Multiple toroidal dipoles governed by mirror-coupled bound states in the continuum in a nanopillar array for a low-threshold nanolaser.

Toroidal dipoles (TDs) exhibit strong field confinement and high-quality factor resonances, making them promising for light-matter interactions, though their weak strength limits applications. However, complex metasurface designs hinder their practical use. Here, we show that a simple nanopillar array supports both electric (ETD) and magnetic (MTD) TD modes governed by a mirror-coupled quasi-bound state in the continuum (BIC) without requiring symmetry breaking, which significantly enhances their quality factors. To further elucidate the characteristics of the ETD and MTD modes, their lasing behaviors are analyzed using a four-level two-electron energy diagram. The results reveal that nanolasers based on these modes exhibit lower thresholds, with calculated values of about ~0.5503 µJ/cm² for the ETD mode and ~0.87 µJ/cm² for the MTD mode. These findings underscore the potential of ETD and MTD modes for efficient nanophotonic applications, offering a promising pathway for the development of low-threshold nanolasers and other advanced photonic devices.