Like something from a science fiction movie, researchers from Kyoto University in Japan, in collaboration with colleagues from the University of Southampton in the UK and Zhejiang University in China, have developed a high-density optical memory device using silica glass.1 “We have demonstrated rewritable optical data storage with a capacity of 300 gigabits per cubic centimeter, or about ten times the capacity of a 12 cm Blu-Ray disk,” says Yasuhiko Shimotsuma who led the research team.

At sufficiently high intensities, a laser can be used to burn dots in glass. The researchers previously showed that lasers with fixed polarization produce dots consisting of a periodic arrangement of alternating dark and bright lines. By passing polarized light through such a dot written in silica glass, the researchers observed that it travels differently depending on the relative orientations of the polarization of the light and the periodic pattern (dark and bright lines). This ‘form birefringence’ phenomenon is the basis of their new memory device. The periodic pattern of the dot can be set to any angle in the silica plane by adjusting the laser polarization during writing, and importantly with regard to possible applications, rewriting the memory with a second laser pulse is possible within a short time after the first pulse.

Fig. 1: Image of a Japanese woodprint of Mount Fuji ‘saved’ in optical memory glass, viewed using an optical microscope (a, unpolarized; b, cross-polarized) and a polarization microscope (c, polarization angle dependence; d, delay dependence).

The memory device has a total of five functional dimensions: the orientation of the periodic pattern of the dot within the plane, the three spatial dimensions of the glass, and the amount by which light of different polarizations is delayed as it travels through the dot. The delay depends on the contrast between the bright and dark periodic lines, and can be adjusted by the number of individual laser pulses used to write the dot.

At present, the experimental system (Fig. 1) is capable of writing and reading dots with a diameter of about 2 µm. “We have written the smallest structures ever created by light in glass, with sizes down to 20 nm,” says Shimotsuma. The researchers believe that the dot size could be made even smaller, leading to even higher memory storage densities.