In recent years, topological insulator and topological semimetal have drawn much attention, due to the novel properties such as robust edge states, chiral anomaly, diverging local density of states and so on. However, the dimension of momentum space is less than three, which limits the exploration the physics in higher dimensions. Take advantage of the concept of synthetic dimension, we can construct higher dimensional spaces to realize more amazing phenomenon including Weyl surface in 5D space, 4D Quantum Hall effect and so on. Some works also focused on probing the topological physics of non-Hermitian systems, which leads to additional novel consequences such as the transformation of the Weyl point into an exceptional ring when introducing a non-Hermitian term.
In this work, we give a flexible platform to investigate the PT-symmetric topological physics in 4D synthetic space. Here, based on 1D photonic crystal with a complex seven-layer unit cell, we construct a 4D synthetic space with two momentum vectors and two geometric parameters. Before introducing the non-Hermitian term, the system shows a 2D Nodal-hyperedge in 4D space. If we add gain and loss in to the PCs but preserving PT symmetry, the 2D hyperedge will be transformed into a 3D exceptional hypersurface, on which the Hamiltonian becomes defective. Such 3D exceptional hypersurface also guarantees the unidirectional reflectionless resonance in 1D PCs even with oblique incidence. Furthermore, we also find the existence of PT-symmetric interface states in our systems, which will give rise to the strong field enhancement, and is quite useful in nonlinear and quantum optics.
Based on the concept of drawing equivalence between different configurations in transformation optics, we introduce a conceptual framework to investigate radiation from accelerating particles using chains of metamaterial atoms with SOI on a metasurface. The framework allows a global geometric picture in visualizing different one-dimensional space-times in general relativity using our two-dimensional metasurface. In particular, chains of metamaterial atoms along different curved lines in generating a common SPP caustic represent the same particle motions observed in difference reference frames, with relative motion through a general-relativistic transformation, such as a Rindler transformation from a reference Minkowski space-time. Such a tool allows us to study particle motions in different space-times in general while the particular geometric understanding provides us unique ways in generating SPP.
Weyl fermions have not been found in nature as elementary particles, but they emerge as nodal points in the band structure of electronic and classical wave crystals. Novel phenomena such as Fermi arcs and chiral anomaly have fueled the interest in these topological points which are frequently perceived as monopoles in momentum space. We demonstrate that generalized Weyl points can exist in a parameter space and we report the first observation of such nodal points in one-dimensional photonic crystals in the optical range. The reflection phase inside the band gap of a truncated photonic crystal exhibits vortexes in the parameter space where the Weyl points are defined and they share the same topological charges as the Weyl points. These vortexes also guarantee the existence of interface states, the trajectory of which can be understood as “Fermi arcs” emerging from the Weyl nodes.
Topological invariant plays a more and more important role in modern physics with the discovery of new materials such as topological insulators. The concept of momentum space topology has also been extended to various photonic systems to realize interesting applications. In this work, a plasmonic interface state is introduced between a photonic crystal and a metasurface which is protected by the Z2 topological mirror symmetry of the photonic crystals. Here we propose a scheme to experimentally measure the topological phase in a photonic system. Using reflection spectrum measurement, we determined the existence of interface states in the gaps, and then obtained the Zak phases. The interface state is excited when the reflection phase matching condition is satisfied. The reflection phase of metasurface can be tuned by changing the structural parameter. The resonance properties of interface state can be manipulated in the process. By manipulating the anisotropic property of the metasurface, we can further tune the polarization of the interface state. Field enhancement induced by the interface state will have important applications in nonlinear and quantum optics.
We demonstrate spin-induced manipulation of surface-plasmon polariton (SPP) by exploiting the plasmonic spin Hall effect. By constructing metasurfaces with plasmonic atoms and varying spin-dependent geometric phase, we establish a holographic interface between an incident plane wave and the SPP on an optical chip. It allows us to gain spin-splitting and flexible control of the shapes and phases of the local SPP orbitals. Furthermore, a linearly polarized incident light with rotating polarization angle can be used to play a motion picture of the orbitals. These investigations provide a feasible route to many applications, including spin-enabled imaging, data storage and integrated optics.
The control of electromagnetic radiation in transformation optical metamaterials brings the development of vast variety of optical devices. Of a particular importance is the possibility to control the propagation of light with light. In this work, we use a structured planar cavity to enhance the thermo-optic effect in a transformation optical waveguide. In the process, a control laser produces apparent inhomogeneous refractive index change inside the waveguides. The trajectory of a second probe laser beam is then continuously tuned in the experiment. The experimental results agree well with the developed theory. The reported method can provide a new approach toward development of transformation optical devices where active all-optical control of the impinging light can be achieved.
A plasmonic cavity composed from metasurface is designed and experimentally demonstrated. Due to the symmetry breaking of the metasurface, the degeneracy of the different polarized cavity states is lifted. It shifts the resonating frequencies of two polarized cavity modes, in which one is blue-shifted and another is red-shifted. Combining with a photothermal effect, we demonstrate that the polarized cavity states can be experimentally tuned by varying the reflection phase of the metasurface through the incident laser intensity. This reported metacavity can be applied in cavity quantum optics, lasers and other light-matter interaction processes.
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