Terahertz Refractometer Based on Extraordinary Optical Transmission for Characterization of Liquid Samples

The use of a metallic metasurface attached to one face of a dielectric window is studied. It is aimed to be a part of a chamber containing some liquid as in industrial machinery, with the metasurface assisting in the monitoring of the optical constants in the terahertz (THz) region of the liquid for the control of industrial processes. The metasurface supports extraordinary optical transmission (EOT) resonances, whose frequency depends on geometric parameters and the refractive index of the surrounding medium. The shift of the EOT resonance allows the estimation of the refractive index of the sample under study. Two metal structures, associated with different fabrication processes, are analyzed: a thick ( ${50} \, \, \mu {m}$ ) free-standing steel metasurface and a thin aluminum-copper one ( ${2}.{5} \, \, \mu {m}$ ) on a substrate. Experiments with a cuvette with such sensing window as one of its walls and filled with different liquids (n-pentane, cyclohexane, and toluene) have shown that the thick metasurface shows a higher sensitivity between 550 and 441 GHz/RIU, despite larger fabrication errors, due to the detrimental effect of the substrate on the resonances. These results prove the feasibility of metasurfaces to enable noncontact monitoring of processes in chemical reactors, plastic extruders, and other manufacturing equipment.