Fig. 1. Schematic illustration of a basic unit of the proposed structure used for electric tuning of plasmonic response in an ultrathin gold nanoribbon array. The thickness, width, and period of the nanoribbons in the array are tGNRA, w, and a, respectively.

Fig. 2. (a) Numerically calculated electron density inside an ITO layer (initial free electron concentration n0=1025  m−3) in a gold-HfO2-ITO structure (inset) as a function of the distance from the ITO/HfO2 interface under the application of various gate voltages. (b) Changes in the optical permittivity of the ITO as a function of the electron density at a wavelength of 1.55 μm. (c) Simulated transmission spectra (given in percentage of the difference in the transmittances between regions with and without plasmonic structures) of plasmonic structures with various GNRA thicknesses; their peak resonance wavelength is tuned to be the same (1.55 μm) by adjusting the nanoribbon width w (see the legend), keeping the period always set to a=3w. (d) Corresponding near-field distributions for active plasmonic structures constructed using GNRAs with thicknesses of 1, 5, and 10 nm at the resonance wavelength of 1.55 μm. (e) Calculated ratios between near-field intensities integrated within the ITO layer and over a rectangular region with the sides separated from the nanoribbon edges by a distance marking 1/e electric field decay from the ITO/air interface [see the dashed white box in the top map in (d) as an example] for the plasmonic structures with various GNRA thicknesses.

Fig. 3. (a) Dependence of the transmission spectra of the electrically tunable plasmonic design (tGNRA=2  nm, w=102  nm, a=306  nm, tITO=3  nm, and tHfO2=2  nm) on the applied gate voltage. (b) Zoomed-in view of (a) in a wavelength range from 1.45 to 1.6 μm. (c) Resonance wavelengths and minimum transmission values for various gate voltages extracted from (b). (d) Variation of the change in the intensity at the transmission dip with the wavelength and corresponding modulation depth upon application of a bias voltage of 1.7 V. (e), (f) Near-field distributions around nanoribbons at the wavelength of 1.55 and 1.516 μm and gate voltages of (e) 0 V and (f) 1.7 V. (g)–(i) Enlarged views (g), (i) of the marked regions in (e) and (f), respectively, together with (h) electric field distribution along the black and red dashed line (1 nm away from the gold edge) in (g) and (i), respectively.

Fig. 4. (a) Dependence of the shift of plasmonic transmission dip on the gate voltage for active plasmonic structure with various nanoribbon thicknesses. (b) Change of the value of the transmission dip as a function of the gate voltage for various nanoribbon thicknesses. Inset: corresponding voltage-dependent absolute values of the magnitude of the transmission dip. (c), (d) Shift of the resonant wavelength as a function of the gate voltage for various HfO2 thicknesses (c), with the corresponding change of the magnitude of the transmission dip (d). Inset: corresponding voltage-dependent absolute values of the magnitude of the transmission dip.

Fig. 5. Permittivity values derived from the Drude model for ITO for various carrier densities. Panels (a) and (b) present the real and imaginary parts of the permittivity, respectively, while panels (c) and (d) show the corresponding real and imaginary parts of the ITO refractive index.

Fig. 6. (a) Real and (b) imaginary parts of the thickness-corrected permittivity of gold for various nanoribbon thicknesses.

Fig. 7. (a) Transmission spectra of an active plasmonic structure with smaller period and width under different gate voltages (tGNRA=2  nm, w=60  nm, a=80  nm, tITO=3  nm, and tHfO2=2  nm). (b) Variation of the change in the intensity at the transmission dip with the wavelength and corresponding modulation depth.

Fig. 8. (a) Schematic of an active plasmonic structure with only a bottom ITO layer used for electric tuning. (b) Corresponding transmission spectra of the active plasmonic structure under different gate voltages (tGNRA=2  nm, w=118  nm, a=354  nm, tITO=3  nm, and tHfO2=2  nm).