Browsing Dept. of Electrical and Electronics Engineering - Ph.D. / Sc.D. by Subject "Ablation cooling"
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Item Open AccessAblation cooled material removal with bursts of ultrafast pulses(Bilkent University, 2016-01) Kerse, M. CanMaterial processing with femto-second pulses allows precise and non-thermal material removal and being widely used in scientific, medical and industrial applications. However, due to low ablation speed at which material can be removed and the complexity of the associated laser technology, where the complexity arises from the need to overcome the high laser induced optical breakdown threshold for e cient ablation, its potential is limited. Physics of the interaction regime hinders a straightforward scaling up of the removal rate by using more powerful lasers due to e ects such as plasma shielding, saturation or collateral damage due to heat accumulation. In analogy to a technique routinely used for atmospheric re-entry of space shuttles since 1950s, ablation cooling, is exploited here to circumvent this limitation, where rapid successions of pulses repeated at ultrahigh repetition rates were applied from custom developed lasers to ablate the target material before the residual heat deposited by previous pulses di use away from the interaction region. This constitutes a new, physically unrecognized and even unexplored regime of laser- material interactions, where heat removal due to ablation is comparable to heat conduction. Proof-of-principle experiments were conducted on a broad range of targets including copper, silicon, thermoelectric couplers, PZT ceramic, agar gel, soft tissue and hard tissue, where they demonstrate reduction of required pulse energies by three orders of magnitude, while simultaneously increasing the ablation e ciency by an order of magnitude and thermal- damage-free removal of brain tissue at 2 mm3/min and tooth at 3 mm3/min, an order-of-magnitude faster than previous results. Item Open AccessTailoring nonlinear temperature profile in laser-material processing(Bilkent University, 2019-03) Kesim, Denizhan KorayAblation cooled material removal opened up great opportunities for understanding nonlinear processes. Especially using lasers as a tool to tailor nonlinear temperature gradient of material and engineering them to achieve effects such as high ablation efficiency, speed, and low collateral damage. Numerical simulations showed such engineering of the temperature gradient of material is possible for any repetition rate falling inside the ablation cooling regime. Two temperature model is used to investigate the effects of repetition rate, pulse energy, and burst duration. Simulations suggest ablation can continue indefinitely as burst duration increases. They also suggest there is an optimum pulse energy for any repetition rate in terms of efficiency of ablation regarding the material. The results of the simulations are confirmed by experiments using lasers with 1.6 GHz, 1.46 GHz, and 13 GHz repetition rate on biological and technical materials. The ablation threshold for a single pulse is lowered 100 times compared to our previous publication. Finally, related studies that can build upon the shown results are presented. A new thin-disk laser oscillator scheme is proposed that implements mode-locking regimes already established in fiber lasers. Dissipative soliton and similariton simulation results are promising for further studies. They can achieve high energy pulses with the help of nonlinear effects instead of limited by it. Then, a new computer generated hologram algorithm is explained where hundreds of layers can be generated from a single hologram. The algorithm utilizes diffusion as a tool to increase the degree of freedom which in turn decreases the cross-talk between layers.