  Dynamic control of circular dichroism in photonic structures is critically
important for compact spectrometers, stereoscopic displays, and information
processing exploiting multiple degrees of freedom. Metasurfaces can help
miniaturize chiral devices but only produce static and limited chiral
responses. While external stimuli are able to tune resonances, their
modulations are often weak, and reversing continuously the sign of circular
dichroism is extremely challenging. Here, we demonstrate dynamically tunable
chiral response of resonant metasurfaces supporting chiral bound states in the
continuum combining them with phase-change materials. Phase transition between
amorphous and crystalline phases allows to control chiral response and vary
chirality rapidly from -0.947 to +0.958 backward and forward via chirality
continuum. Our demonstrations underpin the rapid development of chiral
photonics and its applications.
