Generation of relativistic few-cycle radially polarized mid-infrared pulse in plasma channel

Polarized mid-infrared (mid-IR) pulses have many practical applications, including optical imaging, harmonics generation, and laser-electron acceleration, etc. Although it has been able to generate such pulses based on traditional nonlinear optical methods, extending it to long-wavelength, few-cycle, and relativistic intensity range is still very challenging. Here, we proposed a scheme to generate a relativistic long-wavelength few-cycle and radially polarized mid-IR pulse via the photon deceleration. Three-dimensional particle-in-cell simulations show that, by using a plasma channel, the radially polarized mid-IR pulse can be obtained with a center wavelength of 9.4 μm, a dimensionless intensity of aMIR≈2.5, a 72 fs FWHM duration containing 2.3 optical cycles, and a conversion efficiency of 3.4%. Meanwhile, the beam parameters of the generated radially polarized mid-IR pulse can be effectively controlled by changing the plasma channel depth. The radially polarized pulse can be focused further to excite a stronger plasma wake in a plasma channel with large depth, leading to an increase in the normalized intensity and conversion efficiency of the mid-IR pulse. Such mid-IR pulse sources may offer new opportunities for ultrafast applications and high-field physics, etc.