  Superchiral field is highly important for ultrasensitive detection of chiral
objects and has been intensively explored. Despite the progress, the
construction of electromagnetic field in achiral nanostructures with giant
chirality (C) is fundamentally restricted. Here, we demonstrate a novel
approach to construct superchiral field in achiral dielectric metasurfaces. Due
to the quantum spin-hall effect, each symmetry-protected bound state in the
continuum (BIC) exhibits the ability to support a superchiral field in vicinity
of \Gamma point. By merging two BICs with orthogonal polarizations, all the
criteria for a superchiral field are satisfied and the dramatic enhancement of
chirality becomes more robust. Using typical photonic crystal slabs, we have
produced a superchiral field with enhancement factor (C/C_CPL) orders of
magnitude higher than state-of-the-art in nanostructures. By further
introducing active chiral medium, a chiral mode splitting associated with giant
enhancement of circular dichroism (CD) has been formed by the chiral
light-matter interaction.

Optics