Directive giant upconversion by supercritical bound states in the continuum

Directive giant upconversion by supercritical bound states in the continuum

21 February 2024 | Chiara Schiattarella1, Silvia Romano1, Luigi Sirleto1, Vito Mocella1, Ivo Rendina2, Vittorino Lanzio3, Fabrizio Riminiucci3, Adam Schwartzberg3, Stefano Cabrini3, Jiaye Chen4, Liangliang Liang4, Xiaogang Liu4,5,6,5,6 & Gianluigi Zito1,5
This paper introduces the concept of supercritical coupling, inspired by electromagnetically induced transparency in near-field coupled resonances, to achieve maximum near-field enhancement in photonic bound states in the continuum (BICs). BICs are topologically non-trivial dark modes in open-cavity resonators that have enabled significant advancements in photonics. However, achieving maximum near-field enhancement is challenging due to the need to balance radiative and non-radiative losses. The proposed supercritical coupling approach compensates for the negligible direct far-field coupling with the dark mode by enhancing the near-field coupling between dark and bright modes. This enables a quasi-BIC field to reach the maximum enhancement imposed by non-radiative loss, even when the radiative quality factor is divergent. The experimental design involves a photonic-crystal nanoslab covered with upconversion nanoparticles. Near-field coupling is finely tuned at the nanostructure edge, leading to a coherent upconversion luminescence enhanced by eight orders of magnitude. The emission shows negligible divergence, a narrow width at the microscale, and controllable directivity through input focusing and polarization. This approach has potential applications in light-source development, energy harvesting, and photochemical catalysis. The theoretical model describes the non-Hermitian Hamiltonian of the system, which models transverse electric-like and transverse magnetic-like modes coupled to a single independent radiation channel. The coupled final modes split apart due to strong coupling, with one becoming a perfect dark mode (ideal Friedrich–Wintgen (FW) quasi-BIC) and the other acquiring all radiative losses. The near-field coupling quality factor is defined as \( Q_c = \omega / (2 k_0 c) \), and supercritical coupling is achieved when \( Q_c = \sqrt{Q_0^2 Q_a} \), avoiding the bottleneck of the narrow input radiation channel. Experimental results demonstrate the upconversion emission, showing a significant enhancement factor of 3.6 × 10^4 compared to a bulk sample. The upconversion photons propagate in a plane, forming a microscale coherent beam with a spatial width of less than 100 μm and a divergence of less than 0.07° over a centimetre distance. The supercritical coupling leads to an enhancement of upconversion by eight orders of magnitude. The discussion highlights the mechanism of near-field coupling between the FW quasi-BIC and the bright mode, breaking the limits of single-dark-mode coupling and achieving orders-of-magnitude improvement. The experimental proof of FW quasi-BIC supercritical coupling is provided using chemically and optically stable upconversion nanoparticles, enabling several microscale addressable sources and lasers. The edge enhancement facilitates directive propagation of self-collimated photons with remarkable control, extending beyond conventional self-collimation. The resulting photoluminescence is enhanced by more than eight orders of magnitude, representing one of the highest values achieved with a dielectric resonator.This paper introduces the concept of supercritical coupling, inspired by electromagnetically induced transparency in near-field coupled resonances, to achieve maximum near-field enhancement in photonic bound states in the continuum (BICs). BICs are topologically non-trivial dark modes in open-cavity resonators that have enabled significant advancements in photonics. However, achieving maximum near-field enhancement is challenging due to the need to balance radiative and non-radiative losses. The proposed supercritical coupling approach compensates for the negligible direct far-field coupling with the dark mode by enhancing the near-field coupling between dark and bright modes. This enables a quasi-BIC field to reach the maximum enhancement imposed by non-radiative loss, even when the radiative quality factor is divergent. The experimental design involves a photonic-crystal nanoslab covered with upconversion nanoparticles. Near-field coupling is finely tuned at the nanostructure edge, leading to a coherent upconversion luminescence enhanced by eight orders of magnitude. The emission shows negligible divergence, a narrow width at the microscale, and controllable directivity through input focusing and polarization. This approach has potential applications in light-source development, energy harvesting, and photochemical catalysis. The theoretical model describes the non-Hermitian Hamiltonian of the system, which models transverse electric-like and transverse magnetic-like modes coupled to a single independent radiation channel. The coupled final modes split apart due to strong coupling, with one becoming a perfect dark mode (ideal Friedrich–Wintgen (FW) quasi-BIC) and the other acquiring all radiative losses. The near-field coupling quality factor is defined as \( Q_c = \omega / (2 k_0 c) \), and supercritical coupling is achieved when \( Q_c = \sqrt{Q_0^2 Q_a} \), avoiding the bottleneck of the narrow input radiation channel. Experimental results demonstrate the upconversion emission, showing a significant enhancement factor of 3.6 × 10^4 compared to a bulk sample. The upconversion photons propagate in a plane, forming a microscale coherent beam with a spatial width of less than 100 μm and a divergence of less than 0.07° over a centimetre distance. The supercritical coupling leads to an enhancement of upconversion by eight orders of magnitude. The discussion highlights the mechanism of near-field coupling between the FW quasi-BIC and the bright mode, breaking the limits of single-dark-mode coupling and achieving orders-of-magnitude improvement. The experimental proof of FW quasi-BIC supercritical coupling is provided using chemically and optically stable upconversion nanoparticles, enabling several microscale addressable sources and lasers. The edge enhancement facilitates directive propagation of self-collimated photons with remarkable control, extending beyond conventional self-collimation. The resulting photoluminescence is enhanced by more than eight orders of magnitude, representing one of the highest values achieved with a dielectric resonator.
Reach us at info@study.space
[slides and audio] Directive giant upconversion by supercritical bound states in the continuum