Illuminating matter with petawatt (one quadrillion watts) laser light creates extreme states of plasmas with temperatures exceeding ten million degrees Celsius and pressures exceeding one billion earth atmospheres. These high energy density conditions are driven at the microscopic scale by dense currents of relativistic electrons, oscillating violently in the intense laser fields, as well as the plasma processes arising when these particles lose phase coherence and are injected into bulk target material. Suitably harnessed, this setup opens the way to compact relativistic particle accelerators, laser fusion energy sources, laboratory astrophysics, ultrafast imaging systems, proton cancer therapies, anti-matter creation, high-energy radiation sources and intense high harmonic generation. In this thesis, theoretical models of the ab- sorption of high power laser light by matter are derived, applications of these models are investigated, and simulation tools supporting the diagnosis and implementation of these applications are developed.

In particular, an advanced, relativistically-correct theoretical model of petawatt laser absorption by optically-thick targets is developed, accounting for both ion and electron beam aspects of the interaction. Predictions of the model for the energetic properties of these beams, as well as the dynamical motion of the laser-matter inter- face, are elucidated. Results from high resolution, relativistic, kinetic particle-in-cell simulations using the LSP code are shown to be in good agreement with the model. The theoretical maximum and minimum absorption values in laser-solid interactions are derived from the model in a general fashion, constraining nonlinear absorption processes across the petawatt regime, spanning 10^18 < I_l λ^2_l < 10^23 W μm^2 cm^−2 for intensity I_l and wavelength λ_l. These results are shown to bound several dozens of published experimental and simulation data points, underlining the usefulness of the model. These results are extended to include effects related to heterogeneous plasmas, including relativistically-underdense plasmas relevant to ‘pre-plasma’ situations, and realistic laser spatio-temporal profiles. Our dynamic considerations of absorption processes are then extended to the 10-petawatt scale. In a manner that could support reaching the QED-plasma regime, a mechanism of focusing high power laser light to higher intensities is elucidated. Supporting the measurement and validation of these models, the development of a new simulation tool for understanding high energy density plasmas, based on the proton radiography technique, is detailed.