  We report a decreased surface wettability when polymer films on a glass
substrate are treated by ultra-fast 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 observed 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.
