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 100fs or 20ps is investigated in the 400–800nm wavelength range. The time evolution of the phase transformations has been studied with picosecond resolution real-time reflectivity measurements at a probe wavelength of 514.5nm and also with femtosecond and picosecond pump-probe measurements. Upon picosecond irradiation, three regimes are identified: for wavelengths below ∼550nm and above ∼750nm, the total time to transform between the crystalline and amorphous phases is of the order of 10–24ns while in the intermediate wavelength range of 600–750nm, the transformation time is only ∼650ps. Upon 100fs irradiation, the transformation times are observed to decrease with increasing wavelength with the shortest times of ∼5ns for crystallization and ∼10ns for amorphization, both occurring at 800nm. 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.