All optical sub diffraction multilevel data encoding onto azo polymeric   thin films
  By exploiting photo-induced reorientation in azo-polymer thin films, we demonstrate all-optical polarization-encoded information storage with a scanning near-field optical microscope. In the writing routine, 5-level bits are created by associating different bit values to different birefringence directions, induced in the polymer after illumination with linearly polarized light. The reading routine is then performed by implementing polarization-modulation techniques on the same near-field microscope, in order to measure the encoded birefringence direction. 
Matteo Savoini, Paolo Biagioni, Lamberto Duo', and Marco Finazzi