Almost any physical investigation involves the use of light, or photons, as a probe. High-energy photons are required to probe fundamental quantum processes, and interactions between lasers and matter can generate such photons; a charged particle can absorb laser photons and, in giving up some of its own energy, re-emit the photons at a much higher energy. This has led to great interest in using intense laser facilities to create high-energy photon sources, but there are seemingly unavoidable physical restrictions on their energy. In particular, the generation of high-energy photons leads to the start of an avalanche, or cascade, of electron-positron pairs that, while creating more photons, rapidly draws too much energy from the system, making the source unsustainable. We show that there is in fact a way to overcome such restrictions and create a source of giga-electron-volt (GeV) photons with unprecedented brightness.

The key to our concept is the use of particle-trapping phenomena to initiate a new scenario in which particle cascades and highly nonlinear particle dynamics induce and support each other but, crucially, in a controllable manner. By matching the intensity and duration of the laser pulse, it is possible to induce a cascade that is held at a subcritical level, avoiding significant depletion effects while sustaining photon production. In this scenario, laser radiation is converted into a well-collimated flash of GeV photons. The resulting source has parameters exceeding those provided by existing laser-based sources by several orders of magnitude.

Our concept is feasible for upcoming laser facilities and could enable a new era of experiments in photonuclear and quark-nuclear physics.