Bo Zhen

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.

Topological photonics explores photonic systems that exhibit robustness against defects and disorder, enabled by protection from underlying topological phases. These phases are typically realized in linear optical systems and characterized by their intrinsic photonic band structures. Here we experimentally study Floquet Chern insulators in periodically driven nonlinear photonic crystals, where the topological phase is controlled by the polarization and the frequency of the driving field. Our transient sum-frequency generation measurements reveal strong hybridization of the Floquet photonic bands. The measured spectrum remains gapless under a linearly polarized drive but becomes gapped under a circularly polarized drive. Theoretical analysis confirms that the Floquet gap is topological, characterized by a non-zero Chern number—a consequence of time-reversal symmetry breaking induced by the circularly polarized driving field. This work offers opportunities to explore the role of classical optical nonlinearity in topological phases and their applications in nonlinear optoelectronics.

Understanding the radiative decay of exciton-polaritons is essential for achieving long-lived polaritons - a key prerequisite for enhancing nonlinear and quantum polaritonic effects. However, conventional wisdom - the coupled oscillator model - often oversimplifies polariton radiation as independent emissions from uncoupled excitonic and photonic resonances, overlooking the role of strong exciton-photon coupling in reshaping their radiative behavior. In this work, we present a theoretical framework that goes beyond the conventional coupled oscillator model by fully accounting for the collective and coherent nature of exciton-photon interactions. We demonstrate that these interactions can strongly suppress polariton radiation via destructive interference - both within the excitonic ensemble and between excitonic and photonic radiation channels - giving rise to polaritonic bound states in the continuum with infinitely long radiative lifetimes. Our approach offers a unified description of polariton radiative decay and establishes new design principles for engineering long-lived exciton-polaritons with tailored radiation properties, opening new avenues for nonlinear, topological, and quantum polaritonic applications.

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.

We theoretically analyze Floquet topological crystalline phases in temporally and spatially modulated one-dimensional photonic crystals with second-order optical nonlinearity. We identify a space-time symmetry that governs the degeneracy of photonic crystal band structures. By adjusting a system parameter to break or preserve this symmetry, we achieve a Zak phase transition. We also investigate the physical consequences of this transition, focusing on edge states and filling anomalies. These findings suggest a potential approach to realizing topologically protected strong light confinement.

Strong coupling between light and matter in an optical cavity provides a pathway to giant polariton nonlinearity, where effective polariton-polariton interactions are mediated by materials' nonlinear responses. The pursuit of such enhanced nonlinearity at low optical excitations, potentially down to the single-particle level, has been a central focus in the field, inspiring the exploration of novel solid-state light-matter systems. Here, we experimentally realize extremely nonlinear and robust cavity exciton-polaritons by coupling a charge-tunable MoSe2 monolayer to a photonic crystal nanocavity. We show that the observed polariton nonlinearity arises from increased exciton dephasing at high populations, leading to diminished exciton-photon coupling and ultimately the breakdown of the strong coupling condition. Remarkably, the strong mode confinement of the nanocavity enables all-optical switching of the cavity spectrum at ultralow optical excitation energies, down to ~4 fJ, on picosecond timescales. Our work paves the way for further exploration of 2D nonlinear exciton-polaritons, with promising applications in both classical and quantum all-optical information processing.

We introduce the feature issue of Optical Materials Express on Topological Photonics.

Commercial production of integrated photonic devices is limited by scalability of desirable material platforms. We explore a relatively new photonic material, AlScN, for its use in electro‐optic phase shifting and modulation. Its CMOS‐compatibility could facilitate large‐scale production of integrated photonic modulators, and it exhibits an enhanced second‐order optical nonlinearity compared to intrinsic AlN, indicating the possibility for efficient modulation. Here, we measure the electro‐optic effect in Al _0.80 Sc _0.20 N‐based phase shifters. We utilized the TM0 mode, allowing use of the r ₃₃ electro‐optic coefficient, and demonstrated V _π L around 750 V cm. Since the electro‐optic response is smaller than expected, we discuss potential causes for the reduced response and future outlook for AlScN‐based photonics.