The influence of wavelength on phase transformations induced by picosecond and femtosecond laser pulses in GeSb thin films

Cycling between the crystalline and amorphous phases of 25-nm-thick GeSb films induced by single laser pulses of duration of 100 fs or 20 ps is investigated in the 400-800 nm wavelength range. The time evolution of the phase transformations has been studied with picosecond resolution real-time reflectivity measurements at a probe wavelength of 514.5 nm and also with femtosecond and picosecond pump-probe measurements. Upon picosecond irradiation, three regimes are identified: for wavelengths below ∼550 nm and above ∼750 nm, the total time to transform between the crystalline and amorphous phases is of the order of 10-24 ns while in the intermediate wavelength range of 600-750 nm, the transformation time is only ∼650 ps. Upon 100 fs irradiation, the transformation times are observed to decrease with increasing wavelength with the shortest times of ∼5 ns for crystallization and ∼10 ns for amorphization, both occurring at 800 nm. This behavior is discussed in terms of how the wavelength-dependent refractive index of the phases involved influences the initial supercooling of the molten volume and the subsequent resolidification scenario.