Femtosecond Laser-Induced Type X Structure For Eternal Optical Data Storage

With the rapid development of artificial intelligence,cloud computing and other technologies,the total amount of data worldwide is growing extremely fast,which puts forward higher requirements for data storage technology.Among them,the storage of large amounts of cold data that are not frequently accessed but need to be stored for long periods of time has been a focus of attention.Femtosecond laser permanent optical storage technology uses femtosecond laser generated nano-gratings inside transparent materials to achieve multi-dimensional encoding of data,providing a high-density,low-energy,long-life cold data storage solution,but its storage capacity still needs to be increased.The Type X structure,a new modified structure induced by femtosecond laser with up to 99% transmission rate,can increase the number of storage layers and improve readout accuracy,and is expected to replace nano-gratings as storage units for permanent optical storage with femtosecond laser,thus further increasing storage capacity and improving storage reliability.However,there is little research on Type X structure at home and abroad,especially the evolutionary laws and formation mechanisms of Type X structure are still at the exploration stage,which limits the wider application of Type X structure.In this paper,Type X structures were produced inside quartz glass using femtosecond pulsed laser,their evolution laws were explored by polishing and etching combined with theoretical calculations,and a series of samples of permanent optical storage time capsules using Type X structures as storage units have been produced,the specific research contents include:Firstly,different types of modification were produced by changing the pulse duration,pulse number,single pulse energy and polarization direction of the femtosecond pulse laser.The modified structures were characterized by birefringence microscope and optical microscope,and the evolution law of the retardance and slow axis direction of the modified structures with the laser condition was summarized.Secondly,by polishing,etching the material and characterizing it using Scanning Electron Microscopy(SEM),the evolution law of the morphology of the modified structure was discovered and it was also observed that the stretching direction of the Type X structure is always perpendicular to the laser polarization direction.Meanwhile,the intensity distribution of the femtosecond pulse laser around the nanopores was simulated by COMSOL software.The results show that the femtosecond laser can produce near-field enhancement perpendicular to the polarization direction to the nanopores in Type X structure,the results are in agreement with the experimental ones.Finally,time capsule samples for data storage using Type X structures were made.The information is first represented by ASCII codes,then the decimal codes are converted to hexadecimal,and sixteen Type X structures are generated inside the transparent material by the femtosecond laser,and each code is corresponded to one Type X structure,so that the information can be stored with Type X structures.In addition,taking full advantage of the high transmittance of the Type X structure,multiple layers of data were deposited in the time capsule,ultimately achieving a readout accuracy of up to 99.72%.In this paper,the evolution law and formation mechanism of the new modification Type X induced by femtosecond laser are explored to achieve highcapacity and high-accuracy multi-layer permanent optical storage,providing a new idea for the long-term storage of large amount of cold data and is expected to promote the further development of permanent optical storage technology.