Unidirectional spectral singularity lasing in a defective atomic lattice

We propose an efficient scheme for achieving mode-tunable unidirectional reflection lasing (URL) by establishing a coherent gain atomic system to amplify the probe field and ingeniously designing the one-dimensional (1D) defective atomic lattice. This lattice not only replaces the resonant cavity to provide a distributed feedback mechanism but also breaks the spatial symmetry of the probe susceptibility. Correspondingly, the URL can be characterized by a non-Hermitian degenerate spectral singularity (NHDSS), where the two eigenvalues of the inverse scattering matrix are engineered to satisfy $\lambda _{{S^{-1}}}^{+}\simeq \lambda _{{S^{-1}}}^{-}\rightarrow 0$. This intriguing NHDSS depends on the probe susceptibility and the Bragg condition, both of which can be modulated by adjusting the external optical field and lattice structure, rendering the scheme experimentally feasible. Our approach achieves both nonreciprocity and lasing oscillation in a single system, significantly enhancing the efficiency of optical information transmission and facilitating the integration of active photonic devices into compact quantum networks.