Van der Waals materials are ideally suited for the implementation of high-frequency nanophononic resonators with atomically flat interfaces. Here, we present two versatile van der Waals-based nanophononic architectures: First, we introduce self-supporting nano-domes of WSe_2 as a scalable platform for the simultaneous generation of hundreds of high-quality nanoacoustic resonators with resonance frequencies in the 100 GHz range. Second, we engineer self-supporting nano-drums that reach record-high working frequencies for 2D-semiconductor transducers beyond 1 THz. Through optical pump-probe spectroscopy experiments and photoelastic linear chain model calculations, we gain a detailed understanding of the intricate interplay between phononic mode hybridization across heterostructures, the differences between modes close to the center and edge of the acoustic Brillouin zone, and the temporal structure of the photoelastic response. Both architectures have potential applications in low-cost nanoacoustic probing and the ultrafast modulation of quantum emitters in two-dimensional semiconductors. While nano-drums surpass the THz frequency barrier, nano-domes appear as an accessible, low-cost alternative for developing scalable nanophononic technologies.

The coherent, periodic energy transfer between light- and matter excitations characterizes the strong coupling regime of cavity exciton-polaritons, resulting, in the simplest case, in a Rabi-doublet in the spectral domain. We demonstrate a peculiar regime of strong light-matter coupling, which arises when photonic cavity modes couple to an ultra-thin excitonic mirror. We embed a 12 nm J-aggregated thin film in an open microcavity and tune the coupling strength from weak to the onset of ultrastrong coupling. At resonance, the excitonic mirror selectively changes dielectric to metallic field boundary conditions adding a 2π phase, which links optical cavity modes of different order. Our work gives an exciting perspective to ultra-fast cavity switches and photonic devices based on excitonic optical elements.

The coherent coupling of cavity-confined photons and excitonic matter resonances leads to the formation of cavity polaritons, hybrid light-matter quasi-particles. If multiple exciton resonances couple to the same photonic mode, the resulting polariton constitutes a coherent interaction between matter resonances that can be spatially separated without any direct electronic coupling. In this work, we demonstrate the formation of such a coherent coupling at room temperature using an open optical cavity containing two distinct van der Waals materials - monolayer WS2 and layered quasi-2D halide perovskites (HaPs) - separated by 1.5 μ m. The system forms three polariton branches, with the middle branch possessing nearly equal fractions of both excitons and the photonic mode. White-light reflectivity and luminescence measurements are in good agreement with simulations using a coupled harmonic oscillator and a microscopic Wannier-Hopfield framework. Our results lay the foundation to combine highly complementary degrees of freedom in 2D materials in an in-situ tunable fashion to enable new polaritonic functionalities.

Quasi-2D halide perovskites are chemically synthesized realizations of quantum well stacks with giant exciton oscillator strengths, tunable emission spectra, and very large exciton binding energies. While these features render quasi-2D halide perovskites a promising platform for room-temperature polaritonics, bosonic condensation and polariton lasing in quasi-2D perovskites have so far remained elusive at ambient conditions. Here, we demonstrate room-temperature cavity exciton-polariton condensation in mechanically exfoliated crystals of the quasi-2D Ruddlesden-Popper iodide perovskite (BA)2(MA)2Pb3I10 in an open optical microcavity. We observe a polariton condensation threshold of 0.41 µJ cm−2 per pulse and detect a strong non-linear response. Interferometric measurements confirm the spontaneous emergence of spatial coherence across the condensate with an associated first-order autocorrelation reaching 0.6 with 1 ps coherence time and an effective de Broglie wavelength of 13 µm. Our results lay the foundation for a new class of room-temperature polariton lasers based on quasi-2D halide perovskites with great potential for hetero-integration with other van-der-Waals materials and combination with photonic crystals or waveguides. The authors report the experimental observation of room-temperature condensation of exciton polaritons in quasi-2D layered crystals of halide perovskite, integrated into an open optical microcavity. These materials combine van-der-Waals properties with dominant exciton physics at room temperature.

In a combined experimental and theoretical study, we demonstrate anisotropic polariton transport on the nanoscale in the van der Waals antiferromagnet CrSBr. While effective cavity-polariton formation emerges via the self-hybridization of ultra-high oscillator strength excitons with a thin slab photonic mode, the absence of external mirrors facilitates spectroscopic investigation of these polaritons via cathodoluminescence (CL) on length scales determined by the electron wavelength. This direct access allows us to perform precise charting of the polariton landscape with nanometric resolution, and to probe polariton interference phenomena. The main finding of the work highlights that the coherent polariton transport follows the C_2v symmetry of CrSBr, allowing exclusive transport along the crystallographic a-axis, while no coherent feature is found along the b-axis direction. Our work sets the foundation to use CL spectroscopy in cavity-polaritonics in more advanced landscapes, such as photonic crystals or optical lattices, and establishes the technique as a powerful tool to probe anisotropic expansion and relaxation phenomena on the nanoscale

ABSTRACT Versatile, tunable, and potentially scalable single‐photon sources are a key asset in emergent photonic quantum technologies. In this work, a single‐photon source based on WS 2 micro‐domes, created via hydrogen ion irradiation, is realized and integrated into an open, tunable optical microcavity. Single‐photon emission from the coupled emitter–cavity system is verified via the second‐order correlation measurement, revealing a value of . A detailed analysis of the spectrally selective, cavity enhanced emission features shows the impact of a pronounced acoustic phonon emission sideband, which contributes specifically to the non‐resonant emitter–cavity coupling in this system. The achieved level of cavity–emitter control highlights the potential of open‐cavity systems to tailor the emission properties of atomically thin quantum emitters, advancing their suitability for real‐world quantum technology applications.

Transition metal dichalcogenides represent a versatile platform to study strong light-matter interactions based on excitons and electrons in ordered lattices. Twist-engineering of moiré structures further enables the manipulation of the polaritonic nonlinearities via engineering the exciton landscape on the nanoscale. In this work, we demonstrate in-situ control of the optical saturation-based nonlinearity of moiré exciton-polaritons by phase space restriction via charge doping. Strong exciton-photon coupling is established in a gate-controllable MoTe_2-MoSe_2 heterobilayer, embedded in a spectrally-tunable open cavity. A small gate voltage can effectively lower the necessary polariton density by one order of magnitude to achieve a similar nonlinear saturation effect as in the charge-neutral case. Our microscopic description successfully explains the observed phenomena in the framework of Pauli blocking for the moiré superlattices with charge preoccupation.

Versatile, tunable, and potentially scalable single-photon sources are a key asset in emergent photonic quantum technologies. In this work, a single-photon source based on WS_2 micro-domes, created via hydrogen ion irradiation, is realized and integrated into an open, tunable optical microcavity. Single-photon emission from the coupled emitter-cavity system is verified via the second-order correlation measurement, revealing a g^(2)(τ=0) value of 0.3. A detailed analysis of the spectrally selective, cavity enhanced emission features shows the impact of a pronounced acoustic phonon emission sideband, which contributes specifically to the non-resonant emitter-cavity coupling in this system. The achieved level of cavity-emitter control highlights the potential of open-cavity systems to tailor the emission properties of atomically thin quantum emitters, advancing their suitability for real-world quantum technology applications.

The interaction of a quantum two-level system with a resonant driving field results in the emergence of Rabi oscillations, which are the hallmark of a controlled manipulation of a quantum state on the Bloch sphere. This all-optical coherent control of solid-state two-level systems is crucial for quantum applications. In this work we study Rabi oscillations emerging in a WSe2 monolayer-based quantum dot. The emitter is driven coherently using picosecond laser pulses to a higher-energy state, while photoluminescence is probed from the ground state. The theoretical treatment based on a three-level exciton model reveals the population transfer between the exciton ground and excited states coupled by Coulomb interaction. Our calculations demonstrate that the resulting exciton ground state population can be controlled by varying driving pulse area and detuning which is evidenced by the experimental data. Our results pave the way towards the coherent control of quantum emitters in atomically thin semiconductors, a crucial ingredient for monolayer-based high-performance, on-demand single photon sources.

MoTe_{2} monolayers and bilayers are unique within the family of van der Waals materials since they pave the way toward atomically thin infrared light-matter quantum interfaces, potentially reaching the important telecommunication windows. Here, we report emergent exciton polaritons based on MoTe_{2} monolayers and bilayers in a low-temperature open microcavity in a joint experiment-theory study. Our experiments clearly evidence both the enhanced oscillator strength and enhanced luminescence of MoTe_{2} bilayers, signified by a 38% increase of the Rabi splitting and a strongly enhanced relaxation of polaritons to low-energy states. The latter is distinct from polaritons in MoTe_{2} monolayers, which feature a bottlenecklike relaxation inhibition. Both the polaritonic spin valley locking in monolayers and the spin-layer locking in bilayers are revealed via the Zeeman effect, which we map and control via the light-matter composition of our polaritonic resonances.

Optical resonators are a powerful platform to control the spontaneous emission dynamics of excitons in solid-state nanostructures. We study a MoSe2-WSe2 heterostructure that is integrated in a cryogenic open optical microcavity to gain insights into fundamental optical properties of the emergent interlayer excitons. First, we utilize a low-quality-factor planar open cavity and investigate the modification of the excitonic lifetime as on- and off-resonance conditions are met with consecutive longitudinal modes. Time-resolved photoluminescence measurements revealed a periodic tuning of the interlayer exciton lifetime by 220 ps, which allows us to extract a 0.5 ns free-space radiative lifetime and a quantum efficiency as high as 81.4%±1.4%. We subsequently engineer the local density of optical states by spatially confined and spectrally tunable Tamm-plasmon resonances. The dramatic redistribution of the local optical modes allows us to encounter a significant inhibition of the excitonic spontaneous emission rate by a factor of 3.2. Our open cavity is able to tune the cavity resonances accurately to the emitters to have a robust in situ control of the light-matter coupling. Such a powerful characterization approach can be universally applied to tune the exciton dynamics and measure the quantum efficiencies of more complex van der Waals heterostructures and devices.

Layered perovskites are an emergent class of materials, which feature extraordinarily large light-matter coupling, driven by excitons with binding energies significantly beyond the thermal energy at room-temperature. In this work, widely tunable room-temperature cavity exciton polaritons are demonstrated at the cross-over from the strong coupling to the very strong coupling regime in mechanically exfoliated crystals of quasi-2D Ruddlesden-Popper iodide perovskite (BA)2(MA)2Pb3I10 embedded in an open microcavity. The coupled exciton-cavity system features a Rabi-splitting up to Omega R similar or equal to 155 meV, exceeding the experimentally determined exciton binding energy of E b = 100 +/- 10 meV and thus operates at the onset of the very strong coupling regime, in which the light-matter coupling alters the interaction of electron and hole. This combined experimental-theoretical effort provides a consistent microscopic picture successfully describing the observed peculiar scaling of the Rabi-splitting with an increasing effective cavity length. These findings provide a foundation for future on-chip applications involving tunable polaritonic and nonlinear optical devices based on strongly coupled perovskite systems.

Van der Waals magnets are an emergent material class of paramount interest for fundamental studies in coupling light with matter excitations, which are uniquely linked to their underlying magnetic properties. Among these materials, the magnetic semiconductor CrSBr is possibly a first playground where we can study simultaneously the interaction of photons, magnons, and excitons at the quantum level. Here we demonstrate a coherent macroscopic quantum phase, the bosonic condensation of exciton-polaritons, which emerges in a CrSBr flake embedded in a fully tunable cryogenic open optical cavity. The Bose condensate is characterized by a highly non-linear threshold-like behavior, and coherence manifests distinctly via its first and second order quantum coherence. We find that the condensate's non-linearity is highly susceptible to the magnetic order in CrSBr, and encounters a sign change depending on the antiferro- and ferromagnetic ordering. Our findings open a route towards magnetically controllable quantum fluids of light, and optomagnonic devices where spin magnetism is coupled to on-chip Bose-Einstein condensates.

Controlling exciton relaxation and energy conversion pathways via their coupling to photonic modes is a central task in cavity-mediated quantum materials research. In this context, the light-matter hybridization in optical cavities can lead to intriguing effects, such as modified carrier transport, enhancement of optical quantum yield, and control of chemical reaction pathways. Here, we investigate the impact of the strong light-matter coupling regime on energy conversion, both in relaxation and upconversion schemes, by utilizing a strongly charged MoSe2 monolayer embedded in a spectrally tunable open-access cavity. We find that the charge carrier gas yields a significantly modified photoluminescence response of cavity exciton-polaritons, dominated by an intra-cavity like pump scheme. In addition, upconversion luminescence emerges from a population transfer from fermionic trions to bosonic exciton-polaritons. Due to the availability of multiple optical modes in the tunable open cavity, it seamlessly meets the cavity-enhanced double resonance condition required for an efficient upconversion. The latter can be actively tuned via the cavity length in-situ, displaying nonlinear scaling in intensity and fingerprints of the valley polarization. This suggests mechanisms that include both trion-trion Auger scattering and phonon absorption as its underlying microscopic origin.

We investigate the excitonic properties of epitaxially grown WS2 monolayers, bilayers and multilayers on graphene using monochromatic electron energy loss spectroscopy (EELS) in a scanning transmission electron microscope. This material system is particularly attractive for optoelectronic applications, as direct growth from the gas phase offers a scalable route to wafer-sized heterostructures. The combination of nanometer-scale spatial resolution and high spectral quality in EELS allows for a detailed analysis of layer-dependent excitonic features. To complement the experimental results, we perform ab initio simulations based on density functional theory and the Bethe-Salpeter equation. The experimental spectra reveal a systematic redshift of both A and B excitons at the K-valley─centered near 2.0 and 2.4 eV, respectively─as the number of WS2 layers increases. While such redshifts are often attributed to dielectric screening, our ab initio calculations show that the dominant contribution arises from a subtle lattice mismatch between the lower and upper WS2 layers. We trace this mismatch to the heteroepitaxial alignment of the first WS2 layer to the graphene substrate during the growth process. Our results highlight how nanoscale structural distortions in epitaxial 2D materials can strongly influence key excitonic properties, even in the absence of intentional strain or alloying. By combining nanometer-scale electron spectroscopy with advanced theory, we establish a direct link between atomic structure and excitonic response in realistic, nonidealized heterostructures. These findings underscore the importance of microscopic interface effects in the design and scalable fabrication of exciton-based optoelectronic devices.

Structured optical cavities have advanced as a powerful test bed to study lattice Hamiltonians in general, and topological phenomena in particular. The in situ tuning of topological modes, enabled via substantial modifications of emulated lattice potentials, has remained out of experimental reach due to the commonly utilized monolithic cavity samples. Here, we study the Su-Schrieffer-Heeger (SSH) lattice Hamiltonian, which we emulate in a widely tunable open optical cavity strongly coupled to excitons in an integrated WS2 monolayer. The potential landscape comprises a topological domain boundary hosting a topological, exponentially localized mode at the interface between two lattices characterized by different Zak phases. The mode is spectrally tunable over 80 meV. Moreover, we use the unique tilt tunability of our implementation to transform the SSH lattice into a Stark ladder. This transformation couples the topologically protected defect mode to propagating lattice modes and effectively changes the symmetry of the system. Furthermore, it allows us to directly quantify the Zak-phase difference Delta(Zak) = (1.07 + 0.11)pi between the two topological phases. Our Letter constitutes an important step toward in situ tuning topological lattices to control and guide light on nonlinear chips.

MoTe_2 monolayers and bilayers are unique within the family of van-der-Waals materials since they pave the way towards atomically thin infrared light-matter quantum interfaces, potentially reaching the important telecommunication windows. Here, we report emergent exciton-polaritons based on MoTe_2 monolayer and bilayer in a low-temperature open micro-cavity in a joint experiment-theory study. Our experiments clearly evidence both the enhanced oscillator strength and enhanced luminescence of MoTe_2 bilayers, signified by a 38 % increase of the Rabi-splitting and a strongly enhanced relaxation of polaritons to low-energy states. The latter is distinct from polaritons in MoTe_2 monolayers, which feature a bottleneck-like relaxation inhibition. Both the polaritonic spin-valley locking in monolayers and the spin-layer locking in bilayers are revealed via the Zeeman effect, which we map and control via the light-matter composition of our polaritonic resonances.

Accessing acoustic phonons at high frequencies in nanostructures becomes more and more essential in nanoelectronics, nano- and opto-mechanics and quantum technologies, as phonons can strongly interact with electrons and photons at the nanoscale. In spontaneous Brillouin scattering processes, the scattered photons energy, direction and polarisation are constrained by selection rules for a given input state. These selection rules are usually considered as intrinsic material properties in crystalline solids and the polarisation of the scattered photons depends on the polarisation of the excitation. In this work, we use elliptical optophononic micropillar resonators to control these optical polarisation selection rules. The degeneracy of the optical cavity modes of circular micropillars is lifted due to the elliptical cross-section of the micropillars, leading to two cavity modes orthogonally polarised and split in energy. The optical field polarisation state will depend on both orthogonal cavity modes and their associated polarisation states. Therefore, an incident laser beam linearly polarised along the diagonal axis of the elliptical pillar undergoes a wavelength dependent polarisation rotation. By choosing the polarisation and wavelength of the incident laser, we demonstrate that the polarisation state of the incident and reflected laser and the Brillouin scattering signal are different. In this way, background-free spontaneous Brillouin scattering spectra can be efficiently measured in a cross-polarisation scheme down to 18 GHz. Here, we theoretically and experimentally explore the optimal conditions for the polarisation and wavelength of the incident laser, and the ellipticity of the micropillars, to improve the polarisation-based filtering applied to Brillouin spectroscopy.

We demonstrate evidence of correlated magnetism for exciton-polaritons in a MoSe$_{2}$/WS$_{2}$ moiré heterostructure with near-parallel alignment subject to electron doping. In our experiments, interactions between electrons and moiré excitons are controlled electrostatically by field-effect doping, and the polaritonic regime of strong light-matter coupling is established in an open cryogenic microcavity. Remarkably, at filling fractions around one electron per moiré cell, we observe drastic and nonlinear enhancement of the effective polariton Landé factor as a hallmark of correlated magnetism, which is cavity-controlled via resonance tuning of light and matter polariton constituents. Our work establishes moiré van der Waals heterostructures as an outstanding platform for studies of correlated phenomena in the presence of strong light-matter coupling and many-body phases of lattice-ordered excitons, charges and spins.