Near-infrared linewidth narrowing in plasmonic Fano-resonant metamaterials via tuning of multipole contributions

Abstract

We report on an experimental and computational (multipole decomposition) study of Fano resonance modes in complementary near-IR plasmonic metamaterials. Resonance wavelengths and linewidths can be controlled by changing the symmetry of the unit cell so as to manipulate the balance among multipole contributions. In the present case, geometrically inverting one half of a four-slot (paired asymmetric double bar) unit cell design changes the relative magnitude of magnetic quadrupole and toroidal dipole contributions leading to enhanced quality factor, figure of merit and spectral tuning of the plasmonic Fano resonance. *Email: ranjans@ntu.edu.sg 2 Metamaterials have been a subject of intense interest due to their ability to exhibit optical properties not commonly found in natural materials. By suitably engineering the size and geometry of metamaterials at the sub-wavelength scale, researchers have been able to exploit and manipulate electromagnetic waves to achieve various phenomena such as invisibility cloaking, 1-4 super lenses, 5-8 electromagnetically-induced transparency [9][10][11] , and high quality- Here, we introduce a comparison between complementary nanostructures of paired ADBs in so-called "Fano" and "iFano" configurations (iFano denoting a 180˚ inversion of one ADB relative to the Fano geometry -as shown in Fig. 1), and report that in the iFano configuration, a larger figure of merit and quality factor can be achieved in the infrared regime as compared to the Fano configuration. Numerical simulations attribute the origin of a broad dipolar resonance to the electric dipole, and the asymmetric Fano lineshape is a consequence of the electric dipole interacting with the magnetic quadrupole and the toroidal dipole. Via a decomposition of the multipoles, we find that the magnetic quadrupole competes on the same scale as the toroidal dipole to narrow down the linewidth of the resonance. In addition, the iFano configuration offers large spectral tunability without the need to alter the periodicity or dimensions of the unit cell. The reflectance spectra of fabricated nanostructures were measured using a microspectrophotometer (Jasco MSV-5200) with y-polarized incident light. Numerical simulations were carried out using commercial finite-difference time-domain Maxwell solver software (Lumerical FDTD) assuming plane wave illumination polarized in the y-direction.Periodic boundary conditions were imposed in the x-and y-directions; perfectly matched layers were set in the z-direction so that any incident waves do not reflect at the boundaries but are strongly absorbed. The optical constants of gold were taken from Johnson and Christy 44 and the refractive index of the quartz substrate was set to 1.5.Figs. 2(a, b) show the reflectance spectra measured for y-polarized incident light propagating normal to the plane of the nanostructures. They clearly illustrate that in both configurations, the Fano resonance red-shifts with decreasing asymmetry accompanied by linewidth narrowing.For a pair of symmetric double ...

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“…An approach would be to manipulate the configuration of unit cells in the metamaterial array to mitigate radiative losses. It was reported that by alternating or inverting an asymmetric dipole bars (which constitute to one of the two sub‐unit cells) and placing it adjacent to neighboring asymmetric dipole bars, an enhancement in the Q ‐factor of Fano resonance is achieved. Based on the schematic of dipole–dipole interference, an enhancement in the Q ‐factor of Fano resonance in an alternating unit cell configuration was ascribed to destructive interference of dipole moments contributed by neighboring sub‐unit cells as illustrated in Figure c.…”

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