Kartashov Yaroslav V, Milián Carles, Konotop Vladimir V, Torner Lluis
Opt Lett. 2018 Feb 1;43(3):575-578. doi: 10.1364/OL.43.000575.
We address a 2D parity-time (PT)-symmetric structure built as a chain of waveguides, where all waveguides except for the central one are conservative, while the central one is divided into two halves with gain and losses. We show that such a system admits bound states in the continuum (BICs) whose properties vary drastically with the orientation of the line separating amplifying and absorbing domains, which sets the direction of internal energy flow. When the flow is perpendicular to the chain of the waveguides, narrow BICs emerge when the standard defect mode, which is initially located in the finite gap, collides with another mode in a standard symmetry breaking scenario, and its propagation constant enters the continuous spectrum upon increase of the strength of gain/losses. In contrast, when the energy flow is parallel to the chain of the waveguides, the symmetry gets broken even for a small strength of the gain/losses. In that case, the most rapidly growing mode emerges inside the continuous spectrum and realizes a weakly localized BIC. All BICs found here are the most rapidly growing modes; therefore, they can be excited from noisy inputs and, importantly, should dominate the beam dynamics in experiments.
我们研究了一种二维宇称时间（PT）对称结构，它由一系列波导构成，其中除了中间的波导外，其他所有波导都是保守的，而中间的波导被分为具有增益和损耗的两部分。我们表明，这样的系统允许存在连续谱中的束缚态（BICs），其性质会随着分隔放大和吸收区域的线的方向而急剧变化，该线设定了内部能量流动的方向。当能量流垂直于波导链时，最初位于有限能隙中的标准缺陷模式在标准的对称性破缺情形下与另一个模式碰撞，并且当增益/损耗强度增加时其传播常数进入连续谱，此时会出现窄BICs。相反，当能量流平行于波导链时，即使增益/损耗强度较小，对称性也会被打破。在这种情况下，增长最快的模式出现在连续谱内部并实现一个弱局域化的BIC。这里发现的所有BICs都是增长最快的模式；因此，它们可以从有噪声的输入中被激发，并且重要的是，在实验中应该主导光束动力学。
