Weyl points are isolated degeneracies in reciprocal space that are monopoles of the Berry curvature. This topological charge makes them inherently robust to Hermitian perturbations of the system. ...However, non-Hermitian effects, usually inaccessible in condensed-matter systems, are an important feature of photonic systems, and when added to an otherwise Hermitian Weyl material have been predicted to spread the Berry charge of the Weyl point out onto a ring of exceptional points, creating a Weyl exceptional ring and fundamentally altering its properties. Here, we observe the implications of the Weyl exceptional ring using real-space measurements of an evanescently coupled bipartite optical waveguide array by probing its effects on the Fermi arc surface states and bulk diffraction properties of the two constituent sublattices in an experimental realization of a distributed Berry charge in a topological material.
The polarization, wavelength and power of a light wave can be simultaneously identified by a compact device made from twisted layers of carbon atoms - with a little help from an artificial neural ...network. The problem is exacerbated in moiré materials, whose electronic structure can be affected by minute changes in strain and twist angle. ...although the device is sensitive to multiple light characteristics, practical application of this capability is hampered by the complicated variations in the photovoltage. ...it would be worthwhile to explore similar schemes with other moiré materials in which large nonlinear responses to applied voltage have recently been uncovered9.
Quantum cascade lasers are compact, electrically pumped light sources in the technologically important mid-infrared and terahertz region of the electromagnetic spectrum
. Recently, the concept of ...topology
has been expanded from condensed matter physics into photonics
, giving rise to a new type of lasing
using topologically protected photonic modes that can efficiently bypass corners and defects
. Previous demonstrations of topological lasers have required an external laser source for optical pumping and have operated in the conventional optical frequency regime
. Here we demonstrate an electrically pumped terahertz quantum cascade laser based on topologically protected valley edge states
. Unlike topological lasers that rely on large-scale features to impart topological protection, our compact design makes use of the valley degree of freedom in photonic crystals
, analogous to two-dimensional gapped valleytronic materials
. Lasing with regularly spaced emission peaks occurs in a sharp-cornered triangular cavity, even if perturbations are introduced into the underlying structure, owing to the existence of topologically protected valley edge states that circulate around the cavity without experiencing localization. We probe the properties of the topological lasing modes by adding different outcouplers to the topological cavity. The laser based on valley edge states may open routes to the practical use of topological protection in electrically driven laser sources.
Unusually, this metal is immune to conversion into an insulator by the presence of impurities or distortions on the edge, because its existence is enforced by topological differences between the ...materials on each side of the edge. Since the theoretical prediction6 and experimental discovery7 of topological-insulator materials in 2006 and 2007, respectively, condensed-matter physicists have been scrambling to exploit the unique properties of the materials in applications such as spintronics and fault-tolerant quantum computing.
Unconventional Weyl points (WPs), carrying topological charge 2 or higher, possess interesting properties different from ordinary charge-1 WPs, including multiple Fermi arcs that stretch over a large ...portion of the Brillouin zone. Thus far, such WPs have been observed in chiral materials and acoustic metamaterials, but there has been no clean demonstration in photonics in which the unconventional photonic WPs are separated from trivial bands. We experimentally realize an ideal symmetry-protected photonic charge-2 WP in a three-dimensional topological chiral microwave metamaterial. We use field mapping to directly observe the projected bulk dispersion, as well as the two long surface arcs that form a noncontractible loop wrapping around the surface Brillouin zone. The surface states span a record-wide frequency window of around 22.7% relative bandwidth. We demonstrate that the surface states exhibit a novel topological self-collimation property and are robust against disorder. This work provides an ideal photonic platform for exploring fundamental physics and applications of unconventional WPs.
Recent studies have revealed the counterintuitive possibility that increasing disorder can turn a topologically trivial insulator into a nontrivial insulator, called a topological Anderson insulator ...(TAI). Here, we propose and experimentally demonstrate a photonic TAI in a two-dimensional disordered gyromagnetic photonic crystal in the microwave regime. We directly observe the disorder-induced topological phase transition from a trivial insulator to a TAI with robust chiral edge states. We also demonstrate topological heterostructures that host edge states at interfaces between domains with different disorder parameters.
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