Dynamics of Femtosecond Laser Modification Inside Fused Silica

Femtosecond (fs) laser modification of transparent media provides great potential for the creation of a wide variety of novel optical devices. Progress in the creation of many devices has been limited by a lack of understanding of the mechanism behind the fs-laser modification process. To better understand the nature and behavior of fs-laser modification inside a transparent medium both the short and long time-scale dynamics of the modification process in fused silica were investigated in this thesis research.;To measure short time-scale dynamics of the fs-laser generated plasma inside fused silica a novel, broadband fs pump-probe experiment was built with the broadband probe ranging from 500--1100 nm. In this setup a strongly focused, 800 nm, fs pump pulse is used to create a plasma, which is subsequently interrogated by the weak broadband probe. The time delay between pump and probe can be varied over a 1 ns range with a time resolution of 440 fs.;Probe transmission measurements show that, as the energy of the modifying fs-laser pulse increases, the electron density remains capped at 4.0x10 21 cm-3, which is roughly the critical density associated with the pump wavelength of 800 nm, while the plasma size increases. The spectral shape of the probe transmission suggests that the plasmas generated in the experiments are "collisionless" with electron collision times > 500 fs. The experimental plasmas persist for longer than ∼1 ns after absorption of the fs-laser pulse. Plasma structure and emission characteristics depend on the pulse energy of the modifying fs-laser. At high pulse energies the structure within the plasma exhibits poor stability with very inhomogeneous emission characteristics. At lower pulse energies the fs-laser generated plasmas exhibit a stable "head and tail" structure with two distinct regions of emission: an intense, broad emission centered at 540 nm ("head" region) and a weaker, narrower emission centered at 475 nm ("tail" region).;Long time-scale dynamics were studied by observing changes to fs-laser modified structures under various thermal annealing conditions. Using confocal fluorescence and Raman microscopy, changes in fs-laser induced non-bridging oxygen hole center (NBOHC) defects were monitored through their fluorescence at 650 nm, while changes in the glass network, specifically the concentration of 3-membered Si-O rings, were measured by the relative intensity of the 605 cm-1 Raman peak. NBOHC defects and the Si-O ring distributions of fs-laser modified structures return to levels of unmodified fused silica through thermal annealing. In both cases the required annealing temperatures are far below the glass transition temperature: 300 °C for NBOHCs and 900 °C for 3-membered Si-O rings. The disappearance of these defect and glass network changes is not always accompanied by the removal of the visible modification created by the fs-laser absorption. Under fs-laser conditions that produce "smooth" refractive index changes, suitable for optical waveguide writing, the visible structures return to that of the bulk glass. Under fs-laser conditions that result in visibly "rough" modification that includes cracks and exhibits strong scattering, the visible structure remains constant for annealing temperatures up to 900 °C even when the NBOHC and Si-O ring concentrations return to unmodified values.;The design of the unique broadband pump-probe experiment allows for new, robust measurements of short time-scale plasma dynamics. Such short time-scale dynamics along with measurements of long time-scale dynamics and stability of the fs-laser induced modification improve the understanding of the fs-laser modification process.