Intermediate Phase-Change States with Improved Cycling Durability of Sb₂S₃ by Femtosecond Multi-Pulse Laser Irradiation

Sb₂S₃ has great potential to become a good phase-change material for visible and near-infrared wavebands due to its low loss and high refractive index contrast, so great interest lies in the technology development. This work investigates the intermediate phase-change states and their cycling durabilities by employing a continuous-wave laser for crystallization and a femtosecond laser for amorphization. By considering stratified partial amorphization due to non-uniform intensity distribution along propagation in a film, a double-layer model is proposed to describe intermediate states, which fits experimental data better than the mixture models in effective medium approximation. The phase-change degree is then defined as the ratio of the amorphization depth to the total film thickness, which can be controlled by combination of the pulse energy and the number of pulses in multi-pulse femtosecond laser irradiation for amorphization. The cycling durability is improved by reducing pulse energy and increasing the number of pulses. The experimentally achieved maximum cycling durabilities are 30, 1000, and 7000 cycles for 90%, 60%, and 20% phase-change degrees. The implementation of intermediate states with improved cycling durability may promote the development of Sb₂S₃-based reconfigurable photonic devices.