Photon-Photon Collision Using Extreme Lasers

Dear Colleagues,

Photon–photon collision is a fundamental physical process that has been studied since the very inception of quantum mechanics. Moreover, it is a very famous process, thanks to the Star Wars lightsabers; however, those sabers are fictional and cannot be formulated in real-life scenarios because the collision of two visible photons has a negligibly small, or non-zero, cross section.

When the energies of photons are large enough, an electron–positron pair can be generated in a process that is the reverse motion of electron-position annihilation which generates a pair of photons. For that process to occur, the photon mass has to be comparable or greater than the electron mass. Consequently, the energy of photons is too low for a real pair generation in lasers (infrared, visible, ultraviolet or even soft X-ray).

The impossibility of pair generation in the optical or soft X-ray domain does not imply that the physics of this domain is completely known. The cross section is so small that it seems impossible to obtain any kind of measurement. However, in recent decades, several advancements have been achieved in laser technology, and now the record peak intensity is beyond 10²³ W/cm²; it is anticipated that in the coming years it will be possible to achieve the intensity of 10²⁴ W/cm² . This intensity means an extraordinary concentration of energy, comparable to the density of water, if we can transform all the mass into pure energy.

Such extraordinary progress provides an impetus for the growth of photon–photon collision experiments in the near future. This Special Issue of Photonics calls for papers that address a wide array of questions concerning the development of these experiments:

• What is the best scenario for these experiments? For example, is two counterpropagating petawatt lasers an optimized scenario? Can optical petawatt pulses colliding with soft X-ray lasers (as the XFEL lasers) be an alternative?
• What is the best laser? What is the best pulse length? Is intensity the only relevant parameter?
• Is it possible to develop PW or multi-PW lasers specifically tailored for this purpose?

The advantage of working in low energies for the onset of the electron–positron pair creation is that the vacuum will not be broken with the appearance of new pairs; at the same time, the extreme lasers tension will provide the quantum vacuum with an unprecedented strength. This may be relevant for two reasons. One, to directly measure the vacuum response due to electron–positron virtual pairs in the nonlinear regime. There are two models to describe this response, one presented by Max Born and Leopold Infeld in 1934, and another presented by Werner Heisenberg and Hans Heinrich Euler in 1936. The peculiarity is that they do not coincide in the second-order terms. Although the Heisenberg–Euler model seems to be most popularly accepted by the community, clean experimental evidence of which model is the rightly nonlinear, if any, has not been obtained yet. Moreover, certain kinds of dark matter can modify the response of the quantum vacuum and result in a modification of the photon–photon scattering by adding new couplings beyond the particle physics standard model. In this context, we invite papers that address the following areas:

• Designing laser experimental configurations that can differentiate between these two models;
• The present status of ongoing experiments, either in the commissioning or designing phase, and new experimental problems/solutions found;
• Ultrahigh sensitivity detectors specifically designed for that purpose, such as X-ray ultraprecise polarizers, and single photon detectors;
• Techniques developed for other purposes that can be of interest in photon–photon quantum vacuum experiments;
• Experiments using existing lasers and experimental configurations that would need further laser developments;
• Possibilities to obtain information on the properties of certain kinds of dark matter,the most suitable dark matter candidates that can be seen in such extreme photon bath, advantages of photon fields that cannot generate electron/positron pairs and would mask a hypothetical dark matter signature;
• Comparison with other techniques studying dark matter which also use lasers but not such extreme lasers, and possibilities of combinations and synergies.

Working with such extreme experimental conditions is extraordinarily difficult. Laser technology employs beyond 10 PW peak power, but existing lasers are often too complex because they are designed to be as multipurpose as possible. For this goal, is it necessary to understand the optimized laser (or lasers) configuration, especially when pump–probe scenarios are taken into account. This also represents a detection challenge because it is going to be a very weak source with a lot of possible noise sources. This Special Issue welcomes papers that focus on optimizing the experimental scenarios:

• The best achievable laser or lasers combinations, best frequency combination, and possibility of using multiple beam lasers as alternatives;
• Advantages of using an X-UVV or soft X-ray probe;
• Repetition rate and data management necessary for correct signal conclusive identification inside a huge background noise produced by the extreme laser photons;
• Measurement of the vacuum birefringence or the photon–photon scattering;
• Difficulties present in the experimental set up, such as extreme vacuum needs, large vacuum volumes, and sources of noise;
• Extreme effects from residual electrons, such as radiation reaction, Thompson emission, and pair cascading. Analysis of their characteristic patters in order to reduce the signal-to-noise ratio.

Extreme lasers are now considered as a new alternative to explore photon–photon collisions in the domains where photon energy is so low that pair creation is impossible. However, such a high density of photons can stress the quantum vacuum and induce nonlinear responses, which again become important mechanisms that must to be taken into account while studying elementary particles and dark matter. Papers analyzing or proposing scenarios on how to study these changes, describing the best laser configurations and highlighting the potential experimental difficulties are welcome in this Special Issue.

Precise measurement of photon–photon collisions requires extremely demanding interdisciplinary experiments. Thus, we invite submissions of papers that from a diverse range of fields to shed light on this rather intricate mechanism using varying techniques, including artificial intelligence to extract a weak signal from a noise environment. To conclude, photon–photon scattering means to detect a few scattered photons per pulse—in the best case—out of a background of maybe 1020 photons generated with a multi-petawatt laser.

Prof. Dr. Luis Roso
Guest Editor

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• ultraintense lasers
• photon-photon collisions
• extreme vacuum
• single photon counting
• Breit-Wheeler process
• radiation reaction
• petawatt and exawatt lasers
• vacuum birefringence
• QED Lagrangian
• born-infeld lagrangian
• dark matter searches with laser light
• extreme laser technology
• relativistically driven electrons
• thomson scattering from laser driven electrons