Research Highlight
We demonstrate a quantum stroboscope based on a sequence of identical attosecond pulses that are used
to release electrons into a strong infrared laser field exactly once per laser cycle. The resulting
electron momentum distributions are recorded as a function of time delay between the IR laser and the
attosecond pulse train using a velocity map imaging spectrometer.
>> Article in Physical Review Letters
>> Article in Physical Review Focus
Press release:
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Research Overview
We are performing basic research in a very exciting field at the border between atomic and molecular physics and advanced optics,
nonlinear optics and laser physics: high-order harmonic generation in gaseous media exposed to intense laser fields and its applications.
The harmonic spectrum exhibits an extended "plateau" where consecutive (odd) harmonics have approximately the same intensity.
If the harmonics are emitted in phase, i.e. phase-locked, the temporal structure of the radiation emitted from the medium consists of a “train” of
attosecond pulses separated by half the laser period.
Generation of high-order harmonics in a small cell of rare gas (here argon). The laser creates a plasma which emits the white light.
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Principle for the generation of attosecond pulses using high-order harmonic generation. A typical harmonic spectrum is indicated on the left. If the harmonics are synchronized, their beating leads to a train of attosecond pulses (on the right).
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Generation and characterization of attosecond XUV pulses
We have built a robust and flexible setup for the generation, characterization and compression of attosecond pulses
[R. López-Martens et al.]
. We use thin metallic films
or alternatively broadband XUV mirrors, to filter out a broad spectral range (~30 eV) and to synchronize up to ten consecutive harmonics,
thus achieving on target short attosecond pulses. Our toolbox includes pulse trains with central energy varying from 20 to 90 eV and duration typically between 100 and 300 as.
Experimental setup for the generation, characterization and compression of attosecond pulses.
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Compression of attosecond pulses generated in Ne down to 130 attoseconds (red line) using a Zr filter. The central energy of these pulses is 80 eV.
[E. Gustafsson et al.]
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We can also generate pulse trains with only one pulse per cycle, using a combination of two colors to generate the harmonics. Mixing the fundamental and the second harmonic breaks the inversion symmetry,
odd and even harmonics are generated and the pulse train in general comprises only one pulse per cycle. [J. Mauritsson et al.]
Applications of attosecond XUV pulses
An important part of our research consists in developing applications of attosecond pulses. Our vision is to capture and ultimately control the motion of electrons in atoms, molecules and complex systems.
First applications consisted in measuring the phase of electron wave functions and
in controlling and recording the motion of a free electron wavepacket in presence of a laser field
(see highlight above). Another recent experimental result is the observation of a strong modulation of ionization
when He atoms were excited by attosecond pulses with central energy (23 eV) below the ionization energy (24.5 eV) and probed by an infrared laser field.
The interpretation is that consecutive wave packets excited below threshold (in the Coulomb potential) interfere, and
that the probability of ionization by the IR field is strongly affected by this interference phenomenon.
Interferometry.
Depending on the attosecond timing between the laser cycle and their time of creation,
the electrons acquire positive or negative extra momentum from the laser field. The resulting interference pattern
depends on the phase of the initial orbital. [T. Remetter et al.]
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Wave-packet interferences.
This figure shows the effect of multiple wave packet interferences on the ionization yield. [P. Johnsson et al.]
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Harmonic source development and application
Our group is constantly developing the harmonic source to meet the
requirements of demanding applications such as FEL seeding and coherent
imaging. We use the "low-power" arm of the High-Power Laser Facility as
the basis for a high-intensity high-order harmonic source. Pulse energies
of a few hundred nJ @ 30 eV can be produced routinely in Argon gas.
A time-resolved imaging setup based on the digital in-line holography
method is developed on site. The setup is capable of producing single-shot
images with micrometer resolution.
We also carry out feasibility studies for FEL seeding applications in
cooperation with the MAX-lab National Laboratory.
Experimental setup for application of intense high-order harmonics
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Single shot hologram of a scanning microscope tip and and its reconstruction [J. Schwenke et al.]
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