Tunable ultrafast coherent light in the soft and hard X-ray regions of the spectrum: Phase matching of extreme high-order harmonic generation.

摘要

Using the extreme nonlinear optical process of high harmonic generation, light from an ultrafast infrared laser can be coherently upconverted to generate fully coherent and laser-like beams in the extreme ultraviolet region of the spectrum, with femtosecond-to-attosecond pulse duration. However, progress in extending extreme nonlinear optical techniques into the soft and hard X-ray regions of the spectrum has been hindered to-date, not by the inability of the high harmonic process to generate X-rays, but by the lack of techniques for phase matching this highly nonlinear process. This thesis reports on significant experimental and theoretical advances in two alternative approaches to obtain bright high harmonic emission at keV photon energies. First, using high harmonic generation from ions in a plasma waveguide, it was demonstrated that one can extend significantly the energy range of the generated photons by increasing the laser intensity. At high driving fields, the plasma waveguide prevents laser defocusing, reduces ionization loses, allows to design the ion species independently from the laser dynamics, and provides more uniform propagation conditions amenable to apply quasi phase matching techniques. All-optical quasi phase matching techniques---high-order difference-frequency mixing and grating-assisted phase matching---are proposed in this scheme. Second, in a contrasting regime, using high harmonic generation from weakly-ionized neutral atoms in a hollow waveguide and by increasing the driving laser wavelength, it is shown for the first time that there is no maximum photon energy limit for perfect phase matching of the process. In the past decade prior to our demonstration, it was widely accepted that conventional phase matching of high harmonic generation is limited to about 100 eV. In analogy with the single-atom cutoff energy in a microscopic picture, we introduce the concept of a phase matching cutoff energy in a macroscopic picture. A simple phase-matching cutoff rule is obtained, describing the maximum photon energy that can be fully phase-matched as a function of the laser wavelength. We verify our scaling predictions experimentally by demonstrating phase matching in the soft X-ray region of the spectrum (inside the water window of the spectrum), using ultrafast mid-IR driving laser pulses and extended, high pressure, weakly ionized gas medium. We also show through calculations that scaling of the overall conversion efficiency is surprisingly favorable as the wavelength of the driving laser is increased, making tabletop, fully coherent, multi-keV X-ray sources feasible. The rapidly decreasing microscopic single-atom yield, predicted for harmonics driven by longer-wavelength lasers, is compensated macroscopically by an increased optimal pressure for phase matching and a rapidly decreasing reabsorption of the generated X-rays. In addition, this simple and experimentally attractive regime represents prospects to macroscopically isolate femtosecond-to-attosecond-to-zeptosecond x-ray pulses through phase matching gating. The perfect phase matching and quasi phase matching techniques presented here can provide either broadband or quasi-monochromatic, tunable, ultrafast light in the soft and hard X-ray regions of the spectrum that can be used for applications in microscopy, nanoscience and nanotechnology, X-ray nonlinear optics, as well as wide range of fundamental studies of materials and molecular dynamics.