Ultra-thin perfect absorber employing a tunable phase change material
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(a) The reflection process from a quarter-wave film with low losses (k 2 ≪ n 2) on a perfectly reflecting substrate at normal incidence, showing the partial waves. Many multiple reflections are involved because of the small losses. (b) Phasor addition diagram (the reflected partial waves are represented in the complex plane) demonstrating that a properly engineered quarter wave film on a reflecting substrate can result in zero reflection via destructive interference, corresponding to complete absorption. This occurs for a particular value of k 2, which is relatively small, leading to a small imaginary part of r 0, and corresponding to critical coupling. The phase of the first partial wave r 0 is with respect to the incident wave, but the phase of all of the other partial waves is (c) Reflection process from a highly absorbing (k 2 ∼ n 2), ultra-thin film in a reflecting substrate. (d) Phasor diagram demonstrating that a zero-reflection (and hence perfect absorption) condition is achievable if the complex refractive index of the film has a large imaginary component. In this case, the phase of r 0 deviates significantly from π (the phasor is not along the horizontal axis) and a small number of reflections is sufficient to cancel r 0 and maximize absorption.
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(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.
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(a) Map of the calculated reflectivity as a function of n and k, the real and imaginary parts of the complex refractive index of a uniform dielectric film of 180 nm thickness on sapphire for λ = 11.75 μm. The reflectivity drops to zero for . The black dashed line marks the trajectory of the complex refractive index of VO2 with increasing temperature. The VO2 index passes very close to the minimum reflectivity point in n-k parameter space (black dashed line).
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