Topological photonics/acoustics utilizes classical waves to emulate topologically nontrivial phases originally developed in condensed matter systems such as topological insulators. While the band topology concepts are originally defined in Hermitian context, the classical-wave systems are intrinsically non-Hermitian, due to the inevitable loss and/or deliberately added gain. Here we will introduce some of our recent works in developing acoustic topological states that have no counterparts in condensed matter systems. The first is about non-Hermiticity-driven topological phase transition. This involves the demonstration of topological edge states in a 1D acoustic lattice and topological corner states in a 2D acoustic lattice. The second is about acoustic non-Hermitian skin effect from twisted winding topology. The twisted winding topology consists of two oppositely oriented loops with a contact point in between. We show that this topology dramatically modifies the non-Hermitian skin effect by causing bulk states collapse towards two directions. The contact point corresponds to an extended Bloch-wave-like bulk states. The third is a Floquet higher-order topological insulator realized in a 3D acoustic structure, whose third dimension serves as an effective time-dependent drive. All the above show novel topological physics on the platform of acoustic waves.
We present a theoretical and experimental study of topological modes running along string-like disclinations in two-dimensional electromagnetic lattices. Two-dimensional honeycomb lattices can host lattice defects of five- and seven-membered rings, which serve as the fixed termination points for string-like disclination. The strings act upon the Dirac cone states in a manner analogous to Dirac strings. When a band gap is opened by breaking sublattice symmetry, gap-spanning interface states appear along the strings, similar to topological kink states in valley Hall systems.
But unlike the valley Hall case, the edges can have arbitrary orientations and terminate at fixed points in the bulk. The theoretical and numerical analysis is rigorously verified through microwave experiments.
We predict some novel 2D plasmonic waves as analogues of corresponding hydrodynamic wave phenomena, including plasmonic splashing and V-shaped ship-wakes excited by a swift electron perpendicularly impacting upon and moving parallel above a graphene monolayer, respectively.
2D plasmons have fueled substantial research efforts in the past few years. Recent studies have identified that 2D plasmons exhibit peculiar dispersion that is formally analogous to hydrodynamic deep-water-waves on a 2D liquid surface. Logically, many intricate and intriguing hydrodynamic wave phenomena, such as the splashing stimulated by a droplet or stone impacting a calm liquid surface and the V-shaped ship-wakes generated behind a ship when it travels over a water surface, should have counterparts in 2D plasmons, but have not been studied.
We fill this gap by investigating dynamic excitation of graphene plasmons when a monolayer graphene is perpendicularly impacted by a swift electron, as an analogue of hydrodynamic splashing. A central jet-like rise, called “Rayleigh jet” or “Worthington jet” as a hallmark in hydrodynamic splashing, is demonstrated as an excessive concentration of graphene plasmons, followed by plasmonic ripples dispersing like concentric ripples of deep-water waves. This plasmonic jet, serving as a monopole antenna, can generate radiation as analogue of splashing sound. This is also the first discussion on the space-time limitation on surface plasmon generation.
We then demonstrate a V-shaped plasmonic wave pattern when a swift electron moves parallel above a graphene monolayer, as an analogue of hydrodynamic ship-wakes. The plasmonic wake angle is found to be the same with the Kelvin angle and thus insensitive to the electron velocity when the electron velocity is small. However, the wake angle gradually decreases by increasing the electron’s velocity when the electron velocity is large, and thus transits into the Mach angle, being similar to recent development in fluid mechanics.
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