Detecting Radiation from Strong Electron Acceleration in High-Intensity Laser Fields: From Larmor radiation to the Unruh Effect

The ultra-high fields of high-power short-pulse lasers (beyond 10²⁰ W/cm²) are expected to contribute to improve our understanding of fundamental properties of the quantum vacuum and quantum theory in very strong fields. For example, the neutral QED vacuum breaks down at the Schwinger field strength of 1.3.10¹⁸ V/m, where a virtual e⁺e^- pair gains its rest mass energy over a Compton wavelength and materializes as a real pair. At such an ultra-high field strength, an electron experiences an acceleration of a_S=2.10²⁸ g and hence fundamental phenomena such as the long predicted Unruh effect start to play a role.
The Unruh effect implies that the accelerated electron experiences the quantum vacuum as a thermal bath with the Unruh temperature determined by the amount of the acceleration. In its accelerated frame the electron scatters photons off the thermal bath, corresponding to the emission of an entangled pair of photons in the laboratory frame (see Fig. 1).

In upcoming experiments with intense accelerating fields, we will encounter a set of opportunities to experimentally study the radiation from electrons under extreme fields. Even before the Unruh radiation detection, we should register the copious classical Larmor radiation. The detection of Larmor radiation from linear acceleration and its characterization themselves have never been experimentally carried out to the best of our knowledge, and thus this amounts to a first serious study of physics at extreme acceleration. Moreover, we can study for the first time radiation damping effects, i.e. the back-reaction of the energy loss experienced by an accelerated electron due to its emitted radiation onto its equation of motion (as given by the Lorentz force).

Over the last 100 years many theoretical attempts have been conducted to extend the Lorentz force by suitable terms incorporating radiation damping components like the Landau-Lifshitz radiation. However, so far no consistent description has been found, also due to the fact that in conventional experimental scenarios the effect of radiation damping
is negligibly small. This may change significantly in the high-field regime now accessible with modern high-intensity lasers. Furthermore, the upcoming experiment at the MPQ should be able to confirm or disprove whether the Larmor and  Landau-Lifshitz radiation components may be enhanced by a collective (N²) radiation, if a tightly clumped cluster of N electrons is accelerated. The technique of laser driven dense electron sheet formation by irradiating a thin DLC foil target should provide such a coherent electron cluster with a very high density. If and when such mildly relativistic electron sheets are realized, a counterpropagating second laser can interact with them coherently. Under these conditions enhanced Larmor and Unruh radiation signals may be observed. Detection of the Unruh photons (together with its competing radiation components) is envisaged via Compton polarimetry in a novel highly granular 2D-segmented position-sensitive germanium detector (see Fig. 2).