The self-channeling of ultrashort laser pulses through 20 m of air was demonstrated. The channeled pulse was measured to have 0.75 mJ of energy, a diameter of 80 microm FWHM, and a modulated spectrum. All these values were measured to be fairly constant during the propagation of the pulse. A preliminary model is shown to explain these results.

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Results of laser-induced breakdown experiments in fused silica (SiO2) employing 150 fs–7 ns, 780 nm laser pulses are reported. The avalanche ionization mechanism is found to dominate over the entire pulse-width range. Fluence breakdown threshold does not follow the scaling of Fth∼ √τp, when pulses are shorter than 10 ps. The impact ionization coefficient of SiO2 is measured up to ∼3×108 V/cm. The relative role of photoionization in breakdown for ultrashort pulses is discussed.

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Application of picosecond and femtosecond laser pulses to the controlled ablation of materials represents a relatively unexplored yet important topic in laser processing. Such ultrashort pulses are of potential value in areas of thin-film deposition, micromachining, and surgical procedures. We report here some early results of systematic studies being done from the femtosecond to the nanosecond regime, as an assessment of the problems and benefits associated with various laser pulse durations and their use in processing optically absorbing media. Experimental data and theoretical results of computer simulations are presented and compared for the threshold energies of ablation in gold as a function of pulse width from 10 ns to 100 fs. This work is then extended to include further numerically computed results for gold and silicon on ablation rates, threshold surface temperatures, liquid thicknesses, and vaporization rates as a function of pulse duration throughout the ultrafast regime from tens of femtoseconds to a few hundred picoseconds.

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Purpose To improve triggering efficiency of the prospective respiratory amplitude triggered 4DMRI method and to develop a 4DMRI imaging protocol that can offer T2 weighting for better tumor visualization, good spatial coverage, spatial resolution and respiratory motion sampling within a reasonable amount of time for radiation therapy applications. Methods and Materials The respiratory state splitting (RSS) method and the multi-shot acquisition (MSA) method were analytically compared and validated in a simulation study using the respiratory signals from 10 healthy human subjects. The RSS method was identified to be more effective in improving triggering efficiency. It was implemented in the prospective respiratory amplitude triggered 4DMRI. 4DMRI image datasets were acquired from 5 healthy human subjects. Liver motion was estimated using the acquired 4DMRI image datasets. Results The simulation study showed that the RSS method was more effective in improving triggering efficiency compared to the MSA method. The average reductions in 4DMRI acquisition time were 36% and 10% for the RSS and MSA methods, respectively. The human subject study showed that T2-weighted 4DMRI with 10 respiratory states, 60 slices at a spatial resolution of 1.5×1.5×3.0 mm3 could be acquired in 9-18 minutes, depending on individual's breath pattern. Based on the acquired 4DMRI image datasets, the ranges of peak-to-peak liver displacements among 5 human subjects were 9.0-12.9 mm, 2.5-3.9 mm and 0.5-2.3 mm in superior-inferior, anterior-posterior and left-right directions, respectively. Conclusions We demonstrated that with the RSS method, it was feasible to acquire high quality T2-weighted 4DMRI within a reasonable amount of time for radiation therapy applications.