  We demonstrate that the integration of a phase-change material (PCM) in one
of the two microresonators of a photonic metamolecule, coupled to a bus
waveguide, can lead to unidirectional Fano resonances and to the emergence of a
unidirectional transmission window. The phase change is caused by light-induced
heating and is accompanied by an abrupt increase in the extinction coefficient
of the PCM resonator. Due to the photonic circuit asymmetry, the critical value
of the input light intensity triggering the phase change is strongly dependent
on the input light direction. The latter determines the unidirectional nature
of the emerging transmission window. This effect can be utilized in on-chip
magnetic-free isolators and Q-switches.
