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 μm 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.

在光谱的红外区域中可以使用强的，超短的相干辐射源，这可以通过高谐波生成，“分子指纹”区域中的泵浦探针实验以及开放的相对论区域来生成亚秒级X射线脉冲。等离子体的红外非线性光学。这些应用将受益于中长波长红外区域中的多毫焦耳单周期脉冲。在这里，我们提出了一种新方案，该方案能够基于使用定制的等离子体密度结构的光子频率下移方案，产生5到14μm的可调谐的相对论性强，单周期红外脉冲，转换效率为1.7％。长波长红外脉冲的载波包络相位与驱动激光器的载波包络相位锁定在百分之几以内。