Pulse energy and wavelength-dependent ultrafast dynamics of SnSe₂thin film studied by transient absorption

The ultrafast carrier dynamics in SnSe2 film and their influencing factors are studied using femtosecond transient absorption (TA) spectroscopy. Nano-scale SnSe2 sample films are prepared by pulsed laser deposition on the SiO2 substrate. The observed carrier dynamics are influenced by the film thickness and pulse energy. All carrier ultrafast dynamics can be perfectly fitted with a tri-exponential decay model, which means that the carriers have undergone the process of carrier scattering, carrier recombination and diffusion. The influence of the surface reorganization process and the deep defect state in the material makes the carrier lifetime longer in the thicker film. Moreover, the defect state intensifies the state filling effect, leading to the production of bleaching signal in thin thinner film. With the increase of the pump pulse energy, the carrier lifetime becomes longer. This is due to the influence of the pulse energy on the carrier concentration and the temperature difference between the carrier temperature and the lattice temperature. And the influence of pump pulse energy of band gap reconstruction makes the peak in TA spectrum move with the change of pulse energy. The results reveal the carrier dynamics in SnSe2 thin film at different pump pulse energy and identify the critical role of the pump pulse in photo-generated phenomena.