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Open Access Publications from the University of California

Scholarly Works (13 results)

  • Thesis
  • Peer Reviewed
UC San Diego Electronic Theses and Dissertations (2017)

Uniaxial materials whose axial and tangential permittivities have opposite signs are referred to as indefinite or hyperbolic media. While hyperbolic responses are normally achieved with artificially fabricated nanostructures, hexagonal boron nitride (hBN) naturally possesses this property due to the anisotropic phonons in the mid-infrared. In this dissertation, we studied polaritonic phenomena in hBN using scattering-type scanning near-field optical microscopy (s-SNOM). We performed infrared nano-imaging of highly confined and low-loss hyperbolic phonon polaritons (HP2s) in hBN. The polariton wavelength was shown to be governed by the hBN thickness according to a linear law persisting down to few atomic layers. We have also carried out the tunable hyperbolic response in metastructures comprised of a monolayer graphene deposited on hBN. Electrostatic gating of the top graphene layer allows for the modification of wavelength and intensity of HP2s in bulk hBN. The physics of the modification originates from the plasmon-phonon coupling in the hyperbolic medium. Furthermore, we demonstrated the “hyperlens” for subdiffractional focusing and imaging using a slab of hBN with a record high resolution due to the natural lattice structure in hBN. Finally, we have systematically studied the relative efficiency of polariton emission in two-dimensional materials in the case of HP2S in hBN. We have observed polariton waves launched by various kinds of emitters and compared their relative efficiency and analyzed the origins of the efficiency difference.

    Cover page: Hyperbolic phonon polaritons in hexagonal boron nitride
    • Article
    • Peer Reviewed
    UC Berkeley Previously Published Works (2024)
    The electronic and optical properties of 2D transition metal dichalcogenides are dominated by strong excitonic resonances. Exciton dynamics plays a critical role in the functionality and performance of many miniaturized 2D optoelectronic devices; however, the measurement of nanoscale excitonic behaviors remains challenging. Here, a near-field transient nanoscopy is reported to probe exciton dynamics beyond the diffraction limit. Exciton recombination and exciton-exciton annihilation processes in monolayer and bilayer MoS2 are studied as the proof-of-concept demonstration. Moreover, with the capability to access local sites, intriguing exciton dynamics near the monolayer-bilayer interface and at the MoS2 nano-wrinkles are resolved. Such nanoscale resolution highlights the potential of this transient nanoscopy for fundamental investigation of exciton physics and further optimization of functional devices.
      Cover page: Transient Nanoscopy of Exciton Dynamics in 2D Transition Metal DichalcogenidesCreative Commons 'BY' version 4.0 license
      • Article
      • Peer Reviewed
      UC San Diego Previously Published Works (2018)
      Hexagonal boron nitride (hBN) is a natural hyperbolic material that supports both volume-confined hyperbolic polaritons and sidewall-confined hyperbolic surface polaritons (HSPs). In this work, efficient excitation, control, and steering of HSPs are demonstrated in hBN through engineering the geometry and orientation of hBN sidewalls. By combining infrared nanoimaging and numerical simulations, the reflection, transmission, and scattering of HSPs are investigated at the hBN corners with various apex angles. It is also shown that the sidewall-confined nature of HSPs enables a high degree of control over their propagation by designing the geometry of hBN nanostructures.
        Cover page: Manipulation and Steering of Hyperbolic Surface Polaritons in Hexagonal Boron Nitride
        • Article
        • Peer Reviewed
        UC San Diego Previously Published Works (2017)
        Mid-infrared nanoimaging and spectroscopy of two-dimensional (2D) materials have been limited so far to scattering-type scanning near-field optical microscopy (s-SNOM) experiments, where light from the sample is scattered by a metallic-coated atomic force microscope (AFM) tip interacting with the material at the nanoscale. These experiments have recently allowed imaging of plasmon polaritons in graphene as well as hyperbolic phonon polaritons in hexagonal boron nitride (hBN). Here we show that the high mechanical sensitivity of an AFM cantilever can be exploited for imaging hyperbolic phonon polaritons in hBN. In our imaging process, the lattice vibrations of hBN micrometer-sized flakes are locally enhanced by the launched phonon polaritons. These enhanced vibrations are coupled to the AFM tip in contact with the sample surface and recorded during scanning. Imaging resolution of Δ/20 is shown (Δ being the polaritonic fringes' separation distance), comparable to the best resolution in s-SNOM. Importantly, this detection mechanism is free from light background, and it is in fact the first photonless detection of phonon polaritons.
          Cover page: Mechanical Detection and Imaging of Hyperbolic Phonon Polaritons in Hexagonal Boron Nitride
          • Article
          • Peer Reviewed
          UC San Diego Previously Published Works (2017)
          Using magneto-infrared spectroscopy, we have explored the charge dynamics of (Bi,Sb)2Te3 thin films on InP substrates. From the magneto-transmission data we extracted three distinct cyclotron resonance (CR) energies that are all apparent in the broad band Faraday rotation (FR) spectra. This comprehensive FR-CR data set has allowed us to isolate the response of the bulk states from the intrinsic surface states associated with both the top and bottom surfaces of the film. The FR data uncovered that electron- and hole-type Dirac Fermions reside on opposite surfaces of our films, which paves the way for observing many exotic quantum phenomena in topological insulators.
            Cover page: Faraday Rotation Due to Surface States in the Topological Insulator (Bi1–x Sb x )2Te3
            • Article
            • Peer Reviewed
            UC San Diego Previously Published Works (2018)
            Conventional optical components are limited to size scales much larger than the wavelength of light, as changes to the amplitude, phase and polarization of the electromagnetic fields are accrued gradually along an optical path. However, advances in nanophotonics have produced ultrathin, so-called 'flat' optical components that beget abrupt changes in these properties over distances significantly shorter than the free-space wavelength. Although high optical losses still plague many approaches, phonon polariton (PhP) materials have demonstrated long lifetimes for sub-diffractional modes in comparison to plasmon-polariton-based nanophotonics. We experimentally observe a threefold improvement in polariton lifetime through isotopic enrichment of hexagonal boron nitride (hBN). Commensurate increases in the polariton propagation length are demonstrated via direct imaging of polaritonic standing waves by means of infrared nano-optics. Our results provide the foundation for a materials-growth-directed approach aimed at realizing the loss control necessary for the development of PhP-based nanophotonic devices.
              Cover page: Ultralow-loss polaritons in isotopically pure boron nitride
              • Article
              • Peer Reviewed
              UC San Diego Previously Published Works (2016)
              We use scanning near-field optical microscopy to study the response of hexagonal boron nitride nanocones at infrared frequencies, where this material behaves as a hyperbolic medium. The obtained images are dominated by a series of "hot" rings that occur on the sloped sidewalls of the nanocones. The ring positions depend on the incident laser frequency and the nanocone shape. Both dependences are consistent with directional propagation of hyperbolic phonon-polariton rays that are launched at the edges and zigzag through the interior of the nanocones, sustaining multiple internal reflections off the sidewalls. Additionally, we observe a strong overall enhancement of the near-field signal at discrete resonance frequencies. These resonances attest to low dielectric losses that permit coherent standing waves of the subdiffractional polaritons to form. We comment on potential applications of such shape-dependent resonances and the field concentration at the hot rings.
                Cover page: Imaging of Anomalous Internal Reflections of Hyperbolic Phonon-Polaritons in Hexagonal Boron Nitride
                • Article
                • Peer Reviewed
                UC San Diego Previously Published Works (2017)
                We investigated phonon-polaritons in hexagonal boron nitride - a naturally hyperbolic van der Waals material - by means of the scattering-type scanning near-field optical microscopy. Real-space nanoimages we have obtained detail how the polaritons are launched when the light incident on a thin hexagonal boron nitride slab is scattered by various intrinsic and extrinsic inhomogeneities, including sample edges, metallic nanodisks deposited on its top surface, random defects, and surface impurities. The scanned tip of the near-field microscope is itself a polariton launcher whose efficiency proves to be superior to all the other types of polariton launchers we studied. Our work may inform future development of polaritonic nanodevices as well as fundamental studies of collective modes in van der Waals materials.
                  Cover page: Efficiency of Launching Highly Confined Polaritons by Infrared Light Incident on a Hyperbolic Material
                  • Article
                  • Peer Reviewed
                  UC San Diego Previously Published Works (2018)
                  Conventional optical components are limited to size-scales much larger than the wavelength of light, as changes in the amplitude, phase and polarization of the electromagnetic fields are accrued gradually along an optical path. However, advances in nanophotonics have produced ultra-thin, co-called "flat" optical components that beget abrupt changes in these properties over distances significantly shorter than the free space wavelength. While high optical losses still plague many approaches, phonon polariton (PhP) materials have demonstrated long lifetimes for sub-diffractional modes in comparison to plasmon-polariton-based nanophotonics. We experimentally observe a three-fold improvement in polariton lifetime through isotopic enrichment of hexagonal boron nitride (hBN). Commensurate increases in the polariton propagation length are demonstrated via direct imaging of polaritonic standing waves by means of infrared nano-optics. Our results provide the foundation for a materials-growth-directed approach towards realizing the loss control necessary for the development of PhP-based nanophotonic devices.
                    Cover page: Ultralow-loss polaritons in isotopically pure boron nitride
                    • Article
                    • Peer Reviewed
                    UC San Diego Previously Published Works (2019)
                    An amendment to this paper has been published and can be accessed via a link at the top of the paper.
                      Cover page: Author Correction: Ultralow-loss polaritons in isotopically pure boron nitride
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