Huge gratings promise 50PW pulses

A research collaboration coordinated by Lawrence Livermore National Laboratory (LLNL) has developed new large-scale diffraction gratings that are thought to be capable of delivering ultrashort laser pulses with a power of up to 50 petawatts.

So far the team has produced high-energy pulse compression gratings measuring 85 x 70 cm that will be installed in the L4-ATON laser system at the ELI-Beamlines Facility near Prague in the Czech Republic.

L4-ATON is expected to generate 1.5 kJ, 150 femtosecond pulses - equivalent to 10 petawatts of power - at an unprecedented repetition rate of one shot per minute.

Dielectric gratings
Based on the Nobel-winning chirped-pulse amplification (CPA) technique, laser systems such as L4-ATON must stretch, amplify and compress a high-energy pulse without damaging optical components.

It means that the pulse-compression gratings used must be sufficiently large, efficient and robust to withstand the high photon flux of the laser pulses.

Working alongside collaborators from ELI-Beamlines, Spectra Physics, and Texas-based National Energetics, LLNL's Diffractive Optics Group produced high-energy, low-dispersion (HELD) multi-layer dielectric gratings.

They are said to handle around three times more total energy than current state-of-the-art technology, as LLNL senior laser scientist Hoang Nguyen, leader of the Diffractive Optics Group at the National Ignition Facility, explained:

"The 85-by-70-centimeter HELD gratings, configured at a Littrow angle (the angle of maximum grating efficiency) of 37 degrees, allow for a larger beam width [of] 62.5 centimeters," he said.

"Increasing the beam height to produce a square beam and accounting for the difference in LIDT (laser-induced damage threshold) results in approximately 3.4 times more total energy on the grating compared to the [earlier] high-dispersion, 76.5 degree angle of incidence grating design.

"The HELD grating's excellent uniformity is due to a design with high efficiency over a large range of groove widths and heights and well-controlled dielectric layer thickness."

Diffraction efficiency
LLNL points out that operating at the grating's Littrow angle results in maximum diffraction efficiency and bandwidth but requires tilting the angle of the gratings so the beam reflects slightly upward, or out-of-plane.

The new gratings are based on multilayer dielectric (MLD) material. They comprise a base substrate upon which layers of dielectric mirrors with varying refractive indices are stacked, topped by a layer of ion-etched, light-sensitive photoresist.

The key advantage of MLD gratings is that they substantially enhance the diffraction efficiency of grating compressors over a broad range of wavelengths, as would be found with an ultrashort pulse.

Unlike the metal used in traditional gratings, dielectric materials are non-conducting and are said to absorb 500 times less energy than previous designs.

While the L4-ATON laser beamline will be able to deliver 10 petawatt pulses with the current technology, the LLNL team suggests that by incorporating larger, meter-scale HELD gratings, future systems could hit 20-50 petawatts - opening up even more opportunities for research in plasma and high-energy-density physics, astrophysics, medical diagnostics, industrial processing and more.