Generation of single-cycle relativistic infrared pulses at wavelengths above 20 um from density-tailored plasmas

Ultra-intense short-pulse light sources are powerful tools for a wide range of applications.However,relativistic short-pulse lasers are normally generated in the near-infrared regime.Here,we present a promising and efficient way to generate tunable relativistic ultrashort pulses with wavelengths above 20 um in a density-tailored plasma.In this approach,in the first stage,an intense drive laser first excites a nonlinear wake in an underdense plasma,and its photon frequency is then downshifted via phase modulation as it propagates in the plasma wake.Subsequently,in the second stage,the drive pulse enters a lower-density plasma region so that the wake has a larger plasma cavity in which longer-wavelength infrared pulses can be produced.Numerical simulations show that the resulting near-single-cycle pulses cover a broad spectral range of 10-40 um with a conversion efficiency of～2.1％(～34 mJ pulse energy).This enables the investigation of nonlinear infrared optics in the relativistic regime and offers new possibilities for the investigation of ultrafast phenomena and physics in strong fields.