A remarkably small adjustment to the stacking of two-dimensional materials can dramatically alter how they interact with light. Researchers at City University of Hong Kong and collaborating institutions demonstrate that a one-degree change in the twist angle of an atomically thin moiré system can switch its transmittance from nearly 0%, opaque, to approximately 1%, transparent.
The work reveals that collective moiré exciton states exhibit radiative decay rates that are either strongly enhanced or suppressed depending on their in-plane wavevector, and that these states can be controlled with an applied electric field. This robust system offers a versatile platform for cooperative quantum optics, with potential for applications like single photon storage and switching.
Moiré Exciton Superlattices Enable Tunable Optical Transmittance
Calculations reveal that a moiré exciton lattice, a superlattice formed within layered two-dimensional materials, can switch between nearly complete opacity, a transmittance of approximately zero, and near-total transparency, a transmittance approaching one, with a ∘ adjustment in the twist angle. This sensitivity stems from the cooperative optical response of the moiré excitons themselves, where collective behavior dictates how light interacts with the material. This duality arises from the precise arrangement of excitons within the moiré superlattice, which functions as a network of interacting dipoles.
The localized nature of these moiré excitons, with wavefunctions spanning just a few nanometers, significantly less than the wavelength of visible light, allows for their treatment as point-like dipoles, simplifying the analysis of their cooperative optical response. The optical transition dipole moment is directly proportional to the width of the exciton wavefunction, further influencing the observed optical properties.
The system’s ability to achieve near-zero transmittance is particularly noteworthy, as typical few-layer two-dimensional materials, like monolayer graphene, usually exhibit high transmittance values, around 0.97. The researchers found that zero transmittance occurs at a collective resonance, where the incident light frequency matches the energy of the exciton lattice, effectively halting light propagation. This effect is not merely theoretical; the calculations indicate that a transmittance below 0.2 can be achieved, demonstrating a substantial reduction in light transmission.
This cooperative effect remains stable even with imperfections or non-radiative damping, provided these remain below the collective radiative decay rate. Beyond simply blocking light, the system allows for active control of transmittance through external stimuli. A gate-induced electric field gradient can efficiently switch between the superradiant and subradiant states of the collective moiré excitons, providing a means to dynamically modulate the optical properties of the material.
This switching capability, combined with the sensitivity to twist angle, opens possibilities for creating compact and energy-efficient optical switches and modulators. The ability to manipulate the flow of light at the nanoscale could have implications for integrated photonics and quantum information processing. The theoretical framework employed models each localized moiré exciton as a two-level system with an optical transition dipole, simplifying the calculations of their cooperative optical response.
The total field after scattering with the excitons is determined by a summation over all exciton lattice sites, accounting for the interactions between the incident light and the collective exciton states. This approach allows for precise prediction of the transmittance as a function of twist angle and electric field. The researchers emphasize that the real-space structure of the moiré exciton superlattice is paramount in enabling this efficient light switching.
The extended lifetimes of the subradiant states, potentially orders of magnitude longer than those of individual excitons, also present opportunities for photon storage. By trapping photons within these subradiant states, it may be possible to create a quantum memory capable of preserving quantum information for extended periods. The robustness of these effects against disorder and non-radiative losses further enhances the practicality of this approach.
The work demonstrates that the total optical response of the moiré exciton superlattice is a complex interplay of collective effects. This finding challenges the conventional understanding of light-matter interactions in two-dimensional materials and suggests new avenues for exploring functionalities based on cooperative quantum phenomena.
Real-Space Moiré Structure Reshapes Collective Optical Response
The ability to manipulate light transmission with extreme precision has taken a step forward as researchers demonstrate a near-complete switch between opaque and transparent states in atomically thin materials using a remarkably small adjustment to their stacking. This sensitivity highlights the critical role of real-space structure in governing the collective optical response of these materials, moving beyond traditional understandings focused primarily on reciprocal space. The conventional approach to understanding optical responses in materials typically defines a susceptibility per unit cell, scaling the total response with the number of cells involved in light-matter interaction.
This model relies on proper phase-matching of light beams and assumes reciprocal space structure is dominant, but the work presented challenges this view for moiré excitons. The moiré superlattice constant, when the twist angle is small, can become a significant fraction of the resonant wavelength of light, leading to constructive or destructive interference during light-matter interactions.
This interference fundamentally reshapes the overall optical response, deviating from the simple summation of individual unit cell contributions. The research focuses on cooperative optical effects emerging from the real-space superlattice formed by these moiré excitons, a departure from studies that have largely explored such effects in artificial atom arrays. While isolated atoms benefit from narrow non-radiative linewidths, assembling large, perfectly ordered arrays presents significant experimental hurdles.
These states are not static; gate-induced electric field gradients can efficiently switch between these superradiant and subradiant configurations, offering a pathway to active optical control. This localized nature of the exciton allows for a more precise understanding of how these structures interact with light at the nanoscale. Zero transmittance, a complete blocking of light, occurs at a specific collective resonance condition, demonstrated through simulations with varied twist angles.
This is a departure from conventional models that prioritize reciprocal space structure and opens new avenues for designing materials with tailored optical properties. The ability to finely tune light transmission with such a small angular adjustment or through the application of an electric field gradient positions these moiré systems as promising candidates for advanced photonic devices and quantum optical applications.
Twist Angle and Heterostrain Control Moiré Exciton Properties
A one-degree alteration to the twist angle between stacked two-dimensional materials can transform an atomically thin system from nearly opaque to almost fully transparent, researchers have demonstrated. This dramatic shift in light transmission arises from the collective behavior of moiré excitons, quasi-particles formed by the interaction of electrons and holes, arranged in a superlattice.
The work details how the real-space structure of these moiré systems directly influences their optical properties, moving beyond analyses focused solely on individual excitons. Unlike artificial atomic platforms, these moiré systems naturally form ordered lattices where each site hosts a single exciton, a configuration that simplifies exploration of cooperative quantum optical phenomena.
The researchers found that the cooperative transmittance, denoted as T, can be switched from approximately 0 (opaque) to approximately 1 (transparent) through the application of as little as two percent heterostrain, a deliberate mismatch in the lattice structure, or a one-degree adjustment in the twist angle θ. This level of control is not merely a theoretical possibility; the effect is remarkably robust against imperfections and non-radiative losses, making the moiré system a promising candidate for applications in single photon storage and switching. The team modeled each lattice site as a two-level system, analyzing the electromagnetic interactions among moiré excitons using a Green’s function.
Their analysis revealed that at a fixed light frequency, nearly zero transmission can be achieved. Conversely, full transmission can be toggled by varying the twist angle by less than one degree.
This sensitivity stems from the collective shift, which strongly depends on the moiré lattice spacing, a parameter directly controlled by the twist angle. Applying heterostrain offers an additional method for tuning transmittance, effectively modifying the lattice constant and thus the optical response. “Using heterostrain, one can realize in situ and reversible control over T,” the researchers write. The system’s unique characteristics are rooted in the electromagnetic environment experienced by each exciton, influenced by its neighboring excitons, a phenomenon analogous to the Purcell effect observed in optical cavities.
The researchers emphasize that the optical response of each exciton is strongly influenced by this local electromagnetic environment. This interaction is not simply additive. The collective behavior of the moiré exciton superlattice creates emergent properties that are distinct from those of individual excitons.
The findings demonstrate that the moiré system, despite its atomic thinness, exhibits a level of optical control previously difficult to achieve, offering a new approach to manipulating light at the nanoscale. The robust nature of this control, even in the presence of imperfections, positions moiré excitons as a compelling area of research for future quantum optical devices.
Moiré Superlattices Offer Platform for Cooperative Quantum Optics
Researchers demonstrate that moiré exciton superlattices can transition between nearly opaque and fully transparent states with this minimal adjustment, a feat enabled by the unique arrangement of excitons within the moiré pattern. This duality is directly linked to the in-plane wavevector of the excitons, meaning the direction of their momentum influences how quickly they emit light. The researchers found that these super- and subradiant states are not fixed properties but can be efficiently switched using an applied electric field gradient, providing a dynamic mechanism for controlling the flow of light.
This external control is important for creating active optical devices that can respond to external stimuli. The system’s ability to achieve such precise control is rooted in the way excitons interact within the moiré superlattice. Each localized moiré exciton behaves as a point-like dipole, and the strength of this dipole is proportional to the moiré superlattice constant, .
This relationship, coupled with the ordered arrangement of excitons, allows for strong dipole-dipole interactions even when the superlattice constant is relatively large. The arrangement parallels a single atom coupled to a nanophotonic structure, enabling the cooperative quantum optical functionalities explored in this work. The robustness of this system against imperfections is particularly noteworthy. The cooperative optical effects are largely unaffected by positional disorder, missing excitons, or variations in the strength of individual exciton transitions.
However, the system is more sensitive to variations in exciton resonance frequencies, , tolerating differences on the order of a few hundred GHz. Fortunately, existing two-dimensional materials have demonstrated linewidths as low as tens of GHz at cryogenic temperatures, suggesting that achieving the necessary conditions for cooperative quantum optics is within reach. This means that as the twist angle decreases, the dipole moment increases, strengthening the interaction between excitons.
This scaling relationship, d≈, allows for sustained strong dipole-dipole interactions even with larger superlattice constants. The potential applications of this technology extend beyond simple light switching. The long lifetimes of the subradiant exciton states, potentially extended by orders of magnitude compared to single excitons, make them promising candidates for photon memory. The system’s ability to switch between superradiant and subradiant states further enhances its potential for creating complex optical circuits and devices.
The researchers note that the cooperative optical effects are robust against positional disorder, vacancies of localized moiré excitons and variations in the magnitude of transition dipoles. This control, combined with the system’s inherent robustness, positions moiré excitons as a compelling platform for exploring fundamental quantum optics and developing advanced photonic technologies.
👉 More information
🗞 Cooperative Quantum Optical Effects of Moiré Exciton Superlattices
✍️ Haowei Xu, Wang Yao and Ju Li
🧠 DOI: http://link.aps.org/doi/10.1103/sp3h-pkqx
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