Surface plasmon polariton (SPP)-based methods enable the investigation of biological objects, such as biomolecules and cells, with high sensitivity and spatial resolution. These methods employ various types of SPPs to match the size of the investigated biological object with the penetration depth of an SPP. Large penetration depths are provided by long-range SPPs that are supported by dielectric-metal-dielectric structures with dielectric materials of similar refractive indices (RIs). However, the development of such structures for biosensing is difficult due to the limited availability of dielectrics with a RI close to that of water and desired properties, such as stability and compatibility with relevant fabrication techniques. Here, we describe the development of diffractive structures supporting long-range SPPs. The described structure consists of a low RI dielectric grating with a sine profile with a thin layer of gold. The geometric properties of the structure are optimized using rigorous coupled wave analysis to achieve high sensitivity of the SPPs to bulk RI change and to allow for the excitation of SPPs from both sides of the structure under the normal incidence of light. The laboratory prototypes of the structure were fabricated by creating a grating using soft lithography in a low RI polymer (CYTOP) layer on a glass slide and then coating it with a layer of gold using vacuum deposition. The fabricated structures were characterized experimentally and sensitivity to bulk RI changes was determined and compared with the theoretical predictions. The potential of such structures for SPP biosensing and imaging is also discussed.
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