It is reported that the researchers proved for the first time that the relativistic electrons are driven by low-energy, ultra-short mid-infrared laser pulses. Many applications can use accelerated beams with lower energy and higher repetition rates, such as fast scanning imaging for medical, scientific, and safety purposes. A professor of physics and electrical engineering at the University of Maryland said: “We are currently working to develop a compact, high-repetition rate laser accelerator. That is to say, use the lowest possible laser pointer pulse energy to generate relativistic electrons.”
Compared with previous experiments, the long drive wavelength and low energy femtosecond laser pulses used in this project can easily approach the so-called “critical density” state. Since the critical density varies inversely with the square of the laser wavelength, the gas targets used for MIR laser pulses are 100 times less dense than those used in visible light and NIR, which makes them more difficult to design.
When the mid-infrared green laser pointer pulse of a few millijoules femtoseconds is focused by the curved mirror into a hydrogen jet-the hydrogen jet from the nozzle-then a collimated pulse of relativistic electrons is emitted from the other side of the jet. However, unless the laser achieves a very high intensity, which is much higher than the intensity achievable by using a curved mirror to focus, the above will happen. It is achieved by relatively autofocusing in ionized hydrogen, which shrinks it to a size much smaller than its focal point.
According to opinion, the value of the critical density system is to promote relative autofocus even under low-energy laser pulses. This enhanced high-intensity interaction generates plasma waves, which accelerate some electrons from ionized hydrogen into a forward relative beam. Experiments have shown that without relative autofocusing, electron acceleration cannot be produced. The team’s findings are only the early stages of the development and application of high-repetition rate laser-driven accelerators.