Bumho Kim

Achieving optical nonlinearities at ultralow light intensities in solid-state platforms is essential for advancing nonlinear and quantum photonic technologies. A promising approach involves coupling excitons to photons in optical cavities to create exciton polaritons, where effective photon-photon interactions are mediated by the intrinsic excitonic nonlinearity. However, realizing strong polariton nonlinearities within a scalable architecture remains a significant experimental challenge. Here, we demonstrate highly nonlinear two-dimensional exciton polaritons by coupling a charge-tunable MoSe 2 monolayer to a planar photonic crystal nanocavity. The pronounced excitonic resonance of the monolayer, combined with its seamless integration with the planar nanocavity, facilitates robust exciton-photon hybridization. Remarkably, the strong mode confinement of the nanocavity substantially enhances polariton-polariton interactions, enabling all-optical switching of the cavity spectrum with excitation energies as low as ∼ 4 fJ —several orders of magnitude below previously reported thresholds in 2D exciton-polariton systems. Pump-probe spectroscopy reveals that this switching operates on an ultrafast timescale of a few picoseconds. Our Letter establishes a robust platform for nonlinear 2D polaritonics with broad applications in integrated photonic technologies, including all-optical neuromorphic computing and quantum photonic information processing.

Achieving optical nonlinearities at ultralow light intensities in solid-state platforms is essential for advancing nonlinear and quantum photonic technologies. A promising approach involves coupling excitons to photons in optical cavities to create exciton-polaritons, where effective photon-photon interactions are enhanced by intrinsic exciton interactions in the material. However, realizing strong polariton nonlinearities within a scalable architecture remains a significant experimental challenge. Here, we demonstrate highly nonlinear two-dimensional exciton-polaritons by coupling a charge-tunable MoSe2 monolayer to a planar photonic crystal nanocavity. The pronounced excitonic resonance of the 2D monolayer, combined with its seamless integration with the planar nanocavity, facilitates robust exciton-photon hybridization, leading to the formation of nanocavity exciton-polaritons. Remarkably, the strong mode confinement of the nanocavity substantially enhances polariton-polariton interactions, enabling all-optical switching of the cavity spectrum with excitation energies as low as 4 fJ -- several orders of magnitude below the previously reported thresholds in 2D exciton-polariton systems. We also observe ultrafast polariton dynamics on the order of a few picoseconds via pump-probe spectroscopy. Our work establishes a scalable platform for nonlinear 2D polaritonics with broad applications in integrated photonic technologies, including all-optical neuromorphic computing and quantum photonic information processing.

Atomically thin semiconductor heterostructures provide a two-dimensional (2D) device platform for creating high densities of cold, controllable excitons. Interlayer excitons (IEs), bound electrons and holes localized to separate 2D quantum well layers, have permanent out-of-plane dipole moments and long lifetimes, allowing their spatial distribution to be tuned on demand. Here, we employ electrostatic gates to trap IEs and control their density. By electrically modulating the IE Stark shift, electron-hole pair concentrations above 2 × 10¹² cm⁻² can be achieved. At this high IE density, we observe an exponentially increasing linewidth broadening indicative of an IE ionization transition, independent of the trap depth. This runaway threshold remains constant at low temperatures, but increases above 20 K, consistent with the quantum dissociation of a degenerate IE gas. Our demonstration of the IE ionization in a tunable electrostatic trap represents an important step towards the realization of dipolar exciton condensates in solid-state optoelectronic devices.

Achieving robust and electrically controlled valley polarization in monolayer transition metal dichalcogenides (ML-TMDs) is a frontier challenge for realistic valleytronic applications. Theoretical investigations show that the integration of 2D materials with ferroelectrics is a promising strategy; however, an experimental demonstration has remained elusive. Here, we fabricate ferroelectric field-effect transistors using a ML-WSe2 channel and an Al0.68Sc0.32N (AlScN) ferroelectric dielectric and experimentally demonstrate efficient tuning as well as non-volatile control of valley polarization. We measure a large array of transistors and obtain a maximum valley polarization of similar to 27% at 80 K with stable retention up to 5400 s. The enhancement in the valley polarization is ascribed to the efficient exciton-to-trion (X-T) conversion and its coupling with an out-of-plane electric field, viz., the quantum-confined Stark effect. This changes the valley depolarization pathway from strong exchange interactions to slow spin-flip intervalley scattering. Our research demonstrates a promising approach for achieving non-volatile control over valley polarization for practical valleytronic device applications.

Recent advancements in ferroelectric field-effect transistors (FeFETs) using two-dimensional (2D) semiconductor channels and ferroelectric Al_0.68Sc_0.32N (AlScN) allow high-performance nonvolatile devices with exceptional ON-state currents, large ON/OFF current ratios, and large memory windows (MW). However, previous studies have solely focused on n-type FeFETs, leaving a crucial gap in the development of p-type and ambipolar FeFETs, which are essential for expanding their applicability to a wide range of circuit-level applications. Here, we present a comprehensive demonstration of n-type, p-type, and ambipolar FeFETs on an array scale using AlScN and multilayer/monolayer WSe₂. The dominant injected carrier type is modulated through contact engineering at the metal-semiconductor junction, resulting in the realization of all three types of FeFETs. The effect of contact engineering on the carrier injection is further investigated through technology-computer-aided design simulations. Moreover, our 2D WSe₂/AlScN FeFETs achieve high electron and hole current densities of ∼20 and ∼10 μA/μm, respectively, with a high ON/OFF ratio surpassing ∼10⁷ and a large MW of >6 V (0.14 V/nm).

Interlayer excitons, electron-hole pairs bound across two monolayer van der Waals semiconductors, offer promising electrical tunability and localizability. Because such excitons display weak electron-hole overlap, most studies have examined only the lowest-energy excitons through photoluminescence. We directly measured the dielectric response of interlayer excitons, which we accessed using their static electric dipole moment. We thereby determined an intrinsic radiative lifetime of 0.40 nanoseconds for the lowest direct-gap interlayer exciton in a tungsten diselenide/molybdenum diselenide heterostructure. We found that differences in electric field and twist angle induced trends in exciton transition strengths and energies, which could be related to wave function overlap, moiré confinement, and atomic reconstruction. Through comparison with photoluminescence spectra, this study identifies a momentum-indirect emission mechanism. Characterization of the absorption is key for applications relying on light-matter interactions.

Exciton condensates (ECs) are macroscopic coherent states arising from condensation of electron–hole pairs¹. Bilayer heterostructures, consisting of two-dimensional electron and hole layers separated by a tunnel barrier, provide a versatile platform to realize and study ECs^2,3,4. The tunnel barrier suppresses recombination, yielding long-lived excitons^5,6,7,8,9,10. However, this separation also reduces interlayer Coulomb interactions, limiting the exciton binding strength. Here, we report the observation of ECs in naturally occurring 2H-stacked bilayer WSe₂. In this system, the intrinsic spin–valley structure suppresses interlayer tunnelling even when the separation is reduced to the atomic limit, providing access to a previously unattainable regime of strong interlayer coupling. Using capacitance spectroscopy, we investigate magneto-ECs, formed when partially filled Landau levels couple between the layers. We find that the strong-coupling ECs show dramatically different behaviour compared with previous reports, including an unanticipated variation of EC robustness with the orbital number, and find evidence for a transition between two types of low-energy charged excitations. Our results provide a demonstration of tuning EC properties by varying the constituent single-particle wavefunctions.

Interlayer excitons (ILXs) — electron–hole pairs bound across two atomically thin layered semiconductors — have emerged as attractive platforms to study exciton condensation^1,2,3,4, single-photon emission and other quantum information applications^5,6,7. Yet, despite extensive optical spectroscopic investigations^8,9,10,11,12, critical information about their size, valley configuration and the influence of the moiré potential remains unknown. Here, in a WSe₂/MoS₂ heterostructure, we captured images of the time-resolved and momentum-resolved distribution of both of the particles that bind to form the ILX: the electron and the hole. We thereby obtain a direct measurement of both the ILX diameter of around 5.2 nm, comparable with the moiré-unit-cell length of 6.1 nm, and the localization of its centre of mass. Surprisingly, this large ILX is found pinned to a region of only 1.8 nm diameter within the moiré cell, smaller than the size of the exciton itself. This high degree of localization of the ILX is backed by Bethe–Salpeter equation calculations and demonstrates that the ILX can be localized within small moiré unit cells. Unlike large moiré cells, these are uniform over large regions, allowing the formation of extended arrays of localized excitations for quantum technology.