A Unique Method for Manipulating Atomic Layers to Create Cutting-Edge Materials

Physics and materials science comprehensively understand the interaction of light with naturally occurring materials. However, over the last few decades, scientists have engineered metamaterials capable of interacting with light in unconventional ways, surpassing the inherent physical limitations of naturally occurring materials.

A metamaterial typically consists of arrays of “meta-atoms,” structured at a scale of around a hundred nanometers. While the precise arrangements of these meta-atoms allow for specific light-matter interactions, the large size of meta-atoms compared to regular atoms, which are smaller than a nanometer, has restricted the practical applications of metamaterials.

Bo Zhen’s Team Unveils Groundbreaking Method for Engineering Atomic Structures

In a breakthrough, a collaborative research team led by Bo Zhen at the University of Pennsylvania has introduced a new method. This approach involves engineering the atomic structures of materials by stacking two-dimensional arrays in spiral formations, enabling novel light-matter interactions. This innovative technique overcomes existing technical limitations and opens avenues for advanced lasers, imaging, and quantum technologies. The study’s findings were published in the journal Nature Photonics.

Zhen, a senior author and assistant professor in the School of Arts & Sciences at Penn, explains the analogy: “It’s similar to stacking a deck of cards but twisting each card slightly before adding it to the pile. This twist changes how the entire ‘deck’ responds to light, enabling it to exhibit new properties that individual layers, or traditional stacks, do not possess.”

Bumho Kim, the paper’s first author and a postdoctoral researcher in the Zhen Lab, elucidates that by stacking layers of tungsten disulfide (WS2) and twisting them at specific angles, they introduced screw symmetries.

Kim emphasizes the crucial aspect of this technique, stating, “The key is in manipulating the twist. By twisting the layers at precise angles, you alter the symmetry of the stack. Symmetry, in this context, pertains to the limitations imposed on certain material properties—such as their interaction with light—based on their spatial arrangement.“

Through precise control of the atomic-scale arrangement, the researchers have altered the capabilities of these materials. By manipulating the twist across multiple layers of WS2, they have created 3D nonlinear optical materials.

WS2 Single Layer’s Symmetry and Second-Harmonic Generation Explained by Kim

Kim clarifies that a single layer of WS2 exhibits specific symmetries that enable particular interactions with light, such as second-harmonic generation (SHG), where two photons at a given frequency can interact with the material to produce a new photon at double the frequency.

However, when two layers of WS2 are stacked with a twist angle different from the conventional 0° or 180°, the mirror symmetries present in the single layer are disrupted. This broken mirror symmetry is crucial, leading to a chiral response—an entirely novel aspect not observed in individual layers.

The chiral response is significant because it results from the coupling between the electronic wavefunctions of the two layers, a phenomenon unique to twisted interfaces. Zhen notes an intriguing property: the sign of the chiral nonlinear response flips when the twist angle is reversed, showcasing direct control over nonlinear properties by adjusting the twist angle between layers, a level of tunability with revolutionary potential for designing optical materials with custom responses.

Unraveling SHG Responses in Multi-Layered Stacks with Twist Angles

Progressing from bilayers to trilayers and beyond, the researchers observed how interfacial SHG responses can constructively or destructively interfere based on the twist angles between layers. In stacks with layers in multiples of four, the chiral responses from all interfaces add up, while the in-plane responses cancel out, resulting in a material that exhibits only chiral nonlinear susceptibilities—an achievement requiring precise stacking and twisting of the layers.

The researchers discovered that screw symmetry introduces new selectivity for the light’s electric field in the material, influencing its direction and intensity. Kim highlights how they identified that screw symmetry enables a new type of light generation in twisted four- and eight-layer stacks: counter-circularly polarized third harmonic generation, where light travels in the opposite spiral direction—a quality absent in constituent WS2 monolayers.

In experimental tests, the researchers validated the predicted nonlinearities in various configurations of twisted WS2 stacks. They observed new nonlinear responses and circular selectivity in twisted WS2 stacks, presenting possibilities not found in naturally occurring WS2 and potentially revolutionizing the field of nonlinear optics.

Read the original article on: Phys Org

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