Tunable Relativistic Infrared Pulses from Laser-produced Wakes in Tailored Plasma Structures

The development of intense few-cycle mid-infrared (mid-IR, $\lambda $\textless 5 $\mu $m) laser sources has made significant progress during the past decade, which has opened many opportunities for infrared nonlinear optics, high-harmonic generation and pump-probe experiments in the ``molecular fingerprint'' region. However, even longer carrier wavelength (\textasciitilde 10 $\mu $m) are needed in many applications. It is one of the current challenges facing ultrafast laser technology to generate high-energy, ultra-short long-wave IR (LWIR) pulses, beyond the capability of existing methods. Recently, a new scheme that utilizes asymmetric self-phase modulation (SPM) in a tailored plasma density structure to generate multi-millijoule energy, single-cycle LWIR pulses tunable in a wide spectral range was proposed. Here, we experimentally demonstrate this novel scheme for the first time. An intense single-cycle IR pulse with a central wavelength of 9.4 $\mu $m and energy of 3.4 mJ is generated using a \textasciitilde 580 mJ, 36 fs, 810 nm drive laser. Furthermore, the tunability of the IR wavelength in the range of 3-20 $\mu $m is also demonstrated through simple adjustment of the plasma structure.