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 5: Ultrashort-pulse-driven OPCPA test system using ATLAS. (A) Pulse-front tilt matching of pump and
signal pulses. Maximum gain bandwidth (B) and corresponding transform limited pulse duration (C). Spectrum
transmitted by our chirped-mirror compressor (D) along with pulse reconstructed from SPIDER measurements (E).
lIGhT WAVEFORM SYNThESIS AT MEGAhERTZ
REPETITION RATES
affords promise to produce attosecond pulses at MHz
repetition rate, which, in turn, would open the door for
combining attosecond spectroscopy with coincidence
detection, a long-standing dream of researcher in the
field of attosecond science. By inventing the concept of
chirped-pulse oscillators (CPO) our group has previously
shown how to scale the energy and peak power of
femtosecond pulses from mode-locked oscillators by
lowering their repetition rate from 50-100 MHz by an
order of magnitude [27]. Recently, a collaborative effort
of Akira Ozawa, Thomas Udem (from the group of Prof.
T. Hänsch) and Jens Rauschenberger, Alma Fernandez
and Alexander Apolonskiy from our group coupled
sub-microjoule energy pulses from our Ti:sapphire
chirped-pulse oscillator into a passive build-up cavity to
generate high-order harmonics up to photon energies of
~ 30 eV (wavelength ~ 40 nm) with average powers of
about one microwatt [28]. Implementing the concept
with few-cycle light, which we currently pursue, will
allow extension of frequency combs to the soft-X-ray
regime and attosecond pulses at MHz repetition rates
for attosecond coincidence spectroscopy.
1.3.1.2 lIGhTWAVE ElECTRONICS: ATTOSECOND
CONTROl & SPECTROSCOPY
At the time of the previous progress report, extreme
ultraviolet (XUV) pulses of a duration of 250 as at a
photon energy of about 100 eV, containing some 10 7
photons/pulse and delivered at a 1 kHz repetition rate
have represented the state of the art of attosecond
technology [4]. In our newly-developed AS-1
attosecond beamline [6], these sub-femtosecond pulses
along with their few-cycle NIR drivers have served as
a standard tool for attosecond real-time interrogation
of atomic-scale electron dynamics over the past two
years. Simultaneously, the improved NIR waveforms
reported above have been used to push the frontiers of
attosecond pulse generation and metrology.
ATTOSECOND SPECTROSCOPY
has – in its first implementation [8] – drawn on
electrons leaving the atom after their excitation. Intraatomic
motion has been explored by probing outgoing
electrons. Probing the transient population of shakeup
states by means of strong-field-induced tunnelling
ionization offers another means of interrogating
intra-atomic electron dynamics (Figure 6). If lightfield-induced
electron tunnelling occurs as predicted
by Keldysh some four decades ago, the probability of
setting the electron free from these shake-up states
increases in sub-femtosecond steps near the oscillation
peaks of the NIR laser field, where the binding potential
is suppressed. Matthias Uiberacker, Martin Schultze and
Thorsten Uphues, with collaborators from the group of
Karl Kompa, from Bielefeld and Amsterdam, have now
experimentally verified this long-standing theoretical
prediction, the cornerstone of strong-field theories,
by ionizing neon atoms with a 250-as X-ray pulse and
removing the shake-up electron from neon ions by a
138 Max-Planck-Institut für Quantenoptik • Progress Report 2007/2008