Rydberg exciton-polaritons in a Cu2O microcavity

Ultrafast energizing the parity-forbidden dark exciton in black phosphorus

As conventional electronic materials approach their physical limits, the application of ultrafast optical fields to access transient states of matter captures imagination. The inversion symmetry governs the optical parity selection rule, differentiating between accessible and inaccessible states of matter. To circumvent parity-forbidden transitions, the common practice is to break the inversion symmetry by material design or external fields. Here we report how the application of femtosecond ultraviolet pulses can energize a parity-forbidden dark exciton state in black phosphorus while maintaining its intrinsic material symmetry. Unlike its conventional bandgap absorption in visible-to-infrared, femtosecond ultraviolet excitation turns on efficient Coulomb scattering, promoting carrier multiplication and electronic heating to ~3000 K, and consequently populating its parity-forbidden states. Interferometric time- and angle-resolved two-photon photoemission spectroscopy reveals dark exciton dynamics of black phosphorus on ~100 fs time scale and its anisotropic wavefunctions in energy-momentum space, illuminating its potential applications in optoelectronics and photochemistry under ultraviolet optical excitation.

Opportunities and Challenges of Solid-State Quantum Nonlinear Optics

Nonlinear interactions between photons are fundamentally weak as the photons do not interact directly with each other, and any interaction is mediated by matter. This has motivated researchers over many decades to search for strongly nonlinear materials (by controlling electronic properties) and optical resonators with strong spatial and temporal confinement of light. An extreme form of nonlinear optics is quantum nonlinear optics, where we can realize nonlinear interaction between single photons. Such quantum nonlinear optics is at the heart of any photonic quantum information system including analog quantum simulation and fault-tolerant quantum computing. While engineering light-matter interactions can effectively create photon-photon interactions, the required photon number to observe any nonlinearity are normally very high, where any quantum-mechanical signature disappears. However, with emerging low-dimensional materials and engineered photonic resonators, the photon number can be reduced to reach the quantum nonlinear optical regime. In this review paper, we discuss different mechanisms exploited in solid-state platforms to attain quantum nonlinear optics. We review emerging materials and optical resonator architectures with different dimensionalities. We also present future research directions and open problems in this field.

Subpicosecond Dynamics of Rydberg Excitons Produced from Ultraviolet Excitation of Neutral Cuprite (Cu2O)n Clusters, n < 13

The ultrafast dynamics of subnanometer neutral cuprite clusters (Cu2O)n, n < 13, are examined with pump probe spectroscopy. Upon absorption of an ultraviolet (400 nm) photon, all clusters exhibit a subpicosecond lifetime that we attribute to carrier recombination. Density functional theory (DFT) shows a change in the structural motif between small planar clusters and three-dimensional structures at n = 4. This transition is accompanied by a change in the excited state relaxation behavior, marking the onset for which lifetimes increase gradually with size. Time-dependent DFT calculations show that the excited state lifetimes align with calculated topological parameters and charge carrier delocalization associated with the formation of Rydberg excitons. Terminal Cu atoms are found to be important for the production of Rydberg excitons at the lowest optically allowed excited state. Upon excitation, the electron resides on terminal Cu atoms and the hole becomes delocalized across the remainder of the cluster.

Nonlinear Rydberg exciton-polaritons in Cu2O microcavities

Rydberg excitons (analogues of Rydberg atoms in condensed matter systems) are highly excited bound electron-hole states with large Bohr radii. The interaction between them as well as exciton coupling to light may lead to strong optical nonlinearity, with applications in sensing and quantum information processing. Here, we achieve strong effective photon-photon interactions (Kerr-like optical nonlinearity) via the Rydberg blockade phenomenon and the hybridisation of excitons and photons forming polaritons in a Cu2O-filled microresonator. Under pulsed resonant excitation polariton resonance frequencies are renormalised due to the reduction of the photon-exciton coupling with increasing exciton density. Theoretical analysis shows that the Rydberg blockade plays a major role in the experimentally observed scaling of the polariton nonlinearity coefficient as ∝ n4.4±1.8 for principal quantum numbers up to n = 7. Such high principal quantum numbers studied in a polariton system for the first time are essential for realisation of high Rydberg optical nonlinearities, which paves the way towards quantum optical applications and fundamental studies of strongly correlated photonic (polaritonic) states in a solid state system.

Exciton Polaritons in Emergent Two-Dimensional Semiconductors

The "marriage" of light (i.e., photon) and matter (i.e., exciton) in semiconductors leads to the formation of hybrid quasiparticles called exciton polaritons with fascinating quantum phenomena such as Bose-Einstein condensation (BEC) and photon blockade. The research of exciton polaritons has been evolving into an era with emergent two-dimensional (2D) semiconductors and photonic structures for their tremendous potential to break the current limitations of quantum fundamental study and photonic applications. In this Perspective, the basic concepts of 2D excitons, optical resonators, and the strong coupling regime are introduced. The research progress of exciton polaritons is reviewed, and important discoveries (especially the recent ones of 2D exciton polaritons) are highlighted. Subsequently, the emergent 2D exciton polaritons are discussed in detail, ranging from the realization of the strong coupling regime in various photonic systems to the discoveries of attractive phenomena with interesting physics and extensive applications. Moreover, emerging 2D semiconductors, such as 2D perovskites (2DPK) and 2D antiferromagnetic (AFM) semiconductors, are surveyed for the manipulation of exciton polaritons with distinct control degrees of freedom (DOFs). Finally, the outlook on the 2D exciton polaritons and their nonlinear interactions is presented with our initial numerical simulations. This Perspective not only aims to provide an in-depth overview of the latest fundamental findings in 2D exciton polaritons but also attempts to serve as a valuable resource to prospect explorations of quantum optics and topological photonic applications.

Highly-excited Rydberg excitons in synthetic thin-film cuprous oxide

Cuprous oxide ([Formula: see text]) has recently emerged as a promising material in solid-state quantum technology, specifically for its excitonic Rydberg states characterized by large principal quantum numbers (n). The significant wavefunction size of these highly-excited states (proportional to [Formula: see text]) enables strong long-range dipole-dipole (proportional to [Formula: see text]) and van der Waals interactions (proportional to [Formula: see text]). Currently, the highest-lying Rydberg states are found in naturally occurring [Formula: see text]. However, for technological applications, the ability to grow high-quality synthetic samples is essential. The fabrication of thin-film [Formula: see text] samples is of particular interest as they hold potential for observing extreme single-photon nonlinearities through the Rydberg blockade. Nevertheless, due to the susceptibility of high-lying states to charged impurities, growing synthetic samples of sufficient quality poses a substantial challenge. This study successfully demonstrates the CMOS-compatible synthesis of a [Formula: see text] thin film on a transparent substrate that showcases Rydberg excitons up to [Formula: see text] which is readily suitable for photonic device fabrications. These findings mark a significant advancement towards the realization of scalable and on-chip integrable Rydberg quantum technologies.

Quantum Light from Lossy Semiconductor Rydberg Excitons

The emergence of photonic quantum correlations is typically associated with emitters strongly coupled to a photonic mode. Here, we show that semiconductor Rydberg excitons, which are only weakly coupled to a free-space light mode can produce strongly antibunched fields, i.e., quantum light. This effect is fueled by a micron-scale excitation blockade between Rydberg excitons inducing pair-wise polariton scattering events. Photons incident on an exciton resonance are scattered into blue- and red-detuned pairs, which enjoy relative protection from absorption and thus dominate the transmitted light. We demonstrate that this effect persists in the presence of additional phonon coupling, strong nonradiative decay, and across a wide range of experimental parameters. Our results pave the way for the observation of quantum statistics from weakly coupled semiconductor excitons.

Rydberg polaritons in ReS2 crystals

Rhenium disulfide belongs to group VII transition metal dichalcogenides (TMDs) with attractive properties such as exceptionally high refractive index and remarkable oscillator strength, large in-plane birefringence, and good chemical stability. Unlike most other TMDs, the peculiar optical properties of rhenium disulfide persist from bulk to the monolayer, making this material potentially suitable for applications in optical devices. In this work, we demonstrate with unprecedented clarity the strong coupling between cavity modes and excited states, which results in a strong polariton interaction, showing the interest of these materials as a solid-state counterpart of Rydberg atomic systems. Moreover, we definitively clarify the nature of important spectral features, shedding light on some controversial aspects or incomplete interpretations and demonstrating that their origin is due to the interesting combination of the very high refractive index and the large oscillator strength expressed by these TMDs.