Laser modification of materials at micro- and nanoscale for photonics and information technology

Abstract

In this thesis, I will concentrate on ultrafast laser induced modification in transparent materials, in particular silica glass. About twenty years ago, the subwavelength nanogratings (type II) were observed in silica glass after ultrafast laser writing. This modification exhibits form birefringence， so it has been used for the demonstration of optical components and for multiplexing optical data storage. Although the nanogratings with high thermal and chemical durability enable the elements with high optical damage threshold and five-dimensional (5D) data storage with virtually eternal lifetime, relatively high transmission losses and low writing speeds limit their practical applications.
A new type of birefringent modification (type X) with ultra-high optical transmittance has been demonstrated in silica glass by using suitable ultrafast laser writing parameters. The randomly distributed anisotropic nanopores are responsible for the form birefringence and high transmission from infrared to ultraviolet range. The formation mechanism of type X modification is proposed, which is different from the self-organized mechanism of nanograting formation. Furthermore, different optical elements with ultra-high efficiency were fabricated from the ultraviolet to the infrared range for phase and polarization shaping.
In addition, I demonstrate that elliptically polarized pulses are more efficient to produce anisotropic nanopores based type X modification. This phenomenon is explained as a consequence of the balance between the concentration of nanopores with a maximum at circular polarization and the near-field enhanced anisotropy due to the linear polarisation component. The elliptical polarization writing method not only increases the speed of data recording for 5D optical data storage, but also reduces the manufacturing time for type X based optical elements.
Apart from laser writing parameters, the properties of material are also important in ultrafast laser writing. I have found that the formation of anisotropic nanopores is highly dependent on the grade of silica glass. About five times higher retardance of type X modification can be achieved in a vapor axial deposition (VAD) silica glass sample, compared to the electrically fused silica glass with the same writing parameters. The phenomenon is interpreted in terms of the higher concentration of oxygen deficient defects (ODCs) in fused quartz, which can confine self-trapped holes and prevent the anisotropic nanopores formation. Direct writing of nanopores based geometric phase elements and 5D optical storage require different grades of silica glass. Moreover, laser-written modifications, in particular birefringent modifications, reveal a dependence on the geometry of writing that the modification strength increases from voxel to line structures and multi-line scanned area, which can be explained by free carrier diffusion and reduced electric fields in scanning writing.
To increase the speed of data writing for 5D optical data storage, a single nano-lamella structure has been produced in the silica glass, requiring only a few ultrafast laser pulses to write. An isotropic nanovoid is initially produced with pulse energy above the microexplosion threshold and then reshaped to a nanolamella-like structure via the near-field enhancement effect by weaker pulses to reduce the unwanted thermal effects from high-repetition rate fs pulses. The single nanolamella-like structures are used to improve the writing speed of 5D data storage to ~225 kB/s and a potential data capacity of ~500 TB per disk can be achieved.
Thereafter, I show that an easily overlooked parameter, pulse temporal contrast, is also important for the nanostructuring of silica glass by ultrafast lasers. While nanopores based modifications can be written using a crystalline gain media femtosecond laser with a pulse contrast of 107, such modification cannot be produced by pulses with a nanosecond pedestal and 103 contrast from a fiber laser. The revealed importance of pulse contrast can inspire other studies to improve the efficiency of ultrafast laser processing of various materials.
Finally, I present that polarization-controlled birefringent modification can be induced by ultrafast laser writing in isotropic crystals. Because the slow axis orientation is parallel to the polarization direction of writing laser beam, this modification has different formation mechanism compared to the form birefringence of nanostructures in silica glass. Multiplexed optical data storage with high density is achieved in the crystal and additional dimensions can be encoded by controlling the shape of birefringent voxels.

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Identifiers

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

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