Merged bound states in the continuum for giant superchiral field and chiral mode splitting

Enhanced circular dichroism (CD) signal is crucial for ultrasensitive detection of chiral species. This purpose strongly relies on the construction of superchiral field in achiral resonant nanostructures. Despite the progress, the research on superchiral fields and their related physics is fundamentally restricted. Here, we demonstrate an approach to construct superchiral field in achiral metasurfaces and explore its impact on chiral mode interaction. While the chiral field at individual bound states in the continuum (BIC) is small, all criteria for giant superchiral fields can be satisfied when two BICs with orthogonal polarizations are merged. Consequently, the typical photonic crystal slabs can produce superchiral fields with enhancement factors ($C\text{/}{C}_{\mathrm{CPL}}$) (where CPL is circular polarized light) orders of magnitude higher than state of the art in nanostructures. More interestingly, a type of chiral light-matter interaction has been revealed by introducing chiral active materials. The nearly orthogonal BICs couple one another at the merging point and result in chiral mode splitting.