Unveiling distinct defect state and wavelength dependent saturation absorption enhancement mechanism in Bi1.3In0.7Se3 alloy

Alloying induced mid-gap defect states provide an additional transition pathway for controlling nonlinear optical (NLO) behavior, while the corresponding mechanism is still a challenge due to the increased complexity of multi-process relaxation. This work demonstrated that saturation absorption (SA) improvement of Bi1.3In0.7Se3 alloy is dependent on the excitation wavelength, with an optimal NLO absorption coefficient of −1.16 cm/GW at 500 nm, which is approximately 2 orders of magnitude higher than that of Bi2Se3. The mid-gap defect states introduced by alloying, proved by femtosecond transient absorption spectrum, accelerate the interband carrier recombination time from 19.53 to 1.93 ps, thereby enhancing the SA response in a range from 400 to 1100 nm. Meanwhile, it increases the possibility of excited-state absorption, thereby weakening the SA enhancing response in a range from 850 to 1100 nm. Our work deepens the logical connection between the underlying mechanism of mid-gap defect and the wavelength dependent NLO enhancement.