When two lattices with different angles or periodicities intersect, they create something fascinating: a moiré superlattice. This unique realm is where extraordinary phenomena like superconductivity and optical solitons come to life. At the heart of this realm lies the moiré flatband, a crucial element in shaping advanced interactions between light and matter, such as laser emission and second harmonic generation. In the world of moiré physics and its practical applications, having control over flatbands is a game-changer.
Moiré flatbands are usually generated using specific structures, often manipulated by adjusting rotation angles (known as the magic angle) and spacings (referred to as the magic distance) between the two lattice layers. Recently, a collaborative research team from several universities in China introduced a new method to control moiré flatbands. They achieved this by altering the band offset of two photonic lattices in a parameter space.
As reported in Advanced Photonics Nexus, the team made an intriguing discovery. Instead of flatbands appearing and disappearing as the band offset changes, they found two robust groups of flatbands that remain stable across a wide range of band offsets. This stability makes it easier to control the structural parameters needed to create complex superlattices, opening up fresh possibilities in moiré photonics. By tweaking these structural parameters, they could adjust the resonant frequencies of these robust flatbands, leading to the development of innovative multi-resonant moiré devices.
But how did they achieve this breakthrough? They started with a mismatched silicon-based bilayer moiré superlattice and adjusted the band offset by changing the thickness of one layer of the superlattices. Through careful calculations of the superlattice band structure at different band offsets, they observed that band offset played a crucial role in controlling moiré flatbands, including some that remained stable within a broad range of band offset values.
The key insight here is that creating exceptional moiré superlattices no longer demands precise lattice control. Instead, it allows for the tuning of moiré flatband resonance frequencies by adjusting the band offset. To demonstrate this capability, the researchers thoroughly examined the localized modes originating from the two groups of robust flatbands in finite-sized moiré superlattices, confirming the feasibility of high-quality doubly resonant moiré superlattices.
To understand the mechanism behind robust flatband formation, the authors developed a simple yet effective diagrammatic model based on coupled-mode theory, accounting for the structural characteristics of moiré superlattices. This model revealed both the similarities and differences in the formation of these flatbands. To further validate their findings, the authors incorporated full-wave calculations into the diagrammatic model, successfully predicting the field distribution of these robust flatbands.
This advancement opens up exciting new possibilities in moiré physics. Controlling moiré flatbands by adjusting the band offset in parameter space is a beautifully simple method that holds the key to unlocking complex superlattices and unraveling the mysteries of flatband emergence and disappearance. With control over the frequencies of these flatbands, a realm of multi-resonant and high-quality moiré superlattices emerges.
But there’s more to it—the diagrammatic model isn’t just a tool; it’s a window into the world of flatband formation across various moiré superlattices. This research is poised to inspire future explorations in innovative moiré devices and the captivating field of moiré physics.