The development followed Sharp's announcement last year of a single-layer disk with a "super-resolution functional" film that achieves a 50-Gbyte capacity.
"Sharp has developed a transparent film, which enabled the two-layer disk, doubling the capacity," said a Sharp spokesman. The film is made of a metal oxide material, which features high light transmittance.
Efforts to expand the disk capacity using super resolution functional films have been reported by several research teams.
Matsushita Electric Industrial Co. Ltd. has already developed a 100-Gbyte disk prototype by piling four layers with present BD specification pits and plans to propose a possible extension of Blu-ray Disc format.
Sharp may design its future high-capacity discs to meet Blu-ray Disc specs, said a spokesman.
Sharp's super-resolution functional film enables a blue laser beam to read out smaller pits than its beam spot. In Sharp's prototype, the beam spot is 0.4 micron and pits are 0.1-micron diameter with 0.1-micron interval. The beam covers two pits simultaneously, but the super-resolution functional film lowers its reflection rate when it is heated.
Using the film for the upper layer, Sharp researchers verified the signal read out of both super resolution functional films, which have 0.1-micron diameter pits, about two-thirds of the minimum pit of the Blu-ray Disc pit (0.149 microns).
The researchers are exploring the possibility of piling more layers on the disk.