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Open Access Publications from the University of California

Scholarly Works (4 results)

  • Thesis
  • Peer Reviewed
UC Berkeley Electronic Theses and Dissertations (2013)

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.

    Cover page: Probing Optical 'Dark' Effects in Artificial and Natural Nano-structures
    • Article
    • Peer Reviewed
    UC Berkeley Previously Published Works (2015)
    Optical selection rules fundamentally determine the optical transitions between energy states in a variety of physical systems, from hydrogen atoms to bulk crystals such as gallium arsenide. These rules are important for optoelectronic applications such as lasers, energy-dispersive X-ray spectroscopy, and quantum computation. Recently, single-layer transition metal dichalcogenides have been found to exhibit valleys in momentum space with nontrivial Berry curvature and excitons with large binding energy. However, there has been little study of how the unique valley degree of freedom combined with the strong excitonic effect influences the nonlinear optical excitation. Here, we report the discovery of nonlinear optical selection rules in monolayer WS2, an important candidate for visible 2D optoelectronics because of its high quantum yield and large direct bandgap. We experimentally demonstrated this principle for second-harmonic generation and two-photon luminescence (TPL). Moreover, the circularly polarized TPL and the study of its dynamics evince a sub-ps interexciton relaxation (2p R 1s). The discovery of this new optical selection rule in a valleytronic 2D system not only considerably enhances knowledge in this area but also establishes a foundation for the control of optical transitions that will be crucial for valley optoelectronic device applications such as 2D valley-polarized THz sources with 2p–1s transitions, optical switches, and coherent control for quantum computing.
      Cover page: Nonlinear optical selection rule based on valley-exciton locking in monolayer ws2
      • Article
      • Peer Reviewed
      UC Berkeley Previously Published Works (2016)
      Second-harmonic generation (SHG) has found extensive applications from hand-held laser pointers to spectroscopic and microscopic techniques. Recently, some cleavable van der Waals (vdW) crystals have shown SHG arising from a single atomic layer, where the SH light elucidated important information such as the grain boundaries and electronic structure in these ultra-thin materials. However, despite the inversion asymmetry of the single layer, the typical crystal stacking restores inversion symmetry for even numbers of layers leading to an oscillatory SH response, drastically reducing the applicability of vdW crystals such as molybdenum disulfide (MoS2). Here, we probe the SHG generated from the noncentrosymmetric 3R crystal phase of MoS2. We experimentally observed quadratic dependence of second-harmonic intensity on layer number as a result of atomically phase-matched nonlinear dipoles in layers of the 3R crystal that constructively interfere. By studying the layer evolution of the A and B excitonic transitions in 3R-MoS2 using SHG spectroscopy, we also found distinct electronic structure differences arising from the crystal structure and the dramatic effect of symmetry and layer stacking on the nonlinear properties of these atomic crystals. The constructive nature of the SHG in this 2D crystal provides a platform to reliably develop atomically flat and controllably thin nonlinear media.
        Cover page: Atomically phase-matched second-harmonic generation in a 2D crystal
        • Article
        • Peer Reviewed
        UC Berkeley Previously Published Works (2018)
        Higher-order correlated excitonic states arise from the mutual interactions of excitons, which generally requires a significant exciton density and therefore high excitation levels. Here, we report the emergence of two biexcitons species, one neutral and one charged, in monolayer tungsten diselenide under moderate continuous-wave excitation. The efficient formation of biexcitons is facilitated by the long lifetime of the dark exciton state associated with a spin-forbidden transition, as well as improved sample quality from encapsulation between hexagonal boron nitride layers. From studies of the polarization and magnetic field dependence of the neutral biexciton, we conclude that this species is composed of a bright and a dark excitons residing in opposite valleys in momentum space. Our observations demonstrate that the distinctive features associated with biexciton states can be accessed at low light intensities and excitation densities.
          Cover page: Efficient generation of neutral and charged biexcitons in encapsulated WSe2 monolayers
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