Pump-probe laser reflectivity measurements

To perform in situ pump-probe reflectivity measurements, ~50 nm Ge₁₅Sb₈₅ thin films on silicon substrate are sandwiched between two layers of ZnS–SiO₂ (prepared via RF sputtering). After the deposition, thermal annealing has been performed at 458 K in a glass tube with a heating rate of 5 K/min and various holding times under Ar atmosphere. In situ reflectivity measurements during phase transformation were performed with a static optical tester. Firstly, to crystallize the thin amorphous phase-change film, a focused laser pulse (wavelength = 658 nm) with variable power and duration is applied to a submicron region. Then, in situ reflectivity is recorded by a low-intensity continuous-wave laser (wavelength = 639 nm) at the same position with the pump laser. The relative change in reflectance is ΔR = (R_i − R_f)/R_i where R_i and R_f are initial and final reflectivity, respectively. We tested the replicability of the reflectivity results (Fig. 2a) by measuring a separate set of samples. The two independent measurements show an excellent agreement (Supplementary Fig. ⁴).