We report a decreased surface wettability when polymer films on a glass substrate are treated by ultrafast laser pulses in a back-illumination geometry. We propose that back illumination through the substrate confines chemical changes beneath the surface of polymer films, leaving the surface blistered but chemically intact. To confirm this hypothesis, we measure the phase contrast of the polymer when imaged with a focused ion beam. We observe a void at the polymer-quartz interface that results from the expansion of an ultrafast laser-induced plasma. A modified polymer layer surrounds the void, but otherwise the film seems unmodified. We also use x-ray photoelectron spectroscopy to confirm that there is no chemical change to the surface. When patterned with partially overlapping blisters, our polymer surface shows increased hydrophobicity. The increased hydrophobicity of back-illuminated surfaces can only result from the morphological change. This contrasts with the combined chemical and morphological changes of the polymer surface caused by a front-illumination geometry.

• Received 31 August 2020
• Revised 16 February 2021
• Accepted 6 April 2021

DOI:https://doi.org/10.1103/PhysRevMaterials.5.045201