Exciton-polaritons (EP), half-light half-matter quasiparticles that form in optical cavities, are attractive platforms for creating macroscopic coherent states like BECs. EPs based on organic molecules are of particular interest for realizing such states at room temperature while offering the promise of synthetic tunability. However, the demonstrations of such condensates have been limited to a few specific molecular systems1. Here we report a universal platform for realizing molecular polariton condensates using commercial dyes that solves long standing material challenges. This solution is made possible using a new and programable molecular material called small-molecule, ionic isolation lattices (SMILES) with the potential to incorporate a wide array of molecular fluorophores2. We show EP condensation in rhodamine by incorporating it into SMILES lattice placed in a planar microcavity. The SMILES approach overcomes the major drawbacks of organic molecular photophysical systems such as self-quenching, which sets the foundation for realizing practical polaritonic devices operating at ambient temperatures covering wide spectral range.

Coherent control and manipulation of quantum degrees of freedom such as spins forms the basis of emerging quantum technologies. In this context, the robust valley degree of freedom and the associated valley pseudospin found in two-dimensional transition metal dichalcogenides is a highly attractive platform. Valley polarization and coherent superposition of valley states have been observed in these systems even up to room temperature. Control of valley coherence is an important building block for the implementation of valley qubit. Large magnetic fields or high-power lasers have been used in the past to demonstrate the control (initialization and rotation) of the valley coherent states. Here we demonstrate control of layer-valley coherence via strong coupling of valley excitons in bilayer WS2 to microcavity photons by exploiting the pseudomagnetic field arising in optical cavities owing to the TE-TM splitting. The use of photonic structures to generate pseudomagnetic fields which can be used to manipulate exciton-polaritons presents an attractive approach to control optical responses without the need for large magnets or high intensity optical pump powers.

In addition to their attractive technological applications in photovoltaics and light emitters, the perovskite family of semiconductors has recently emerged as an excellent excitonic material for fundamental studies. Specifically, the 2D hybrid organic-inorganic perovskite (HOIP) offers the added advantage of room temperature investigations owing to their large exciton binding energy. In this work, we strongly couple excitons in 2D HOIP crystals to planar microcavity photons sustaining exciton-polaritons under ambient conditions resulting in a Rabi splitting of 290 meV. Dark excitons directly pump the polariton branch along its dispersion in resonance with the Stokes shifted emission state (radiative pumping), creating a high density of polaritons at higher in-plane momentum ( k || ). We further probe the nonlinear polariton dispersion dynamics at varying input laser fluence, which indicates efficient polariton-polariton scattering and decay to k || = 0 from higher k || . The observation of Stokes shift-assisted energy exchange of dark states with lower polaritons coupled with evidence of efficient polariton-polariton scattering makes 2D HOIPs an attractive platform to study exciton-polariton many-body physics and Bose-Einstein like condensation (BEC) at room temperature.

We investigate the role of radiative pumping and cavity Q factor in stimulated scattering and room temperature cooling of exciton-polaritons in a red fluorescent protein coupled to a planar microcavity approaching a condensate phase transition.

Transition metal dichalcogenides (TMDCs) have been in the limelight for the past decade as a candidate for several optoelectronic devices and as a versatile test bed for various fundamental light-matter interaction phenomena thanks to their exceptional linear optical properties arising from their large binding energy, strong spin-orbit coupling, and valley physics in the monolayer (ML) limit. They also boast strong nonlinear properties fortified by excitonic responses in these systems. However, the strong second-order nonlinear responses are mostly restricted to the ML limit, owing to crystal symmetry requirements, posing several limitations in terms of smaller interaction length and lower damage threshold. Here we demonstrate a self-hybridized exciton-polariton system in bulk WSe2 that allows us to relax the crystal symmetry rules that govern second-order nonlinearities. The demonstrated polariton system shows intense second harmonic generation (SHG) when the fundamental wavelength is resonant with the lower polariton, with an efficiency comparable to the one from a ML WS2 when excited at the same fundamental wavelength and intensity. We model this phenomenon by considering a system with alternating second-order susceptibilities under an asymmetric electric field profile determined by the polariton mode. Helicity-resolved polarization experiments show a very similar nonlinear response as the one from a ML, where the helicity of the SHG flips with respect to the fundamental harmonic. This polaritonic system offers a platform to leverage a robust second-order nonlinear response from centrosymmetric systems, while at the same time allowing access to third-order nonlinearity inherent in strongly coupled systems.

We report the first demonstration of strong coupling of an interlayer exciton (IE). This results in an order of magnitude enhancement in nonlinear optical response compared to the more common intralayer exciton -polaritons.

We demonstrate nonlinear increase in polariton density and thermalization at room temperature using a red emitting fluoroscent protein, mscarlett. Such fluoroscent proteins are an attractive platform to study polariton condensation and even realize condensate lattices.

Realizing nonlinear optical response in the low photon density limit in solid-state systems has been a long-standing challenge. Semiconductor microcavities in the strong coupling regime hosting exciton-polaritons have emerged as attractive candidates in this context. However, the weak interaction between these quasiparticles has been a hurdle in this quest. Dipolar excitons provide an attractive strategy to overcome this limitation but are often hindered by their weak oscillator strength. The interlayer dipolar excitons in naturally occurring homobilayer MoS$_2$ alleviates this issue owing to their hybridization of interlayer charge transfer exciton and intralayer B exciton. Here we demonstrate the formation of dipolar exciton polaritons in bilayer MoS$_2$ resulting in unprecedented nonlinear interaction strengths. A ten-fold increase in nonlinearity is observed for the interlayer dipolar excitons compared to the conventional A excitons. These highly nonlinear dipolar polaritons will likely be a frontrunner in the quest for solid-state quantum nonlinear devices.

Fluorescent proteins (FPs) have recently emerged as a serious contender for realizing ultralow threshold room temperature exciton-polariton condensation and lasing. Our contribution investigates the thermalization of FP microcavity exciton-polaritons upon optical pumping under ambient conditions. We realize polariton cooling using a new FP molecule, called mScarlet, coupled strongly to the optical modes in a Fabry Perot cavity. Interestingly, at the threshold excitation energy (fluence) of 9 nJ/pulse (15.6 mJ/cm2), we observe an effective temperature, Teff 350 +/- 35 K close to the lattice temperature indicative of strongly thermalized exciton-polaritons at equilibrium. This efficient thermalization results from the interplay of radiative pumping facilitated by the energetics of the lower polariton branch and the cavity Q factor. Direct evidence for dramatic switching from an equilibrium state into a metastable state is observed for the organic cavity polariton device at room temperature via deviation from the Maxwell-Boltzmann statistics at k = 0 above the threshold. Thermalized polariton gases in organic systems at equilibrium hold substantial promise for designing room temperature polaritonic circuits, switches, and lattices for analog simulation.

Polaritons in organic systems has shown the potential to modify chemical properties and to mediate long-range energy transfer between individual chromophores, among other capabilities. Here, we demonstrate that strong coupling and formation of organic exciton-polaritons can be used to selectively tune the isomer emission of organic molecules. By taking advantage of their delocalized and hybrid character, polaritons emerging in the strong coupling regime open a new relaxation pathway that allows for an efficient funneling of the excitation between the molecular isomers. We implement this by strong coupling to trans-DCS (E-4-dimethylamino-4′cyanostilbene)molecules, which present two isomers in different amounts when immersed in a polymer matrix. Thanks to this new relaxation pathway, the photoexcitation that is first shared by the common polaritonic mode is then selectively funneled to the excited states of one of the isomers, recognizing pure emission from the isomeric states that do not contribute to emission under normal conditions.

Realizing nonlinear optical response in the low photon density limit in solid state systems has been a long-standing challenge. Semiconductor microcavities in the strong coupling regime hosting light-matter quasiparticles called exciton-polaritons have emerged as an attractive candidate in this context. However, the weak interaction between these quasiparticles has been a hurdle in this quest. Two-dimensional transition metal dichalcogenides (TMDCs) owing to their inherently large oscillator strength and the wide array of excitonic complexes they host present an opportunity to address this challenge. Among the different excitations supported by TMDCs, a prime candidate is the interlayer excitons that form in heterostructures of TMDCs. Due to the spatial separation of the electron and holes in different layers, they have a permanent dipole moment making them interact stronger. This advantage is often diminished by their poor oscillator strength making them unsuitable for realizing polaritons. The recent discovery of interlayer excitons in naturally occurring homobilayer MoS$_2$ alleviates this issue owing to their hybrid characteristics arising from the interlayer charge transfer state and intralayer B exciton. Here we demonstrate the strong coupling of interlayer excitons in bilayer MoS$_2$ with cavity photons resulting in unprecedented nonlinear interaction strengths. A ten-fold increase in nonlinearity is observed for the interlayer excitons compared to the conventional A excitons which have been used extensively for strong coupling studies. The measured interaction strength in the weak pump limit is $\sim(100\pm 2)$ \textmu eV \textmu m$^{2}$. The observed nonlinear response is attributed to a combination of both exciton-exciton interaction and saturation due to phase space-filling.

Biswajit Datta, Mandeep Khatoniar, 2 Prathmesh Deshmukh, 2 Rezlind Bushati, 2 Simone De Liberato, Stephane Kena Cohen, and Vinod M. Menon 2, a) Department of Physics, City College of New York, New York, NY, USA Department of Physics, Graduate Center of the City University of New York (CUNY), New York, NY, USA School of Physics and Astronomy, University of Southampton, Southampton, UK Department of Engineering Physics, École Polytechnique de Montréal, Montréal, Quebec, Canada

The robust spin and momentum valley locking of electrons in two-dimensional semiconductors make the valley degree of freedom of great utility for functional optoelectronic devices. Owing to the difference in optical selection rules for the different valleys, these valley electrons can be addressed optically. The electrons and excitons in these materials exhibit valley Hall effect, where the carriers from specific valleys are directed to different directions under electrical or thermal bias. Here we report the optical valley Hall effect where the light emission from the valley polarized excitons in monolayer WS2 propagates in different directions owing to the preferential coupling of excitonic emission to the high momentum states of the hyperbolic metamaterial. The experimentally observed effects are corroborated with theoretical modeling of excitonic emission in the near field of hyperbolic media. The demonstration of the optical valley Hall effect using a bulk artificial photonic media without the need for nanostructuring opens the possibility of realizing valley-based excitonic circuits operating at room temperature.

Abstract Transition Metal Dichalcogenides (TMDCs) have been in the limelight for the past decade as a candidate for several optoelectronic devices, and as a versatile test bed for various fundamental light-matter interaction phenomena thanks to their exceptional linear optical properties arising from their large binding energy, strong spin-orbit coupling and valley physics in the monolayer (ML) limit. They also boast strong non-linear properties fortied by excitonic responses in these systems. However, the strong second order non-linear responses are mostly restricted to the ML limit owing to crystal symmetry requirements, posing several limitations in terms of smaller interaction length and lower damage threshold. Here we demonstrate a self-hybridized exciton-polariton system in bulk WSe2 that allows us to relax the crystal symmetry rules that govern second order non-linearities. The demonstrated polariton system shows intense Second Harmonic Generation (SHG) when the fundamental wavelength is resonant with the lower polariton, with an efficiency comparable to the one from a ML WS2 when excited at the same fundamental wavelength and intensity. We model this phenomenon by considering a system with alternating second- order susceptibilities under an asymmetric electric field profile determined by the polariton mode. Helicity resolved polarization experiments show very similar non-linear response as the one from a ML where the helicity of the SHG flips with respect to the fundamental harmonic. This polaritonic system offers a platform to leverage robust second order non-linear response from centrosymmetric systems, while at the same time allowing access to third-order non-linearity inherent in strongly coupled systems.

Supplementary information on Optical Analog of Valley Hall Effect of 2D Excitons in Hyperbolic Metamaterial

We study thermalization of exciton-polaritons in 2D hybrid organic-inorganic perovskites strongly coupled to a planar microcavity. The integrated PL exhibits a bottleneck effect and the extracted lattice temperature shows cooling behavior of up to 35 meV.

We demonstrate significant enhancement (> 40%) in valley coherence of localized excitons in monolayer WS 2 via strain engineering. The observed enhancement is attributed to the suppression of inter valley scattering due to strain induced potential.

Amorphous molecular solids are inherently disordered, exhibiting strong exciton localization. Optical microcavities containing such disordered excitonic materials have been theoretically shown to support both propagating and localized exciton‐polariton modes. Here, the ultrastrong coupling of a Bloch surface wave photon and molecular excitons in a disordered organic thin film at room temperature is demonstrated, where the major fraction of the polaritons are propagating states. The delocalized exciton‐polariton has a group velocity as high as 3 × 10 7 m s –1 and a lifetime of 500 fs, leading to propagation distances of over 100 µm from the excitation source. The polariton intensity shows a halo‐like pattern that is due to self‐interference of the polariton mode, from which a coherence length of 20 µm is derived and is correlated with phase breaking by polariton scattering. The demonstration of ultralong‐range exciton‐polariton transport at room temperature promises new photonic and optoelectronic applications such as efficient energy transfer in disordered condensed matter systems.

We report enhanced second harmonic generation (SHG) from centrosymmetric bulk WSe2. This enhancement arises due to pumping in resonance with the Exciton-Polariton modes formed in self-hybridized bulk WSe2.