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High sensitivity of SPR with microplasmonic structures

The plasmonic nature of discontinuous thin films with micro-patterned structures such as triangles and hole arrays present distinct optical properties with a Kretschmann surface plasmon resonance (SPR) instrument. Au microstructures were prepared with a modified nanospheres lithography (NSL) method using 3.2 μm spheres which gives 1.8 μm triangles and hole arrays with hole diameter ranging from 2.5 to 0.5 μm. The sensitivity to refractive index in thin film is increased by up to 45% with using microhole arrays instead of continuous film. A transition in the microstructure aspect from triangles to hole arrays with large hole diameter affects the spectral aspect of the SPR active band. Triangles present a characteristic broad transmission maximum band while in hole arrays, a broad and weak absorption band first appears for large holes, which sharpens and increases in intensity as the hole diameter decreases. Moreover, the SPR penetration depth is tuned between 230 and 30 nm as the microstructure aspect shifts from continuous film, to small hole arrays and to isolated triangles. Thus, these new plasmonic properties were observed in microhole arrays excited in Kretschmann SPR configuration, which are spectrally similar to continuous film. These can significantly improve the existing SPR sensing methods.