Imaginary coupling induced Dirac points and group velocity control in non-reciprocal Hermitian Lattice

TL;DR

This paper introduces a novel mechanism using imaginary coupling in non-reciprocal Hermitian lattices to control light group velocity and induce topological phase transitions, enabling bifurcation light transmission.

Contribution

It presents a new approach to manipulate light group velocity via imaginary coupling in non-reciprocal photonic lattices with topological properties.

Findings

Real energy spectrum with Dirac points in the lattice

Topological phase transition at Dirac points

Controlled bifurcation light transmission

Abstract

We propose a mechanism to achieve the group velocity control of bifurcation light via an imaginary coupling effect in the non-reciprocal lattice. The physical model is composed of two-layer photonic lattices with non-reciprocal coupling in each unit cell, which can support a real energy spectrum with a pair of Dirac points in the first Brillouin zone due to the Hermicity. Furthermore, we show that the systems experience topological phase transition at the Dirac points by tuning the coupling strength, allowing the existence of topological edge states on the left or right boundaries of respective lattice layers. By adjusting the imaginary coupling and the wave number, the group velocity of the light wave can be manipulated, and bifurcation light transmission can be achieved both at the Dirac points and the condition without the group velocity dispersion. Our work might guide the design of photonic directional couplers with group velocity control functions.