Lasing in strongly resonant metasurfaces with fluorescent dyes: quasi-dark and bright optical states and the reabsorption effect.

Abstract

A light-emitting metasurface for highly efficient lasing in the visible is proposed and thoroughly theoretically studied. The metasurface comprises split-square slots etched in a thin TiO₂ layer and periodically arrayed in a subdiffractive lattice. The slots are filled with highly fluorescent rhodamine 6G (R6G) molecules in a methanol matrix, providing optically pumped gain under pulsed excitation. Lasing is enabled by a slot-type quasi-bound state in the continuum (qBIC) offering enhanced light-matter interaction. Through a meticulous design process, the lasing threshold is minimized by mitigating reabsorption losses and effectively coupling the pump light to a bright mode exhibiting significant angular dispersion. This enables relaxed resonant pumping conditions at the desired wavelength by appropriately choosing the incidence angle. To assess the lasing response, a rigorous temporal coupled-mode theory (CMT) framework is employed, which is adapted to organic dye gain media accounting for the critical reabsorption effect. The proposed metasurface laser has a very low lasing threshold of 1 kW/cm² and remarkably high efficiency of 15% at 570 nm.