The investigation of the interaction of matter with laser pulses by now has reached a level at which fundamental aspects such as the quantum nature of both light and matter, relativity and couplings among the involved particles have become key issues and substantial challenges alike. Theory helps to explore the effects of extremely strong fields which are not yet reached in current experiments but may be available in the near future.

When intense laser fields interact with matter, the electrons strongly couple to the outer field and thus efficiently absorb energy from the field. Thereby they can become so fast that the relativistic effects play an important role. This requires the search for solutions of the time-dependent Schrödinger and Dirac equations, including electron-electron correlations. In particular, the electrons of neighbouring atoms can be coupled causing interesting phenomena like energy transfer reactions in molecules or quantum gases.

Collective effects in atomic systems exposed to intense laser fields are mediated via freed electrons. Driven back and forth by the intense field, they can convert their energy into coherent ultrashort soft X-ray pulses when recolliding with their parent ion. At very high intensities the electron is driven away from the atom by the “light pressure” preventing the recollision. New theoretical concepts with counter-propagating laser pulses circumvent this effect and enable the generation of even more energetic X-ray pulses.
Tightly focused, extremely strong crossed laser beams permit the direct acceleration of light atomic ions, that are released from a target and pre-accelerated by a laser beam. The resulting ion beam reaches the intensity, energy and quality as required for medical applications.

The great success of quantum optical methods with atoms in the optical frequency regime prompts the question whether similar techniques could also be applied to nuclei. With new light sources such as the X-ray free-electron laser (XFEL) and efficient acceleration facilities coming into reach, nuclear quantum dynamics and nuclear quantum optics are becoming increasingly relevant. This on the one hand would promote preparation, control, and detection in nuclear physics, but on the other hand would open the door for coherent and non-classical effects in X-ray science. Theoretical studies explore collective effects that may provide novel X-ray sources and methods to control their properties.

Another topic is the temporal evolution of quark-gluon plasmas (QGP), that can be created in high-energy collisions for an extremely short time at the size of a nucleus. The QGP emits gamma rays, but at some intermediate time not in all directions so that double pulses may be observed. This state of matter is similar to that of the Universe right after the Big Bang where temperatures were so high, that the constituents of atomic nuclei, the protons and neutrons, were split into their constituents, the quarks and gluons.

Division Keitel

Theory of Collective and Relativistic Quantum Dynamics in Strong Laser Fields (pdf)

In the Institute’s laser laboratories, phase-controlled laser pulses as short as 5 fs at intensities of up to about 10¹⁶ W/cm² are routinely available for experiments. Even shorter pulses of some attoseconds (1 as = 10^–18 s) duration are generated by special nonlinear optical techniques. The resulting coherent high-harmonic radiation in the extreme UV range can then be combined with broadband infrared/visible pulses from the main Ti:Sapphire laser.

Isolated as well as double and triple attosecond pulses are produced and used to probe gaseous atomic and molecular samples by interferometric methods. In pump-probe measurements, the time delay between two pulses can be precisely adjusted on attosecond time scales. Combined with spectroscopy or imaging detectors, this allows for direct and time-resolved observation (and control) of nuclear and electronic quantum motion in chemical reactions. The MPIK is one of the five MPG core-partners of the “Max Planck Centre for Attosecond Science”.

In addition, MPIK researchers use the ultraviolet and X-ray pulses of the free-electron lasers in Hamburg (FLASH), Japan (SCSS) and Stanford (LCLS). They also contribute to the future extreme light infrastructure, ELI.

Division Ullrich    Group Pfeifer

Ultra-Short Laser Pulses: "Chemistry" in Slow Motion (pdf)
Free-Electronen Lasers: Physics with Ultra-Short and Super-Brilliant X-Ray Pulses (pdf)