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
In another study, Vladislav Yakovlev has investigated the
possibility of coherent superposition of laser-driven SXR
harmonics in successive sources by one and the same
laser pulse. His prediction of the feasibility of quasi-
A
B
Figure 10: High-order harmonic generation from
successive sources. (A) Schematic illustration of a
sequence of harmonic sources formed by gas jets.
The sources are arranged along the optical axis of
a focused laser beam and pumped by one and the
same laser pulse. In our first proof-of-concept study
we have employed only two sources (depicted in
black), further ones can be added for scaling the
SXR harmonic yield in future experiments (depicted
in grey). (B) Increasing the density of atomic dipole
emitters tends to increase the coherent harmonic
yield. However, increasing density causes the dipole
oscillators to increasingly dephase along the laser
propagation direction. In a single source, this leads to
saturation of the harmonic yield at an atomic density
corresponding to a backing pressure of ~ 40 mbar,
red curve (calculated) and squares (measured).
Splitting the generation medium into two equal
sections and moving them apart so that the phase
of the atomic dipole oscillations gets shifted by in
the focused laser beam allows the atomic density to
be increased by a factor of two (at backing pressure
of ~ 80 mbar), leading to saturation at a factor of
four higher harmonic intensities (blue curve and
diamonds).
phase-matched SXR harmonic generation by a focused
laser beam in a gas medium of modulated density
has meanwhile been confirmed by our experimental
observation of constructive and destructive interference
between the harmonic signals originating from two
successive sources, see Figure 10 [32]. Our proof-ofconcept
study opens the prospect of enhancing the
photon flux of SXR harmonic sources to levels enabling
researchers to tackle a range of applications in physical
as well as life sciences.
Should attosecond pulse generation be scalable to
photon energies of several kiloelectronvolts, attosecond
X-ray diffraction might allow, one day, 4-dimensional
microscopy with attosecond temporal and picometre
spatial resolution. Drawing on the pioneering work of
Ahmed Zewail, we also pursue this long-term goal by
advancing ultrafast electron diffraction. To this end,
Ernst Fill, Laszlo Veisz, and Sasha Apolonskiy have
presented a novel concept of an electron gun for
generating few-femtosecond- to sub-femtosecondduration
electron bunches [33]. The basic idea is
to utilize a DC acceleration stage combined with a
microwave cavity, the time-dependent field of which
generates an electron energy chirp for bunching at the
target. To reduce space charge broadening, the number
of electrons in the bunch is reduced and the gun is
operated at a MHz repetition rate. In the absence of
space charge (one electron per bunch), the duration
of the generated electron wavepacket may potentially
be shortened below 1 fs. The team around Ernst Fill,
Martin Centurion and Peter Reckenthäler, have also
devised a technique for measuring the bunch duration
with potentially attosecond resolution [34] and have
also performed preliminary time-resolved experiments
with (multi-)electron bunches, which allowed them to
image dynamics changes in the density distribution of a
laser-induced plasma [35].
1.3.1. 3 hIGh-FIElD ATTOSECOND SCIENCE: RElA-
TIVISTIC ElECTRON CONTROl
As described in Chapter 1.3.1.1, we pursue scaling of
waveform-controlled few-cycle laser pulse generation to
relativistic intensities in the expectation that controlling
the motion of relativistic electrons with attosecond
precision in time and nano- to micrometre precision in
space paves the way towards scaling attosecond pulse
generation to higher flux and/or photon energies and
may allow the development of compact, laboratory
sources of brilliant X-rays. Simultaneously with our
efforts aiming at laser development, in collaboration
with Dieter Habs and his group, we are also conducting
relevant proof-of-concept studies drawing on our
laser sources currently available. Two routes are being
pursued at MPQ: the development of a compact X-ray
free electron laser seeded by laser-accelerated electron
bunches and coherent harmonic XUV/X-ray pulse
generated from relativistically-driven surfaces. In this
142 Max-Planck-Institut für Quantenoptik • Progress Report 2007/2008