Mediated Interactions and Photon Bound States in an Exciton-Polariton Mixture

Non-Perturbative Determination of Isotope-induced Anomalous Vibrational Physics

In general, vibrational physics has been well described by quantum perturbation theory to provide footprint characteristics for common crystals. However, despite weak phonon anharmonicity, the recently discovered cubic crystals have shown anomalous vibrational dynamics with elusive fundamental origin. Here, we developed a non-perturbative ab initio approach, in together with spectroscopy and high-pressure experiments, to successfully determine the exact dynamic evolutions of the vibrational physics for the first time. We found that the local fluctuation and coupling isotopes significantly dictate the vibrational spectra, through the Brillouin zone folding that has been previously ignored in literature. By decomposing vibrational spectra into individual isotope eigenvectors, we observed both positive and negative contributions to Raman intensity from constitutional atoms (10B, 11B, 75As or 31P). Importantly, our non-perturbative theory predicts that a novel vibrational resonance appears at high hydrostatic pressure due to broken translational symmetry, which was indeed verified by experimental measurement under a pressure up to 31.5 GPa. Our study develops fundamental understandings for the anomalous lattice physics under the failure of quantum perturbation theory and provides a new approach in exploring novel transport phenomena for materials of extreme properties.

Strongly Interacting Bose Polarons in Two-Dimensional Atomic Gases and Quantum Fluids of Polaritons

Polarons are quasiparticles relevant across many fields in physics: from condensed matter to atomic physics. Here, we study the quasiparticle properties of two-dimensional strongly interacting Bose polarons in atomic Bose–Einstein condensates and polariton gases. Our studies are based on the non-self consistent T-matrix approximation adapted to these physical systems. For the atomic case, we study the spectral and quasiparticle properties of the polaron in the presence of a magnetic Feshbach resonance. We show the presence of two polaron branches: an attractive polaron, a low-lying state that appears as a well-defined quasiparticle for weak attractive interactions, and a repulsive polaron, a metastable state that becomes the dominant branch at weak repulsive interactions. In addition, we study a polaron arising from the dressing of a single itinerant electron by a quantum fluid of polaritons in a semiconductor microcavity. We demonstrate the persistence of the two polaron branches whose properties can be controlled over a wide range of parameters by tuning the cavity mode.

Moiré-Induced Optical Nonlinearities: Single- and Multiphoton Resonances

Moiré excitons promise a new platform with which to generate and manipulate hybrid quantum phases of light and matter in unprecedented regimes of interaction strength. We explore the properties in this regime, through studies of a Bose-Hubbard model of excitons coupled to cavity photons. We show that the steady states exhibit a rich phase diagram with pronounced bistabilities governed by multiphoton resonances reflecting the strong interexciton interactions. In the presence of an incoherent pumping of excitons we find that the system can realize single- and multiphoton lasers.

Polaritons in an Electron Gas—Quasiparticles and Landau Effective Interactions

Two-dimensional semiconductors inside optical microcavities have emerged as a versatile platform to explore new hybrid light–matter quantum states. A strong light–matter coupling leads to the formation of exciton-polaritons, which in turn interact with the surrounding electron gas to form quasiparticles called polaron-polaritons. Here, we develop a general microscopic framework to calculate the properties of these quasiparticles, such as their energy and the interactions between them. From this, we give microscopic expressions for the parameters entering a Landau theory for the polaron-polaritons, which offers a simple yet powerful way to describe such interacting light–matter many-body systems. As an example of the application of our framework, we then use the ladder approximation to explore the properties of the polaron-polaritons. Furthermore, we show that they can be measured in a non-demolition way via the light transmission/reflection spectrum of the system. Finally, we demonstrate that the Landau effective interaction mediated by electron-hole excitations is attractive leading to red shifts of the polaron-polaritons. Our work provides a systematic framework to study exciton-polaritons in electronically doped two-dimensional materials such as novel van der Waals heterostructures.

Attractive and Repulsive Exciton-Polariton Interactions Mediated by an Electron Gas

Realizing strong photon-photon interactions in a solid-state setting is a major goal with far reaching potential for optoelectronic applications. Using Landau's quasiparticle framework combined with a microscopic many-body theory, we explore the interactions between exciton-polaritons and trions in a two-dimensional semiconductor injected with an electron gas inside a microcavity. We show that particle-hole excitations in the electron gas mediate an attractive interaction between the polaritons, whereas the interaction between trions and polaritons mediated by the exchange of an electron can be either repulsive or attractive. These mediated interactions are intrinsic to the quasiparticles and are also present in the absence of light. Importantly, they can be tuned to be more than an order of magnitude stronger than the direct polariton-polariton interaction in the absence of the electron gas, thereby providing a promising outlook for nonlinear optical components. Finally, we compare our theoretical predictions to two recent experiments.