The dielectric response of transparent conductive oxides near the bulk plasmon frequency is characterized by a refractive index less than vacuum. In analogy with x-ray optics, it is shown that this regime results in total external reflection and air-guiding of light. In addition, the strong reduction of the wavevector in the ITO below that of free space enables a new surface plasmon polariton mode which can be excited without requiring a prism or grating coupler. Ultrafast control of the surface plasmon polariton mode is achieved with a modulation amplitude reaching 20%.Epsilon-Near Zero (ENZ) materials are a class of optical materials characterized by a real part of the dielectric function close to zero. ENZ materials are of interest for a range of applications including tailoring of directional emission and radiation phase patterns, [1][2][3] air-guiding of electromagnetic waves, [4] and electromagnetic tunnelling devices. [5][6][7] While a lot of effort is aimed at achieving an ENZ response using artificial metamaterial resonators, some naturally occurring materials also show a strong reduction of the permittivity below that of vacuum. An example of naturally occurring low-index materials are noble metals where the optical permittivity ǫ is governed by the collective motions of the free electron gas known as bulk plasmons. According to the Drude model, the permittivity is given bywhere γ denotes the damping rate of the free electrons and the plasma frequency ω pl is given by ω pl = (N e 2 /ǫ 0 m) 1/2 . Around the (screened) bulk plasmon frequency ω bp ≡ ω pl / √ ǫ ∞ , the real part of the permittivity shows a transition from negative to positive values. Noble metals have carrier densities N exceeding 10 22 cm −3 , therefore their bulk plasmon plasma frequency is located in the UV region. In contrast, highly doped semiconductors typically have electron densities below 10 19 cm −3 and can be well described by the Drude model with a bulk plasmon plasma frequency in the THz range. Transparent conducting oxides (TCOs), with an electron density inbetween that of bulk metals and doped semiconductors, show a bulk plasmon frequency in the near-infrared. The resulting combination of a metal-like response in the infrared and a dielectric optical response in the visible region has stimulated application of TCOs as transparent electrical contacts and as heat reflecting windows. Recently, metal oxides such as indium-tin oxide (ITO) and aluminium-doped zinc oxide have received interest for their plasmonic response in relation to metamaterials and transformation optics. [8,9] The plasma frequency can be tuned by controlling the electron density using electrical or optical methods, opening up opportunities for nearinfrared electro-optic or optical modulators [7,10,11] and sensing devices. [12,13] Pioneering studies by Franzen and co-workers have investigated the plasmonic response of ITO and ITOgold hybrid structures in the metallic (negative epsilon) regime of ITO. [13] Next to a conventional surface plasmon polariton mode f...
We present numerical simulations of low aspect ratio gallium phosphide nanowires under plane wave illumination, which reveal the interplay between transverse and longitudinal antenna-like resonances. A comparison to the limiting case of the semiconducting sphere shows a gradual, continuous transition of resonant electric and magnetic spherical Mie modes into Fabry-Pérot cavity modes with mixed electric and magnetic characteristics. As the length of the nanowires further increases, these finite-wire modes converge towards the leaky-mode resonances of an infinite cylindrical wire. Furthermore, we report a large and selective enhancement or suppression of electric and magnetic field in structures comprising two semiconducting nanowires. For an interparticle separation of 20 nm, we observe up to 300-fold enhancement in the electric field intensity and an almost complete quenching of the magnetic field in specific mode configurations. Angle-dependent extinction spectra highlight the importance of symmetry and phase matching in the excitation of cavity modes and show the limited validity of the infinite wire approximation for describing the response of finite length nanowires toward glancing angles.
Figure S1 Scattering maps from antennas on racetrack. a-e, SEM images of top sections of racetracks with different numbers of nanoantennas varying from 1 to 5, respectively. Scale bar in the left one is applicable to the other 4 images. f-j, Dark-field images of the 5 racetracks transmitting suppressed modes with 1 to 5 nanoantennas on their horizontal sections, respectively. k-o, Measured scattering spectra from the 1-to 5-nanoantenna arrays on the racetrack. The arrows in each spectrum denote the wavelengths at which the Dark-field images in f-j were collected. p-t, Calculated far-field patterns projected from 1-to 5-nanoantenna arrays overlapping with standing wave anti-nodes in single straight waveguides and the corresponding near field E y distributions near the antennas.
The ultrafast concentration of electromagnetic energy in nanoscale volumes is one of the key features of optical nanoantennas illuminated at their surface plasmon resonances. Here, we drive the insulator to metal phase transition in vanadium dioxide (VO2) using a laser-induced pumping effect obtained by positioning a single gold nanoantenna in proximity to a VO2 thermochromic material. We explore how the geometry of the single nanoantenna affects the size and permittivity of the nanometer-scale VO2 regions featuring phase transition under different pumping conditions. The results reveal that a higher VO2 phase transition effect is obtained for pumping of the longitudinal or transversal localized surface plasmon depending on the antenna length. This characterization is of paramount importance since the single nanoantennas are the building blocks of many plasmonic nanosystems. Finally, we demonstrate the picosecond dynamics of the VO2 phase transition characterizing this system, useful for the realization of fast nano-switches. Our work shows that it is possible to miniaturize the hybrid plasmonic-VO2 system down to the single-antenna level, still maintaining a controllable behavior, fast picosecond dynamics, and the features characterizing its optical and thermal response.
Metal oxides are materials of great interest for a wide range ofapplications in electrochemistry, catalysis, sensing, and electronics.We show that highly doped transparent conducting oxides can becombined with plasmonics to enhance functionality, or can be usedas novel plasmonic materials exhibiting resonances in the midinfrared.
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