Enhanced upconversion in one-dimensional photonic crystals: A simulation-based assessment within realistic material and fabrication contraints

This paper presents a simulation-based assessment of the potential for improving the upconversion efficiency of β-NaYF4:Er3+ by embedding the upconverter in a one-dimensional photonic crystal. The considered family of structures consists of alternating quarter-wave layers of the upconverter material and a spacer material with a higher refractive index. The two photonic effects of the structures, a modified local energy density and a modified local density of optical states, are considered within a rate-equation-modeling framework, which describes the internal dynamics of the upconversion process. Optimal designs are identified, while taking into account production tolerances via Monte Carlo simulations. To determine the maximum upconversion efficiency across all realistically attainable structures, the refractive index of the spacer material is varied within the range of existing materials. Assuming a production tolerance of σ = 1 nm, the optimized structures enable more than 300-fold upconversion photoluminescence enhancements under one sun and upconversion quantum yields exceeding 15% under 30 suns concentration

Enhanced upconversion via plasmonic near-field effects: Role of the particle shape

The large energy-density enhancements, associated with the near-field of plasmonic metal nanoparticles (MNPs), can potentially be utilized to increase the efficiency of nonlinear processes such as upconversion (UC). A drawback of employing metallic structures for UC applications is luminescence quenching, i.e. the transfer of energy from the upconverter material to the metal, where it is dissipated as heat. In this study, a rate-equation model is applied to study the interplay between near-field enhancement and luminescence quenching for a range of different geometries. It is found that while shapes that incorporate pointy features and/or narrow gaps support stronger near-field enhancements, they also suffer more severely from luminescence quenching. Due to the strong correlation between the two effects, the predicted enhancement in UC luminescence is similar across all considered geometries ranging from 1 to 3. Our results indicate that the near-field of plasmonic MNPs might not be suitable for increasing UC efficiency

Bragg stacks enhancing upconversion for photovoltaics: A theoretical and experimental analysis

To enhance upconversion, we optimize Bragg stacks considering changes of local field, density of states and dynamics of β-NaYF4:Er3+. We experimentally characterize the impact on upconversion luminescence, spectrally resolved and power-dependent

Soft thermal nanoimprint of PMMA doped with upconverter nanoparticles

Upconversion of low-energy photons into higher energy photons is a non-linear effect that can be strongly enhanced by increasing the intensity of impinging light. Such an enhanced intensity can also be achieved by near-field optical effects in photonic structures. In this paper, we investigate photonic structures in the form of linear gratings with varying periods realised in poly-methyl-methacrylate (PMMA) layers using soft stamps in a thermal nanoimprint process. Master structures were fabricated using e-beam lithography and reactive ion etching in silicon. These master structures were replicated into bi-layer polydimethylsiloxane (PDMS) stamps. PMMA layers with embedded β-NaYF4:25% Er3 + upconverter nanoparticles were applied to glass substrates using spin coating. The PDMS stamps were used to imprint the PMMA layer with very accurate pattern fidelity. The upconversion luminescence around 980 nm was enhanced more than three times by the photonic structure under 1523 nm laser excitation with 0.43 ± 0.02 W/cm2 irradiance, in comparison to a reference sample containing the same amount of upconverter material