Modern laser technology can now consistently produce conditions of incredibly high energy density, which is providing a number of exciting new regimes of physics to research. Studies of these include areas of research identified as some of five grand challenges by the recent DOE BESAC report “Directing Matter and Energy: Five Challenges for Science and the Imagination” . The report highlights the understanding of non-equilibrium systems, the development of fundamentally new methods for the complete quantum control of electron dynamics, understanding emergent phenomena, better design and tailoring of materials, and nanotechnology. High intensity laser-matter interactions are naturally almost always far from equilibrium, and hence research in this area will lead to better understanding of non-equilibrium systems. The technologies that develop from high-power lasers, including particle acceleration and radiation generation will ultimately result in new methods for control of electron dynamics, and will result in advanced diagnostics for materials, such as temporal resolution of atomic processes.
Chirped pulse amplification (CPA), first conceived in the mid-1980s [Strickland OC 1985], now routinely allows incredibly short laser pulses to be amplified to petawatt powers. This method stretches the laser pulse in time, using diffraction gratings to ‘chirp’ the pulse, so that amplification to high energy does not damage the gain medium. The pulse is then recompressed, which further increases the power dramatically. Over the past ten years this advancement in optical engineering has driven the development of ultra-high intensity, ultra-fast laser systems at national laboratories and universities around the world. Terawatt facilities are now common in universities, petawatt class lasers are now found at a number of government laboratories, and 10-100 PW lasers are being proposed for the near future [ELI]. One particularly exciting prospect, enabled by this technology, is the production of compact sources of high quality energetic particle and radiation beams having ultra-short durations. Recent advances are projected to have a major impact on many diverse and inter-related technologies, some with direct relevance to areas of important national need, including homeland security, renewable energy, and advanced medical treatment and diagnosis.
For presently operating laser systems, the focused intensity can reach over 1022 Wcm-2 [LaserFocus World], allowing ultra-relativistic plasma physics to be accomplished.The parameter that parameterizes the physics of high-field matter interactions is the normalized vector potential, a0 = eE/mecω0,where E is the laser electric field strength, me is the electron mass and ω0 is the laser frequency. The magnitude of the momentum of an electron due to the accelerating field, normalized to mec, is approximately given by a0. This means that interactions can be classed as non-relativistic for a0 <1, for a0 = 1 the kinetic energy of an electron is equal to its rest mass, and for a0> 1, the interaction can be considered to be relativistic. For laser intensities exceeding 1022 Wcm-2, and with 1 µm wavelength, which is typical of current technologies, the vector potential approaches a0 = 100, and the interaction can therefore be considered ultra-relativistic.
Under these conditions, the physics of matter interactions is not well understood, as exotic effects such as radiation damping [Dirac Proc. Roy. Soc. 1938, Spohn EPL 2000, Medina JPA 2006, Rohrlich PRD 1999] start to become important, and the propagation of radiation is strongly affected by the change in the optical properties induced by the strong field. The most significant change, compared with lower intensity interactions, is that of the complete expulsion of the electrons, and eventually ions, from the focal region[Sun POF 1987, Pukhov PPCF 2004, Lu POP 2006] due to the ponderomotive force of the laser [P Brown PR 1964, Quesnel PRE 1998]. The electromagnetic fields found in plasmas, due to these density perturbations, can be extremely high and are promising for use as a next generation of particle accelerators [Mangles PT Roy. Soc. 2006]. Tunable, short duration radiation sources based on free electron lasers or harmonic generation will provide useful diagnostics for many areas of scientific research. In addition, the intensities of the next generation of lasers [ELI] would reach a regime where the energy densities are so high that production of exotic matter could test quantum electrodynamics(QED) theory [Piazza Rev. Mod. Phys. 2012]. Notably, at I~ 1029 Wcm-2, the electric field is sufficient to separate electron-positron pairs from the zero-point energy vacuum fluctuations [Schwinger PR 1951].