Ultrafast Sources of Intense Radiation

Abstract

Exploration at the frontiers of modern physics depends on electromagnetic radiation with almost unimaginable properties. Attosecond pulses freeze the motion of electrons. Petawatt beams accelerate particles to relativistic velocities in femtoseconds. Brilliant x-rays capture the interior structure of proteins. Lasers and laser-like sources of coherent radiation with extreme intensity, wavelength, and pulse duration promise further groundbreaking advances in both fundamental and applied science, yet surpassing current capabilities requires new methods for generating and manipulating high-intensity light.

This dissertation presents a series of experimental, computational, and theoretical advances towards the development of plasma-based sources of extreme radiation with a focus on relativistic high-order harmonic generation (HHG) from plasma mirrors for high-energy extreme ultraviolet and x-ray generation and plasma-mediated parametric amplification for high-power lasers.

In particular, this work offers the following contributions to laser-plasma interaction physics. A detailed experimental characterization of ultrafast plasma mirror performance over a broad range of parameters provides spectral and spatial measurements of second, third, and fourth harmonic generation for varied intensity and contrast, demonstrates relativistic harmonic generation, and relates high-order harmonic generation to plasma-mirror mechanical stability. Key features of the relativistic HHG spectrum are explained by a model for the synchrotron-like motion of plasma electrons, which includes the dynamics of the electron bunch formation and quantifies the efficiency limits and scaling of the process. Harmonic generation dramatically improves for two-color and multi-color driving beams, with a strong dependence on the exact waveform shape; the mechanism for this enhancement arises from the sub-cycle interplay between laser and plasma fields. This also leads to predicted performance improvements for cascaded plasma mirror systems. The Brillouin mechanism outperforms Raman scattering for x-ray amplification and plasma amplification in the pump depletion regime tolerates substantial incoherence. These two observations offer a route towards x-ray free electron lasers as pumps for high-power x-ray amplifiers. A powerful scattering mechanism in electron-positron plasmas is identified, along with a new mechanism for stimulated scattering in strongly-magnetized plasmas, unique in its combination of high instability growth rate and small frequency downshift. Taken together, these advances suggest a plasma-centered path towards the next generation of extreme light sources.