Quantum Physics in Strong External Fields

The interaction of electromagnetic radiation with matter marked the starting point for the development of quantum theory a century ago. Nowadays the interaction of photons with quantum systems is still a very active research area, due to the availability of high-intensity lasers from the near-infrared to the x-ray frequency range. They allow to explore novel phenomena in atomic physics, where the application of lasers has a long and fruitful tradition, and also to build bridges into other areas of physics which are characterized by higher energy scales, such as nuclear physics or quantum electrodynamics. The enormous photon densities in high-intensity laser pulses enable electrons to interact with many laser photons simultaneously, giving rise to largely enhanced energy absorption from the field via multiphoton processes. Against this background, our theory group studies quantum effects in strong laser fields from a broad variety of physical areas. Apart from the dense photonic environments that are created by powerful lasers, we are also interested in correlated quantum processes that occur in dense atomic environments.

a) Interatomic Phenomena and Dynamic Electron Correlations

Fundamental atomic processes, such as photoionization or recombination, can be modified substantially when a neighbor atom at relatively close distance is present. For example, ionization can proceed through an indirect pathway then, where the neighbor atom - which belongs to a different atomic species - is first resonantly photoexcited and, afterwards, transfers the excitation energy radiationlessly to the other atom, leading to its ionization (see Fig. 1). The latter step, representing a two-center Auger process, is commonly known as interatomic Coulombic decay. Due to its resonant character, the two-center ionization mechanism can be remarkably strong and even dominate over the usual direct ionization. Photoionization and related interatomic phenomena have been studied intensively in recent years, both theoretically and experimentally. They allow to probe dynamic two-center electron correlations on microscopic length and very short time scales. Our group is particularly interested in interatomic ionization, recombination and scattering phenomena and develops corresponding theoretical descriptions [1-3].


Figure 1: Scheme of two-center resonant photoionization (from [2]).

[1] A. Eckey, A.Jacob, A. B. Voitkiv and C. Müller, Resonant electron scattering and dielectronic recombination
     in two-center atomic systems, Phys. Rev. A 98, 012710 (2018)
[2] B. Najjari, A. B. Voitkiv und C. Müller, Two-center resonant photoionization, Phys. Rev. Lett. 105, 153002 (2010)
[3] C. Müller, A. B. Voitkiv, J. R. Crespo López-Urrutia und Z. Harman, Strongly enhanced recombination
     via two-center electronic correlations, Phys. Rev. Lett. 104, 233202 (2010)

b) Spin Dynamics in Intense Laser Fields

When an electron is subject to a laser field, its spin performs a precessive motion. However, after the laser pulse has passed, the electron spin returns to its initial orientation. Permanent spin transitions can be induced by counterpropagating laser beams through the Kapitza-Dirac effect [6]. Bichromatic laser fields turn out to be particularly useful, where the electron spin can be controlled at moderate intensities by a proper choice of the polarization geometry of the fields [5]. A suitable arrangement of three pairs of counterpropagating laser fields even allows to build a spin-polarizer for free electrons [4] (see Fig. 2). Noteworthy, the possible existence of a Stern-Gerlach-like device for free (rather than bound) electrons has been under debate since the early days of quantum mechanics. Photoionization in high-intensity laser fields can be sensitive to the electron spin, as well. The characteristic antisymmetry of the wave function plays a role in the strong-field ionization of two-electron systems like helium [7], and interesting spin dynamics occurs in electron removal from highly-charged ions in super-intense laser fields [8].


skizze fig

Figure 2: Arrangement of laser waves that acts as a spin-polarizing interferometric beam splitter
for free electrons – in close similarity to a Stern-Gerlach magnet (from [4]).


[4] M. M. Dellweg and C. Müller, Spin-polarizing interferometric beam splitter for free electrons,
     Phys. Rev. Lett. 118, 070403 (2017)
[5] M. M. Dellweg and C. Müller, Controlling electron spin dynamics in bichromatic Kapitza-Dirac scattering
    by the laser field polarization, Phys. Rev. A 95, 042124 (2017) [Editor's Suggestion]
[6] S. Ahrens, H. Bauke, C. H. Keitel, C. Müller, Spin Dynamics in the Kapitza-Dirac Effect
     Phys. Rev. Lett. 109, 043601 (2012)
[7] M. Klaiber, E. Yakaboylu, C. Müller, H. Bauke, G. G. Paulus and K. Z. Hatsagortsyan, Spin dynamics in
     relativistic ionization with highly charged ions in super-strong laser fields, J. Phys. B 47, 065603 (2014)
[8] D. Zille, D. Seipt, M. Möller, S. Fritzsche, S. Gräfe, C. Müller and G. G. Paulus,
     Spin-dependent rescattering in strong-field ionization of helium, J. Phys. B 50, 065001 (2017)

c) Laser-induced Decay of the Quantum Vacuum

In laser fields of extremely high intensity, the quantum vacuum can decay into electron-positron pairs. This process represents a striking example of Einstein‘s equivalence between mass and energy, since the purely electromagnetic energy of (many) photons is transformed into matter. For sizeable pair production probabilities, the laser field strength must approach the critical scale where the electric work performed by the laser field on an elementary charge along a Compton wavelength is of the order of the electron rest energy. While the highest fields that can nowadays be reached in laboratory still lie substantially below the critical value, it is possible to enhance the pair yields by applying suitable field configurations (e.g. short-pulse [9] or bichromatic fields [10]) or combining high-intensity laser fields with relativistic particle beams [11]. Our theoretical predictions can help paving the way for corresponding experiments at upcoming high-field laser facilities. - Analogies to QED vacuum decay at much lower energies can be tested in special condensed-matter systems like graphene where electrons in the valence band behave like relativistic particles. The closest similarity is achieved in bandgap graphene where the Dirac-like quasiparticles possess a non-zero mass [12].


Figure 3: „Fingerprint of the quantum vacuum“: Momentum distribution
of positrons produced in a bichromatic electric field (from [10b]).


[9] M. J. A. Jansen and C. Müller, Strong-field Breit-Wheeler pair production in short laser pulses:
     Identifying multiphoton interference and carrier-envelope-phase effects, Phys. Rev. D 93, 053011 (2016)
[10] M. J. A. Jansen and C. Müller, Strongly enhanced pair production in combined high- and low-frequency
      laser fields, Phys. Rev. A 88, 052125 (2013); I. Akal, S. Villalba-Chavez and C. Müller, Electron-positron
      pair production in a bifrequent oscillating electric field, Phys. Rev. D 90, 113004 (2014)
[11] K. Krajewska, C. Müller, and J. Z. Kaminski, Bethe-Heitler pair production in ultrastrong short laser pulses,
      Phys. Rev. A 87, 062107 (2013); S. Augustin and C. Müller, Interference effects in Bethe-Heitler pair creation
      in a bichromatic laser field, Phys. Rev. A 88, 022109 (2013) [Editor's Suggestion]
[12] I. Akal, R. Egger, C. Müller and S. Villalba-Chavez, Low-dimensional approach to pair production in
      an oscillating electric field: Application to bandgap graphene layers, Phys. Rev. D 93, 116006 (2016)

d) Optical Signatures of Hidden Particles

Some advanced theories of particle physics predict the existence of new very light particles, which hitherto have remained unobserved because they interact only very weakly. Examples are axions (or axion-like particles), minicharged particles and paraphotons (see Fig. 4). Considerable experimental efforts to search for these hypothetical particles are being carried out worldwide. High-precision polarimetric techniques are applied since the existence of hidden particles would modify the birefringence and dichroism of the quantum vacuum which is polarized by a strong external field. We explore the discovery (or exclusion) potential that high-intesity laser pulses might have in the search for axion-like particles, minicharges and paraphotons [13-15]. Corresponding studies impose constraints on the allowed range of particle parameters, such as their mass and charge. Nonlinear optics strong external fields thus represents a complementary low-energy route to unravel the basic building blocks of matter in our universe.


Figure 4: Photon-paraphoton oscillation mediated by
a vacuum polarization loop formed by virtual minicharged particles.


[13] S. Villalba-Chavez, T. Podszus and C. Müller, Polarization-operator approach to optical
      signatures of axion-like particles in strong laser pulses, Phys. Lett. B 769, 233 (2017)
[14] S. Villalba-Chavez, S. Meuren and C. Müller, Minicharged particles search by
      strong laser pulse-induced vacuum polarization effects, Phys. Lett. B 763, 445 (2016)
[15] S. Villalba-Chavez and C. Müller, Light dark matter candidates in intense laser pulses I:
      paraphotons and fermionic minicharged particles, JHEP 06, 177 (2015)