After two decades of efforts at several physics institutes around the world, researchers at the Center for Relativistic Laser Science (CoReLS) at the Institute of Basic Sciences (IBS) in South Korea, have achieved laser intensity unprecedented with their experimental setup. The laser was able to produce a power of 1023 watts (100 trillion watts) concentrated on a square centimeter! A world first, which will allow the exploration of new phenomena occurring under extreme physical conditions.
Ultra-high intensity lasers are important research tools, used in several scientific fields, in particular for the exploration of new physical phenomena. Since the demonstration of a laser with an intensity of 1022 W / cm² by a team from the University of Michigan in 2004, the realization of a laser with an intensity greater than 1023 W / cm² has been studied and tested for almost 20 years.
Obtaining such a level of ultra-high laser intensity requires two key elements: a laser with extremely high output power, and focusing the laser beam on the smallest point possible. While continuous wave lasers are limited to an intensity of the order of megawatt, much higher peak output power (of the order of petawatt) is possible in pulsed laser systems, by delivering energy to a timescale as short as a femtosecond (10-15 second).
A long-standing effort bringing together several innovative techniques
To develop the most powerful laser in the world, several ultra-high power laser installations with output powers of 10 PW (petawatts; 1015 watts) and above, such as ELI (EU), Apollon (France), EP-OPAL (United States) and SEL (China), have been built or are in the pipeline. A recent study from Osaka University even proposed a prototype for an exawatt class laser (1018 watts). At the same time, the CoReLS laser team has been operating a 4-PW laser system since 2016. This year, in April 2021, they finally reached the record of 1023 W / cm² by tightly focusing the multi-PW laser beam. The detailed results have been published in the journal Optica.
Several special techniques were employed to accomplish this feat. The intensity of the power was maximized using a focusing optic called an off-axis parabolic mirror, which was used to focus a 28cm laser beam onto a spot only 1.1 micrometers wide. Such narrow focusing limited by diffraction can only be obtained with a clean laser beam without wavefront distortion. The CoReLS laser team therefore made their PW laser beam as clean as possible by using a set of deformable mirrors to correct for the wavefront distortion of the PW laser.
The CoReLS 4-PW laser is a very high power Ti: sapphire femtosecond laser, based on the chirped pulse amplification (CPA) technique. A low energy femtosecond laser pulse from the frontal was stretched into a nanosecond pulse by a pulse stretcher. The initial laser pulse was then amplified to 4.5 J (joules) by the two power amplifiers, then to 112 J by the two booster amplifiers.
The size of the laser beam has increased along the beam path thanks to a series of beam wideners: 25 mm just after the power amplifiers, 65 mm at the input of the 1st booster amplifier, 85 mm at the input of the 2nd booster amplifier, and 280 mm at the input of the pulse compressor. In the pulse compressor, the laser pulse was recompressed to 20 fs (femtosecond), which allowed its maximum power to increase to 4 PW after compression.
Correct optical aberrations
In order to compensate for the wavefront distortion of the PW laser beam, two deformable mirrors were used in the PW laser beam line. The first deformable mirror (DM1), with a diameter of 100 mm, was installed after the final booster amplifier, its role being to correct the wavefront distortion accumulated between the front end and the expander. of final beam.
The second deformable mirror (DM2), with a diameter of 310 mm, was installed after the pulse compressor. Its role is to correct the additional aberrations induced by the large aperture optics of the pulse compressor, the beam distribution line and the target zone. In the target chamber, the PW laser beam was tightly focused using an off-axis parabolic mirror with an f / 1.1 focal length, which has an effective focal length of 300mm.
For imaging and characterization of the focused point, the focused beam was collimated by an objective lens. It was then split into two beams with a beam splitter for focal spot and wavefront characterization. A camera was used to track the focal point of the transmitted laser beam, and a wavefront sensor was used to measure the wavefront of the reflected laser beam.
Deepen our knowledge of physics and the Universe
” This work has shown that the CoReLS PW laser is the most powerful laser in the world. With the highest laser intensity ever achieved, we can tackle challenging new areas of experimental science, including strong field quantum electrodynamics (QED), which has been addressed primarily by theorists. We can explore new physical problems of electron-photon scattering (Compton scattering) and photon-photon scattering (Breit-Wheeler process) in the nonlinear regime. This type of research is directly related to various astrophysical phenomena that occur in the universe and can help us to further expand our horizon of knowledge. », Explains Professor NAM Chang Hee, director of CoReLS.
