Low-loss birefringent modification of transparent materials for 5D data storage

Abstract

In this thesis, my focus revolves around the induced modifications in silica glass using femtosecond laser technology, exploring their properties and practical applications, particularly within the realm of 5D optical data storage. Various modification types, including isotropic refractive index increase (type 1), self-assembled nanogratings or nanoplanes (type 2), flattened nanopores (type X), single nanolamella-like structures (type S), and lamella-shaped structures (type 2m), are introduced, with their generation contingent on laser writing parameters.
Error-free 5D optical data storage in silica glass is achieved based on ultralow-loss flattened nanopores. This birefringent modification exhibits exceptional transmission, surpassing 99% in the visible range, facilitating the practical recording and retrieval of 100 layers of multibit digital data with 100% readout accuracy. My research delves into optimizing data storage capacity, writing speed, and data retrieval accuracy through a thorough investigation of different parameters.
It has been observed that for type X modification, as the scanning speed increases, the retardance first increases and then decreases and the amount of increase becomes larger with a higher numerical aperture, which prove advantageous for data storage, achieving a demonstrated speed of 72 kB/s. Additionally, single pulses are proven capable of inducing type X modifications in nanoporous glass, thereby restricting the data writing speed solely by the repetition rate of the laser. Furthermore, the adoption of acoustic-optic deflector elevates the writing speed to approximately 250 kB/s utilizing single nanolamella-like structures in silica glass.
Beyond laser writing parameters, material properties play a pivotal role in ultrafast laser writing. Notably, the formation of anisotropic nanopores is heavily contingent upon the grade of silica glass. A vapor axial deposition (VAD) silica glass sample achieves about fivefold higher retardance in type X modification compared to electrically fused silica glass using identical writing parameters. However, for data storage applications, higher readout accuracy is observed when writing in electrically fused silica glass.
Lastly, I highlight the significance of an often-overlooked parameter: pulse temporal contrast in nanostructuring silica glass using ultrafast lasers. While nanopore-based modifications are feasible with a crystalline gain media femtosecond laser with a pulse contrast of 10^7, pulses with a 200 ps pedestal and 10^3 contrast from a fibre laser fail to produce such modifications.

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Identifiers

ORCID for Huijun Wang: orcid.org/0000-0002-2368-7699

ORCID for Gholamreza Shayeganrad: orcid.org/0000-0002-5225-2176

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