Quantum Photonics and cavity-QED in two-dimensional materials

Transition metal dichalcogenide (TMD) monolayers and their heterostructures, represent a new class of two-dimensional (2D) semiconductors, exhibiting features such as a valley or layer pseudo-spin degree of freedom, ultra-strong Coulomb interactions leading to tightly bound excitons and unprecedented electrical control of optical properties. Aided by an impressive improvement in sample quality and device fabrication techniques, it is now routine to generate a moire superlattice for electrons as well as for excitons, to effect strongly correlated states and to realize re-configurable p-i-n diode structures using electrical gating. We propose to exploit these developments for implementing novel quantum optics experiments in TMD heterostructures. A common theme for many of the projects we propose is the enhancement of polariton interactions to reach the polariton blockade regime. To this end, we will make use of the advances of the forerunner project, including the observation of correlated Mott state of electrons in twisted homobilayer structures, the demonstration of interactions between polaritons that are dressed by electrons to form attractive-polaron (AP)-polaritons in time-resolved pump-probe experiments and tunable quantum confinement of excitons using electric fields.Optical excitation of a single confined electron, termed trion, forms a direct counterpart of a two-level atom. When the electrons form a correlated Mott-Wigner state, we obtain a near perfect lattice of electrons with < 20 nanometer spacing. AP excitations out of this sub-wavelength array of localized electrons realize a moire mirror with enhanced optical nonlinearities. In twisted bilayer structures with coherent hole tunneling, the existence of a layer pseudo-spin degree of freedom allows us to realize electromagnetically induced transparency (EIT): in such a system, each incident photon driving an AP transition of one layer results in inter-layer charge transfer, thereby forming ground-state dipolar excitons with a large Bohr radius. When embedded in a microcavity this EIT scheme would lead to interacting dark-state polaritons and form a solid-state analog of the Rydberg blockade effect. An alternative method to generate strongly interacting optically active dipolar excitons is to use the dc-Stark effect arising from large and inhomogeneous in-plane electric fields to spatially confine neutral excitons. The resulting one-dimensional (1D) excitons should couple nonperturbatively to cavity modes to form 1D polaritons. We estimate a dramatic enhancement of photonic interactions in this system that would allow not only for strong polariton blockade but also for the realization of a Tonks-Girardeau gas of polaritons.In a completely new direction for the group, we propose to explore the effect of cavity-modified electromagnetic vacuum fluctuations on the ground state of a polarizable medium. Our efforts focus on embedding a thin-layer of paraelectric strontium titanate (STO) inside a terahertz cavity. We investigate the theoretically predicted transverse-optical phonon mode softening, leading to a cavity induced paraeletric-to-ferroelectric phase transition.The research we propose to carry out will explore new materials for quantum optics and cavity-QED. Observation of strong polariton blockade effect will constitute a breakthrough not only for efforts aimed at investigating strongly correlated photons but also for the realization of arrays of identical single-photon sources. Exploration of the ultra-strong coupling regime of cavity-QED with STO on the other hand will demonstrate the use of vacuum fluctuations to modify equilibrium material properties in the absence of classical electromagnetic fields.