Tailoring radiative and nonradiative losses of chiral quasi-bound states in the continuum in plasmonic nanohole arrays

Plasmonic systems are typically characterized by a trade-off between sub-wavelength field confinement and dissipative losses that limit the Q-factor of optical resonances. In this work, we study chiral quasi-bound states in the continuum (q-BICs) in a metallic nanohole array with broken-symmetry holes, focusing on the role of losses in a wide spectral range for the q-BIC resonance frequency. We disentangle the contributions of radiative vs ohmic losses to the Q-factor, showing that both of them are reduced at longer resonance wavelengths, and relating this behavior to the change in field confinement. As a result of the reduction of both kinds of losses, we demonstrate that high circular dichroism, high Q-factor, and strong local-field enhancement can be simultaneously achieved; therefore, the chiral q-BIC investigated here effectively breaks the trade-off between field confinement and losses. The results can be exploited to realize high-Q chiral resonances with enhanced radiation-matter interaction.