Symmetry Engineering in Heterometallic TMDC Mo_(1-x)Nb_xSe₂: A Systematic Route Toward Enhanced Nonlinear Optical...

Semiconducting MoSe₂ is susceptible to photon reabsorption, while metallic NbSe₂ 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_xSe₂, 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 NbSe₂. 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.