<p>Thermal radiation is one of the fundamental modes of heat transfer. Emissive pattern of ordinary materials is usually diffuse, incoherent and broadband. Coherent, directional and wavelength-selective thermal radiation can be engineered using photonic metamaterials and metasurfaces. Much of the research in the past to tailor emission spectrum was focused on the use of static metamaterials.&nbsp; Development of active infrared metamaterials for controlling thermal radiation has received significant attention in recent years owing to their potential applications wherein emitted power can be modulated dynamically. As a main alternative to use of phase-change materials to control emissivity, modulation of emissivity by carrier injection is an attractive scheme to achieve dynamic control over thermal emission as the emissive power can be altered without changing the temperature. This approach has a potential to operate at speeds much higher than typical thermal cycling times.</p><p>Indium Arsenide (InAs) is a suitable III-V type semiconductor that has a tunable refractive index around 3.6 µm wavelength range through the injection of charge carriers. Since the change in emissivity is directly related to the change in refractive index, modulation of emissivity with bare InAs is weak. We show that, this can be significantly enhanced by carefully designing resonances around the index-tunable region of the spectrum. We propose an n-InAs based guided mode resonance grating that exhibits high-Q resonances in mid-IR range with highly selective directional emission around 3.6 µm close to 45 deg angle of incidence. The metasurface is designed such that top metal contacts and bottom layer of reflective substrate can be utilized for carrier injection by applying DC voltage. In this manner, it would be possible to independently bring about a change in refractive index in each InAs stripe. High-Q resonances ensure that emissivity peaks are highly sensitive to a change in refractive index. This allows dynamic control of emissivity possible even with a small change in refractive index. We study spectral and directional control of emissivity using the design presented. We numerically demonstrate two schemes of dynamic emissivity control. In the first approach, we use an index perturbation with alternate increase and decrease in refractive index across InAs grating stripes. Such an asymmetric index perturbation brings about phase modulation of guided modes within the dielectric layer. Formation of bandgaps as a result of this modulation leads to splitting of emissivity peaks that can be dynamically controlled by applied voltage.&nbsp; In the second scheme, a constant change in refractive index can be introduced across the grating. This effectively changes the wavelength of propagation withing the dielectric and causes a shift in resonant frequencies across all wavevectors. With this method, it can be shown that directional control of thermal emission is possible. This also enables a continuous tuning of emissive wavelength.</p>

Dynamic Control of Emissivity in Guided Mode Resonance Grating Through Index Perturbation

Paper Proceedings: https://asmedigitalcollection.asme.org/IMECE/IMECE2021/volume/85543

ASME 2021 International Mechanical Engineering Congress and Exposition (IMECE2021)

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