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Application of space-time structured light to controlled high-intensity laser matter interactions in point and line target geometries
Meier, Amanda K.
Meier, Amanda K.
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2015
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2016-09-22
Abstract
With ultrashort laser pulses, nonlinear effects can be observed with low energy in each pulse. The broad bandwidth that makes it possible to produce a short pulse also introduces new degrees of freedom for manipulating the beam. The different frequency components that make up the bandwidth can be thought of as individual Gaussian beamlets that travel at different directions through optical elements due to dispersion or spatial chirp. These distortions are usually minimized in laser alignments yet manipulation of the of the Gaussian beamlets can be useful in axial localization and pulse front tilt (PFT), which is called simultaneous spatial and temporal focusing (SSTF) and is useful in many applications. In order to use SSTF, we need to characterize the pulse in both the spatial and spectral domains. We have developed a novel Sagnac shearing interferometer which combines spatial and spectral interference. The combination of divergence and spatial shear results in a local angle between the beams which can be extracted from the interference pattern using spatially resolved spectral interferometry by Fourier analysis. A spatial inversion allows our design to be extended to characterize a coupled spatio-temporal distortion, spatial chirp. We have developed techniques to control relative PFT for focused beams. We use a single pass grating compressor as a passively stable pump-probe experiment stemming from diffractive optics, where the +/-1 diffracted orders from a transmission grating pair are focused by an off-axis parabola which crosses the pump beams to form an index grating at the focus that is probed by the zero order. The experiment can be aligned for overlap of the pulse front tilt across the entire focal spot. This PF overlap can be applied in nonlinear mixing processes, such as harmonic generation or four wave mixing, to characterize semiconductor samples or ionization dynamics. We have also extended SSTF to a cylindrical geometry in Bessel-Gauss and vortex beams. Our novel setup for producing radial SSTF double passes a Gaussian beam through an axicon to produce a collimated ring beam that is then focused to a Bessel zone. Including a vortex mask in the beam gives a phase singularity on axis which creates a higher order Bessel-Gauss, corresponding to the vortex mode order. Circular gratings were also designed to extend SSTF to a cylindrical geometry as well as utilize the pulse front matching technique mentioned above. The Bessel zone with vortex singularity allows for high intensity walls that ionize causing the index of refraction to be higher in the core than the cladding, therefore allowing beam guiding. We modeled the waveguide geometry to optimize mode coupling. Radial SSTF could be used to guide high intensity beams with application to guide high harmonics.
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