Upconversion phosphors produce luminescence at wavelengths shorter than that of the excitation source. The intrinsically poor quantum efficiency of upconversion nanocrystals (UCNC), particularly at low excitation power densities, is the major limitation for many potential applications of these materials. Plasmonic enhancement is one of the primary strategies being pursued to improve the efficiency of upconversion. UC brightness can be significantly enhanced in the vicinity of noble metal nanostructures. The “plasmonic strategy”, however, has its own set of limitations. Plasmonic enhancement is based on near-field interactions between the UC particles and the metal surface, and is, therefore, effective only for particles within approximately 100 nm of the metal surface. Here, we show that it is possible to design patterned metal surfaces which exploit the diffractive and reflective properties to enhance the efficiency of upconversion. The primary advantage of this strategy is that the enhancement resulting from these optical effects can be extended to sample volumes far outside the near-field range of the metal surface. Also, because the enhancement depends mainly on the reflective and diffractive properties of the substrates, it is not necessary to use noble-metal coatings such as gold and silver. We also show that the enhancement mechanisms are effective primarily due to the non-linear nature of UC mechanisms and would show only modest improvement for single-photon excitation processes.

Beyond the near-field: Enhancing the efficiency of upconversion-luminescence nanomaterials using the diffractive and reflective properties of structured metal surfaces

255th National Meeting (2018)

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