Ultrafast Carrier Dynamics of Undoped and Ho³⁺-Doped α-Bismuth Oxide Microrods

We report the femtosecond two-color pump–probe studies on bismuth oxide microrods. The undoped data show three distinct decays: (1) fastest decay (τ 1 ∼ 0.3–0.5 ps) attributed to intraband nonlinear effects (2) intermediate two-body decay (τ 2 ∼ 3 ps) attributed to the trap-assisted Auger recombination; and (3) slow single-body decay (τ 3 ∼ 36 ps) attributed to the Shockley–Read–Hall recombination. Upon doping with Ho 3+, the sample shows an additional slow decay (τ 4 ∼ 12 μs), attributed to the Shockley–Read–Hall recombination due to the trap states formed by the doping, along with the three decays observed in the undoped sample. The imaginary parts of third-order susceptibility values for both doped and undoped samples were estimated from the pump–probe studies, and the values are found to decrease inversely with pump fluence suggesting the plausible contribution from hot electrons.