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Photonic structures can be designed to tailor luminescence properties of materials, which becomes particularly interesting for non-linear phenomena, such as photon upconversion. However, there is no adequate theoretical framework to optimize photonic structure designs for upconversion enhancement. Here, we present a comprehensive theoretical model describing photonic effects on upconversion and confirm the model’s predictions by experimental realization of 1D-photonic upconverter devices with large statistics and parameter scans. The measured upconversion photoluminescence enhancement reaches 82 ± 24% of the simulated enhancement, in the mean of 2480 separate measurements, scanning the irradiance and the excitation wavelength on 40 different sample designs. Additionally, the trends expected from the modeled interaction of photonic energy density enhancement, local density of optical states and internal upconversion dynamics, are clearly validated in all experimentally performed parameter scans. Our simulation tool now opens the possibility of precisely designing photonic structure designs for various upconverting materials and applications., A theoretical framework to optimize photonic structure designs for upconversion enhancement is lacking. Here, the authors present a comprehensive theoretical model and confirm the model’s predictions by experimental realisation of 1D-photonic upconverter devices with large statistics and parameter scans.

Upconversion of sunlight with energy below the band gap of a solar cell is a promising technique for enhancing the cell efficiency, simply by utilizing a larger part of the solar spectrum. The present topical review addresses this concept and discusses the material properties needed for an efficient upconversion process with focus on both silicon and organic solar cells. To design efficient upconverters, insight into topics such as quantum-optics, nano-optics, numerical modeling, optimization, material fabrication, and material characterization is paramount, and the necessary concepts are introduced throughout the review. Upconversion modeling is done using rate equations, while optical modeling is done by solving Maxwell's equations using the finite element method. Topology optimization is introduced and used to generate geometries of gold nanoparticles capable of greatly enhancing the upconversion yield. Fabrication and experimental characterization methods are discussed. Some recent results are presented and finally the possibility of designing upconverting materials capable of increasing the short-circuit current in a solar cell is discussed.

Modifications induced by ultrashort laser pulses have been investigated in crystalline Ge 2 Sb 2 Te 5 thin films. The observations comprise optical and structural changes, amorphization, and ablation. By combining optical microscopy and cross-sectional scanning- and transmission electron microscopy, it is found that the threshold for permanent change of the optical properties of Ge 2 Sb 2 Te 5 – responsible for the reduced optical reflectivity – is slightly lower than that for amorphization; no further change in reflectivity is seen upon amorphization. The laser-fluence thresholds for amorphization and change of the optical properties both show a strong dependence on film thickness that can be explained thermally by two-temperature simulations. In the case of sufficiently thick films, two distinct low- and high-fluence ablation thresholds are found. The mechanisms of the ablation thresholds are discussed, and it is concluded that ablation in the low-fluence regime proceeds by photomechanical spallation.

We investigate photonic effects of optimized 1D photonic structures on embedded upconverting core-shell nanoparticle layers in a thorough comparison of simulation and experiment regarding parameters relevant for photovoltaic applications.

The upconversion luminescence (UCL) of colloidal lanthanide-doped upconversion nanocrystals (UCNCs) can be improved either by precise encapsulation of the surface by optically inert shells around the core, by an alteration of the nearby environment via metal nanoparticles, or by a combination of both. Considering their potential importance in crystalline silicon photovoltaics, the present study investigates both effects for two-dimensional arrangements of UCNCs. Using excitation light of 1500 nm wavelength, we study the variation in the upconversion luminescence from an Er3+-doped NaYF4 core as a function of the thickness of a NaLuF4 shell in colloidal solutions as well as in spin-cast-assisted self-assembled monolayers of UCNCs. The observed UCL yields and decay times of Er3+ ions of the UCNCs increase with increasing shell thickness in both cases, and nearly no variation in decay times is observed in the transition of the UCNCs from solution to film configurations. The luminescence efficiency of the UCN...

A new method for dose regularization in optimization-based proximity-effect-correction is proposed. In contrast to the commonly adopted approach of adding penalty terms to the objective function, a modified scheme is demonstrated where dose regularization is achieved via filtering and projection techniques. The resulting dose patterns are simple and two-toned, and can thus readily be applied in production. Furthermore, existing extensions developed in the context of topology optimization that build on top of the filtering framework, such as robust optimization and strict length scale control, can be adopted directly. The validity of the scheme is assessed in experiments, where the resolvable feature size of the considered 30 kV electron-beam lithography system is decreased from around 100 nm to a few tens of nm. A Python implementation of the scheme is made freely available.

Spectral conversion of the sunlight has been proposed as a method for enhancing the efficiency of photovoltaicvdevices, which are limited in current production by the mismatch between the solar spectrum and the wavelength range for efficient carrier generation. For example, the photo current can be increased by conversion of two low-energy photons (below the band gap of the absorber) to one higher-energy photon (i.e. upconversion). In this paper, we will review our ongoing activities aimed at enhancing such spectral-conversion processes by employing appropriately designed plasmonic nanoparticles. The nanoparticles serve as light-concentrating elements in order to enhance the non-linear upconversion process. From the theoretical side, we approach the optimization of nanoparticles by finite-element modelling of the plasmonic near fields in combination with topological optimization of the particle geometries. Experimentally, the nanostructures are formed by electronbeam lithography on thin films of Er3+-containing transparent materials, foremost TiO2 made by radio-frequency magnetron sputtering, and layers of chemically synthesized NaYF4 nanoparticles. The properties of theupconverter are measured using a variety of optical methods, including time-resolved luminescence spectroscopy on erbium transitions and spectrally resolved upconversion-yield measurements at ∼1500-nm-light excitation.The calculated near-field enhancements are validated using a technique of near-field-enhanced ablation by tunable, ultrashort laser pulses.

This paper presents a simulation-based assessment of the potential for improving the upconversion efficiency of β-NaYF

Quantitative measurements of the electric near-field distribution of star-shaped gold nanoparticles have been performed by femtosecond laser ablation. Measurements were carried out on and off the plasmon resonance. A detailed comparison with numerical simulations of the electric fields is presented. Semi-quantitative agreement is found, with slight systematic differences between experimentally observed and simulated near-field patterns close to strong electric-field gradients. The deviations are attributed to carrier transport preceding ablation.

We propose a simplified rate-equation model for the 1500 nm to 980 nm upconversion in Er$^{3+}$. The simplifications, based on typical experimental conditions as well as on conclusions based on previously published more advanced models, enable an analytical solution of the rate equations, which reproduces known properties of upconversion. We have compared the model predictions with intensity-dependent measurements on four samples with different optical properties, such as upconversion-luminescence yield and the characteristic lifetime of the $^4I_{13/2}$ state. The saturation of the upconversion is in all cases well-described by the model over several orders of magnitude in excitation intensities. Finally, the model provides a new measure for the quality of upconverter systems based on Er$^{3+}$ -- the saturation intensity. This parameter provides valuable information on upconversion parameters such as the rates of energy-transfer upconversion and cross-relaxation. In the present investigation, we used the saturation intensity to conclude that the differences in upconversion performance of the investigated samples are mainly due to differences in the non-radiative relaxation rates., Comment: 8 pages, 6 figures, to be submitted to PRB

A two-particle model is proposed which enables the assessment of particle-particle interactions in large, sparse arrays of randomly distributed plasmonic metal nanoparticles of arbitrary geometry in inhomogeneous environments. The two-particle model predicts experimentally observed peak splittings in the extinction cross section spectrum for randomly distributed gold nanocones on a TiO2:Er3+ thin film with average center-to-center spacings of 3-5 diameters. The main physical mechanism responsible is found to be interference between the incident field and the far-field component of the single-particle scattered field which is guided along the film.

The resource quality and the temporal generation pattern of variable renewable energy sources vary significantly across Europe. In this paper spatial distributions of renewable assets are explored which exploit this heterogeneity to lower the total system costs for a high level of renewable electricity in Europe. Several intuitive heuristic algorithms, optimal portfolio theory and a local search algorithm are used to find optimal distributions of renewable generation capacities that minimise the total costs of backup, transmission and renewable capacity simultaneously. Using current cost projections, an optimal heterogeneous distribution favours onshore wind, particularly in countries bordering the North Sea, which results in average electricity costs that are up to 11% lower than for a homogeneous reference distribution of renewables proportional to each country's mean load. The reduction becomes even larger, namely 18%, once the transmission capacities are put to zero in the homogeneous reference distribution. Heuristic algorithms to distribute renewable capacity based on each country's wind and solar capacity factors are shown to provide a satisfactory approximation to fully optimised renewable distributions, while maintaining the benefits of transparency and comprehensibility. The sensitivities of the results to changing costs of solar generation and gas supply as well as to the possible cross-sectoral usage of unavoidable curtailment energy are also examined., 16 pages, 14 figures

The potential for increasing the upconversion efficiency of β-NaYF4:Er3+ using 1D photonic crystals is investigated. Within realistic material and fabrication constraints, simulations predict up to 300-fold luminescence enhancement at one sun.

Self-organizing nanopatterns can enable economically competitive, industrially applicable light-harvesting platforms for thin-film solar cells. In this work, we present transparent solar cell substrates having quasi-periodic uniaxial nanowrinkle patterns with high optical haze values. The self-organized nanowrinkle template is created by controlled heat-shrinking of metal-deposited pre-stretched polystyrene sheets. A scalable UV-nanoimprinting method is used to transfer the nanopatterns to glass substrates on which single-junction hydrogenated amorphous silicon p-i-n solar cells are subsequently fabricated. The structural and optical analyses of the solar cell show that the nanowrinkle pattern is replicated throughout the solar cell structure leading to enhanced absorption of light. The efficient broadband light-trapping in the nanowrinkle solar cells results in very high 18.2 mA/cm2 short-circuit current density and 9.5% energy-conversion efficiency, which respectively are 35.8% and 39.7% higher than the values obtained in flat-substrate solar cells. The cost- and time-efficient technique introduces a promising new approach to customizable light-management strategies in thin-film solar cells.

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.

One-dimensional multi-component Fermi or Bose systems with strong zero-range interactions can be described in terms of local exchange coefficients and mapping the problem into a spin model is thus possible. For arbitrary external confining potentials the local exchanges are given by highly non-trivial geometric factors that depend solely on the geometry of the confinement through the single-particle eigenstates of the external potential. To obtain accurate effective Hamiltonians to describe such systems one needs to be able to compute these geometric factors with high precision which is difficult due to the computational complexity of the high-dimensional integrals involved. An approach using the local density approximation would therefore be a most welcome approximation due to its simplicity. Here we assess the accuracy of the local density approximation by going beyond the simple harmonic oscillator that has been the focus of previous studies and consider some double-wells of current experimental interest. We find that the local density approximation works quite well as long as the potentials resemble harmonic wells but break down for larger barriers. In order to explore the consequences of applying the local density approximation in a concrete setup we consider quantum state transfer in the effective spin models that one obtains. Here we find that even minute deviations in the local exchange coefficients between the exact and the local density approximation can induce large deviations in the fidelity of state transfer for four, five, and six particles., 12 pages, 7 figures, 1 table, final version

Upconversion enhancement of 1500 nm light has been achieved using geometrically optimized Au nano-discs placed in close vicinity of Er+3 ions in a TiO2 matrix. A finite element modeling is compared to the experimental findings.

In this letter, we present a comparative experimental–simulation study of Au-nanodisc-enhanced upconversion of 1500 nm light in an Er3+ doped TiO2 thin film. The geometry of the Au nanodiscs was guided by finite-element simulations based on a single nanodisc in a finite computational domain and controlled experimentally using electron-beam lithography. The surface-plasmon resonances (SPRs) exhibited a well-known spectral red shift with increasing diameter, well explained by the model. However, an experimentally observed double-peak SPR, which resulted from inter-particle interactions, was expectedly not captured by the single-particle model. At resonance, the model predicted a local-field enhancement of the upconversion yield, and experimentally, the luminescence measurements showed such enhancement up to nearly 7 fold from a nanodisc with 315 nm diameter and 50 nm height. The upconversion enhancement agreed qualitatively with the theoretical predictions, however with 3–5 times higher enhancement, which w...