Led by Bo Zhen from the School of Arts & Sciences, a team of researchers has innovatively created materials by artificially manipulating and layering two-dimensional atomic “sheets.” These materials interact with light and matter in ways distinct from their original 2D atomic compositions, heralding new possibilities in laser, imaging, and quantum technology development.
This group of physicists at the School of Arts & Sciences discovered that twisting stacks of tungsten disulfide offers fresh methods to control light.
Understanding how light interacts with natural materials has been a staple of physics and materials science. Recently, however, researchers have developed metamaterials, which have unique interactions with light, surpassing the capabilities of natural materials.
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Metamaterials: Challenges and Opportunities
Metamaterials consist of “meta-atom” arrays, engineered into desired nanostructures. The organization of these meta-atoms allows for precise light-matter interaction control. Nevertheless, the substantial size of meta-atoms, compared to regular atoms, has restricted the practical utility of metamaterials.
Pioneering Metamaterial Research
Bo Zhen’s team at the University of Pennsylvania has now introduced a groundbreaking method for manipulating material atomic structures. By spirally stacking two-dimensional arrays, they harness novel light-matter interactions. This technique propels metamaterials beyond existing limitations, opening pathways for advanced lasers, imaging systems, and quantum technologies. Their work is detailed in Nature Photonics.
Zhen likens this process to stacking a deck of cards but with a twist in each layer. This twist alters the entire stack’s response to light, manifesting properties unseen in individual layers or traditional stacks.
An illustration depicts light passage through twisted tungsten disulfide, altering the light’s color and orientation, a feature unique to this engineered material.
Insights into Tungsten Disulfide Manipulation
Bumho Kim, a postdoctoral researcher in Zhen’s lab, notes that by layering tungsten disulfide (WS2) and introducing specific twists, they create screw symmetries.
Kim emphasizes the importance of twist control, altering the stack’s symmetry and consequently, its interaction with light.
By adjusting atomic arrangements, the team has redefined material capabilities, leading to the development of 3D nonlinear optical materials.
The Role of Chiral Responses
Kim explains that a single WS2 layer has specific symmetries allowing certain light interactions, like second-harmonic generation (SHG). However, stacking two WS2 layers with unconventional twist angles breaks these symmetries, resulting in a chiral response—a novel phenomenon not present in individual layers.
This chiral response arises from the electronic wavefunctions’ coupling in twisted interfaces.
Advancing Nonlinear Properties
Zhen highlights that reversing the twist angle inversely affects the chiral nonlinear response. This discovery indicates the potential for tailor-made optical materials through simple adjustments in layer twists.
Exploring beyond bilayers, the team observed that interfacial SHG responses could amplify or negate each other, dependent on the twist angles.
In four-layer stacks, the chiral responses cumulatively manifest, while in-plane responses negate each other, creating materials exclusively exhibiting chiral nonlinear susceptibilities—a result achievable only through precise layer manipulation.
The Significance of Screw Symmetry
The team discovered that screw symmetry introduces selectivity in the material’s interaction with light’s electric field. Kim notes this symmetry enables novel light generation in four- and eight-layer twisted stacks, a characteristic absent in standard WS2 monolayers.
Verifying the Technique and Its Impacts
Experimental tests confirmed the inherent nonlinearities and circular selectivity in various twisted WS2 stack configurations. These findings, showcasing unique nonlinear responses in engineered WS2 stacks,
