Towards stimulated cyclotron emission in Dirac systems

Benjamin BENHAMOU

The use of terahertz (THz) radiation is rapidly expanding, from fundamental research to applications in communications, security, and non-destructive testing. However, a major technological gap persists as no compact solid-state source can continuously cover the 1–5 THz range. This PhD addresses this challenge by exploring the physical properties of Dirac materials and the fundamental mechanisms that could enable such new THz sources. The experimental work focuses on magneto-spectroscopic studies of semiconductor heterostructures, particularly cyclotron (Landau) emission. While magneto-absorption spectroscopy probes transitions between Landau levels via photon absorption, Landau emission involves radiative recombination from higher to lower Landau levels (LLs). This mechanism enabled the only LL laser ever demonstrated -the p-Ge laser-capable of tunable THz emission through magnetic field. However, its reliance on strong fields and cryogenic conditions limits its use to laboratory applications. HgTe-based quantum wells (QWs), discovered in the 2000s as Dirac and topological materials, exhibit strong Landau emission across the full THz spectrum at much lower magnetic fields. The first achievement of this PhD was to show that the Dirac phase allows electrical tuning of the emission frequency via gate-controlled carrier density modulation. Further experiments under intense crossed electric and magnetic fields aimed to explore streaming or pseudo-relativistic regimes, but instead revealed the strong non-parabolicity of the band structure through hot-carrier dynamics. This behavior underlined a key limitation of II–VI semiconductors as their sensitivity to temperature and fabrication constraints makes it difficult to reach the strong electric fields required for population inversion. An alternative path to stimulated emission lies in exploiting strong light–matter coupling, leading to hybrid quasiparticles known as polaritons. These can condense into a single quantum state and emit coherent light upon decay. A THz Fabry–Perot cavity was thus designed, resonant with the cyclotron transition of HgTe QWs, to generate Landau polaritons. Their formation was confirmed by magneto-reflectivity experiments. We subsequently demonstrated Landau-polariton electroluminescence and showed that the system operates close to the lasing threshold. Increasing the cavity quality factor, while maintaining small effective mode volumes, was identified as the most promising route toward polariton lasing and a compact THz emitter. Finally, the work expands to III–V semiconductor systems, which are less temperature-sensitive, technologically mature, and can support high carrier mobilities. Landau emission was observed in InSb- and GaAs-based QWs, and an exotic InAs/GaInSb superlattice was studied. This system hosts a three-dimensional Dirac phase and emerges as a highly promising platform for developing a Landau-level laser capable of bridging the longstanding THz gap.