Relativistic electron mirrors from high intensity laser nanofoil interactions

The reflection of a laser pulse from a mirror moving close to the
speed of light could in principle create an X-ray pulse with
unprecedented high brightness owing to the increase in photon
energy and accompanying temporal compression by a factor of
$4\gamma^2$, where $\gamma$ is the Lorentz factor of the mirror.
While this scheme is theoretically intriguingly simple and was
first discussed by A. Einstein more than a century ago, the
generation of a relativistic structure which acts as a mirror is
demanding in many different aspects. Recently, the interaction of a
high intensity laser pulse with a nanometer thin foil has raised
great interest as it promises the creation of a dense, attosecond
short, relativistic electron bunch capable of forming a mirror
structure that scatters counter-propagating light coherently and
shifts its frequency to higher photon energies. However, so far,
this novel concept has been discussed only in theoretical studies
using highly idealized interaction parameters. This thesis
investigates the generation of a relativistic electron mirror from
a nanometer foil with current state-of-the-art high intensity laser
pulses and demonstrates for the first time the reflection from
those structures in an experiment. To achieve this result, the
electron acceleration from high intensity laser nanometer foil
interactions was studied in a series of experiments using three
inherently different high power laser systems and free-standing
foils as thin as 3nm. A drastic increase in the electron energies
was observed when reducing the target thickness from the micrometer
to the nanometer scale. Quasi-monoenergetic electron beams were
measured for the first time from ultrathin ($\leq$5nm) foils,
reaching energies up to ~35MeV. The acceleration process was
studied in simulations well-adapted to the experiments, indicating
the transition from plasma to free electron dynamics as the target
thickness is reduced to the few nanometer range. The experience
gained from those studies allowed proceeding to the central goal,
the demonstration of the relativistically flying mirror, which was
achieved at the Astra Gemini dual beam laser facility. In this
experiment, a frequency shift in the backscatter signal from the
visible (800nm) to the extreme ultraviolet (~60nm) was observed
when irradiating the interaction region with a counter-propagating
probe pulse simultaneously. Complementary to the experimental
observations, a detailed numerical study on the dual beam
interaction is presented, explaining the mirror formation and
reflection process in great depth, indicating a $>10^4$ fold
increase in the backscatter efficiency as compared to the expected
incoherent signal. The simulations show that the created electron
mirrors propagate freely at relativistic velocities while
reflecting off the counter-propagating laser, thereby truly acting
like the relativistic mirror first discussed in Einstein's thought
experiment. The reported work gives an intriguing insight into the
electron dynamics in high intensity laser nanofoil interactions and
constitutes a major step towards the coherent backscattering from a
relativistic electron mirror of solid density, which could
potentially generate bright bursts of X-rays on a micro-scale.