Ferroelectric catastrophe: beyond nanometre-scale optical resolution

The optical diffraction limit is rigidly determined as a simple equation of wavelength a, and lens numerical aperture NA ( < 1): A/2/NA. In this paper, we report that Ag5.8In4.4Sb61.OTe28.s and Ge2Sb2Te5 chalcogenide thin films, which are typical of optical recording materials used in digital versatile discs (DVDs), enable a resolution of under a,/ 10 due to their ferroelectric properties. In the Ag5.81n4.4Sb61.oTe28.8 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 R3m) to a rhombohedral structure of R32 or R3m. In Ge2Sb2Te5, 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. Research and development of ultrahigh-density optical data storage with terabyte capacity has been carried out over the last decade by applying near-field optics [1-5]. Several methods have been proposed, but most recently, two methods: using a solid immersion lens (SIL) [6, 7] and a super-resolution near-field structure (super-RENS) 18-10] have both been used to achieve a capacity of nearly 0.1 Tbits in-2. In super-RENS especially, the progress of this research has been incredibly fast. Kim et al recently reported a signal carrier-