Nano-optical imaging of exciton-plasmon polaritons in WSe2/Au heterostructures.

We report a nano-optical imaging study of exciton-plasmon polaritons (EPPs) in WSe2/Au heterostructures with scattering-type scanning near-field optical microscopy (s-SNOM). By mapping the interference fringes of EPPs at various excitation energies, we constructed the dispersion diagram of the EPPs, which shows strong exciton-plasmon coupling with a sizable Rabi splitting energy (∼0.19 eV). Furthermore, we found a sensitive dependence of the polariton wavelength (λp) on WSe2 thickness (d). When d is below 40 nm, λp decreases rapidly with increasing d. As d reaches 50 nm and above, λp drops to 210 nm, which is over 4 times smaller than that of the free-space photons. Our simulations indicate that the high spatial confinement of EPPs is due to the strong localization of the polariton field inside WSe2. Our work uncovers the transport properties of EPPs and paves the way for future applications of these highly confined polaritons in nanophotonics and optoelectronics.

Thermodynamically Driven Formation of Intercalated Cu Carpets from Supported Cu Pyramids on MoS2.

Thermodynamic and kinetic analyses based on our first-principles density functional theory calculations are used to interpret the experimentally observed formation of Cu carpets intercalated under the top layer of a 2H-MoS2 substrate. Spontaneous Cu transport from Cu pyramids on top of the MoS2 substrate through surface point defects to the growing Cu carpet is shown to be driven by a slightly lower chemical potential for the Cu carpet. We demonstrate that the competition between a preference for a thicker Cu carpet and the cost of elastic stretching of the top MoS2 layer results in a selected Cu carpet thickness. We also propose that Cu transport occurs primarily via vacancy-mediated diffusion through constricting point defect portals.

Imaging Anisotropic Waveguide Exciton Polaritons in Tin Sulfide.

In recent years, novel materials supporting in-plane anisotropic polaritons have attracted a great deal of research interest due to their capability of shaping nanoscale field distributions and controlling nanophotonic energy flows. Here we report a nano-optical imaging study of waveguide exciton polaritons (EPs) in tin sulfide (SnS) in the near-infrared (near-IR) region using scattering-type scanning near-field optical microscopy (s-SNOM). With s-SNOM, we mapped in real space the propagative EPs in SnS, which show sensitive dependence on the excitation energy and sample thickness. Moreover, we found that both the polariton wavelength and propagation length are anisotropic in the sample plane. In particular, in a narrow spectral range from 1.32 to 1.44 eV, the EPs demonstrate quasi-one-dimensional propagation, which is rarely seen in natural polaritonic materials. A further analysis indicates that the observed polariton anisotropy originates from the different optical band gaps and exciton binding energies along the two principal crystal axes of SnS.

Near-field spectroscopic investigation of dual-band heavy fermion metamaterials.

Broadband tunability is a central theme in contemporary nanophotonics and metamaterials research. Combining metamaterials with phase change media offers a promising approach to achieve such tunability, which requires a comprehensive investigation of the electromagnetic responses of novel materials at subwavelength scales. In this work, we demonstrate an innovative way to tailor band-selective electromagnetic responses at the surface of a heavy fermion compound, samarium sulfide (SmS). By utilizing the intrinsic, pressure sensitive, and multi-band electron responses of SmS, we create a proof-of-principle heavy fermion metamaterial, which is fabricated and characterized using scanning near-field microscopes with <50 nm spatial resolution. The optical responses at the infrared and visible frequency ranges can be selectively and separately tuned via modifying the occupation of the 4f and 5d band electrons. The unique pressure, doping, and temperature tunability demonstrated represents a paradigm shift for nanoscale metamaterial and metasurface design.