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Register a new userSymmetry-protected photonic chiral spin textures by spin-orbit coupling
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Chiral spin textures are researched widely in condensed matter systems and show potential for spintronics and storage applications. Along with extensive condensed-matter studies of chiral spin textures, photonic counterparts of these textures have been observed in various optical systems with broken inversion symmetry. Unfortunately, the resemblances are only phenomenological. This work proposes a theoretical framework based on the variational theorem to show that the formation of photonic chiral spin textures in an optical interface is derived from the systems symmetry and relativity. Analysis of the optical systems rotational symmetry indicates that conservation of the total angular momentum is preserved from the local variations of spin vectors. Specifically, although the integral spin momentum does not carry net energy, the local spin momentum distribution, which determines the local subluminal energy transport and minimization variation of the square of total angular momentum, results in the chiral twisting of the spin vectors. The findings here deepen the understanding of the symmetries, conservative laws and energy transportation in optical system, construct the comparability in the formation mechanisms and geometries of photonic and condensed-matter chiral spin textures, and suggest applications to optical manipulation and chiral photonics.
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Dirac semimetal (DSM) is a phase of matter, whose elementary excitation is described by the relativistic Dirac equation. Its parity-time symmetry enforces the linear-dispersed Dirac cone in the momentum space to be non-chiral, leading to surface states connected adiabatically to a topologically trivial surface state. Inspired by the flavor symmetry in particle physics, we theoretically propose a massless chiral Dirac equation linking two Weyl fields with the identical chirality by assuming SU(2) isospin symmetry, independent of the space-time rotation exchanging the two fields. Dramatically, such symmetry is hidden in certain solid-state spin-1/2 systems with negligible spin-orbit coupling, where the spin degree of freedom is decoupled with the lattice. Therefore, it cannot be explained by the conventional (magnetic) space group framework. The corresponding system is called chiral DSM. The four-fold degenerate Dirac fermion manifests linear dispersion and a Chern number of +2/-2, leading to a robust network of topologically protected Fermi arcs throughout the Brillouin zone. For material realization, we show that the transition-metal chalcogenide CoNb3S6 with experimentally confirmed collinear antiferromagnetic order is ideal for chiral DSM. Our work unprecedentedly reveals a condensed-matter counterpart of the flavor symmetry in particle physics, leading to further possibilities of emergent phenomena in quantum materials.
We generate experimentally a honeycomb refractive index pattern in an atomic vapor cell using electromagnetically-induced transparency. We study experimentally and theoretically the propagation of polarized light beams in such photonic graphene. We demonstrate that an effective spin-orbit coupling appears as a correction to the paraxial beam equations because of the strong spatial gradients of the permittivity. It leads to the coupling of spin and angular momentum at the Dirac points of the graphene lattice. Our results suggest that the polarization degree plays an important role in many configurations where it has been previously neglected.
High-order topological phases, such as those with nontrivial quadrupole moments, protect edge states that are themselves topological insulators in lower dimensions. So far, most quadrupole phases of light are explored in linear optical systems, which are protected by spatial symmetries or synthetic symmetries. Here we present Floquet quadrupole phases in driven nonlinear photonic crystals (PhCs) that are protected by space-time screw symmetries. We start by illustrating space-time symmetries by tracking the trajectory of instantaneous optical axes of the driven media. Our Floquet quadrupole phase is then confirmed in two independent ways: symmetry indices at high-symmetry momentum points and calculations of the nested Wannier bands. Our work presents a general framework to analyze symmetries in driven optical materials and paves the way to further exploring symmetry-protected topological phases in Floquet systems and their optoelectronic applications.
Chirality refers to a geometric phenomenon in which objects are not superimposable on their mirror image. Structures made of nano-scale chiral elements can display chiroptical effects, such as dichroism for left- and right- handed circularly polarized light, which makes them of high interest for applications ranging from quantum information processing and quantum optics to circular dichroism spectroscopy and molecular recognition. At the same time, strong effects have been challenging to achieve even in synthetic optical media and chiroptical effects for light with normal incidence has been speculated to be prohibited in lossless, thin, quasi-two-dimensional structures. Here, we report on our experimental realization of a giant chiroptical effect in a thin monolithic photonic crystal mirror. Unlike conventional mirrors, our structure selectively reflects only one spin state of light, while preserving its handedness, with a near unity level of circular dichroism. The operational principle of the photonic-crystal mirror relies on Guided Mode Resonance (GMR) with simultaneous excitation of leaky TE and TM Bloch modes in the photonic crystal slab. Such modes are not reliant on the suppression of their radiative losses through the long-range destructive interference and even small areas of the photonic-crystal exhibit robust circular dichroism. Despite its simplicity, the mirror strongly surpasses the performance of earlier reported structures and, contrary to a prevailed notion, demonstrates that near unity reflectivity contrast for the opposite helicities is achievable in a quasi-two-dimensional structure.
Chiral quantum optics has attracted considerable interest in the field of quantum information science. Exploiting the spin-polarization properties of quantum emitters and engineering rational photonic nanostructures has made it possible to transform information from spin to path encoding. Here, compact chiral photonic circuits with deterministic circularly polarized chiral routing and beamsplitting are demonstrated using two laterally adjacent waveguides coupled with quantum dots. Chiral routing arises from the electromagnetic field chirality in waveguide, and beamsplitting is obtained via the evanescent field coupling. The spin- and position-dependent directional spontaneous emission are achieved by spatially selective micro-photoluminescence measurements, with a chiral contrast of up to 0.84 in the chiral photonic circuits. This makes a significant advancement for broadening the application scenarios of chiral quantum optics and developing scalable quantum photonic networks.
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