Phenomena of ultrafast laser material modification with respect to spatio-temporal couplings of the laser pulse

Abstract

The nano-structuring of transparent media with subpicosecond laser pulses has attracted significant interest due to its unique applications. In contrast to nanosecond pulses where the energy introduced to the lattice is absorbed, leading to melting/boiling of the material around the focal volume, femtosecond lasers can alter material properties of the glass at high pressures without excessive production of heat, modifying the structures with sub-micron resolution. Permanent modifications can then be induced without strong collateral damage. Although femtosecond pulses are beneficial for material processing, short pulse durations and broad spectral bandwidths require a novel approach to femtosecond pulse control.

It is well known that laser induced modification depends on fluence, wavelength and polarization. Another dependence of material modification is the spatio-temporal properties of the ultrashort pulse. These spatio-temporal couplings give rise to intrinsic nonlinear optical phenomena, which are well known in experiment but otherwise lack a clear explanation. While the formation mechanisms with respect to the nano-structuring of transparent media is still under debate, a better understanding of the nonlinear optical phenomena that affects the formation would provide insight into the physics of ultrafast light-matter interaction.

In this thesis, the origin and thorough investigation of spatio-temporal induced phenomena are reported. By controlling the spatio-temporal couplings separately, I demonstrate complete control of all of the dependencies with the use of prism compressors and grating compressors and discuss the intricacies behind the control of the spatio-temporal couplings with complete characterization of the pulse. By investigating two of the main phenomena associated with spatio-temporal couplings, which give rise to a directional dependence when writing in the bulk (“quill-writing effect”) and a photosensitive anisotropy (“blade effect”), a more thorough understanding of the light-matter interaction is demonstrated and reported.

I demonstrate that spatio-temporal couplings are inherent for all ultrafast laser systems with chirped-pulse amplification and result in a strongly anisotropic light-matter interaction. I identify angular dispersion in the focus as the main cause for the anisotropic photosensitivity coming from the spatially chirped pulse, which shows to yield a 200% increase in modification strength. With tighter focusing (NA ≥ ~0.4), this non-paraxial effect leads to a more apparent manifestation of spatio-temporal couplings in photo-induced modification. I control the anisotropy and exploited it as a new degree of freedom in tailoring laser induced modification in transparent material. A non-paraxial field structure analysis near the focus is conducted, with an elliptical optical beam, to provide insight on the origin of the photosensitive anisotropy.

After a complete identification of the spatio-temporal properties of the electric field, the quill-effect was confirmed to be due to pulse front tilt in the focus with a direct comparison with other major spatio-temporal couplings including wavefront rotation. I reveal that the non-reciprocity during femtosecond laser writing in transparent media induces either an isotropic damage-like structure or a self-assembled nanostructure depending on the movement direction of the beam - known as the “quill-writing effect.” I also identify the switching of the modification regime from the formation of isotropic damage-like to anisotropic grating-like structures observed when the translation of the beam is in the direction of the tilt and is qualitatively described in terms of the first-order phase transition in the irradiated volume of a transparent dielectric.

The structural evolution from void modification to self-assembled nanogratings in fused silica for moderate (NA > 0.4) focusing conditions is also discussed in this thesis. Void formation appears before the geometrical focus after the initial few pulses with nanogratings gradually occurring at the top of the induced structures after subsequent irradiation. Nonlinear Schrödinger equation-based simulations are conducted to simulate the laser fluence, intensity and electron concentration in the regions of modification. Comparing the experiment with simulations, the voids form due to cavitation in the regions where electron concentration exceeds 10²⁰ cm^-3 but remains below critical. In this scenario, the energy absorption is insufficient to reach the critical electron concentration that was once assumed to occur in the regime of void formation and nanogratings, shedding light on the potential formation mechanism of nanogratings.

In-situ observations of harmonic generation during the ultrafast laser writing is presented to better understand the underlying physics that occur during the process of nanograting formation. Second and third harmonic generation is observed, with third harmonic distributed as two lobes following the polarization orientation of the electric field, identified as Cherenkov Third Harmonic. These harmonics are observed and correlated with the different regimes of material modification to understand whether they are part of the nanograting formation or corollary to give insight on the formation mechanisms of the self-assembled nanostructures.

Finally, I discuss the work on the concept of an on-axial simultaneous spatio-temporal focusing with the use of a simple polarization dependent circular grating for the purpose of material modification using the expertise of the group on polarization gratings. The design and theoretical validation of the technique is reported in this thesis with the potential of further work in perfecting it for material modification and chirped-pulse amplification applications.

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