Symmetry Engineering in Heterometallic TMDC Mo _(1−x) Nb _x Se ₂ : A Systematic Route Toward Enhanced Nonlinear Optical Performance

ABSTRACT Semiconducting MoSe 2 is susceptible to photon reabsorption, while metallic NbSe 2 yields low optical damage thresholds, each limiting their nonlinear optical (NLO) performance. Combining these contrasting traits, this study demonstrates that equimolar heterometallic Mo (1−x) Nb x Se 2 , undergoes structural reconfiguration that substantially enhances NLO responses, as revealed by second harmonic generation (SHG) and nonlinear optical absorption (NOA) measurements. Pronounced SHG is observed in odd‐layer samples across 840–1020 nm, with a 32.5% higher optical damage threshold than NbSe 2 . Further, anomalous SHG persists in even layers, attributed to strain‐induced symmetry breaking and emergence of non‐zero Berry curvature under substantial alloying. This rare behavior distinguishes the alloy from its binaries and enables integration into piezoelectric, ferroelectric, and spin‐valley‐coupling platforms. Furthermore, straindriven bandstructure modifications also yield prominent saturable absorption (SA) at 1 µm, as demonstrated by the generation of ultrafast‐ultrashort laser pulses of 2.292 ps with repetition rate of 11.21 MHz in Yb‑doped fiber laser system and exceptional reverse‐SA (RSA) at 1.5 µm, exhibiting intrinsic “selftriggered” optical‐limiting (10.21% and 18.15% improvements over respective binaries), suitable for laserprotection devices. Overall, the study provides a systematic strategy to engineer and enhance NLO functionalities in binary 2D materials, facilitating the development of functional NLO photonics devices.