Direct laser micro-fabrication by adaptive optics

Ultrafast lasers are widely used for the precise three-dimensional micro-fabrication inside transparent materials. The resolution of fabricated features depends upon the size of the focal spot, and the process efficiency depends upon the generation of short pulses at the focus. The performance is detrimentally affected by the presence of phase aberrations, which can be introduced by the optics of the system or by refraction at the surface of material. Efficiency can also be affected by other forms of aberration that are related to the ultra-short pulses, such as pulse front distortion or material dispersion.
Adaptive optics has in the past been introduced into laser material processing in order to overcome the problems caused by phase aberrations. However, there are related phenomena specific to systems using ultrashort pulses that have not been extensively studied, nor have benefitted from the application of adaptive optics. This thesis concerns the development of theoretical and practical techniques to address these issues and improve the performance of laser fabrication system. New applications are enabled with this technology.
Spatiotemporal modelling of laser focal intensity distribution is enabled by adopting the Fourier optics. The effects of phase aberrations and sample dispersion on the ultrafast laser focusing are studied in detail. Simulations for both conventional and temporal laser focusing methods are presented. I explore the significance of individual aberration and sample dispersion to investigate the benefit of their compensation in various practical scenarios.
A new method in controlling the ultrafast laser pulses - pulse front adaptive optics - is introduced. With the combination of a deformable mirror (DM) and a spatial light modulator (SLM), quadratic shape pulse fronts with different magnitudes are created. The results are demonstrated through wave-front sensor and auto-correlator measurements. In addition, I introduce the first application of this method into the pulse front correction for a two-photon microscope. An improvement of 1.4 times in the two-photon signal beyond the phase correction is demonstrated.
A series of new applications are enabled by adopting the adaptive optics into laser fabrication. In the adaptive laser fabrication in diamond, graphitic wires with any three-dimensional shapes are enabled. The resistivity of the wires is demonstrated to be reduced by more than 100 times compared to the previous reports. Non-reciprocal effects in the diamond fabrication are studied in detail. The influence of laser parameters, light polarization and pulse front tilt are investigated. I also explore several more advanced applications in the diamond fabrication. Specifically, initial results of the alternating conductor, micro-capacitor and electrode arrays for radiation detector are presented. Additionally, I discuss micro-fabrication of three dimensional funnel structures in silica glass for applications in neuroscience research.

Keywords: Laser micro-fabrication
Optics and Photonics
Laser material processing
Diamond
Adaptive optics
Ultrafast laser phonomenon
Advisor: Booth, M