Strong light-matter coupling in van der Waals materials

In recent years,two-dimensional(2D)van der Waals materials have emerged as a focal point in materials research,drawing increasing attention due to their potential for isolating and synergistically combining diverse atomic layers.Atomically thin transition metal dichalcogenides(TMDs)are one of the most alluring van der Waals materials owing to their exceptional electronic and optical properties.The tightly bound excitons with giant oscillator strength render TMDs an ideal platform to investigate strong light-matter coupling when they are integrated with optical cavities,providing a wide range of possibilities for exploring novel polaritonic physics and devices.In this review,we focused on recent advances in TMD-based strong light-matter coupling.In the foremost position,we discuss the various optical structures strongly coupled to TMD materials,such as Fabry-Perot cavities,photonic crystals,and plasmonic nanocavities.We then present several intriguing properties and relevant device applications of TMD polaritons.In the end,we delineate promising future directions for the study of strong light-matter coupling in van der Waals materials.

Speckle spatial correlations aiding optical transmission matrix retrieval:the smoothed Gerchberg–Saxton single-iteration algorithm

The estimation of the transmission matrix of a disordered medium is a challenging problem in disordered photonics.Usually,its reconstruction relies on a complex inversion that aims at connecting a fully controlled input to the deterministic interference of the light field scrambled by the device.At the moment,iterative phase retrieval protocols provide the fastest reconstructing frameworks,converging in a few tens of iterations.Exploiting the knowledge of speckle correlations,we construct a new phase retrieval algorithm that reduces the computational cost to a single iteration.Besides being faster,our method is practical because it accepts fewer measurements than state-of-the-art protocols.Thanks to reducing computation time by one order of magnitude,our result can be a step forward toward real-time optical imaging that exploits disordered devices.

Polarization shaping of Poincare´ beams by polariton oscillations

We propose theoretically and demonstrate experimentally the generation of light pulses whose polarization varies temporally to cover selected areas of the Poincare´sphere with both tunable swirling speed and total duration(1 ps and 10 ps,respectively,in our implementation).The effect relies on the Rabi oscillations of two polariton polarized fields excited by two counter-polarized and delayed pulses.The superposition of the oscillating fields result in the precession of the Stokes vector of the emitted light while polariton lifetime imbalance results in its drift from a circle of controllable radius on the Poincare´sphere to a single point at long times.The positioning of the initial circle and final point allows to engineer the type of polarization spanning,including a full sweeping of the Poincare´sphere.The universality and simplicity of the scheme should allow for the deployment of time-varying full-Poincare´polarization fields in a variety of platforms,timescales,and regimes.

Microcavity exciton polaritons at room temperature

The quest for realizing novel fundamental physical effects and practical applications in ambient conditions has led to tremendous interest in microcavity exciton polaritons working in the strong coupling regime at room temperature.In the past few decades,a wide range of novel semiconductor systems supporting robust exciton polaritons have emerged,which has led to the realization of various fascinating phenomena and practical applications.This paper aims to review recent theoretical and experimental developments of exciton polaritons operating at room temperature,and includes a comprehensive theoretical background,descriptions of intriguing phenomena observed in various physical systems,as well as accounts of optoelectronic applications.Specifically,an in-depth review of physical systems achieving room temperature exciton polaritons will be presented,including the early development of ZnO and GaN microcavities and other emerging systems such as organics,halide perovskite semiconductors,carbon nanotubes,and transition metal dichalcogenides.Finally,a perspective of outlooking future developments will be elaborated.

Superluminal X-waves in a polariton quantum fluid

In this work,we experimentally demonstrate for the first time the spontaneous generation of two-dimensional exciton-polariton X-waves.X-waves belong to the family of localized packets that can sustain their shape without spreading,even in the linear regime.This allows the wavepacket to maintain its shape and size for very low densities and very long times compared to soliton waves,which always necessitate a nonlinearity to compensate the diffusion.Here,we exploit the polariton nonlinearity and uniquely structured dispersion,comprising both positive-and negative-mass curvatures,to trigger an asymmetric four-wave mixing in momentum space.This ultimately enables the self-formation of a spatial X-wave front.Using ultrafast imaging experiments,we observe the early reshaping of the initial Gaussian packet into the X-pulse and its propagation,even for vanishingly small densities.This allows us to outline the crucial effects and parameters that drive the phenomena and to tune the degree of superluminal propagation,which we found to be in close agreement with numerical simulations.

High-speed flow of interacting organic polaritons

The strong coupling of an excitonic transition with an electromagnetic mode results in composite quasi-particles called exciton polaritons,which have been shown to combine the best properties of their individual components in semiconductor microcavities.However,the physics and applications of polariton flows in organic materials and at room temperature are still unexplored because of the poor photon confinement in such structures.Here,we demonstrate that polaritons formed by the hybridization of organic excitons with a Bloch surface wave are able to propagate for hundreds of microns showing remarkable third-order nonlinear interactions upon high injection density.These findings pave the way for the study of organic nonlinear light–matter fluxes and for a technologically promising route of the realization of dissipation-less on-chip polariton devices operating at room temperature.

Quantum hydrodynamics of a single particle

Semiconductor devices are strong competitors in the race for the development of quantum computational systems.In this work,we interface two semiconductor building blocks of different dimensionalities with complementary properties:(1)a quantum dot hosting a single exciton and acting as a nearly ideal single-photon emitter and(2)a quantum well in a 2D microcavity sustaining polaritons,which are known for their strong interactions and unique hydrodynamic properties,including ultrafast real-time monitoring of their propagation and phase mapping.In the present experiment,we can thus observe how the injected single particles propagate and evolve inside the microcavity,giving rise to hydrodynamic features typical of macroscopic systems despite their genuine intrinsic quantum nature.In the presence of a structural defect,we observe the celebrated quantum interference of a single particle that produces fringes reminiscent of wave propagation.While this behavior could be theoretically expected,our imaging of such an interference pattern,together with a measurement of antibunching,constitutes the first demonstration of spatial mapping of the self-interference of a single quantum particle impinging on an obstacle.