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We investigate the macroscopic physics of noncollinear high harmonic generation (HHG) at high pressures. We make the first experimental demonstration of phase matching of noncollinear high-order-difference-frequency generation at ionization fractions above the critical ionization level, which normally sets an upper limit on the achievable cutoff photon energies. Additionally, we show that noncollinear high-order-sum-frequency generation requires much higher pressures for phase matching than single-beam HHG does, which mitigates the short interaction region in this geometry. We also dramatically increase the experimentally realized cutoff energy of noncollinear circularly polarized HHG, reaching photon energies of 90 eV. Finally, we achieve complete angular separation of high harmonic orders without the use of a spectrometer.

We investigate the macroscopic physics of noncollinear high harmonic generation (HHG) at high pressures. We make the first experimental demonstration of phase matching of noncollinear high-order-difference-frequency generation at ionization fractions above the critical ionization level, which normally sets an upper limit on the achievable cutoff photon energies. Additionally, we show that noncollinear high-order-sum-frequency generation requires much higher pressures for phase matching than single-beam HHG does, which mitigates the short interaction region in this geometry. We also dramatically increase the experimentally realized cutoff energy of noncollinear circularly polarized HHG, reaching photon energies of 90 eV. Finally, we achieve complete angular separation of high harmonic orders without the use of a spectrometer.

We generate angularly isolated beams of circularly polarized extreme ultraviolet light through the first implementation of non-collinear high harmonic generation with circularly polarized driving lasers. This non-collinear technique offers numerous advantages over previous methods, including the generation of higher photon energies, the separation of the harmonics from the pump beam, the production of both left and right circularly polarized harmonics at the same wavelength and the capability of separating the harmonics without using a spectrometer. To confirm the circular polarization of the beams and to demonstrate the practicality of this new light source, we measure the magnetic circular dichroism of a 20 nm iron film. Furthermore, we explain the mechanisms of non-collinear high harmonic generation using analytical descriptions in both the photon and wave models. Advanced numerical simulations indicate that this non-collinear mixing enables the generation of isolated attosecond pulses with circular polarization., Article

In this project, we studied the linear and nonlinear propagation of structured beams of ultrafast pulses. We have been taking advantage of the special characteristics of spatially-chirped beams, in which the frequency components cross each other at the focus of a lens. In this configuration, the pulse focuses temporally and spatially at the same time. We developed a theory for understanding the beam propagation, verified aspects of this theory experimentally, developed a suite of diagnostics to test the alignment of these beams. We also demonstrated application of these beams to cutting biological material. In the project we investigated the spatio-temporal properties of cross-polarized wave generation and its application to pulse characterization. We developed a four-wave mixing apparatus in which the spatial chirp and pulse front is controlled. We extended our knowledge of beam propagation to Bessel-Gauss beams, and developed a technique to produce a uniform axial line focus by shaping only the spatial phase of the starting beam. Finally, we showed for the first time that Kerr-lens modelocking can be achieved in a Ti: sapphire oscillator pumped only by laser diodes., The original document contains color images.