Ultra-narrowband metamaterial absorbers for multispectral infrared microsystems

• Online Access: http://hdl.handle.net/2047/D20316374

Infrared (IR) absorption spectroscopy is a powerful technique to identify and study chemicals or objects of various kinds in a non-disruptive way. Recently, the demand for high performance and compact IR spectroscopy systems has been steadily growing due to the advent of Internet of Things and the burgeoning development of miniaturized sensors. The key challenge lies in realizing multispectral absorber arrays that are lithographically defined and integrated on the same chip, with a minimized footprint. This challenge has been tackled in the study of metamaterials absorbers, the artificial materials composed of an array of subwavelength structures that manipulate electromagnetic waves to achieve extraordinary light absorption properties. The metal-insulator-metal (MIM) IR absorbers, in particular, are characterized by the near-unity absorptance with lithographically tunable peak absorption wavelength and spectral selectivity in an ultra-thin form factor, suitable for the implementation of miniaturized spectroscopic IR microsystems. Nevertheless, there has been no accurate analytical model to guide the design of MIM IR absorbers with ultra-narrow absorption bandwidth and near-unity absorption, while meeting all the stringent requirements for high-resolution multispectral IR microsystems. This thesis presents the modeling and characterization of ultra-narrowband MIM IR absorbers based on the modified circuit model capable of accurately predicting the spectral responses of MIM IR absorbers. The simultaneous excitation of electric and magnetic resonances in the MIM IR absorbers is modeled via the addition of coupling capacitance to the existing RLC circuit branch, resulting in the accurate description of the effective surface impedance. The model is experimentally validated in the mid-wavelength IR spectral range (λ = 3~7 μm) and exploited for the first demonstration of a narrowband MIM IR absorber that exhibits performance approaching the predicted physical limits: full-width at half-maximum ≈ 3% and near-unity absorption (η > 99.7%) at 5.83 μm wavelength, while independent of incident angle and polarization of the impinging IR radiation. Furthermore, the two novel plasmonically-enhanced IR microsystems, enabled by the high performance MIM IR absorbers, are also presented: multispectral resonant IR detectors and zero-power micromechanical photoswitches. The optimized ultra-narrowband MIM IR absorbers are proven to be the ideal candidates for the implementation of such new class of multispectral IR microsystems with the unprecedented performance beyond what the existing technologies can achieve.