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Probing Optical 'Dark' Effects in Artificial and Natural Nano-structures

  • Author(s): Ye, Ziliang
  • Advisor(s): Zhang, Xiang
  • et al.

In the nano-scale world, many interesting optical effects cannot be detected in the far field zone by linear probes. Nevertheless, they are promising to bring numerous novel optical functionalities while some already play dominant roles in determining the overall optical responses. This dissertation presents a consistent endeavor to study these fascinating optical dark effects, using a wide range of tools including, but not limited to, the nearfield scanning optical microscopy, the nonlinear spectroscopy/microscopy and the photocurrent scanning microscopy. The studied systems are classified into two categories: artificial plasmonic structures and natural two dimensional transition metal dichalcogenide.

Among the plasmonic artificial structures, we focus on plasmonic antennas. By fine-tuning the nearfield coupling between them, we demonstrate a range of classical analogs to many interesting atomic effects, such as electromagnetic induced transparency and anti-Hermitian coupling induced superradiance. Both of them are dark to the far field, but can serve as effective color sorters below the diffraction limit within the near field zone. Employing the resonance nature of plasmonic antenna, we demonstrate for the first time the photonic spin Hall effect at the metasurface. A large spin orbit coupling induced deflecting is observed even at the normal incidence condition. Also, with nearfield microscopy, we map the mode profile of the hybrid waveguide. The optical mode is found to be highly confined within the gap between the metal and dielectric layer, which has a good potential to become the future platform for integrated active optical components.

In the study of 2D transition metal dichalcogenide, we discover a series of dark excitonic resonance with a set of selection rules complimentary to the linear transition, provided by two-photon absorption process. These resonances are originated from a very large binding energy in a unique 2D exciton model. A TMDC exciton based light emitting device is further demonstrated as a potential application. With the nonlinear probes, both the crystal orientation and an edge state resonance are identified, which has an otherwise zero or negligible response to the linear probe. Finally, we extend the study to explore the valley Hall effect in TMDC. Different photocurrent mechanisms are investigated and a laser polarization dependent transverse photocurrent is observed.

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