Understanding the physical fundamentals of photon Bose-Einstein condensates in semiconductor optical microcavities

• Applicant: Privatdozent Dr. Axel Pelster
• Subject Area: Optics, Quantum Optics and Physics of Atoms, Molecules and Plasmas ; Theoretical Condensed Matter Physics
• Term: since 2025
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 563751692

The proposed project aims to push the frontier of understanding the physical fundamentals of photon Bose-Einstein condensates (BECs) in the platform of semiconductor laser microcavities. Based on the synergy between state-of-the-art experiments in Wrocław and theory in Kaiserslautern, the binational project tackles key questions of this new research field in the realm of quantum fluids of light: Explain why spectral temperatures measured as a function of the driving current deviate from the device temperature. Determine how strong photons in semiconductor microcavities effectively interact and unravel the physical nature of this interaction. Investigate whether the photon BEC in semiconductor microcavities can be understood within the Berezhinskii-Kosterlitz-Thouless or the Kardar-Parisi-Zhang universality classes. Determine the timescales describing the dynamics of a photon BEC in a quench experiment. Work out microscopic and mean-field models to understand why photon BECs can occur in vertical-cavity surface-emitting lasers (VCSELs). These goals are achieved with a continuous exchange between theory and experiment. In the experimental part near- and far-field spectroscopic methods as well as coherence measurements are used. Condensates are prepared in box potentials in current-driven experiments. And a quench experiment is performed by using a semiconductor optical amplifier to design an excitation ramp across the critical point in order to test the universal dynamical behaviour. In the theoretical part a Lindblad master equation for the coherent and dissipative dynamics of photonic and matter degrees of freedom is derived. Approximate solutions determine steady-state and dynamic properties interpolating between photon BEC and laser-like behaviour. And this microscopic modelling also allows to deduce stochastic Gross-Pitaevskii equations and rate equations, which leads a simplified mean-field description of these open-dissipative systems. In summary, the project addresses timely questions at the research frontier of photon BEC. It will provide a theoretical framework to describe photon BECs in semiconductors and their thermodynamic as well as nonequilibrium properties are measured. With this the project expands the understanding of photonic condensates based on the new semiconductor platform. And with tackling the question of the photon interaction strength in this system it works towards achieving photonic superfluidity.

• DFG Programme: Research Grants
• International Connection: Poland, United Kingdom
• Cooperation Partners: Professor Dr. Maciej Pieczarka; Professorin Dr. Marzena Szymanska