Correlation of Energy-Band Structure and Slow-Saturable-Absorption Characteristics for 2D or Bulk TeSe₂

Laser modulation with the saturable absorption characteristics of two-dimensional (2D) materials has emerged as a research hotspot in recent years. However, to date, there has been little work truly explaining the macroscopic laser modulation phenomena of 2D materials based on their microscopic electronic band structures. In this work, TeSe2 saturable absorbers with two different thicknesses are employed in solid-state lasers operating at wavelengths of 1063.57 and 1928.10 nm, revealing distinct behaviors. For the 1063.57 nm laser, modulation with 2-layer TeSe2 results in a narrower pulse width of 48.75 ns. Conversely, for the 1928.10 nm laser, modulation with bulk TeSe2 achieves a narrower pulse width of 396.75 ns. To elucidate the differences in modulation effects, a combination of theoretical and experimental approaches is adopted. In theory, band structures and other relative parameters for both 2-layer and bulk TeSe2 are calculated based on first-principles. The three-level system for nonlinear absorption is settled. The 1063.57 nm laser saturable absorption of 2-layer TeSe2 is involved with fewer levels than that of the bulk one, indicating a narrower pulse, considering the natural broadening mechanism. Conversely, for the saturable absorption of 1928.10 nm laser, stronger lattice vibration broadening in 2-layer TeSe2 results in wider Q-switched pulses than the bulk one.