This work presents a theoretical investigation of an active diffraction grating of the Parity-Time (PT) symmetric architecture. The analytical study of the free-space mode propagation in the grating structure indicates the unique bifurcation property due to the PT-symmetry modulation. It is shown that both the gain/loss contrast and the lattice constant parameters are critical factors to modulate the photonic system in between the PT-symmetry to the symmetry-broken phases. Furthermore, numerical simulations via the Rigorous Coupled-Wave Analysis (RCWA) method discover the existence of a unique Spectral Singularity (SS) phenomenon in this PT grating structure which is corresponding to a non-trivial single-mode and near-zero bandwidth photonic resonant emission. Also, the guiding procedure for fulfilling SS modes is found to be related to the unique formation of the scattering matrix applied in the PT-symmetric diffraction gratings. This theoretical work takes a fresh look into the active PT-symmetric diffraction gratings focusing on the discovery of new free-space emission modes rather than the commonly studied unidirectional properties, which could contribute to the development of novel low-threshold and super-coherent laser devices.
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