Directive giant upconversion by supercritical bound states in the continuum

Area of Science:

• Photonics
• Quantum Optics
• Condensed Matter Physics

Background:

• Photonic bound states in the continuum (BICs) are topologically non-trivial dark modes in open-cavity resonators.
• Achieving maximum near-field enhancement in BICs is challenging due to the need to match radiative and non-radiative losses.

Purpose of the Study:

• To propose and demonstrate the concept of supercritical coupling for enhanced near-field effects in BICs.
• To enable quasi-BIC fields to reach maximum enhancement limited by non-radiative loss, irrespective of radiative quality factor.

Main Methods:

• Utilized supercritical coupling, inspired by electromagnetically induced transparency, where near-field coupling compensates for negligible far-field coupling.
• Experimentally implemented using a photonic-crystal nanoslab coated with upconversion nanoparticles.
• Tuned near-field coupling at the nanostructure edge.

Main Results:

• Observed coherent upconversion luminescence enhanced by eight orders of magnitude.
• Demonstrated quasi-BIC fields reaching maximum enhancement limited by non-radiative loss.
• Achieved emission with negligible divergence, narrow spectral width, and controllable directivity.

Conclusions:

• Supercritical coupling provides a novel pathway to maximize near-field enhancement in BICs.
• The demonstrated approach offers significant potential for advancements in light-source development, energy harvesting, and photochemical catalysis.