2. Key Laboratory of Information Storage System Ministry of Education of China, Huazhong University of Science and Technology, Wuhan, Hubei 430074, China
Overview: Human beings are entering a big data era, which has significantly boosted the current digital economy and society. According to an estimation by the International Data Corporation (IDC), the information generated and consumed is nearly doubled every two year. Human being have already generated data onto an amount of 35 ZB (1 ZB=1000 EB=1000, 000 PB=1000, 000, 000 TB=1000, 000, 000, 000 GB) globally in 2017 and in the year of 2020 the total amount will reach 44 ZB. However, all current data storage technologies and mediums can only store less than half of this amount, which means most of the data will be forcibly lost if without breakthrough in high-capacity storage technologies. The infrastructure of the current information technology and the sustainability of the current information economy has been constantly challenged by the thirst for more storage capacities as well as low energy consumption. These challenges set a fundamental obstacle to the longevity and sustainability of the current information technology. Known for its green features, optical data storage is regarded as an excellent candidate for long-term data archiving. However, Ernst Abbe set a fundamental barrier that limits the smallest feature size of a recording cell to approximately half of the wavelength, leading to a capacity of hundreds of Gigabytes per disc. This capacity limitation could be overcome by implementing multiplex technology. This technology enables the potential for storing more than one bit of data in a single memory cell. It can be applied to materials which exhibit sensitivity to not only the intensity but also other parameters of light like polarization, wavelength, and fluorescence. Limited by the material response, only five multiplex dimensions have been achieved in gold nanorods embedded polymer and fused silica glass. The nanogratings, generated by femtosecond laser writing in fused silica, behave as a uniaxial optical crystal with negative birefringence. The two parameters of birefringence, the slow axis orientation and retardation can be independently controlled by the polarization and intensity of the incident laser beam. Thanks to the effect of multi-photon excitation, 3D space of the medium volume can be simultaneously utilized by focusing femtosecond laser in fused silica. Such memory, encoding data in 5 dimensions, is capable of recording 360 TB data per disc for billions of years. It is believed that 5D optical data storage based on nanogratings in fused silica opens a new era of eternal data storage.