(a) Experimental setup. A sapphire substrate coated with h = 180 nm of VO₂ is placed on a temperature-controlled stage mounted inside an infrared (IR) microscope and illuminated at normal incidence using a mid-IR source. A mercury-cadmium-telluride (MCT) detector is used to collect the reflected light. (b) Experimental reflectivity spectrum at temperatures from 297 K to 360 K. At 343 K, the reflectivity drops to ∼0.0025 at λ = 11.6 μm. (c) Experimental reflectivity from the sample at λ = 11.6 μm as a function of increasing (red) and then decreasing (blue) temperature. A ∼5 K hysteresis is seen in the reflectivity. Inset: Normalized dc resistance of the VO₂ thin film sample as a function of temperature showing nearly four orders of magnitude of change in the resistance and hysteretic behavior. (d) Calculated reflectivity spectrum at temperatures from 295 K to 360 K using experimental values for the complex refractive indices of VO₂ (Ref. 17) and sapphire.²⁶ The reflectivity of bare sapphire is shown in black.