Fig. 1. Sketch of principle of a CRM boosting the E-field of a light wave. The yellow dot indicates the CRM focus in vacuum.

Fig. 2. Physics of plasma mirrors. (a) Plasma mirrors specularly reflect an incident ultraintense laser beam. At ultrahigh intensities, this laser field ((b), field E(t) in the upper graph, spectrum in the lower graph) drives a periodic relativistic oscillation of the plasma surface. This induces a Doppler effect on the reflected beam, resulting in a periodically distorted reflected waveform ((c), upper graph), the spectrum of which consists of a comb of high-order harmonics ((c), lower graph). (d) Snapshots of the electron density at the plasma mirror surface (in a Lorentz-boosted frame where the laser is normally incident on the plasma), at different times in a laser optical cycle (see white labels), revealing two key effects: first, the relativistic oscillation of the plasma surface; second, the spatial curvature of this surface induced by the radiation pressure of the incident laser field. From a PIC simulation with a laser intensity I = 10²² W/cm².

Fig. 3. Raw image of an experimental harmonic spectrum (orders ≈10 (left) to 40 (right)) generated upon reflection of the UHI 100 TW laser (CEA Saclay) on a relativistic plasma mirror.

Fig. 4. Measured temporal intensity profile of the attosecond pulses produced by the ROM mechanism (superposition of harmonic orders 9 to 14). From Ref. [53].

Fig. 5. Principle of intensity boosting using a p-CRM. The plasma surface curvature is induced either by radiation pressure, or by more demanding schemes such as preshaped solid targets (e.g., micro-off-axis parabolas).

Fig. 6. (a) Spectral and (b)–(d) temporal effects of the reflection of an ultraintense laser field on a p-CRM, from PIC simulations. The spectral rectification effect is highlighted in (a), by comparing the harmonic spectrum right after the p-CRM (black) and at the p-CRM focus (red). (b)–(d) Comparison of the waveforms of the incident field (b), of the field right after reflection from the p-CRM (c) and of the field right at the centre of the p-CRM focus (d). The strong difference between (c) and (d) is due to the spectral rectification effect evidenced in (a). All fields are plotted in units of the Schwinger field E_s, and the intensity of the incident laser is I = 10²² W/cm².

Fig. 7. Light intensity as a function of peak power P of the driving laser. Grey curve: maximum intensity achievable from the laser itself. Black curve: with intensity boost, curvature of the p-CRM induced by radiation pressure. Red curve: optimized curvature leading to focusing of the harmonics at their diffraction limit. The red star indicates the present intensity record achieved with a 4 PW laser^[34]. The different grey areas indicate the estimated ranges of light intensities that could be reached, depending on the class of power of the lasers used to irradiate relativistic plasma mirrors.