近日，意大利国家研究委员会应用科学与智能系统研究所的Gianluigi Zito及其研究小组与新加坡国立大学的Xiaogang Liu等人合作并取得一项新进展。经过不懈努力，他们实现连续域中超临界束缚态的定向巨大上转换。相关研究成果已于2024年2月21日在国际权威学术期刊《自然》上发表。

该研究团队提出了超临界耦合的概念，其从接近Friedrich–Wintgen条件的近场耦合共振的电诱导透明中获得灵感。当暗模和亮模之间的近场耦合补偿了与暗模之间可忽略的直接远场耦合时，就发生了超临界耦合。这使得准BIC场能够达到由非辐射损失施加的最大增强，即使辐射质量因子是发散的。该实验设计由覆盖上转换纳米粒子的光子晶体纳米板组成。

研究人员在纳米结构边缘对近场耦合进行了微调，观察到相干上转换发光增强了8个数量级。发射发散可以忽略不计，在微尺度下宽度很窄，通过输入聚焦和偏振可以控制方向性。这种方法与各种物理过程相关，在光源开发、能量收集和光化学催化方面具有潜在的应用前景。

据悉，连续域光子束缚态(BICs)嵌入在具有发散辐射质量因子的自由空间波的光谱中，是开放腔谐振器中拓扑上非平凡的暗模式，使光子学取得了重要进展。然而，实现最大的近场增强尤其具有挑战性，因为这需要匹配辐射和非辐射损失。

附：英文原文

Title: Directive giant upconversion by supercritical bound states in the continuum

Author: Schiattarella, Chiara, Romano, Silvia, Sirleto, Luigi, Mocella, Vito, Rendina, Ivo, Lanzio, Vittorino, Riminucci, Fabrizio, Schwartzberg, Adam, Cabrini, Stefano, Chen, Jiaye, Liang, Liangliang, Liu, Xiaogang, Zito, Gianluigi

Issue&Volume: 2024-02-21

Abstract: Photonic bound states in the continuum (BICs), embedded in the spectrum of free-space waves with diverging radiative quality factor, are topologically non-trivial dark modes in open-cavity resonators that have enabled important advances in photonics. However, it is particularly challenging to achieve maximum near-field enhancement, as this requires matching radiative and non-radiative losses. Here we propose the concept of supercritical coupling, drawing inspiration from electromagnetically induced transparency in near-field coupled resonances close to the Friedrich–Wintgen condition. Supercritical coupling occurs when the near-field coupling between dark and bright modes compensates for the negligible direct far-field coupling with the dark mode. This enables a quasi-BIC field to reach maximum enhancement imposed by non-radiative loss, even when the radiative quality factor is divergent. Our experimental design consists of a photonic-crystal nanoslab covered with upconversion nanoparticles. Near-field coupling is finely tuned at the nanostructure edge, in which a coherent upconversion luminescence enhanced by eight orders of magnitude is observed. The emission shows negligible divergence, narrow width at the microscale and controllable directivity through input focusing and polarization. This approach is relevant to various physical processes, with potential applications for light-source development, energy harvesting and photochemical catalysis.

DOI: 10.1038/s41586-023-06967-9

Source: https://www.nature.com/articles/s41586-023-06967-9