High-order mode structure and spectral interferometry in ultrathin foils: underpinning physics of transparency-enhanced ion acceleration

The spatio-temporal and polarisation properties of intense light is important in wide-ranging topics at the forefront of intense light-matter interactions, including laser-driven particle acceleration. In the context of experiments to optimize transparency-enhanced ion acceleration in expanding ultrathin foils, we investigate the polarisation and temporal properties of intense light measured at the rear of the target. An effective change in the angle of linear polarisation of the light results from a superposition of coherent radiation, generated by a directly accelerated bipolar electron distribution, and the light transmitted due to the onset of relativistic self-induced transparency. Simulations show that the generated light has a high-order transverse electromagnetic mode structure in both the first and second laser harmonics that can evolve on intra-pulse time-scales. The mode structure and polarisation state vary with the interaction parameters, opening up the possibility of developing this approach to achieve dynamic control of structured light fields at ultrahigh intensities [1]. We also report on frequency-resolved optical gating measurements of the light which demonstrate a novel and simple approach to diagnose the time during the interaction at which the foil becomes transparent to the laser light. This is a key parameter for optimising ion acceleration in expanding ultrathin foils. Coherent transition radiation produced at the foil rear interferes with laser light transmitted through the foil producing spectral fringes. The fringe spacing enables the relative timing of the onset of transmission with respect to the transition radiation generation to be determined. This self-referencing approach to spectral interferometry provides a route to optically controlling and optimising ion acceleration from ultrathin foils undergoing transparency [2]. [1] M.J. Duff et al., Scientific Reports 10, 105 (2020) [2] S.D.R. Williamson et al., Phys. Rev. Applied 14, 034018 (2020)