Ferroelectric catastrophe: beyond nanometre-scale optical resolution

The optical diffraction limit is rigidly determined as a simple equation of wavelength λ and lens numerical aperture NA ( ): λ/2/NA. In this paper, we report that Ag 5.8 In 4.4 Sb 61.0 Te 28.8 and Ge 2 Sb 2 Te 5 chalcogenide thin films, which are typical of optical recording materials used in digital versatile discs (DVDs), enable a resolution of under λ/10 due to their ferroelectric properties. In the Ag 5.8 In 4.4 Sb 61.0 Te 28.8 film it was found that this optical super-resolution can be observed between 350 and 400 °C, resulting in a second phase transition from a hexagonal (A7 belonging to ) to a rhombohedral structure of R 32 or R 3 m . In Ge 2 Sb 2 Te 5 , on the other hand, the temperature range is much wider, between 250 and 450 °C, which is also due to a second phase transition from a NaCl-type fcc to a hexagonal structure.