Al/SiOx/Al single and multiband metamaterial absorbers for terahertz sensor applications

Abstract

Abstract. To increase the sensitivity of uncooled thermal sensors in the terahertz (THz) spectral range (1 to 10 THz), we investigated thin metamaterial layers exhibiting resonant absorption in this region. These metamaterial films are comprised of periodic arrays of aluminum (Al) squares and an Al ground plane separated by a thin silicon-rich silicon oxide (SiO x ) dielectric film. These standard MEMS materials are also suitable for fabrication of bi-material and microbolometer thermal sensors. Using SiO x instead of SiO 2 reduced the residual stress of the metamaterial film. Finite element simulations were performed to establish the design criteria for very thin films with high absorption and spectral tunability. Single-band structures with varying SiO x thicknesses, square size, and periodicity were fabricated and found to absorb nearly 100% at the designed frequencies between three and eight THz. Multiband absorbing structures were fabricated with two or three distinct peaks or a single-broad absorption band. Experimental results indicate that is possible to design very efficient thin THz absorbing films to match specific applications.

Metamaterial Resonant Absorbers for Terahertz Sensing

“…The thermal capacitance is proportional to an objects volume; thus, a thinner dielectric has a smaller volume, which leads to a desirable design parameter in maximizing the sensor's speed of operation [16]. An over-etch layer, which is a result of fabrication limitations, found to have an average thickness of 100 nm.…”

“…The SiO x dielectric thickness (1.1 µm) was a critical geometric input variable to maximize absorption characteristics and was established from past research in the NPS SRL [16][17][18]. Since one desired application for these unit cells is their integration into future designs of thermal sensors in the form of focal plane arrays, it is important to maintain the dielectric thickness as thin as possible to minimize the thermal capacitance.…”

Terahertz metamaterial absorbers with an embedded resistive layer

“…However, when these approaches extend to THz spectral range, the entire structure becomes relatively thick. This affects the thermal time constant or, in the case of placing the ground plane on the substrate, could reduce the absorption within the membrane [15,16]. Additionally, one could simply attempt to use the bolometric layer as the ground plane of the metamaterial, however, the high resistivity of such materials negatively affects the absorption.…”

“…These films exhibit optical properties not found in their constituent materials [14]. Of particular interest for THz thermal sensors is the fact that they can absorb nearly 100% of the incident radiation near a tunable resonant frequency, resulting in a very efficient absorbing membrane to be used with narrowband illumination sources (such as THz-QCLs) [15,16]. Additionally, these metamaterial films can be constructed from materials already used in microbolometer fabrication due to the fact that the absorption characteristics are primarily controlled through the metamaterial geometry rather than the constituent materials.…”

“…Additionally, these metamaterial films can be constructed from materials already used in microbolometer fabrication due to the fact that the absorption characteristics are primarily controlled through the metamaterial geometry rather than the constituent materials. Although not the only possible configuration [14], a periodic array of metal squares, separated from a metal ground plane by a dielectric spacer, can accomplish this in the THz range with fabrication-friendly materials and relatively small dielectric thickness [15]. In addition to THz absorption, the metamaterial absorber should also contain a layer with a high temperature coefficient of resistance (TCR) for measuring temperature change due to the absorption.…”