Effect of Microsphere Size on Infrared Selective Emission of Hole-Structured Metamaterial

The infrared (IR) sensor applied to the modern IRST system detects radiation signature emitted from the target, and the IR signal in the 5-8 μm band cannot reach the sensor due to the atmospheric transmission window effect. Therefore, it is important to reduce the IR signal in the “detection band” to prevent from being attacked. In particular, the long-wavelength infrared (LWIR) signal in 8-12 μm is dominantly generated on the aircraft’s fuselage surface, which is relatively cold compared to plume. In order to reduce the IR signal generated from fuselage, it is necessary to design a surface that reduces the signal in the detectable LWIR band and selectively emits the signal to the undetected band. For the purpose of selective absorption/emission in the IR band, dielectric multilayered materials or metal-dielectric-metal (MDM) layered metamaterials are being actively studied. In case of MDM metamaterials, many researchers mainly used lithography or nanoimprinting fabrication methods. However, these processes are expensive, time consuming, and difficult to process over a large area. For this reason, we suggested a cost and time efficient and large-area fabrication method for IR selective emitter of hole-structured MDM layer using PS microsphere. We designed hole-structured periodic metamaterials using the hexagonal packing of microspheres. For monolayer hexagonal packing of PS microsphere, we used sodium dodecyl sulfate (SDS) solution. The hole-structured MDM metamaterial mainly changes the absorptivity/emissivity characteristics depending on the hole diameter, hole pitch, and dielectric material’s thickness. To analyze the optical properties of the metamaterial according to the microsphere’s diameter, we fixed the other parameters, which are dielectric thickness and hole diameter, and then, we conducted numerical simulation using COMSOL Multiphysics®. In addition, we measured the reflection and emission of fabricated specimen using FT-IR and compared with the numerical simulation results. As the diameter of the PS microsphere increased, the pitch between the particles in the hexagonal packed monolayer increased, and accordingly, the hole pitch of the metamaterial also increased. Since the resonance frequency of the MDM structure can change with different hole pitch, we finally confirmed that our metamaterials can controlled selective emission characteristics into the undetected band with different microsphere diameters.