Dissertation Title: “DIRECT LASER WRITING BELOW THE DIFFRACTION LIMIT BY EXPLORING MULTI-PULSE-INDUCED PHYSICS”
Ultrafast laser ablation has enabled high-precision processing of a wide range of materials including metals, semiconductors, dielectrics, and polymers. Several laser nanostructuring methods exist, including those based on optical near-fields, special material properties, surface plasmons, and multiphoton absorption (MPA). Among these methods, the MPA method has the potential for nanoscale direct laser writing by using a simple experimental setup. However, the understanding of the fundamental mechanism involved in the laser ablation process is still incomplete, and it remains challenging to obtain a feature size much smaller than the diffraction-limited spot size. The goal of this research is to understand how ultrafast laser pulses interact with different types of materials (including metal, dielectrics, and semiconductors), and to look for a repeatable method to achieve feature size deep below the diffraction limit. To this end, this dissertation describes novel approaches that enable the localization of laser energy at micron to nanoscale utilizing laser-matter interaction under unusual exposure conditions.
Spatiotemporally separated double pulses are used to control the free-electron dynamics during laser ablation and self-trapped-excitons are found to be significant for localizing the spatial distribution of electron density when the delay between pulses is much longer than the free-electron lifetime. Moreover, the same relationship between laser-induced feature size and laser energy is found in metal, semiconductors, and dielectrics. The minimum controllable feature size is the smallest for the material with a larger bandgap, and this feature size highly depends on how well the laser energy is controlled.
To produce nano/sub-micron feature size, an “ultrashort pulse burst” is used to achieve energy localization by tuning the delay between pulses. An experimental setup consisting of a fourfold Michelson interferometric system is built and characterized. Laser ablation experiments on fused silica are conducted and nano/sub-micron pits on the surface of fused silica samples are produced. The formation of these structures is attributed to the surface defects that provide energy levels within the bandgap through which selective excitation is achieved by choosing an excitation pathway that favors a long carrier lifetime. This result provides another means for tailoring surface structures at the sub-micron and nanoscale.
Major: Optics and Photonics PhD
BS: 2013, Electronic Science and Technology, Harbin Institute of Technology
MS: 2015, Optical Engineering, Harbin Institute of Technology
MS: 2020, Optics and Photonics, University of Central Florida
Committee in Charge:
Dr. Xiaoming Yu, Chair
Dr. Aravinda Kar
Dr. M. J. Soileau
Dr. Michael Chini
Approved for distribution by Xiaoming Yu, Committee Chair, on April 19th, 2022.
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