Self-Referencing Plasmonic Array Sensors
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Self-Referencing Plasmonic Array Sensors Reza Kohandani 1 & Simarjeet S. Saini 1 Received: 16 October 2019 / Accepted: 22 March 2020 # Springer Science+Business Media, LLC, part of Springer Nature 2020
Abstract A self-referencing plasmonic platform is proposed and analyzed. By introducing a thin gold layer below a periodic twodimensional nano-grating, the structure supports multiple modes including localized surface plasmon resonance (LSPR), surface plasmon resonance (SPR), and Fabry-Perot resonances. These modes get coupled to each other creating multiple Fano resonances. A coupled mode between the LSPR and SPR responses is spatially separated from the sensor surface and is not sensitive to refractive index changes in the surrounding materials or surface attachments. This mode can be used for self-referencing the measurements. In contrast, the LSPR dominant mode shifts in wavelength when the refractive index of the surrounding medium is changed. The proposed structure is easy to fabricate using conventional lithography and electron beam deposition methods. A bulk sensitivity of 429 nm/RIU is achieved. The sensor also has the ability to detect nanometer thick surface attachments on the top of the grating. Keywords Self-referencing . Surface plasmon resonance . Nano-grating
Introduction Over the last decade, surface plasmonic sensors have been demonstrated in the area of biochemical sensing [1], medical diagnostics [2], environmental monitoring [3], food safety [4], water testing [5], and homeland security [6]. In recent years, periodic metallic structures have been used to generate Fano resonances resulting from the interference between the plasmonic mode and broad optical modes [7–10]. This results in narrowing of the resonance, increasing the quality factor (Q), and increasing the wavelength shift due to refractive index change, thereby increasing the sensitivity of the sensors [10–12]. Periodic nanostructures like split-ring resonators [13], ring nanocavities [14], disk nanocavities [15], nanoclusters [16], and crescents [17] allow for the excitation of localized surface plasmons. Sensitivities of up to 725 nm/ RIU have been realized in these structures [18], and surface attachment of nanometer-scale layers can be detected making these sensors very promising in sensing-related applications [19]. A figure of merit (FOM) defined as sensitivity (S)
* Reza Kohandani [email protected] 1
Department of Electrical and Computer Engineering, University of Waterloo, 200 University Avenue West, Waterloo, ON N2L 3G1, Canada
divided by the spectral linewidth (Δλ) has been used to compare different structures [20]. Using Fano resonances, the FOM of 72/RIU has been demonstrated [15]. In a previous work, a plasmonic sensor was demonstrated which also created vivid structural colors [21]. The colors changed as the refractive index of the surrounding medium changed, and detection could be achieved by using a simple camera. The structure employed a two-dimensional periodic structure on top of a conducting layer creating
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