Abstract
Bragg resonances caused by the same transverse modes can always play a major role in periodic waveguides when the period is larger than the average radius. Because of higher-order mode cutoffs, the related Bragg gaps can be identified as interactions between different spatial harmonics of the fundamental mode, and the first Bragg gaps are more intensive than the higher ones. When we alter the parameters of the periodic waveguide, especially, decrease the period, the first transverse mode can be involved in Bragg gaps. Here, we demonstrate a direct mode-stopband interaction between the first mode and the second Bragg gap, that an extraordinary passband arises in the original second Bragg gap and splits the bandgap into two. Furthermore, the extraordinary passband is mainly composed of a pure first mode, which effectively suppresses the transmission of the fundamental one. We have also investigated the influence of wall profiles on the transmission and mode purity, and have found that the defined shape factor of wall profiles is proportionally related to the width of both pass and stop bands. The results could benefit not only the understanding of wave phenomena but also the applications in mode generators, filters, and so on.
Highlights
Periodic structures are constituted by identical components which are arranged in an array or in a lattice
More attention has been paid to determine the frequency bandgaps of periodic structures since the bandgap is an important feature of periodic structures, and it can be applied in photonic bandgap materials which can act like a kind of “semiconductors.”4 a lot of topics has been developed in physics and technologies, such as invisibility cloak,5 negative refraction,6,7 and thermal management
With the finite element method (FEM), we show the simulated transmissions of periodic waveguides in Fig. 5 for the sinusoidal, rectangular, and triangular wall profiles by the blue solid, red dashed, and green dash-dot lines, respectively
Summary
Periodic structures are constituted by identical components which are arranged in an array or in a lattice. The study of frequency bandgaps is crucial for periodic corrugated waveguides. If the interference is caused by interactions between two different transverse modes, it will result in nonBragg resonance. Both types of resonances will lead to bandgaps, and they have been reported in recent works.. We will investigate interactions between the transverse mode and Bragg gap in periodically corrugated waveguides, and demonstrate that they can efficiently generate a single first mode and suppress the fundamental ones.
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