Photon acceleration is the phenomenon whereby a light wave changes color when propagating through a medium whose index of refraction changes in time. This concept can be used to describe the spectral changes experienced by electromagnetic waves when they propagate in spatially and temporally varying plasmas. In this paper the detection of a large-amplitude laser-driven wake field is reported for the first time, demonstrating photon acceleration. Several features characteristic of photon acceleration in wake fields, such as splitting of the main spectral peak and asymmetries between the blueshift and redshift for large shifts, have been observed. The experiment is modeled using both a novel photon-kinetic code and a three-dimensional particle-in-cell code. In addition to the wide-ranging applications in the field of compact particle accelerators, the concept of wave kinetics can be applied to understanding phenomena in nonlinear optics, space physics, and fusion energy research.

Iron oxide Fe₂O₃ nano-particle powder is investigated by laser-induced breakdown spectroscopy (LIBS). Laser ablation of loose Fe₂O₃ powder (density 0.43 g cm⁻³) leads to the ejection of particles and generates a plasma channel along the laser beam path. LIBS spectra are obtained by spatially integrated and time-gated measurement in flushed N₂ gas background. A simple powder delivery system allows for investigation of the nano-particle powder directly without any pre-treatment and fixation of the loose material. The Saha–Boltzmann analysis of spectra yields plasma temperature T = (10250 ± 200) K and electron density N_e = (11 ± 3) × 10¹⁶ cm⁻³ for the loose Fe₂O₃ powder and for pressed Fe₂O₃ powder pellets that are investigated for comparison. LIBS analysis of more than 20 different powder samples establishes calibration curves for Al, Si, Ni and Mn element impurities. The limits of detection derived from such curves are 8, 15, 66 and 860 ppm for Al, Si, Ni and Mn, respectively.