Low-Energy Laser Pulse Generates Relativistic Electron Beams

Relativistic electrons driven by low-energy, ultrashort mid-infrared (MIR) laser pulses have been demonstrated for the first time, according to researchers. Many applications could use a lower energy and higher repetition rate accelerated beam, such as rapid-scan imaging for medical, scientific and security purposes.

“We’re trying to develop laser-driven accelerators that are extremely compact and have a high repetition rate,” said professor Howard Milchberg, a professor of physics and electrical engineering at the University of Maryland. “That means using as low a laser pulse energy as possible to generate relativistic electrons.”

Compared to prior experiments, the long-driver-wavelength low-energy femtosecond laser pulses used in this project resulted in easy access to what is called the “critical density” regime. Because the critical density varies inversely as the square of the laser wavelength, gas targets used for MIR laser pulses can be up to 100 times less dense than those used in the visible and NIR, making them far less difficult to engineer.

“When a few-millijoule femtosecond mid-IR laser pulses are focused by a curved mirror into a hydrogen gas jet — a stream of hydrogen puffing out of a nozzle — a collimated pulse of relativistic electrons beams out the other side of the jet,” said Milchberg. “However, this can't happen unless the laser achieves an extremely high intensity — much higher than achievable by focusing with the curved mirror alone. It does so by relativistic self-focusing in the ionized hydrogen gas so that it collapses to a size much smaller than its focal spot.”

The value of being in the critical density regime, according to Milchberg, is that it promotes relativistic self-focusing even for low energy laser pulses. This boosted high-intensity interaction generates plasma waves that accelerate some of the electrons from the ionized hydrogen into a forward-directed relativistic beam.

Experiments demonstrated that electron beams were present for powers such that the characteristic self-focusing length in the plasma was shorter than the gas jet width, showing that electron acceleration cannot occur without relativistic self-focusing.

Relativistic self-focusing is an extreme example of the process of self-focusing in nonlinear optics, but with
the added bonus of accelerated relativistic particles generated from the nonlinear medium. Even with only
20 mJ of MIR laser energy, the laser in these experiments could significantly exceed the threshold for relativistic self-focusing, giving rise to relativistic multifilamentation. The team observed multiple relativistic electron beamlets associated with these filaments.

The team's findings represent early steps toward the development and application of high repetition rate laser driven accelerators.

“In particular, long wavelength femtosecond lasers are especially promising, as they can access the relativistic nonlinear regime of free electrons surprisingly easily,” said Milchberg.

The presentation entitled Laser wakefield acceleration with mid-IR laser pulses will take place at Frontiers in Optics + Laser Science APS/DLS (FIO + LS), an Optical Society event to be held September 17-21, 2017 at the Washington Hilton in Washington, DC.