Indefinite metacavities (IMCs) made of hyperbolic metamaterials show great advantages in terms of extremely small mode volume due to large wave vectors endowed by the unique hyperbolic dispersion. However, quality (Q$Q$) factors of IMCs are limited by Ohmic loss of metals and radiative loss of leaked waves. Despite the fact that Ohmic loss of metals is inevitable in IMCs, the radiative loss can be further suppressed by leakage engineering. Here we propose a mirror coupled IMC structure which is able to operate at Fabry–Pérot bound states in the continuum (BICs) while the hyperbolic nature of IMCs is retained. At the BIC point, the radiative loss of magnetic dipolar cavity modes in IMCs is completely absent, resulting in a considerably increased Q$Q$ factor (>90$>90$). Deviating from the BIC point, perfect absorption bands (>0.99$>0.99$) along with a strong near-field intensity enhancement (>1.8×104$>1.8\times {10}^4$) appear when the condition of critical coupling is almost fulfilled. The proposed BICs are robust to the geometry and material composition of IMCs and anomalous scaling law of resonance is verified during the tuning of optical responses. We also demonstrate that the Purcell effect of the structure can be significantly improved under BIC and quasi-BIC regimes due to the further enhanced Q$Q$ factor to mode volume ratio. Our results provide a new train of thought to design ultra-small optical nanocavities that may find many applications benefitting from strong light–matter interactions.

中文翻译：

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耦合到镜子的不定元腔：具有反常共振尺度的连续体中的束缚态

由双曲超材料制成的不定元腔（IMC）由于独特的双曲色散赋予大波矢量，在极小的模式体积方面表现出巨大的优势。然而，IMC 的质量 ( Q$Q$ ) 因数受到金属的欧姆损耗和泄漏波的辐射损耗的限制。尽管IMC中金属的欧姆损耗是不可避免的，但辐射损耗可以通过泄漏工程进一步抑制。在这里，我们提出了一种镜像耦合 IMC 结构，它能够在连续体 (BIC) 中的法布里-珀罗束缚态下运行，同时保留 IMC 的双曲性质。在 BIC 点，IMC 中磁偶极腔模式的辐射损耗完全不存在，导致Q$Q$因子显着增加 ( > 90$>90$ )。偏离BIC点，当几乎满足临界耦合条件时，会出现完美的吸收带（ > 0.99$>0.99$）以及强烈的近场强度增强（> 1.8 × 10 4 ）。$>1.8\times {10}^4$所提出的 BIC 对于 IMC 的几何形状和材料成分具有鲁棒性，并且在光学响应调谐过程中验证了共振的反常缩放定律。我们还证明，由于Q$Q$因子与模式体积比的进一步增强，结构的 Purcell 效应在 BIC 和准 BIC 体系下可以得到显着改善。我们的研究结果提供了设计超小型光学纳米腔的新思路，这种超小型光学纳米腔可能会发现许多受益于强光与物质相互作用的应用。