Quasi-PT Symmetry: A Pathway to Better Laser Performance

Semiconductor laser diodes (LDs) are key enablers in various applications, including optical communication, material processing, and healthcare, because of their compact size, efficiency, and ease of integration. However, increasing LD output power while preserving spatial and spectral beam quality remains a challenge, often limited by nonlinear effects and filamentation. Established solutions, such as photonic crystals in VCSELs or tapered/antiguiding designs in EELDs, boost power and beam quality but inflate fabrication complexity and cost, hindering widespread adoption. [1]. Recently, non-Hermitian photonics has emerged as a paradigm for engineering the spatial distribution of optical gain and loss. Parity-time-symmetric (PTS) lasers, in particular, can achieve single-mode operation in both longitudinal and transverse directions. More advanced approaches, combining non-Hermitian engineering, supersymmetry, and topological physics, have been proposed for large-area laser arrays, yet realistic power levels and high beam quality remain elusive, often relying on pulsed pumping or exhibiting spurious passive-waveguide emission [2], [3]. Here, we present a systematic theoretical and experimental comparison of PTS and quasi-PTS (q-PTS) EELDs (see Fig. 1). Although both architectures achieve comparable output powers under similar pumping conditions, only q-PTS lasers maintain excellent beam quality (M<sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> < 1.5) at power levels near 0.5 W, surpassing PTS devices (M<sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> ≈ 4). Moreover, power conversion efficiency remains unaffected in q-PTS structures, indicating a minimal trade-off between filtering higher-order modes and preserving overall device efficiency. Equally important, these benefits are achieved without adding complexity or additional fabrication steps beyond a conventional EELD process. These findings challenge the prevailing notion that spatial filtering must be enforced by PTS, which can introduce side lobes that degrade beam quality. Instead, q-PTS leverages stronger coupling among higher-order modes while leaving the fundamental mode unperturbed, striking an optimal balance between mode selectivity and practicality. Our results, therefore, offer a roadmap for large-area single-mode EELDs that deliver both high power and high beam quality, translating non-Hermitian concepts from proof-of-concept to real-world impact.