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Cun-Zheng Ning (Shenzhen, China):
Haken’s quantum field theoretical understanding of semiconductors and lasers and its present-day impact
Prof. Haken was among the earliest few who applied the then-new
quantum field theory (QFT) to understand physical processes in semiconductors in the 1950s and lasers in the 1960s. The first decade of
his scientific career was devoted to the QFT treatment of non-metallic
solids. His long-lasting impacts are reflected by popular terms such as
the Haken Potential for excitons and Feynman-Haken Path Integral for
calculating the ground state energy of polarons. The second decade
of his career started at Stuttgart. It was devoted to the newly invented laser whose fundamental understanding, as he quickly realized,
required extending the known QFT to include noise and dissipation.
In the process, he established the full quantum theory for open
systems and laid the foundation for Synergetics. His laser theory not only
explained or predicted many phenomena in lasers but also provided a
general framework for the understanding of problems whenever light-matter interaction is involved. While his first two decades focused
on the QFT treatment of semiconductors or light field respectively,
a proper description of semiconductor optics requires the QFT treatment of both semiconductors and optical field self-consistently. This
task turns out to be as challenging as it is rewarding when Coulomb
interaction is included and remains an active field of research today,
continued by generations of his students. This talk will cover aspects
of Prof. Haken’s early contributions and some recent progress.
Maciej Pieczarka (Wroclaw, Poland):
Bose-Einstein condensation of photons in vertical-cavity
surface-emitting lasers
Professor Haken pioneered the development of the quantum theory of
lasers and discovered that lasing action can be viewed as a
nonequilibrium second-order phase transition. This visionary and broader view
inspired many to find a link between lasing and the Bose-Einstein condensation (BEC) of photons. It appears that the worlds of lasers and
BEC are deeply intertwined, as BEC was found in dye-filled microcavities [1] and, more recently, in semiconductor lasers [2].
I will present our demonstration of photon BEC phase transition in
a real-world device - a Vertical-Cavity Surface-Emitting Laser (VCSEL) [2]. Besides distinctive differences from the complete thermal
equilibrium, we show that photons in a VCSEL follow the equation
of state for an ideal bosonic gas. We argue that photon BEC can be
a much more common phenomenon in laser physics than previously
anticipated.
[1] J. Klaers et al., Nature 468, 545 (2010).
[2] M. Pieczarka et al., Nature Photonics 18, 1090 (2024).
Milan Radonjic (Hamburg, Germany):
Photons in a dye-filled cavity: quantum-optical system interpolating between Bose-Einstein condensates and laser-like
states
It is well known that photons in a dye-filled cavity exhibit a Bose-Einstein condensate (BEC) of light [1]. We generalize the microscopic
non-equilibrium Keeling-Kirton model [2] of such a system by carefully considering the interplay of coherent and dissipative dynamics
within the Lindblad master equation framework pioneered by Hermann Haken in his theory of lasers [3]. The resulting equations of
motion of both photonic and matter degrees of freedom are then used
to study the steady state properties of the system. We demonstrate
that this system can interpolate between photon BEC and laser-like
states, depending on whether the dissipative or coherent influence of
the environment is dominant [4]. In the former case, we show that the
cavity modes of different energies are essentially uncorrelated. In the
laser-like regime, some cavity mode acquires macroscopic occupation,
while the populations of other cavity levels strongly deviate from the
Bose-Einstein distribution. Additionally, the steady state contains a
rather high degree of correlations between the different cavity modes.
[1] J. Klaers, J. Schmitt, F. Vewinger, and M. Weitz, Nature 468, 545 (2010).
[2] P. Kirton and J. Keeling, Phys. Rev. Lett. 111, 100404 (2013).
[3] H. Haken, Laser Theory (Springer-Verlag, Berlin, 1970, 1984).
[4] M. Radonjic, W. Kopylov, A. Balaz, and A. Pelster, New J. Phys. 20, 055014 (2018).
Eckehard Schöll (Berlin, Germany):
From laser physics to nonlinear dynamics and synergetics
Hermann Haken was a pioneer of laser physics and developed the first
full quantum theory of the laser [1]. He interpreted the laser transition
as a nonequilibrium phase transition [2], and found that this is a special
case of a much wider class of open systems driven far from thermodynamic equilibrium. Based upon this observation he founded the field
of synergetics which deals with systems composed of many subsystems
like atoms, molecules, photons, cells, etc., and shows that cooperation
of the subsystems leads to spatial, temporal, or functional structures
by self-organization [3]. He demonstrated that the semiclassical laser
equations are mathematically equivalent to the Lorenz equation derived from fluid dynamics [4], exhibiting higher instabilities and chaos,
like many other nonlinear dynamical systems in physics, chemistry, biology, medicine, and even economics, sociology and psychology. This
has given rise to a plethora of new phenomena in nonequilibrium system widely studied during the past five decades. Coherence resonance
is just one example which was first discovered by Haken [5], and later
studied in various systems ranging from lasers to the brain.
[1] H. Haken, Laser Theory, Springer (1970, 1984).
[2] R. Graham, and H. Haken, Z. Phys. 237, 31 (1970).
[3] H. Haken, Synergetics, An Introduction, Springer (1977).
[4] H. Haken, Phys.Lett. 53A,77 (1975).
[5] G. Hu, T. Ditzinger, C.-Z. Ning, and H. Haken, Phys. Rev. Lett. 71, 807 (1993).
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Picture Gallery
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The gallery depicts some highlights of Session Q 34 at the DPG Spring Meeting in Bonn on March 12, 2025. The 13 pictures stem
from Sejung Yong.
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