Marija Vranic, a professor at Instituto Superior Técnico, together with PhD student Óscar Amaro, are part of the list of researchers who recently published (and received cover honours) a study by an international team in the prestigious scientific journal Nature Photonics. Both are members of the Group of Lasers and Plasmas at the Institute for Plasmas and Nuclear Fusion (IPFN) and were responsible for the theoretical framework that guided the experiment.
The study was conducted at the Centre for Relativistic Laser Science (CoReLS) in South Korea, employing one of the world’s most powerful laser systems to demonstrate this phenomenon known as Nonlinear Compton scattering (in which a high-energy electron absorbs multiple photons, emitting a single gamma-ray photon), demonstrated by colliding a high-energy electron beam with a high-power laser pulse. In this experiment – a thousand times brighter than previous records at this energy scale – the researchers used a laser beam so intense that it approaches the so-called Schwinger limit – an intensity of light so strong that it can ‘boil’ the vacuum of space, creating pairs of matter and antimatter.
Although the highest laser intensity achieved to date (in this very laser) is still a million times below this physical limit, the method used, based on Einstein’s Theory of Relativity, allowed the scientists to reach 50 per cent of this limit in the electron’s reference frame, triggering fascinating non-linear quantum effects.
‘Here we have a naturally adjustable frequency source that can be used for various purposes, for example to see very small objects and very rapid changes’, explains Marija Vranic. The observation could pave the way for better visualisation of what is ‘normally difficult to see directly’, such as biological processes or images of the quality of critical parts (such as aircraft parts) before installing them, or for observing the state of old nuclear reactors without opening them up.
‘This study has a fundamental part: it verifies an effect that has been theoretically predicted for decades, and it has significant implications for applications – the photon beam generated has promising characteristics for use as a radiation source in medical and biological imaging, security, and quality control”, she adds.
The work of the team she leads ‘focuses on the extreme states of plasmas, where quantum effects coexist at the same time as collective effects’. ‘This extreme regime exists naturally in space, but could be achieved in the laboratory in the future with the most powerful lasers in the world. This experiment is an important step along that path,’ the researcher explains.
The research that led to the article: understanding quantum electrodynamics
This research work is part of a global effort to better understand quantum electrodynamics (QED) in strong fields – a branch of physics that usually deals with phenomena found in extreme astrophysical environments, such as pulsars, magnetars, supernovae and black holes. It is based on previous experiments, such as the one carried out at SLAC (Stanford Linear Acceleration Centre, United States) in 1996, but the new method uses only a laser, without the need for a large accelerator. Further experiments are planned in top laboratories such as DESY (Deutsches Elektronen-Synchrotron, Germany), SLAC and future installations of multi-petawatt lasers in various parts of the world.
In the experiment, the laser was split into two beams. The first beam was focussed on a gas-filled chamber, accelerating electrons to almost (99.999999%) the speed of light. The second beam, lasting just 20 femtoseconds (20 millionths of a billionth of a second), was directed to collide with the accelerated electrons.
The precision required for this collision is impressive: the beams were synchronised in a region just a few micrometres wide, with an overlap of just 10 femtoseconds. This precise control allowed the electrons to ‘dance’ in the field of the powerful laser, absorbing up to 400 photons at once and emitting high-energy gamma rays.
The researchers carefully characterised this energy, verified the signatures predicted by the theory and compared the experimental results with analytical models and large-scale simulations carried out on supercomputers. The agreement between experiment and simulation results confirmed, for the first time, the occurrence of non-linear Compton scattering and allowed the team to deduce the intensity of the colliding laser by extracting its ‘fingerprint’ from the gamma-ray signals.
In the media:
Cientistas portugueses colaboraram na “primeira observação de novo efeito físico” (Público)
Cientistas do Técnico colaboram na “1.ª observação de novo efeito físico” (Notícias ao minuto)
