Graduate Exam Abstract

Abstract: The Nobel Prize-winning chirped pulsed amplification technique was devised to address limitations posed by the laser-induced damage threshold (LIDT) of dielectric optical coatings. Consequently, research has centered on improving the LIDT of high index, high band gap optical coatings, particularly those based on hafnium oxide. The research into UV coatings capable of enduring billions of laser shots unscathed finds its most profound significance within the forefront of laser-driven inertial confined fusion energy (IFE) research. The overarching trajectory of fusion energy's scalability hinges upon the ability of optical coatings to withstand repeated irradiation over protracted durations. The implementation of high LIDT optical coatings is limited by the ability to engineer amorphous oxide layers with higher band gap without sacrificing high index of refraction. Typically, hafnium oxide is the standard for high index material. Unlike most high index materials (i.e TiO2 ), HfO2 has a high index of refraction and a high band gap. Meanwhile, SiO2 and Al2O3 have high band gap, high LIDT, but a low index of refraction. This illustrates the fundamental engineering tradeoff in laser damage mitigation. To optimize the index band gap tradeoff, mixtures of hafnia and a high band gap oxide have been proposed. Investigating mixed oxide systems is a promising research direction, where the needs of high energy lasers can be met without sacrificing coating thickness.

This thesis investigates a variety of metal oxide mixture thin films as the high index material in an optical coating. Primarily, the optical properties, laser damage performance, amorphous morphology, and electronic state analysis of hafnium oxide mixtures are investigated versus mixing of SiO2, Al2O3. This section of the thesis reports on the parameter space available to these metal oxide mixtures and their effects on 355nm, nanosecond, LIDT. Specifically, the changes in index of refraction and optical band gap as dopant concentration increases. It was discovered that the LIDT of hafnia improves with alumina or silica incorporation, with significant performance improvement for pure alumina or highly doped HfSiOx films, at the cost of refractive index. Comparison between e-beam evaporated (EBE) and ion beam sputtered (IBS) coatings indicates higher multi-shot LIDT in the EBE coating and higher single shot LIDT in the IBS coating. These results are supported by a focused study on the effect of laser conditioning. The use of raster scan laser conditioning was shown to improve the LIDT in sputtered films, but not as much as the improvement demonstrated in literature on e-beam coatings. Finally, investigations into the role of substrate interfacial defects were performed. It was revealed that dry etching the residual CeO polishing contamination drastically improves the LIDT of AR coatings, which has profound implications for the interpretation of LIDT data in the literature.

Adviser: Carmen Menoni
Co-Adviser: N/A
Non-ECE Member: Justin Sambur, Chemistry
Member 3: Jesse Wilson, Electrical and Computer Engineering
Addional Members: N/A

Publications:
N/A

Program of Study:
ECE673
ECE506
ECE546
ECE673
ECE574
ECE441
MSE502A
MSE502D