Ultrafast laser pulse processing of the nanolayered crystalline mineral muscovite
Ultrafast, laser-matter interactions at the femtoseconds time scale, uses a pulse duration shorter than the electron-to-ion energy exchange time. The latter is typically, depending on the material properties, a few picoseconds. This leads to electron heating (cold ablation) being the dominant phenomenon responsible for the material modification using fs laser processing. This allows superior non-thermal and high-spatial-resolution processing of a material. However, it has been used to mostly study the response of standard materials such as gold (and similar metals), sapphire (and similar dielectrics) and PMMA (and similar polymers). There has been a lack of application in complex material systems such as layered minerals. Muscovite mica is a nanolayered, atomically flat, naturally occurring mineral with a complex composition and transparent dielectric properties. No study in the literature to date has attempted a systematic study of ultrafast laser pulse processing of muscovite. A preliminary study, prior to this thesis research, indicated muscovite was a material of interest for study.
In this dissertation, we document the first systematic study of single femtosecond laser pulse processing of muscovite mica. The response of muscovite is novel and unconventional compared to other standard transparent crystalline dielectrics. Laser processing at 800 nm wavelength and 150 fs pulse duration is found to lead to a systematic range of laser modification topologies as a function of fluence, including bulk removal in muscovite. However, at 515 nm wavelength and 190 fs laser pulse parameter, muscovite is observed to respond like a polymer. This thesis identifies the most likely reasons for this unexpected response. It underlines that, based on our observations and analysis, the explanation of the response of muscovite to femtosecond laser pulse irradiation requires the combination of a thorough knowledge of the femtosecond laser processing across a range of materials and the results of sophisticated characterization techniques. To establish the comprehensive understanding of the subject, the dissertation is spread over 7 chapters. Chapter 1 introduces the fundamentals of femtosecond laser pulse absorption in materials classified as metals, semiconductors, and dielectrics. It also includes the brief introduction of absorption mechanism in polymers and its response to ultrafast laser pulse processing. This chapter also highlights the contrast in the response of these materials to femtosecond laser pulse (s) irradiation. The fundamental understanding developed in this chapter is pivotal to appreciate the novel results presented in this thesis. Chapter 2 discusses the structure of muscovite mica with properties important in the dissertation's context. This chapter also includes some preliminary experimental results, to establish the nature of muscovite. These results do not feature in the major results chapters.
Chapter 3 presents the first important results on the response of muscovite mica upon femtosecond laser pulse processing at 800 nm. It comprises the manuscript titled, ‘Single-femtosecond-laser-pulse interaction with mica’. This published document highlights the different regimes of modification obtained at the muscovite surface upon single femtosecond laser pulse irradiation of several fluences. In this manuscript, we also propose optical surface profilometry (OSP) as a viable tool to characterize the laser-modified sites (esp. complex topologies) with sub-nanometer resolution. In chapter 4, we extend the OSP characterization to calculate and study the volume of small and complex laser pulse processed regions. We use knowledge of the calculated volume as an indicator to transition between several underlying physical processes in laser pulse matter interaction. We document the results in the published manuscript titled, ‘Micro-volumetric analysis of complex fs laser processed sites using optical surface profilometry (OSP)’.
Another significant result is that we identified the response of muscovite to be highly wavelength dependent. Hence, chapter 5 elaborates on the polymer like response of muscovite upon laser pulse processing at 515nm in the manuscript titled, ‘Polymer like response of muscovite upon 515 nm femtosecond laser pulse processing’. This chapter compares the response of muscovite to 800 nm (cited in chapter 3) and 515 nm wavelength single femtosecond laser pulse irradiation. Chapter 6 focuses on the restructuring and migration of elements in the femtosecond laser pulse processed surface volume of the muscovite. Time of flight-secondary ion mass spectroscopy (ToF-SIMS), is an underutilized but powerful technique for the characterization of the laser pulse processed surface volume. We use it to study, qualitatively, the migration of elements within the muscovite laser processed volume close to the surface. Chapter 4-6 report the major original results of the thesis.
Chapter 7 contains a collective discussion of the results presented in chapters 3-6. It links the fundamental chapters 1-2 to the documented results. It also discusses future potential theoretical and experimental studies to develop understanding of the physics behind the laser pulse processing of such a complex material.
