A metal-insulator-metal waveguide-based plasmonic refractive index sensor for the detection of nanoplastics in water

A metal-insulator-metal waveguide-based square-ring resonator plasmonic refractive index sensor is designed and optimized for achieving high sensitivity. The sensitivity of the sensor critically depends on the physical dimension and the geometrical parameters of the resonator. Systematic studies on varying geometrical parameters of the resonator reveal that the sensitivity increases with the number of concentric square-rings. Moreover, the full-width-half-maxima of the resonance line is found to increase with the number of square rings. Importantly, variations in the coupling length affect the transmitted intensity as well as the full-width-half-maxima of the resonance spectra in a characteristic fashion. An initial exploration of the optimized sensor for nanoplastic detection for a range of volume fractions 0.15625-0.625% shows a systematic linear increase in the resonance wavelength with changing refractive index of the surrounding medium. This offers the possibility of ultrasensitive detection of extremely small change <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mrow><mml:mo>(</mml:mo> <mml:mo>∼</mml:mo> <mml:mn>0.00025</mml:mn> <mml:mo>)</mml:mo></mml:mrow> </mml:math> in the local refractive index as the signature of a minute level of plastic contamination. This was achieved by using an optimized sensor design with a sensitivity of 2700 nm/RIU and a full-width-half-maxima of 333 nm. Results presented in the paper demonstrate the considerable promise of the proposed plasmonic refractive index sensor towards nanoplastic detection.