The dynamics of energetic particles in strong electromagnetic fields can be heavily influenced by the energy loss arising from the emission of radiation during acceleration, known as radiation reaction. When interacting with a high-energy electron beam, today’s lasers are sufficiently intense to explore the transition between the classical and quantum radiation reaction regimes. We present evidence of radiation reaction in the collision of an ultrarelativistic electron beam generated by laser-wakefield acceleration ($ϵ>500\mathrm{MeV}$) with an intense laser pulse ($a_0>10$). We measure an energy loss in the postcollision electron spectrum that is correlated with the detected signal of hard photons ($\gamma$ rays), consistent with a quantum description of radiation reaction. The generated $\gamma$ rays have the highest energies yet reported from an all-optical inverse Compton scattering scheme, with critical energy ${ϵ}_{\mathrm{crit}}>30\mathrm{MeV}$.

The description of the dynamics of an electron in an external electromagnetic field of arbitrary intensity is one of the most fundamental outstanding problems in electrodynamics. Remarkably, to date, there is no unanimously accepted theoretical solution for ultrahigh intensities and little or no experimental data. The basic challenge is the inclusion of the self-interaction of the electron with the field emitted by the electron itself—the so-called radiation reaction force. We report here on the experimental evidence of strong radiation reaction, in an all-optical experiment, during the propagation of highly relativistic electrons (maximum energy exceeding 2 GeV) through the field of an ultraintense laser (peak intensity of $4\times {10}^{20}\mathrm{W}/{\mathrm{cm}}^2$). In their own rest frame, the highest-energy electrons experience an electric field as high as one quarter of the critical field of quantum electrodynamics and are seen to lose up to 30% of their kinetic energy during the propagation through the laser field. The experimental data show signatures of quantum effects in the electron dynamics in the external laser field, potentially showing departures from the constant cross field approximation.

Electron–positron pair plasmas represent a unique state of matter, whereby there exists an intrinsic and complete symmetry between negatively charged (matter) and positively charged (antimatter) particles. These plasmas play a fundamental role in the dynamics of ultra-massive astrophysical objects and are believed to be associated with the emission of ultra-bright gamma-ray bursts. Despite extensive theoretical modelling, our knowledge of this state of matter is still speculative, owing to the extreme difficulty in recreating neutral matter–antimatter plasmas in the laboratory. Here we show that, by using a compact laser-driven setup, ion-free electron–positron plasmas with unique characteristics can be produced. Their charge neutrality (same amount of matter and antimatter), high-density and small divergence finally open up the possibility of studying electron–positron plasmas in controlled laboratory experiments.

We report on the generation of a narrow divergence (${\theta }_{\gamma }<2.5\mathrm{mrad}$), multi-MeV ($E_{\max }\approx 18\mathrm{MeV}$) and ultrahigh peak brilliance ($>1.8\times {10}^{20}\mathrm{photons}{\mathrm{s}}^{-1}{\mathrm{mm}}^{-2}{\mathrm{mrad}}^{-2}$ 0.1% BW) $\gamma$-ray beam from the scattering of an ultrarelativistic laser-wakefield accelerated electron beam in the field of a relativistically intense laser (dimensionless amplitude $a_0\approx 2$). The spectrum of the generated $\gamma$-ray beam is measured, with MeV resolution, seamlessly from 6 to 18 MeV, giving clear evidence of the onset of nonlinear relativistic Thomson scattering. To the best of our knowledge, this photon source has the highest peak brilliance in the multi-MeV regime ever reported in the literature.

The acceleration of ions from ultrathin foils has been investigated by using 250 TW, subpicosecond laser pulses, focused to intensities of up to $3\times {10}^{20}\mathrm{W}{\mathrm{cm}}^{-2}$. The ion spectra show the appearance of narrow-band features for protons and carbon ions peaked at higher energies (in the $5–10\mathrm{MeV}/\mathrm{\text{nucleon range}}$) and with significantly higher flux than previously reported. The spectral features and their scaling with laser and target parameters provide evidence of a multispecies scenario of radiation pressure acceleration in the light sail mode, as confirmed by analytical estimates and 2D particle-in-cell simulations. The scaling indicates that monoenergetic peaks with more than $100\mathrm{MeV}/\mathrm{\text{nucleon}}$ are obtainable with moderate improvements of the target and laser characteristics, which are within reach of ongoing technical developments.

The generation of ultrarelativistic positron beams with short duration (${\tau }_{e^{+}}\simeq 30\mathrm{fs}$), small divergence (${\theta }_{e^{+}}\simeq 3\mathrm{mrad}$), and high density ($n_{e^{+}}\simeq {10}^{14}–{10}^{15}{\mathrm{cm}}^{-3}$) from a fully optical setup is reported. The detected positron beam propagates with a high-density electron beam and $\gamma$ rays of similar spectral shape and peak energy, thus closely resembling the structure of an astrophysical leptonic jet. It is envisaged that this experimental evidence, besides the intrinsic relevance to laser-driven particle acceleration, may open the pathway for the small-scale study of astrophysical leptonic jets in the laboratory.