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
which are shown in Figure 7, indicate sub-femtosecond
emission time for both types of electrons. Closer
inspection has yielded a discernible temporal shift
between the two streaking spectrograms: careful data
analysis reveals a delay in emission of the core electrons
with respect to the conduction-band electrons of about
110 attoseconds. This means that the former reach the
surface – from the several-Angstrom depth which they
can escape from – some 110 as later than the latter. The
analysis of Pedro Echenique has revealed that only half
of this delay can be accounted for by the lower kinetic
energy of core electrons, the other half is attributed to
a larger effective mass of these electrons. This proof-ofconcept
study opens the door for direct time-domain
access to a wide range of hyperfast electron processes
in solids and surfaces, such as e.g. charge transfer in
host-guest systems or within molecular assemblies on
surfaces, charge screening and e-e scattering in metals,
or collective motion in surface plasmons.
After being successfully demonstrated in atoms and
solids, attosecond spectroscopy now remains to be
extended to molecular systems. As a matter of fact,
the valence electronic states are typically separated by
one or more electronvolts, indicative of a hyperfast
motion of electron wavepackets composed of such
states. The few-femtosecond-duration broadband
deep UV and VUV pulses reported in Chapter 1.3.1.1
lend themselves to launching electron wavepackets
on molecular orbitals with substantial probability,
whilst synchronized attosecond XUV pulses would
ideally suit for probing the unfolding electronic and
structural dynamics in molecules. Both of these tools
are derived from waveform-controlled few-cycle NIR
pulses, our basic tool, which ensures attosecond
synchronism between the deep-UV/VUV and XUV
pulses. In order that these tools become available for
real-time observation of hyperfast electronic motion
as well as complex, intertwined electronic and nuclear
dynamics in molecules, Marcus Fieß, Wolfram Helml,
and Eleftherios Goulielmakis – under the guidance of
Reinhard Kienberger – have developed a next-generation
attosecond beamline: AS-2, which accommodates two
frequency-conversion assemblies, for attosecond XUV
and few-femtosecond UV/VUV pulse generation. Both
beams can be recombined on target with attosecond
timing accuracy.
Encouraged by the successful extension of attosecond
spectroscopy to solids [10], a team led by Reinhard
Kienberger: Elisabeth Magerl, Adrian Cavalieri, and Ralph
Ernstorfer – in close cooperation with our MAP partners:
Peter Feulner, Dietrich Menzel and Johannes Barth from
the Technische Universität München, are developing
the world’s first ultrahigh vacuum beamline for
attosecond surface and solid-state spectroscopy: AS-3.
We are hopeful that measurement in AS-2 and AS-3
will make several contributions to the highlights of our
forthcoming progress report.
ADVANCING TIME-RESOlVED METROlOGY INTO
ThE SUB-100-AS DOMAIN
is indispensable for direct time-domain insight into
electron correlations on the atomic scale. These govern
or affect such fundamental processes as the intraatomic
energy transfer between electrons, the response
of atomic electron systems to external influence, or its
rearrangement following the sudden loss of one or more
electrons. Detailed insight into the electronic response
of atomic-scale solid-state structures to strong external
fields of infrared or visible light, e.g. via non-adiabatic
tunneling is required for the development of solid-state
lightwave electronics and will also rely on sub-100-as
temporal resolution.
By confining optical field ionization to a single, wellcontrolled
light oscillation period, the sub-1.5-cycle NIR
laser pulses described in Chapter 1.3.1.1 along with
novel broadband XUV multilayer mirrors developed
by Michael Hofstetter and Ulf Kleineberg have recently
permitted our team, Eleftherios Goulielmakis, Martin
Schultze and coworkers, to generate isolated sub-100as
pulses of XUV light [29], the measured XUV pulse
profile and the NIR electric field waveform are shown
in Figure 8. Detailed evaluation of the measurement
has been performed with a new algorithm developed
by Justin Gagnon and Vladislav Yakovlev [30]. The
quasi-monocycle driving field benefits attosecond pulse
generation in several ways. Abrupt onset of ionization
within a single half cycle minimizes the backgroundfree
electron density at the instant of harmonic pulse
emission, minimizing thereby distortion of the driving
wave and its dephasing with the generated harmonics
during propagation. Hence coherent build-up of the
harmonic emission over extended propagation is
maximized. In addition, order-of-magnitude variation of
the ionization probability between adjacent half-cycles
creates unique conditions for single sub-100-as pulse
emission without the need for sophisticated gating
techniques.
On the measurement side, improved resolution results
from three (almost equally) important advances: (i)
shorter XUV pulse duration, (ii) higher signal-to-noise
ratio (S/N) and/or shorter overall measurement time due
to the increased XUV photon flux, and (iii) the feasibility
of stronger streaking before the onset of the NIR-fieldinduced
ionization in attosecond streaking or enhanced
S/N due to reduced number of tunnelling steps in
attosecond tunnelling spectroscopy.
Consequences of these favourable conditions include the
routine generation of isolated sub-100-as XUV pulses
(at a carrier photon energy of about 80 eV), for details
140 Max-Planck-Institut für Quantenoptik • Progress Report 2007/2008