Controlling ripples' periodicity using temporally delayed femtosecond laser double pulses

We demonstrate the capability to control the ripple periodicity on polycrystalline ZnO films by applying temporally delayed femtosecond double pulses. It is shown that there is a characteristic pulse separation time for which one can switch from low- to high- spatial-frequency ripple formation. Results are interpreted based on the relation of the characteristic delay time with the electron-phonon relaxation time of the material. Our results indicate that temporal pulse shaping can be advantageously used as a mean to control the periodic nanoripples' formation and thus the outcome of laser assisted nanofabrication process, which is desirable for the applications of nanopatterned transparent semiconductors.