Dynamic control of unidirectional guided resonances in PT-symmetric bilayer gratings

Unidirectional guided resonances (UGRs) enable on-chip optical modes that radiate exclusively to one side of the structure without relying on bulky mirrors, offering a compact route for directional light emission. However, most UGRs reported so far are fixed in their radiation angle and limited to off-vertical directions, which restricts their flexibility in practical photonic systems. In this work, we propose a PT-symmetric bilayer grating that supports a pair of upward- and downward-radiating UGRs, whose radiation angles and directionality can be continuously tuned by adjusting the gain–loss contrast. As the modulation strength increases, both modes shift smoothly in momentum space, and the unidirectional efficiency of the upward-radiating one is progressively enhanced, which, at a specific gain-loss value, reaches the exact vertical (Γ-point) direction, a highly desirable configuration for surface-emitting devices and on-chip light extraction. Furthermore, by simply reversing the structural misalignment or interchanging gain and loss, the radiation direction can be flipped from upward to downward. Finally, to facilitate experimental implementation, we demonstrate that similar tunability can be achieved by varying the interlayer distance or by using a purely lossy structure, eliminating the need for active gain in some configurations. Our design provides a versatile, experimentally accessible platform for dynamically controlled single-side radiation, with potential applications in beam steering, optical interconnects, and integrated photonic circuits.