MEMS Micro-Mirror Switch for Thermal Emissivity Control

Controlling infrared (IR) emissivity is of great importance for a range of applications, including IR camouflage, smart windows, personal thermal management, radiative cooling, and thermophotovoltaics. Many applications require active switching and control of emissivity. Sevreal different approaches including phase transition, thermo-optical modulation and electronic tuning have been demonstrated recently to achieve dynamic emissivity control. While these approaches show promising performance, future systems would benefit from a larger contrast in emissivity switching and from utilizing a broader spectral bandwidth.
Here, we demonstrate an active high-contrast modulation of thermal radiation in an ultrabroad 5-14 um spectral range. Our approach consists of a combination of a metasurface thermal emitter combined with a micro-mirror MEMS switch. The micro-mirrors have feature sizes ranging from 25 to 150 µm (fabricated by standard microfabrication techniques). Each micro-mirror is comprised of a 240 nm bi-layer SiO₂/Al structure. When mirrors are in a closed position (OFF-state), Al surface is exposed and serves as a low emissivity surface. In an open position (ON-state), a higher emissivity underlying metasurface is exposed. The metasurface design is a multilayer structure consisting of a 100 nm ITO atop of a 500 µm SiO₂ substrate backed with an Al backreflector. Here ITO serves as an antireflection coating and as the transparent electrode. We test the performance of fabricated devices by performing FTIR measurements. Spectral analysis shows that in an ON state, the average emissivity (ε_on) is ~0.69 in a broad 5 to 14 µm infrared window. In an OFF state, the average emissivity (ε_off) is ~ 0.26, as a result, high contrast is achieved. Upon applying an electrical bias of ~ 120V the MEMS switch is activated, and mirrors close. Our tests further show that such MEMS micro-mirror arrays can be operated with >1kHz rate. In summary, we have demonstrated a broadband and high contrast switching of thermal emissivity with micro-mirror integrated metasurfaces. Such a thermal radiation switch could find applications in smart windows, thermal cameras, and radiative cooling.