(A) Photoisomerization scheme of chromophore substituents from the protonated merocyanine (MCH+, Left) to SP (Right) forms in the methylenebis(acrylamide) cross-linked p(AAm-co-AAc) hydrogel. (B) Photographs of chromophore-containing p(AAm-co-AAc) hydrogel monoliths employed in experiments. (C) UV-visible absorbance spectra demonstrating reversible isomerization of MCH+ (absorption λmax = 420 nm) to SP (λmax = 320 nm) in solution. (D) Experimental setup (Top) to probe laser self-trapping due to photoinduced local contraction of the hydrogel, schematically depicted on the Bottom (see also Movie S1). A laser beam is focused onto the entrance face of the hydrogel while its exit face is imaged onto a CCD camera.