Unveiling Distinct Ultrafast Photocarrier Dynamics and Tailored Nonlinear Optical Absorption in Phase-Engineered Two-Dimensional MoTe.

Polymorphic two-dimensional (2D) transition metal dichalcogenides (TMDCs) exhibit diverse properties for optoelectronic applications. Here, utilizing phase-engineered MoTe as a prototypical platform, we comprehensively explored its ultrafast and nonlinear optical properties to complete the fundamental framework of phase-dependent optical phenomena in 2D TMDCs. Starting with the phase-selective synthesis of 2H- and 1T'-MoTe with tailored thicknesses, we revealed their distinct photocarrier relaxation mechanisms using intensive power-/temperature-/thickness-dependent transient absorption spectra (TAS). Rapid electron-electron scattering and interband recombination dominated in the metallic 1T' phase, while slower defect trapping and phonon-mediated processes prevailed in the 2H phase, attributed to intrinsic differences in carrier concentration and band structure. Furthermore, we correlated the observed relaxation characteristics with nonlinear saturable absorption (SA) performance by integrating TAS and micro--scan on identical flakes and revealed that prolonged photocarrier lifetimes and high linear absorbance contributed to SA enhancement via excited-state population regulation. Guided by this principle, an obvious MoTe SA improvement in the underperforming near-infrared region was achieved simply by increasing its thickness. Surprisingly, both phases exhibited high nonlinear coefficients of 10-10 cm GW (400-1100 nm), superior to most 2D materials. Our findings enrich phase-tunable photophysics in 2D TMDCs and deliver effective optimization strategies for ultrafast photonics and optoelectronics.