Efficient generation of relativistic near-single-cycle mid-infrared pulses in plasmas

Ultrashort intense optical pulses in the mid-infrared (mid-IR) region are very important for broad applications ranging from super-resolution spectroscopy to attosecond X-ray pulse generation and particle acceleration. However, currently, it is still difficult to produce few-cycle mid-IR pulses of relativistic intensities using standard optical techniques. Here, we propose and numerically demonstrate a novel scheme to produce these mid-IR pulses based on laser-driven plasma optical modulation. In this scheme, a plasma wake is first excited by an intense drive laser pulse in an underdense plasma, and a signal laser pulse initially at the same wavelength (1 micron) as that of the drive laser is subsequently injected into the plasma wake. The signal pulse is converted to a relativistic multi-millijoule near-single-cycle mid-IR pulse with a central wavelength of ~5 microns via frequency-downshifting, where the energy conversion efficiency is as high as approximately 30% when the drive and signal laser pulses are both at a few tens of millijoules at the beginning. Our scheme can be realized with terawatt-class kHz laser systems, which may bring new opportunities in high-field physics and ultrafast science. A method to produce ultra-short and intense pulses of mid-infrared laser light could open new opportunities in applications ranging from super-resolution spectroscopy to attosecond X-ray pulse generation and particle acceleration. The procedure is proposed and numerically demonstrated by researchers in China and the UK led by Su-Ming Weng and Zheng-Ming Sheng at Shanghai Jiao Tong University in China. The method is based on the interaction of two pulses of laser light with a plasma – a gaseous mix of ions and electrons. The researchers also describe procedures to flexibly tune the resulting mid-infrared pulses by adjusting the plasma and lasers. If the current simulations are confirmed in practical tests, the method could overcome difficulties found in using standard optical techniques to generate high intensity mid-infrared pulses of close to single-cycle duration.