Metamaterials with broken symmetry and their applications




Sun, Liuyang

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Conventionally, the development of optical technology has often relied on the discovery of new natural materials with desirable optical properties for specific tasks, e.g. birefringence for polarizer, or fluorite for dispersionless lens. Meanwhile, the typical feature dimensions of the optical components are much larger than the wavelength of light. For example, the radius of a curved lens surface is on the order of centimeters. Recently, instead of ‘discovering’ new materials with proper optical properties, great efforts have been dedicated to ‘designing’ artificial materials of desirable optical properties. Such artificial materials are known as metamaterials, which are made from assemblies of chosen dielectric and/or metallic components. Optical metamaterials have anomalous optical properties beyond what are naturally available in their basic constituent materials, e.g., optical magnetism and negative refractive index. With advanced nano fabrication and imaging technologies, researchers are able to make metamaterials in unprecedented ways. New designs and applications of metamaterials are being developed at rapid pace. However, challenges remain in various aspects. For example, mechanisms underlying the novel properties of complicated metamaterials are not understood thoroughly. How can one converge to effective designs of metamaterials while facing an enormous parameter space. In this thesis, we explore intentional symmetry breaking as a guiding principle for designing metamaterials. We experimentally demonstrate their properties and applications. In the first example, we first theoretically explore the light-matter interaction for a single metal nanoparticle (NP) and metamolecules which are clusters of NPs. The NPs can be assembled in such a way that they exhibit strong magnetic response at optical frequencies, a property absent in natural materials. While enhancement of the optical magnetism has previously been demonstrated by breaking the symmetry of the metamolecule, a fundamental understanding of the mechanisms is lacking. By fabricating the metamolecule of suitably designed broken symmetries, we separated the magnetic moment and magneto-electric contributions to the magnetic resonance in the optical frequency regime for the first time. By choosing uniform and spherical NPs, and by using angle- and polarization-resolved scattering measurements, we separate the different contributions to optical magnetism. The demonstrated capability of controlling different contributions to the magnetic resonance is expected to stimulate future development of metamolecules and metamaterials with exotic optical properties. As the second example, we designed and fabricated metasurfaces with a broken mirror symmetry. We succeeded in applying such metasurface to manipulate the valley degree of freedom in monolayer transition metal dichalcogenides (TMDCs). Valley refers to the energy extrema points in the energy bands. Typically, valley does not coupling to any external field specifically. By coupling to surface plasmon polaritons along asymmetric grooves on a metasurface, valley excitons in monolayer MoS2are spatially separated at room temperature an important step toward valleytronic applications. The combination of metasurfaces and 2D materials enables conceptually novel hybrid photonic devices that may be used to control exciton/spin/valley transport in unprecedented ways and to engineer quantum light emitters. We also demonstrated a molecule chirality sensor with zeptomole detection sensitivity based on two coupled metasurfaces lacking reflection symmetry. Chirality of biomedical molecule is strongly related to their pharmacological effects, such as potency and toxicity. We exploited the interaction between enantiomers and metamaterials with specifically designed symmetry. By placing the specimen onto a chiral metamaterial, we found that the molecular chiral signal can be enhanced by a factor of 100. The spectra obtained from enantiomers of opposite handedness show opposite spectral bending, providing a way to reveal the molecular chirality. This work presents a novel method for advanced biomedical detection with ultrathin planarized nanophotonic device. In conclusion, we have been able to theoretically analyze and experimentally realize and characterize asymmetric metamaterials to explore both fundamental physics and practical applications, oftentimes guided by symmetry considerations. We briefly discuss remaining challenges in the plasmonic metamaterial fields and possible solutions



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