This study introduces a novel approach to achieve giant upconversion by utilizing supercritical coupling in photonic systems. The concept of supercritical coupling is inspired by electromagnetically induced transparency (EIT) in near-field coupled resonances near 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, enabling a quasi-BIC field to reach 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, with near-field coupling finely tuned at the nanostructure edge. This results in a coherent upconversion luminescence enhanced by eight orders of magnitude, with negligible divergence, narrow width, and controllable directivity through input focusing and polarization. The approach has potential applications in light-source development, energy harvesting, and photochemical catalysis. The study demonstrates that a Friedrich–Wintgen quasi-BIC can be achieved through supercritical coupling, which overcomes the negligible direct far-field coupling with the quasi-BIC and restores the maximum level of enhancement imposed by the non-radiative loss. The results show that upconversion photons propagate in 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 combination of supercritical coupling leads to an enhancement of upconversion by eight orders of magnitude. The study also highlights the importance of non-radiative losses in determining the cavity enhancement and the role of supercritical coupling in achieving maximum enhancement. The results demonstrate the potential of this approach for various physical processes and applications in nanophotonics.This study introduces a novel approach to achieve giant upconversion by utilizing supercritical coupling in photonic systems. The concept of supercritical coupling is inspired by electromagnetically induced transparency (EIT) in near-field coupled resonances near 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, enabling a quasi-BIC field to reach 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, with near-field coupling finely tuned at the nanostructure edge. This results in a coherent upconversion luminescence enhanced by eight orders of magnitude, with negligible divergence, narrow width, and controllable directivity through input focusing and polarization. The approach has potential applications in light-source development, energy harvesting, and photochemical catalysis. The study demonstrates that a Friedrich–Wintgen quasi-BIC can be achieved through supercritical coupling, which overcomes the negligible direct far-field coupling with the quasi-BIC and restores the maximum level of enhancement imposed by the non-radiative loss. The results show that upconversion photons propagate in 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 combination of supercritical coupling leads to an enhancement of upconversion by eight orders of magnitude. The study also highlights the importance of non-radiative losses in determining the cavity enhancement and the role of supercritical coupling in achieving maximum enhancement. The results demonstrate the potential of this approach for various physical processes and applications in nanophotonics.