Ultrafast lasers enable non-contact, waste free, precise material removal. We have demonstrated single-digit nanometer precision polishing of optical material using a femtosecond laser. For ultrafast-laser-based waveguide writing, we study the underlying physics behind nonlinear optical dynamics during the femtosecond laser processing of crystalline materials. Unidirectional pulse propagation equation simulation is carried out to study the evolution of energy, fluence, plasma generation, and beam waist of a femtosecond pulse along the propagation direction under different energy and focusing conditions. Waveguides having a loss of 0.21 dB/cm are obtained. A Nd:YAG based waveguide laser with lasing threshold of 50 mw was demonstrated.
Femtosecond laser ablation has a wide variety of applications, from re-shaping the cornea of the eye to micro-machining electronic devices. It is imperative to understand the dynamics of ablation from energy absorption to surface vaporization. In this work, time- and space-resolved microscopy is used to analyze the ablation dynamics induced by femtosecond laser pulses in single-crystalline silicon. These dynamics are revealed by capturing the surface images generated with probe pulse reflection at a variable delay time relative to the pump pulse. When the peak fluence of the incident laser pulse is near the ablation threshold, the transient surface reflectivity initially changes from low to high due to electron-hole plasma formation and then exhibits dynamic Newton-ring patterns with increasing numbers of fringes. When the peak fluence exceeds two times the ablation threshold, surface reflectivity first increases, then significantly decreases (to a level lower than the initial value), and increases again with a growing Newton-ring pattern resulting from thermally induced material expansion. Finally, a crater is formed with two side bumps within which the absorption depth is reduced with spatial fluence level over two times the ablation threshold. The time-resolved silicon images and analyses describing the evolution of transient reflectivity and morphological will be presented.
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