capability of our microscopic theory will enable systematic screening and design of material systems for photonic spintronics and optical technologies.
Geometric or Berry phases are fundamental manifestations that appear in many areas of physics. They arise from the geometry of the space describing the properties of multicomponent wave fields. An important example for electromagnetic waves is the spin-redirection Berry phase associated with the evolution of the spin direction. Because this effect has traditionally been studied in isotropic media where the spin is aligned with the ray trajectory, it has become commonly assumed that this spin-redirection Berry phase requires nonplanar rays. Here, we show that a spin-redirection phase can in fact arise along a planar ray if the spin evolves along the ray. We expose this effect through the singular example of a moving unmagnetized plasma, and demonstrate how this behavior can more generally arise from a finite transverse spin. In identifying this new spin-redirection mechanism, our work not only provides the tools to discover additional manifestations of spin-orbit interactions (SOIs) in nature, but also uncovers supplemental degrees of freedom to harness SOIs to control light.
Abstract We demonstrate that a nonlinear spin–orbit interaction (SOI) emerges in cold Rydberg atomic gases, where the orbital characteristics (spatial structure) of light fields become coupled to their polarization (spin). This SOI originates from the giant, spatially inhomogeneous nonlinear optical response arising from the strong, long-range interactions between Rydberg atoms. By extending the theoretical framework beyond the conventional paraxial approximation, we derive a light propagation equation that rigorously incorporates such inhomogeneous nonlinearity. Our study reveals that the SOI is driven by the gradient of the inhomogeneous nonlinear response and therefore vanishes in linear or homogeneous media. When two orthogonal light fields co-propagate, the SOI generates novel nonlinear energy transfer and pronounced structural deformation, which are not captured without considering the SOI or within the paraxial approximation. Furthermore, owing to its direct dependence on the gradient of the nonlocal nonlinearity, the SOI is inherently mode-dependent. As a consequence, the structural deformation becomes particularly significant for light fields with odd parity symmetry. Our investigation establishes novel pathways for exploring the nonlinear SOI and examining inhomogeneous nonlinear optical responses beyond the paraxial approximation in cold Rydberg atomic gases.
The physics of optical vortices, also known as twisted light, is now a well-established and a growing branch of optical physics with a number of important applications and significant interdisciplinary connections.Optical vortex fields of widely varying forms and degrees of complexity can be realised in the laboratory by a host of different means.The interference between such beams with designated orbital angular momenta and optical spins (the latter is associated with wave polarisations) can be structured to conform to various geometrical arrangements.The focus of this review is on how such tailored forms of light can exert a controllable influence on atoms with which they interact.The main physical effects involve atoms in motion due to application of optical forces.The now mature area of atom optics has had notable successes both of fundamental nature and in applications such as atom lasers, atom guides and Bose-Einstein condensates.The concepts in atom optics encompass not only atomic beams interacting with light, but atomic motion in general as influenced by optical and other fields.Our primary concern in this review is on atoms in structured light where, in particular, the twisted nature of the light is made highly complex with additional features due to wave polarisation.These features bring to the fore a variety of physical phenomena not realisable in the context of atomic motion in more conventional forms of laser light.Atoms near resonance with such structured light fields become subject to electromagnetic fields with complex polarisation and phase distributions, as well as intricately structured intensity gradients and radiative forces.From the combined effect of optical spin and orbital angular momenta, atoms may also experience forces and torques involving an interplay between the internal and centre of mass degrees of freedom.Such interactions lead to new forms of processes including scattering, trapping and rotation and, as a result, they exhibit characteristic new features at the micro-scale and below.A number of distinctive properties involving angular momentum exchange between the light and the atoms are highlighted, and prospective applications are discussed.Comparison is made between the theoretical predictions in this area and the corresponding experiments that have been reported to date.
Optical spin–orbit interaction (SOI), which can be used to simultaneously control the spin and orbital angular momentum of light, is important for both classical and quantum information applications. In linear and nonlinear optics, the SOI of light has been extensively explored in both artificial structures and conventional optical crystals. However, optical SOI in quantum nonlinear optical processes, such as spontaneous parametric downconversion (SPDC), has not been studied before. Here, we experimentally demonstrate that optical SOI in the SPDC process can be realized through a nonlinear crystal with threefold rotational symmetry. Two-photon quantum states with controlled angular momentum can be generated through the symmetry selection rules in nonlinear optics and the SOI of the pump wave. The proposed methodology may facilitate the generation and control of spin and orbital angular momentum of entangled photons.
Metamaterials have been a major research area for more than two decades now, involving artificial structures with predesigned electromagnetic properties constructed from deep subwavelength building blocks. They have been used to demonstrate a wealth of fascinating phenomena ranging from negative refractive index and epsilon-near-zero to cloaking, emulations of general relativity effects, and super-resolution imaging, to name a few. In the past few years, metamaterials have been suggested as a new platform for quantum optics, and several pioneering experiments have already been carried out with single photons. Here, we employ a dielectric metasurface to generate entanglement between spin and orbital angular momentum of single photons. We demonstrate experimentally the generation of the four Bell states by utilizing the geometric phase arising from the photonic spin-orbit interaction. These are the first experiments with entangled states with metasurfaces, and as such they are paving the way to the new area of quantum metamaterials.