Exciton-dielectric mode coupling in MoS2 nanoflakes visualized by cathodoluminescence

Two-dimensional (2D) transition metal dichalcogenides (TMDCs), possessing unique exciton luminescence properties, have attracted significant attention for use in optical and electrical devices. TMDCs are also high refractive index materials that can strongly confine the electromagnetic field in nanoscale dimensions when patterned into nanostructures, thus resulting in complex light emission that includes exciton and dielectric resonances. Here, we use cathodoluminescence (CL) to experimentally visualize the emission modes of single molybdenum disulfide (MoS2) nanoflakes and to investigate luminescence enhancement due to dielectric resonances in nanoscale dimensions, by using a scanning transmission electron microscope. Specifically, we identify dielectric modes whose resonant wavelength is sensitive to the shape and size of the nanoflake, and exciton emission peaks whose energies are insensitive to the geometry of the flakes. Using a four-dimensional CL method and boundary element method simulations, we further theoretically and experimentally visualize the emission polarization and angular emission patterns, revealing the coupling of the exciton and dielectric resonant modes. Such nanoscopic observation provides a detailed understanding of the optical responses of MoS2 including modal couplings of excitons and dielectric resonances which play a crucial role in the development of energy conversion devices, single-photon emitters, and nanophotonic circuits with enhanced light-matter interactions.

Silicene dynamic optical response in the presence of external electric and exchange fields

We investigate the dynamic optical transition of monolayer silicene in the presence of external electric and exchange fields within the low-energy tight-binding model. Applying external electric and exchange fields breaks the silicene band structure spin and valley degeneracies. Three phases of silicene corresponding to different strengths of perpendicular electric field with respect to the spin-orbit coupling (∆z < ∆so, ∆z = ∆so and ∆z > ∆so) are considered. We obtain the spinand valley-dependent optical responses to the incoming circularly polarized light using the Kubo formula. We show and discuss how the magnitude and direction of the transverse and longitudinal optical responses of such a system change with the electric and exchange fields. Our calculations suggest that the intraband part of the longitudinal optical response as well as the initial point of the interband part have strong dependencies on the exchange field. Furthermore, we show that one of the spin subbands plays a dominant role in the response to polarized light. Depending on the type of incident light polarization, the dominant subband may change. Our results shed light on the relation between silicene dynamic optical responses and externally applied fields.

Electromagnetic response in spiral magnets and emergent inductance

Emergent electromagnetism in magnets originates from the strong coupling between conduction electron spins and those of noncollinear ordered moments and the consequent Berry phase. This offers possibilities to develop new functions of quantum transport and optical responses. The emergent inductance in spiral magnets is an example recently proposed and experimentally demonstrated, using the emergent electric field induced by alternating currents. However, the microscopic theory of this phenomenon is missing, which should reveal factors to determine the magnitude, sign, frequency dependence, and nonlinearity of the inductance L. Here we theoretically study electromagnetic responses of spiral magnets by taking into account their collective modes. In sharp contrast to collinear spin-density wave, the system remains metallic even in one dimension, and the canonical conjugate relation of uniform magnetization and phason coordinate plays an essential role, determining the properties of L. This result opens a way to design the emergent inductance of desired properties.

Anisotropic optical responses of layered thallium arsenic sulfosalt gillulyite

Multi-element two-dimensional (2D) materials hold great promise in the context of tailoring the physical and chemical properties of the materials via stoichiometric engineering. However, the rational and controllable synthesis of complex 2D materials remains a challenge. Herein, we demonstrate the preparation of large-area thin quaternary 2D material flakes via mechanical exfoliation from a naturally occurring bulk crystal named gillulyite. Furthermore, the anisotropic linear and nonlinear optical properties including anisotropic Raman scattering, linear dichroism, and anisotropic third-harmonic generation (THG) of the exfoliated gillulyite flakes are investigated. The observed highly anisotropic optical properties originate from the reduced in-plane crystal symmetry. Additionally, the third-order nonlinear susceptibility of gillulyite crystal is retrieved from the measured thickness-dependent THG emission. We anticipate that the demonstrated strong anisotropic linear and nonlinear optical responses of gillulyite crystal will facilitate the better understanding of light-matter interaction in quaternary 2D materials and its implications in technological innovations such as photodetectors, frequency modulators, nonlinear optical signal processors, and solar cell applications.

Polarization-dependent optical responses in natural 2D layered mineral teallite

Multi-element layered materials enable the use of stoichiometric variation to engineer their optical responses at subwavelength scale. In this regard, naturally occurring van der Waals minerals allow us to harness a wide range of chemical compositions, crystal structures and lattice symmetries for layered materials under atomically thin limit. Recently, one type of naturally occurring sulfide mineral, ternary teallite has attained significant interest in the context of thermoelectric, optoelectronic, and photovoltaic applications, but understanding of light-matter interactions in such ternary teallite crystals is scarcely available. Herein, polarization-dependent linear and nonlinear optical responses in mechanically exfoliated teallite crystals are investigated including anisotropic Raman modes, wavelength-dependent linear dichroism, optical band gap evolution, and anisotropic third-harmonic generation (THG). Furthermore, the third-order nonlinear susceptibility of teallite crystal is estimated using the thickness-dependent THG emission process. We anticipate that our findings will open the avenue to a better understanding of the tailored light-matter interactions in complex multi-element layered materials and their implications in optical sensors, frequency modulators, integrated photonic circuits, and other nonlinear signal processing applications.