Study: Semiconductor laser shows properties of a Bose-Einstein condensate of photons
The State Research Center OPTIMAS at the University of Kaiserslautern-Landau focuses on promoting research at the interface of optics and materials science. Kaiserslautern theoretical quantum physicist and OPTIMAS member Private Lecturer Axel Pelster is contributing to this with a current research project, which was developed in collaboration with experimental colleagues from the Technische Universität Berlin and the Universities of Łódź and Wrocław. It shows that the light from a laser diode can also have the properties of a Bose-Einstein condensate (BEK) of photons. The corresponding publication has appeared in the renowned journal "Nature Photonics".
Bose-Einstein condensation: atoms versus photons
When bosons are cooled, their indistinguishability leads to impressive quantum effects. In particular, the wave functions of individual bosons overlap in the ground state and form a single macroscopic wave that describes the collective coherent behavior of the particles. This macroscopic quantum phenomenon of Bose-Einstein condensation was realized in 1995 in bosonic alkali atoms in magneto-optical traps at temperatures close to absolute zero. In 2010, the University of Bonn succeeded in doing the same with photons, the quanta of light, in a microcavity filled with dye at room temperature. If one observes the spectrum of the photons leaving the microcavity, it corresponds to a Bose-Einstein distribution. Surprisingly, this is the thermodynamic equilibrium state of an actually open-dissipative many-body quantum system. Since the photons are constantly absorbed and emitted by the dye molecules, the photon gas thermalizes, while at the same time the pumping of the microcavity with a laser compensates for the losses through the mirrors. An experiment on Bose-Einstein condensates of photons in microcavities filled with dye was also recently set up in the Kaiserslautern working group of OPTIMAS member Professor Georg von Freymann. As part of the Collaborative Research Center OSCAR, he and Axel Pelster are investigating the emergent phases of lattice structures of driven, dissipative photon gases in a joint project.
VCSEL
"My experimental colleagues led by Professor Maciej Pieczarka from the Wrocław University of Technology now asked themselves whether such Bose-Einstein condensates of photons could also occur in lasers," reports Axel Pelster. To clarify this, they systematically investigated VCSELs (acronym for Vertical-Cavity Surface-Emitting Lasers). This is a laser diode in which the light is emitted perpendicular to the plane of the semiconductor chip. This special near-infrared semiconductor laser is the key technology for a rapidly growing global market whose turnover is expected to exceed 2 billion euros this year. For example, VCSELs are used in laser printers, fiber optic data transmission, facial recognition for unlocking smartphones, contactless payment and navigation for autonomous vehicles.
Laser or Bose-Einstein condensate?
The core of the VCSEL consists of a quantum well with an n-doped GaAs region located inside a microcavity. Absorption and emission of photons reflected by the two mirrors leads to the creation and annihilation of electrons and holes in the conduction and valence band of the quantum well. The detuning, i.e. the energy difference between the photon energy and the excitation energy of the quantum well, is crucial for the functioning of the VCSEL. It can be changed via the distance between the mirrors, as this determines the photon energy. In the case of negative (positive) detuning, precise experimental investigations show that the light emerging from the VCSEL has the properties of a laser (BEKs). In the case of positive detuning, it is found that so many electron-hole pairs are excited during the lifetime of the photons that effective thermalization to a Bose-Einstein distribution can take place. However, in contrast to the microcavities filled with dye, the resulting spectral temperature is significantly lower than the ambient temperature. A deeper understanding of this astonishing observation is still pending.
For the Bose-Einstein condensation of photons, it is also crucial that the longitudinal component of the wave vector is determined by the boundary conditions at the mirrors, while there are no restrictions in the transverse direction. For sufficiently long-wavelength transverse excitations, the three-dimensional relativistic dispersion relation of massless photons is therefore approximately similar to that of a two-dimensional gas of non-relativistic massive particles. The resulting effective mass corresponds to about one hundred-thousandth of the mass of an electron. By determining the density of states of the photons in the VCSEL experiments, the effective two-dimensionality of the photon gas could be confirmed.
Outlook
The publication discussed here appeared in "Nature Photonics" at the same time as a similar study conducted independently of it, in which the experiments were carried out under the auspices of Imperial College in London. Both research papers impressively demonstrate that photonic Bose-Einstein condensates can also occur in semiconductor lasers. On the one hand, this opens up the possibility of realizing greater photon-photon interaction strengths through a suitable choice of semiconductor material. In the future, it may be possible to demonstrate the superfluidity of a weakly interacting photon gas by observing quantized vortices. On the other hand, the natural integration of a Bose-Einstein condensate of photons into semiconductor technology promises potentially interesting applications.
Bibliographic information on the published study:
Bose-Einstein condensation of photons in a vertical-cavity surface-emitting laser
M. Pieczarka, M. Gębski, A.N. Piasecka, J.A. Lott, A. Pelster, M. Wasiak, and T. Czyszanowski
Nature Photonics, Published online 12. August 2024
https://doi.org/10.1038/s41566-024-01478-z
Bose-Einstein condensation of light in a semiconductor quantum well microcavity
R.C. Schofield, M. Fu, E. Clarke, I. Farrer, A. Trapalis, H.S. Dhar, R. Mukherjee, T.S. Millard, J. Heffernan, F. Mintert, R.A. Nyman, and R.F. Oulton
Nature Photonics, Published online 12. August 2024
https://doi.org/10.1038/s41566-024-01491-2
For further information:
Priv.-Doz. Dr. Axel Pelster
Phone: +49 (0)631 205 2270
E-mail: axel.pelster(at)rptu.de