Magnetized plasma rotator for relativistic mid-infrared pulses via frequency-variable Faraday rotation

Optical rotators based on the Faraday effect have been widely used in optical systems, such as optical isolation and circulators. However, due to the limitation of crystals, the application of such optical rotators in high-power lasers has been severely hindered. Here, we propose a novel plasma rotator based on the frequency-variable Faraday rotation (FVFR) in a compact manner, achieved by driving the magnetized underdense plasma with a relativistic linearly polarized laser. In the magnetized plasma, the drive laser undergoes photon deceleration and relativistic Faraday rotation, leading to the generation of relativistic polarization-tunable mid-infrared (mid-IR) pulse with intensity $\ge {10}^{16}$ W cm^-2 and a spectral width of 5-25 μm. With different magnetic fields, the polarization angle of the generated mid-IR pulse can be well controlled. Especially, one can obtain a circularly polarized mid-IR pulse with the spatial average polarization degree of $\ge 0.94$ at a suitable external magnetic field. The robustness of the rotator has been well demonstrated through comprehensive three-dimensional particle-in-cell simulations across a wide range of laser and plasma parameters. Such a rotator via FVFR is valid from mid to far-infrared and even THz waveband, offering new opportunities for strong-field physics, attosecond science, laboratory astrophysics, etc, and paving the way for relativistic plasma magneto-optics and future relativistic plasma optical devices.