双周期结构上连续介质中传播束缚态的条件鲁棒性
Conditional robustness of propagating bound states in the continuum on biperiodic structures
摘要：
对于夹在两个均匀介质之间的周期性结构，连续介质中的束缚态（BIC）是具有辐射连续介质中的频率的引导布洛赫模式。光学BIC已经发现了许多应用，主要是因为它们产生具有超高品质因数的谐振。如果周期性结构具有相关的对称性，则BIC可以具有与相同频率的传入和传出传播波以及兼容的波矢量的对称性失配，并且被认为是受对称性保护的。具有非零布洛赫波矢的双折射晶体已经在许多高度对称的周期结构中被发现。它们不受通常意义上的对称性的保护（即，不存在对称性失配），但是它们中的一些似乎依赖于对称性来实现它们的存在和鲁棒性。在本文中，我们表明，低频传播BIC（只有一个辐射通道）的双周期结构与反转对称的周期平面和反射对称的垂直方向是鲁棒的保序结构扰动。换句话说，当双周期结构稍微扰动时，传播的BIC以稍微不同的频率和稍微不同的布洛赫波矢量继续其存在，从而保持反转和反射对称性。我们的研究增强了对周期结构上BIC的理论理解，并为它们的应用提供了有用的指导。
英文摘要：
For a periodic structure sandwiched between two homogeneous media, a bound state in the continuum (BIC) is a guided Bloch mode with a frequency in the radiation continuum. Optical BICs have found many applications, mainly because they give rise to resonances with ultra-high quality factors. If the periodic structure has a relevant symmetry, a BIC may have a symmetry mismatch with incoming and outgoing propagating waves of the same frequency and compatible wavevectors, and is considered as protected by symmetry. Propagating BICs with nonzero Bloch wavevectors have been found on many highly symmetric periodic structures. They are not protected by symmetry in the usual sense (i.e., there is no symmetry mismatch), but some of them seem to depend on symmetry for their existence and robustness. In this paper, we show that the low-frequency propagating BICs (with only one radiation channel) on biperiodic structures with an inversion symmetry in the plane of periodicity and a reflection symmetry in the perpendicular direction are robust to symmetry-preserving structural perturbations. In other words, a propagating BIC continues its existence with a slightly different frequency and a slightly different Bloch wavevector, when the biperiodic structure is perturbed slightly preserving the inversion and reflection symmetries. Our study enhances theoretical understanding for BICs on periodic structures and provides useful guidelines for their applications.
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