Mirrorless unidirectional lasing with inversionless gain in asymmetric ladder atomic Bragg lattices
Phys. Rev. A 114, 043712 – Published 9 October, 2026
DOI: https://doi.org/10.1103/5kj5-r5t2
Abstract
Direction-selective distributed feedback can bring weak optical gain to the threshold of mirrorless reflection-type nonreciprocal lasing. We demonstrate this mechanism in a one-dimensional asymmetric Bragg atomic lattice of coherently driven, incoherently pumped ladder-type atoms. The pump controls the probe gain, while the two-section lattice combines an off-Bragg pass-band section with a Bragg stop-band section to form asymmetric Bragg feedback. Using a transfer-matrix description, we formulate the threshold condition in terms of unit-cell and supercell Bloch modes. The unidirectional lasing onset occurs when a left-reflection spectral singularity is matched to a destructive-interference dip in the right-reflection channel; equivalently, the inverse scattering-matrix eigenvalues coalesce and vanish at a non-Hermitian degenerate spectral singularity, which identifies the linear lasing threshold rather than the actual nonlinear steady-state laser output. The threshold is therefore a cooperative non-Hermitian effect of weak gain compensation, Bragg feedback, and direction-selective interference, rather than a direct consequence of material gain alone. Tuning the pump and coupling fields allows this mechanism to operate in both population-inversion and inversionless-gain regimes, showing that population inversion is not required when Bragg feedback and destructive interference are simultaneously satisfied. Finite-bandwidth pulse calculations connect the resulting dip–peak spectrum to direction-dependent temporal response. These results establish incoherent pumping as an effective control parameter for threshold engineering in active asymmetric Bragg atomic lattices.
Physics Subject Headings (PhySH)
- Atom lasers
- Atom optics
- Atomic, optical & lattice clocks
- Classical optics
- Lasers
- Non-reciprocal propagation
- Optical coherence
- Optical pumping
- Optical transient phenomena
- Optics & lasers
- Quantum coherence & coherence measures
- Quantum communication
- Quantum information processing
- Quantum information theory
- Quantum optics
