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Title: Directive giant upconversion by supercritical bound states in the continuum
Authors: Chiara Schiattarella
Silvia Romano
Luigi Sirleto
Vito Mocella
Ivo Rendina
Vittorino Lanzio
Fabrizio Riminucci
Adam Schwartzberg
Stefano Cabrini
Jiaye Chen
Liangliang Liang 
Xiaogang Liu 
Gianluigi Zito
Keywords: Upconversion
Bound states in the continuum
Issue Date: 21-Feb-2024
Publisher: Springer Nature
Citation: Chiara Schiattarella, Silvia Romano, Luigi Sirleto, Vito Mocella, Ivo Rendina, Vittorino Lanzio, Fabrizio Riminucci, Adam Schwartzberg, Stefano Cabrini, Jiaye Chen, Liangliang Liang, Xiaogang Liu, Gianluigi Zito (2024-02-21). Directive giant upconversion by supercritical bound states in the continuum. Nature 626 : 765–771. ScholarBank@NUS Repository.
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.
Source Title: Nature
ISBN: 0028-0836
ISSN: 1476-4687
DOI: 10.1038/s41586-023-06967-9
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