  Hollow-core toroidal micro-cavities are obtained by self-rolling of
double-layer (polyvinyl pyridine/polystyrole) polymer films. Rolling of the
bilayer is due to preferential swelling of the polyvinyl pyridine in water
solution of dodecyl benzene sulfonic acid. The tube formation proceeds from a
circular opening in the film made by photolithography or by mechanical
scratching. The toroid equilibrium dimensions are determined by the balance of
the elastic energy relaxation via the film scrolling and the work of the
in-plane stretching that is due to increasing radius of the toroid. The
principal features of the micro-toroid formation process are captured by a
simple analytical model. The inner walls of the cavities can be made metal
coated. To this aim, the polymer bilayer can be metallized by vacuum sputtering
prior to lithographic patterning and rolling of the bilayer. The toroids with
metallic inner surface are promising for the future research as IR-frequency
range resonators.
