Plasmonic Mode Analysis on Asymmetric ITO/Au/ITO Hyperbolic Metamaterials Using Prism-Coupled Spectroscopic Ellipsometry

Abstract

Various trilayers (ITO-Au-ITO) of a constant total thickness of 43 nm, with different bottom and top ITO film thicknesses, have been prepared by magnetron sputtering. The effects of the relative position of the inserted gold layer on the trilayers’ plasmonic properties have been systemically investigated. To study their optical reflectance changes caused by the surface plasmon resonance, total internal reflection ellipsometry (TIRE) has been employed. Two reflectance minima, corresponding to the pseudo-Brewster (PB) and bulk plasmon polariton (BPP), are observed from the reflectance spectra. The impacts of the light incident angles on the behaviors of these modes have been monitored. The measured reflectivity spectra match well with the simulated reflectivity spectra based on the Fresnel formalism. For the ITOt/Au/ITOb = 5/3/35 sample, where the Au layer is positioned closest to the sampling liquid, the BPP mode becomes the sharpest (based on the amplitude and half-width) as compared to other samples, indicating that the closer the ITOt/Au interface is to the sampling liquid, the stronger the observable plasmonic resonance effect is. Furthermore, it is found that the dip position of this BPP mode is independent of the angle of incidence but is sensitive to the refractive index of the sampling liquid. Our results show that the dip position shifts to a shorter wavelength with increasing refractive index of the sampling liquid. This means that the trilayer can act as a refractive index sensor. By linear regression, the obtained sensing accuracy is up to 552 nm/RIU. This facilitates the potential application of this trilayer structure as a surface plasmon-enhanced refractive index sensor.