Nowadays, the using of big data is reshaping our lives via artificial intelligence (AI) and internet of things (IoT) by penetrating education medical care, business, entertainment, and so on. Industrial companies around the world are sparing no effort to collect much data to obtain market conditions, competitors, and logistics information for profits and have long created TB- or even PB-scale information. Meanwhile, consumers are integrating social media, entertainment, and real-time personalized services on mobile devices to connect with friends and shop online. According to the International Data Corporation (IDC), there is an explosive growth in global data, which is estimated to reach 175 zettabytes (ZBs) by 2025. However, the disparity between the amount of digital data and the available storage capacities is enlarging. Most importantly, data storage accounts for 1% of global electricity consumption and exert enormous pressure on resources and environments. However, none of the current medium is capable to meet the requirements. In especial, the cold data storage that is for culture, history, scientific research and these important but infrequently used data is urgently calling for long-term and high capacity medium.

Therefore, we are challenged with the arduous task of developing next-generation data storage technologies, where femtosecond laser direct writing for eternal data storage offers a practical solution with low energy consumption, long lifetime, and high capacity. With multiplexing degrees of freedom, this technology’s achievable limit capacity could reach 360 TB/disc. Furthermore, accelerated aging measurements show that nanograting has unprecedentedly high stability, including thermal stability up to 1000 ℃ and a practically unlimited lifetime.

We reviewed the research progress of femtosecond laser direct writing for eternal data storage. At first, we introduce the interaction between femtosecond laser and materials by reviewing three types of modification. On this basis, the concept and basic physical mechanism of femtosecond laser permanent optical storage were introduced. Then, we reviewed the development of 3D optical storage and 5D optical storage, as well as the structure formation mechanism in detail. Next, we introduced the high-density storage of over 100 layers and fast data recording at a speed of 100 kB/s via a single channel (potential MB/s via multichannel). At the final, based on electronic field continuity conditions at the nanoscale, we calculated the theoretical bottleneck and physical limit of optical storage by femtosecond laser direct writing.

Femtosecond laser direct writing inside hard materials for permanent optical storage provides an unexceptionable solution for cold data storage to meet the demands of big data era. However, there are still some significant scientific and technical problems that must be addressed between the laboratory and the industrial application. For instance, volumes of nanograting must be minimized, and the dot and layer spacing must be reduced to increase the storage density. Moreover, fast writing with fewer pulses and new data readout algorithms for accurate and fast data readout are required. We firmly believe this technology will support every aspect of our lives and bring huge economic benefits to society in the future.