A novel method of filtering out atoms and small particulates, emitted from a laser plasma EUV radiation source, has been developed and experimentally characterized. The method consists of elimination of debris species by an optically transparent assembly of foils positioned in a buffer gas environment near the source. A high trapping efficiency is achieved due to retardation and scattering of particles in the gas and subsequent deposition on the foils. The method imposes no limitations of the radiation acceptance angle. The foil trap technique, a debris suppression method universally applicable for different EUVL radiation sources, has been investigated in combination with a fast rotating laser plasma target. A target unit with a disk edge velocity of up to 500 rn/s enabling nearly full elimination of large particulates, served as a source of different debris components for experiments on foil trapping atoms and sub-micron particulates. An integrated suppression coefficient of 500 has been measured for debris with sizes of up to a micrometer using a pilot trap cooled down to -90 °C. Extrapolation of this data to conditions when debris of sub-micron size only is produced, resulted in a suppression coefficient of 2000.
We demonstrate a highly elongated (aspect ratio over 500:1) optical breakdown in water produced by a single pulse of a picosecond laser focused with a combination of an axicon and a lens. Locations of the proximal and distal ends of the breakdown region can be adjusted by modifying radial intensity distribution of the incident beam with an amplitude mask. Using Fresnel diffraction theory we derive a transmission profile of the amplitude mask to create a uniform axial intensity distribution in the breakdown zone. Experimentally observed dynamics of the bubbles obtained with the designed mask is in agreement with the theoretical model. A system producing an adjustable cylindrical breakdown can be applied to fast linear or planar dissection of transparent materials. It might be useful for ophthalmic surgical applications including cataract surgery and crystalline lens softening.
Transparent biological tissues can be precisely dissected with ultrafast lasers using optical breakdown in the tight focal zone. Typically, tissues are cut by sequential application of pulses, each of which produces a single cavitation bubble. We investigate the hydrodynamic interactions between simultaneous cavitation bubbles originating from multiple laser foci. Simultaneous expansion and collapse of cavitation bubbles can enhance the cutting efficiency, by increasing the resulting deformations in tissue, and the associated rupture zone. An analytical model of the flow induced by the bubbles is presented and experimentally verified. The threshold strain of the material rupture is measured in a model tissue. Using the computational model and the experimental value of the threshold strain one can compute the shape of the rupture zone in tissue resulting from application of multiple bubbles. With the threshold strain of 0.7 two simultaneous bubbles produce a continuous cut when applied at the distance 1.35 times greater than that required in sequential approach. Simultaneous focusing of the laser in multiple spots along the line of intended cut can extend this ratio to 1.7. Counterpropagating jets forming during collapse of two bubbles in materials with low viscosity can further extend the cutting zone-up to approximately a factor of 1.5.
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