Room-temperature exciton-polaritons with two-dimensional WS2

Two-dimensional transition metal dichalcogenides exhibit strong optical transitions with significant potential for optoelectronic devices. In particular they are suited for cavity quantum electrodynamics in which strong coupling leads to polariton formation as a root to realisation of inversionless lasing, polariton condensationand superfluidity. Demonstrations of such strongly correlated phenomena to date have often relied on cryogenic temperatures, high excitation densities and were frequently impaired by strong material disorder. At room-temperature, experiments approaching the strong coupling regime with transition metal dichalcogenides have been reported, but well resolved exciton-polaritons have yet to be achieved. Here we report a study of monolayer WS$_2$ coupled to an open Fabry-Perot cavity at room-temperature, in which polariton eigenstates are unambiguously displayed. In-situ tunability of the cavity length results in a maximal Rabi splitting of $\hbar \Omega_{\rm{Rabi}} = 70$ meV, exceeding the exciton linewidth. Our data are well described by a transfer matrix model appropriate for the large linewidth regime. This work provides a platform towards observing strongly correlated polariton phenomena in compact photonic devices for ambient temperature applications.Comment: 12 pages, 6 figure

Polaritons unidimensionnels dans les microfils de Zno (vers la dégénérescence quantique dans les gaz de polaritons unidimensionnels)

Dans cette thèse, nous avons étudié les propriétés expérimentales des polaritons unidimensionnels dans les microfils de ZnO dans le but d'étudier le régime de dégénérescence quantique des polaritons à haute température et en régime de confinement de basse dimensionnalité. ZnO est en effet un matériau semiconducteur à grand gap dans lequel l'exciton bénéficie d'une très forte énergie de liaison qui garantit leur stabilité à température ambiante. D'autre part, la géométrie en "fil" de section hexagonale et de diamètre micrométrique confine les modes photoniques et les rend unidimensionnels. On montre que l'interaction entre l'exciton et ces modes photoniques est en régime de couplage fort, et que les polariton-excitoniques qui en résultent sont eux aussi en régime de confinement unidimensionnel. Cette thèse propose une étude détaillée de la physique de ces polaritons 1D. Dans un premier temps, on démontre que le régime de couplage fort unidimensionnel est conservé jusqu'à température ambiante avec une très grande énergie de Rabi de 300 meV pour une largeur de raie typique 75 fois plus faible. Cette faible largeur de raie, même à température ambiante, est une conséquence inattendue de la grande énergie de Rabi en comparaison de l'énergie maximum des phonons dans ZnO. Cet effet isole très efficacement les polaritons des vibrations thermiques du réseau. Nous nous sommes intéressés aussi à une structure similaire: les microfils de GaN. Dans ces fils, on profite d'une zone fortement dopée pour comparer expérimentalement le spectre en régime de couplage faible et en régime de couplage fort dans le même fil. Nous avons ensuite étudié les propriétés des gaz de polaritons dans les microfils de ZnO sous forte excitation dans le but d'atteindre le régime de dégénérescence quantique 1D. Nous démontrons qu'un régime de laser à polaritons est atteint à basse température en régime de couplage fort dans une situation inédite où les polaritons sont à 97% excitoniques. Cette propriété est comprise grâce à une étude détaillée des propriétés de relaxation des excitons vers les états de polaritons en régime de faible et forte excitations. Cette thèse donne les bases de la compréhension des polaritons unidimensionnels dans les microfils de ZnO. Les propriétés observées montrent que les microfils de ZnO sont particulièrement adaptés à l'étude des gaz de polaritons dégénérés 1D à haute température.In this thesis, we have studied the experimental properties of one-dimensional polaritons in the ZnO microwires in order to study the quantum degenerate regime in a one-dimensional polariton gas at high temperature. ZnO is a wide gap semiconductor material in which the exciton is stable at room temperature thanks to its high binding energy. The "wire" geometry with a micrometric scale hexagonal cross-section results in a one-dimensional confinement of the light. The ZnO excitons and these photonic modes are in the strong coupling regime resulting in new light-matter eigenstates called exciton-polaritons which are, as well, in the one-dimensional confinement regime. This thesis provides a detailed study of the Physics of these 1D polaritons. As a first step, we demonstrate that the one-dimensional strong coupling regime is preserved up to room temperature with a very high Rabi splitting of 300 meV for a typical linewidth 75 times smaller. This small linewidth, even at room temperature, is an unexpected consequence of the high Rabi energy compared to the maximum phonon energy in ZnO. This effect efficiently isolates the polaritons from the thermal fluctuations of the lattice. We have studied as well a similar structure: the GaN microwires. Because of a highly doped part in these wires, it is possible to compare experimentally the spectrum in weak and strong coupling regime in a single wire. We have studied the properties of such polariton gas in ZnO microwires in the high excitation regime in order to reach the 1D quantum degeneracy limit. We have demonstrated that the polariton lasing regime is obtained at low temperature in the strong coupling regime and that it exhibits an unusual situation: the lasing polariton mode is made up of 97% of exciton. This property is understood thanks to a detailed study of the relaxation properties of the excitons towards the polariton states below and above the polariton lasing threshold. This thesis provides the basics to understand the one-dimensional polaritons in ZnO microwires. The properties described in this thesis demonstrate that the ZnO microwires are particularly suitable for the study of 1D degenerate polariton gas at room temperature.SAVOIE-SCD - Bib.électronique (730659901) / SudocGRENOBLE1/INP-Bib.électronique (384210012) / SudocGRENOBLE2/3-Bib.électronique (384219901) / SudocSudocFranceF

Strong exciton-photon coupling in large area MoSe2 and WSe2 heterostructures fabricated from two-dimensional materials grown by chemical vapor deposition

Two-dimensional semiconducting transition metal dichalcogenides embedded in optical microcavities in the strong exciton-photon coupling regime may lead to promising applications in spin and valley addressable polaritonic logic gates and circuits. One significant obstacle for their realization is the inherent lack of scalability associated with the mechanical exfoliation commonly used for fabrication of two-dimensional materials and their heterostructures. Chemical vapor deposition offers an alternative scalable fabrication method for both monolayer semiconductors and other two-dimensional materials, such as hexagonal boron nitride. Observation of the strong light-matter coupling in chemical vapor grown transition metal dichalcogenides has been demonstrated so far in a handful of experiments with monolayer molybdenum disulfide and tungsten disulfide. Here we instead demonstrate the strong exciton-photon coupling in microcavities composed of large area transition metal dichalcogenide/hexagonal boron nitride heterostructures made from chemical vapor deposition grown molybdenum diselenide and tungsten diselenide encapsulated on one or both sides in continuous few-layer boron nitride films also grown by chemical vapor deposition. These transition metal dichalcogenide/hexagonal boron nitride heterostructures show high optical quality comparable with mechanically exfoliated samples, allowing operation in the strong coupling regime in a wide range of temperatures down to 4 Kelvin in tunable and monolithic microcavities, and demonstrating the possibility to successfully develop large area transition metal dichalcogenide based polariton devices