Relativistic single-cycle tunable infrared pulses generated from a tailored plasma density structure

The availability of intense, ultrashort coherent radiation sources in the infrared region of the spectrum is enabling the generation of attosecond X-ray pulses via high-harmonic generation, pump-probe experiments in the 'molecular fingerprint' region and opening up the area of relativistic infrared nonlinear optics of plasmas. These applications would benefit from multi-millijoule single-cycle pulses in the mid-to long-wavelength infrared region. Here, we present a new scheme capable of producing tunable relativistically intense, single-cycle infrared pulses from 5 to 14 mu m with a 1.7% conversion efficiency based on a photon frequency downshifting scheme that uses a tailored plasma density structure. The carrier-envelope phase of the long-wavelength infrared pulse is locked to that of the drive laser to within a few per cent. Such a versatile tunable infrared source may meet the demands of many cutting-edge applications in strong-field physics and greatly promote their development.