Collective states in semiconductors

We explore a broad variety of quantum collective states, from nuclear and electronic spins to indirect excitons and polaritons, and investigate their collective dynamics, correlations, elementary excitations and phase transitions.

Compared to their constituting individual particles (spins, excitons, polaritons), these collective states are shaped by particle-particle interactions, quantum statistics, and/or connections with a reservoir.

They may be driven into exotic equilibrium states, or strongly out-of-equilibrium schemes. We have developed a broad variety of non-linear spectroscopy techniques, and hyperspectral imaging, allowing us to access their spatial and temporal dynamics.

These activities are funded by :

• IXTASE (ANR project, 2021-2024) : Indirect eXciTons for emerging quAntum StatEs
• DINAMITE (“Quantum Technology in Region Occitanie” project, 2021-2024) : DIpolar excitons hosted by Nitride-bAsed heterostructures for eMergIng quantum staTEs funded PhD student : Rémi Aristégui
• NEWAVE (ANR project, 2022-2025) : New concepts for waveguide micro- and nano-lasers
• Comb-on-GaN (Labex Ganext project, 2020-2024) : Integrated photonics for fluids of light, from visible to near-infrared

Indirect excitons :

Indirect excitons in coupled quantum wells are long-living quasiparticles, explored in the studies of collective quantum states. At low density, decreasing the temperature of the exciton liquid leads to BEC. At higher densities and low temperatures a phase separation is predicted between Bose-Einstein and Bardeen-Cooper-Schrieffer -like condensates.

The figure shows the phase diagram of an electron-hole system in temperature/density plane. Left and bottom scales stand for GaAs-based systems, right and upper scale for GaN and ZnO.

Room temperature polariton lasers and polariton condensates :

Polariton condensates in microcavities are a rich model system for the physics of quantum collective states, inspired by atomic condensates. GaN and ZnO are semiconductor materials with strong excitons, in terms of oscillator strength and binding energy. They are thus particularly interesting for the generation and control of polariton condensates, which we study at room temperature.

Achieving high-quality planar ZnO microcavities has long been a significant challenge. As early as 2011, we realized the first ZnO polariton laser (at T=120K) in a hybrid microcavity (ZnO active layer, dielectric mirrors, and nitride on silicon, Q=450). In a radically different approach, we demonstrated polariton condensation at room temperature in a ZnO microcavity with dielectric mirrors (Q>2000, with a Rabi splitting of 250 meV). We measured the complete phase diagram of the polariton laser and studied new microcavities on silicon substrates.

More recently, we performed 2D imaging, both near-field and far-field, of the ballistic propagation of the polariton condensate, which we compared to numerical simulations of the spatial dynamics of the condensate.

Currently, we are exploring a new geometry: the polariton waveguide. This effort aims to achieve a polariton laser operating under electrical injection at room temperature and to control the formation of condensates that generate optical soliton pulses.