Experimental observation of multi-mode chiral edge state

Abstract

Electromagnetic (EM) topological states [1, 2] have been drawing lots of attentions in both physical and engineering field for their good performance in optical isolator, circulator, etc‥ As a resemblance of signal carrier in modern communication, the excited EM chiral edge state (CES) is immune to the impurity of transmission path without any backward scattering, providing a potential for future transoceanic communication with no require of repeaters of amplifiers. Mimicking modulation and demodulation of FDM, in this work we realize a resemble function in photonic system by constructing multi-mode CES. Different from most of the presented work with only one CES mode, in this paper we design and experimentally observed a two-mode CES. Such multi-mode CES resembles frequency division multiplexing (FDM). If we consider real space position in PC as the time and the wave vector as frequency. By designing a magnetic photonic crystal (MPC) with square lattice, we obtained eight Dirac points existing at non-high symmetry point in Brillioun zone due to mirror symmetry [3] by keeping time-reversal symmetry. It is demonstrated all Dirac points are unbuckled to open a full gap in the effect of bias magnetic field. Once the MPC is truncated by metal cladding, two-mode edge states appear inside the gap as shown. At an operating frequency, two EM waves are excited with different wavelengths (wavevector) but the same transmission direction. The experimental data verify such two-mode edge states are unidirectional and robust against the scatterings from disorders of various kinds. By introducing the Fast Fourier Transform (FFT) algorithm in EM system, we successfully obtain the band structure of CES basing on the field profile measured. Later, imitating the demodulation in communication engineering, we theoretically propose a method to eliminate one of modes by introducing two feed with same magnitude and phase but located at different positions.