Attosecond Control and Measurement: Lightwave Electronics

1 . 3 AT T O S E C O N D A N D H I G H - F I E L D P H Y S I C S D I V I S I O N
Figure 13: Gigaelectronvolt-scale laser wake field accelerator at MPQ. G: GRENOUILLE pulse diagnostics, C1-C5:
cameras; D1, D2: diodes; L1, L2: scintillating screens; S: spectrometer; W: wedge; OAP: off-axis parabola; CAP:
capillary waveguide. The inset shows the incoming laser focus at the capillary entrance.
relativistic plasma, which shortens its duration into the
required domain. Longer-than-optimal driver pulses
compromise efficiency as well as reproducibility, and
result in copious amounts of low-energy electrons
accompanying the mono-energetic emission with an
exponentially-decaying spectrum, forming a “thermal”
background. By using the multi-terawatt, sub-10-fs
pulses from LWS-10, Karl Schmid, Laszlo Veisz and
coworkers, in close cooperation with Ulrich Schramm,
Dieter Habs and collaborators from the Univ. Düsseldorf
have recently demonstrated the first electron accelerator
based on high-density plasma waves driven with
laser pulses that fit in one half of the plasma period
(Figure 11) [14]. Direct “impulsive” excitation of the
plasma wave permits mono-energetic electron
acceleration virtually free from a thermal background
for the first time. In our experiments, 5-terawatt, 8femtosecond
laser pulses yield electron bunches up to
energies of 25 MeV (Figure 12). The flux of low-energy
electrons dramatically reduced as compared to earlier
experiments also manifests itself in a strongly-reduced
secondary radiation emerging from the accelerator and
offers the potential for enhancing efficiency and stability
with more intense driver pulses.
The electron energy can be further boosted by laser-driven
plasma-wake-field acceleration (LWFA) in a discharge
capillary to the gigaelectronvolt range, as demonstrated
by Wim Leemans and coworkers at Berkeley in 2006.
Jens Osterhoff, Antonia Popp, Zsuzsanna Major, Stefan
Karsch and coworkers, in close co-operation with Dieter
Habs and his group, have recently also demonstrated
LWFA to the GeV frontier from a discharge capillary
waveguide provided by Simon Hooker from Oxford
University by driving the accelerator with 0.75 J, 40fs,
800-nm pulses from ATLAS, MPQ’s multiterawatt
Ti:Sa laser, see Figure 13 [12].
Quasi-monoenergetic electron beams with energies as
high as 500 MeV have been detected, with features
reaching up to 1 GeV, albeit with large shot-to-shot
fluctuations. The beam divergence and the pointing
stability in this case are around or below 1 mrad and
8 mrad, respectively. These shot-to-shot electron-bunch
parameter variations are greatly suppressed by utilizing
the capillary as a gas-vessel without pre-ionization by an
external electrical discharge. In this regime of operation
quasi-monoenergetic electron beams of up to ~200
MeV in energy have been observed (Figure 14). These
beams emitted within a low-divergence cone of mrad
FWHM display unprecedented shot-to-shot stability
in energy (2.5% RMS), pointing (1.4 mrad RMS) and
charge (16% RMS) owing to a highly reproducible gasdensity
profile within the interaction volume (Figure15)
[13]. The excellent stability of bunch parameters affords
promise for the potential ability of laser-accelerated
electron bunches to meet the stringent criteria for
seeding free electron lasers.
INTENSE ATTOSECOND XUV/SXR PUlSES
might allow triggering as well as probing ultrafast
electron dynamics with attosecond pulses, i.e.
attosecond XUV-pump/XUV-probe spectroscopy,
which has not been possible so far. Relativistic laserplasma
interactions have been identified as a promising
approach to achieving this goal. Recent experiments
confirmed that relativistically-driven overdense plasmas
are able to convert infrared laser light into harmonic
XUV radiation with unparalleled efficiency and that the
generation technique is scalable towards hard X-rays.
In a recent experiment (Figure 15) Yutaka Nomura,
Rainer Hörlein, Sergey Rykovanov, George Tsakiris and
coworkers, in collaboration with the teams of Matt Zepf
from Belfast and Dimitris Charalambidis from Heraklion,
have recently succeeded in obtaining conclusive
experimental evidence for the phases of the XUV
harmonics emanating from the interaction that they are
144 Max-Planck-Institut für Quantenoptik • Progress Report 2007/2008