Active infrared tuning of metal-insulator-metal resonances by VO2 thin film

Abstract

VO₂ is a promising phase change material offering a large contrast of electric, thermal, and optical properties when transitioning from semiconductor to metallic phase. Here we show that a hybrid metamaterial obtained by proper combination of a VO₂ layer and a nanodisk gold array provides a tunable plasmonic gap resonance in the infrared range. Specifically, we have designed and fabricated a metal-insulator-metal gap resonance by inserting sub-wavelength VO₂ film between a flat gold layer and a gold nanodisk resonator array. The resonance of the hybrid metamaterial is centered in the useful 3-5 μm range when VO₂ is in its semiconductor state. The experimental study highlights a monotonical spectral tuning of the resonance when increasing temperature up to 50 °C above the room temperature, providing a continuous resonance shift of almost 1 μm in the mid-infrared range. Wavelength range and intensity tunability can be further optimized by modifying the thicknesses of the layers and metamaterial parameters.

Keywords: Metamaterials; Phase change materials; Plasmonics; VO2.

Fig. 1

(a) 3D schematic of the unit cell of the metamaterial on CaF₂ substrate. (b) SEM image of the sample.

Fig. 2

(a) Experimental spectra showing the resonance tuning of the metamaterial with the temperature. (b) Simulation using Bruggeman formalism and Looyenga mixing rule; f is the fitting parameter which represents the fraction of metallic VO₂ during the heating.^,.

Fig. 3

(a) Simulated and experimental absorption intensity at the resonant wavelength for the purely semiconductor VO₂. Blue, green and red stars on the graph represent the total simulated absorption when the fitting parameter f is equal to 0 (semiconductor-like), 0.6 (intermediate) and 1 (metal-like). Heating increases reflection, hence decreasing absorption, as shown in (b): 3D distribution of the absorption density at f = 0, f = 0.6 and f = 1. (c) Magnetic field intensity as a function of the volume parameter f; the strong magnetic field at the resonant wavelength and f = 0, is gradually switched off as f increases.

Fig. 4

(a) Simulated and experimental results for the resonant wavelength as a function of the fitting parameter f (simulation, left axis) and temperature (experiment, right axis), respectively. The blue and the red stars represent the simulated resonant wavelength for the fitting parameter f = 0 (semiconductor-like state) and f = 1 (metal-like state). (b) Distribution of the electric field in the xy-plane positioned 10 nm above the Au nanodisk: resonant wavelengths and the enhancement red-shift with the increase of the factor f. (c) xz-monitor of the simulated magnetic field intensity at resonant wavelengths for f = 0 (4.72 μm) and f = 1 (5.95 μm).