Online process diagnostics of ultrafast laser modifications and energy transport mechanisms in the volume of dielectrics and semiconductors

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We propose to develop a unique online process characterization and optimization facility for the observation and analysis of microscopic, ultrafast laser-induced phenomena deep inside transparent and opaque dielectrics and semiconductors. The proposed non-destructive approach will enable the study of heat accumulation, thermal and non-thermal melting, and resulting mass transport and morphology changes of the processed materials. This characterization facility will combine the unique ability to quantitatively detect transient refractive index changes and induced temperature distributions with sub-picosecond temporal resolution. The knowledge obtained experimentally of the ultrafast process dynamics will provide important data for the fundamental description of the laser-matter interaction and the subsequent energy transfer processes e.g. melting and resolidification dynamics. The proposed experimental approach is based on optical pump-probe broadband interference microscopy of the processed material utilizing ultra-short pulsed white-light continuum emission and multiline mid-IR radiation as illumination sources. This research will address fundamental aspects of ultrafast laser-matter interaction processes and the subsequent energy transfer within the material, as well as providing solutions to many other technological problems with a very broad impact. The intellectual merit follows from the process understanding during in-volume irradiation of dielectrics and semiconductors with ultrafast laser radiation. The ability to precisely control the energy deposition into the material will enable the study of metastable aggregation states and different regimes of the near threshold laser-materials processing, the study of thermally induced stresses and defects, as well as heat accumulation effects. Compared to previous work in the field, for the first time the optical phase change and associated thermo-physical parameters will be measured in real time, based on experimental data obtained in a spectral detection range from visible to mid-IR. These crucial advancements will allow us to address technological aspects of existing and novel laser-assisted fabrication techniques e.g. for waveguide writing in optical glasses, fusion welding of similar and dissimilar materials, and in-volume selective etching of dielectrics and semiconductors. The engineering challenges of the online process control will be addressed as well. In pursuit of these goals, real-time computational analysis and feedback-based process optimization techniques will be utilized. The broader impact of the proposed online process diagnostics and control technique will be significant since fabrication of integrated photonic, electronic and microfluidic devices with existing approaches relies heavily on physical and chemical processes having low reproducibilty. As a consequence the gain in process understanding and control for in-volume laser processing of dielectrics and semiconductors will revolutionize wide sectors of today’s micro-manufacturing technology in photonics, electronics, sensing and life sciences. In particular, the development of medical lab-on-chip applications e.g. implantable diagnostic and therapeutic micro-devices will significantly impact our society and healthcare system. The educational program associated with this proposal will involve student participation, including students from underrepresented groups, in various aspects of optics, laser engineering and materials science, and incorporation of the research results into graduate and upper-undergraduate courses. Pertinent visual-learning and web-based tools will be developed to integrate the research and education activities.