Recently, plasma optics have gained significant attention due to their high damage threshold, making them ideal for ultrafast, ultraintense lasers. Novel concepts for plasma-based photonic devices have been proposed, including plasma mirrors, plasma lenses, plasma gratings, plasma wave plates, and plasma polarizers. For example, plasma-based polarizers have been demonstrated at Lawrence Livermore National Laboratory using low-density gas targets. However, polarization optics based on solid, dense plasmas have not been widely explored. In this paper, we propose a new design for plasma polarization optics based on overdense, nanometer-thin foils. We investigate this concept using particle-in-cell (PIC) simulations. By carefully adjusting plasma parameters, the nanometer-thin foil behaves like a linear polarizer or a quarter-wave plate. This behavior is driven by the inhomogeneity of the plasma density distribution resulting from the interaction, as confirmed by three-dimensional PIC simulations.

We investigate plasma polarization optics using the epoch code, conducting both two-dimensional (2D) and three-dimensional (3D) simulations. A nanometer-thin foil composed of protons and electrons is placed in the simulated region, with dimensions of 50λ×20λ for the 2D simulation and 20λ×10λ×10λ for the 3D simulation. These regions are divided into 50000×500 and 4000×500×500 grids, respectively. A circularly polarized laser with 800 nm wavelength and a 2.5 μm spot size is focused onto the target and propagates along the x-axis, with peak intensities of 1×10²⁰, 5×10²⁰, 9×10²⁰ W/cm². The electron density of the targets is set to 50nc, 100nc, and 150nc, with target thickness varying from 0.01λ to 0.40λ. The polarization of the transmitted laser is calculated by integrating the laser energy along the E_y and E_z components, and the phase is estimated from the mean phase difference between the local maxima of the electric field. The 2D electron density distribution from the 3D simulations is extracted by lineout along the center of the y- and z-axes and the 1D electron density distribution along the x-axis is extracted from the center of the 2D electron density distribution.

2D simulations show that when a circularly polarized laser passes through a 0.1λ thick, 150nc target, the electric field along the z-axis is significantly suppressed, resulting in an extinction ratio of about 0.84. The transmitted laser then becomes nearly linearly polarized along the y-axis. Further parameter scans with varying thickness and electron density show that the phase difference and polarization undergo dramatic changes in the relativistically induced transparency (RIT) region where laser transmission drops to about 1%. At RIT, the phase difference between E_y and E_z reaches its maximum, and the polarization increases sharply from a relatively small value before saturating when the target thickness or electron density reaches a specific value. In addition, the incident angle plays a crucial role in determining the phase difference. By carefully adjusting the target’s thickness, electron density, and twist angle, we can achieve significant phase delay between the electric fields along the y- and z-axes. A phase delay of π2 is achieved when the circularly polarized laser passes through plasma with a thickness of 0.045λ and an electron density of 150nc. The polarization transitions from circular to linear, with an angle of 45° to the y-axis.

In this paper, we propose a new design for plasma polarization optics based on overdense nanometer-thin foils, and the polarization behavior of these foils is explored using both 2D and 3D PIC simulations. A linear polarizer or quarter-wave plate is demonstrated under specific parameters. We believe these results will be of great interest to the community and could influence the state-of-the-art in the field of high-field laser science.