Small Absorption Cross Section Research Articles

Broadband Near-Infrared Light Excitation Generates Long-Lived Near-Infrared Luminescence in Gallates.

Persistent luminescence describes the phenomenon whereby luminescence remains after the stoppage of excitation. Recently, upconversion persistent luminescence (UCPL) phosphors that can be directly charged by near-infrared (NIR) light have gained considerable attention due to their promising applications ranging from photonics to biomedicine. However, current lanthanide-based UCPL phosphors show small absorption cross sections and low upconversion charging efficiency. The development of UCPL phosphors faces challenges due to the lack of flexible upconversion charging pathways and poor design flexibility. Herein, we discovered a lattice defect-mediated broadband photon upconversion process and the accompanying NIR-to-NIR UCPL in Cr-doped zinc gallate nanoparticles. The zinc gallate nanoparticles can be directly activated by broadband NIR light in the 700-1000 nm range to produce persistent luminescence at about 700 nm, which is also readily enhanced by rationally tailoring the lattice defects in the phosphors. This proposed UCPL phosphor achieved a signal-to-background ratio of over 200 in bioimaging by efficiently avoiding interference from autofluorescence and light scattering. Our work reported a lattice defect-mediated photon upconversion phenomenon, which significantly expands the horizons for the flexible design of UCPL phosphors toward broad applications ranging from bioimaging to photocatalysis.

Analysis of the atomic ratio of H and Zr effect on the neutronics parameters of ZrH moderated space nuclear reactor

Zirconium hydride (ZrH) is an ideal moderator for space nuclear reactors due to its exceptional properties, including high hydrogen content, small neutron absorption cross section, and high operating temperature. The primary goal of this study is to examine how the atomic ratio of H to Zr (H/Zr ratio) influences the neutronics parameters of a ZrH moderated space nuclear reactor, with a focus on establishing a reliable reference for ensuring the optimal safety and minimization of such reactors. The neutronics calculation based on the ZrH moderated space nuclear reactor named Topaz-II is performed using the Reactor Monte Carlo code (RMC code) with the ENDF/VII cross-section database. The effects of the H/Zr ratio were studied with a particular focus on the initial keff, the burnup, the temperature reactivity coefficient and the criticality safety. The results show that with the increase of the H/Zr ratio, the initial keff increases while the drums’ worth decreases. The Moderator Temperature Coefficient (MTC) is a positive value that rises as the H/Zr ratio increases. In the dropping accidents, the reactor full of voids with seawater is more serious, and the introduced reactivity decreases with the increase of the H/Zr ratio.

Phase-field simulation of high-temperature corrosion of binary Zr-2.5Sn alloy

Due to the small neutron absorption cross section and excellent thermal creep performance, zirconium alloy is one of the most important cladding materials for fuel rods in commercial fission reactors. However, quantitative analysis of the effects of temperature and grain boundaries on the corrosion microstructure evolution of zirconium alloys is still needed. The establishing of a phase field simulation for the corrosion process of polycrystalline zirconium alloy and the systematical investigating of the thermodynamic influence are both very important. In this study, the phase field model of the corrosion process in zirconium alloys is developed by combining corrosion electrochemistry through calculating the interfacial energy at the metal-oxide and oxide-fluid boundaries. Then the model is used to investigate the uniform corrosion behavior on the surface of Zr-2.5Sn alloy, which demonstrates that the corrosion kinetic curve follows a cubic rule. Subsequently, the influence of temperature on the corrosion thickening curve of zirconium alloy is examined, and good agreement between simulation and experimental results is achieved. It is observed that during early stage of oxide layer formation, there is a high growth rate with minimal temperature dependence; however, as the oxide layer thickness increases, temperature becomes a significant factor affecting its growth rate, with higher temperatures resulting in faster corrosion rates. Furthermore, the effect of polycrystalline zirconium alloy matrices on corrosion rate is investigated, revealing that the grain boundaries accelerate oxide layer thickening due to enhanced oxygen diffusion rates. At metal-oxide interface, O<sup>2–</sup> bands are formed in areas with higher O<sup>2–</sup> concentration along these grain boundaries towards the metal matrix, which mainly influences oxidation-corrosion rate during the initial oxidation stage.

Continuous Condensed Triplet Accumulation for Irradiance-Induced Anticounterfeit Afterglow.

Afterglow room-temperature emission that is independent of autofluorescence after ceasing excitation is a promising technology for state-of-the-art bioimaging and security devices. However, the low brightness of the afterglow emission is a current limitation for using such materials in a variety of applications. Herein, the continuous formation of condensed triplet excitons for brighter afterglow room-temperature phosphorescence is reported. (S)-(-)-2,2'-Bis(diphenylphosphino)-1,1'-binaphthyl ((S)-BINAP) incorporated in a crystalline host lattice showed bright green afterglow room-temperature phosphorescence under strong excitation. The small triplet-triplet absorption cross-section of (S)-BINAP in the whole range of visible wavelengths greatly suppressed the deactivation caused by Förster resonance energy transfer from excited states of (S)-BINAP to the accumulated triplet excitons of (S)-BINAP under strong continuous excitation. The steady-state concentration of the triplet excitons for (S)-BINAP reached 2.3 × 10-2 M, producing a bright afterglow. Owing to the brighter afterglow, afterglow detection using individual particles with sizes approaching the diffraction limit in aqueous conditions and irradiance-dependent anticounterfeiting can be achieved.

Downshifting luminescence amplification by multi-phase energy harvesting

Lanthanide-based luminescent nanoprobe with the second near-infrared (NIR-II) emission under NIR excitation is widely used in deep tissue bioimaging, biological detection, drug delivery, cancer therapy and information storage due to their unique advantages, such as deep penetration depth, tunable lifetime, and emission wavelength. However, due to the small absorption cross-section and low brightness, the practical application of NIR-II luminescent lanthanide nanoprobe remains a formidable challenge. Here, we present an unconventional nanoparticle design that incorporates co-sensitization along with an additional exterior energy harvesting mechanism. This design expands the energy capture pathway, maximizes the NIR photon absorption capability of as-synthesized nanocrystals, and facilitates energy hopping within Yb3+ sublattices, resulting in remarkably amplified downshifting NIR-II emission from Yb3+ at 980 nm under 808 nm excitation. This study introduces a sophisticated heterogeneous nanostructure that can bring about significant improvements in their photophysical properties, offering promising prospects in the field.

A general design of pyridinium-based fluorescent probes for enhancing two-photon microscopy

Two-photon absorbing fluorescent probes have emerged as powerful imaging tools for subcellular-level monitoring of biological substances and processes, offering advantages such as deep light penetration, minimal photodamage, low autofluorescence, and high spatial resolution. However, existing two-photon absorbing probes still face several limitations, such as small two-photon absorption cross-section, poor water solubility, low membrane permeability, and potentially high toxicity. Herein, we report three small-molecule probes, namely MSP-1arm, Lyso-2arm, and Mito-3arm, composed of a pyridinium center (electron-acceptor) and various methoxystyrene “arms” (electron-donor). These probes exhibit excellent fluorescence quantum yield and decent aqueous solubility. Leveraging the inherent intramolecular charge transfer and excitonic coupling effect, these complexes demonstrate excellent two-photon absorption in the near-infrared region. Notably, Lyso-2arm and Mito-3arm exhibit distinct targeting abilities for lysosomes and mitochondria, respectively. In two-photon microscopy experiments, Mito-3arm outperforms a commercial two-photon absorbing dye in 2D monolayer HeLa cells, delivering enhanced resolution, broader NIR light excitation window, and higher signal-to-noise ratio. Moreover, the two-photon bioimaging of 3D human forebrain organoids confirms the successful deep tissue imaging capabilities of both Lyso-2arm and Mito-3arm. Overall, this work presents a rational design strategy in developing competent two-photon-absorbing probes by varying the number of conjugated “arms” for bioimaging applications.

Squaraine Dye-Sensitized Upconversion with Enhanced Stability and Minimized Aggregation-Caused Quenching.

Upconversion nanoparticles (UCNPs) doped with lanthanides have limited brightness due to their small absorption cross section to light. However, using organic sensitizers can significantly enhance their light absorption ability. Unfortunately, the practical application of organic sensitizers has been hindered by poor stability and aggregation-caused quenching (ACQ). To address these issues, we developed a novel squaraine-based dye, SQ-739, for sensitizing upconversion luminescence (UCL). This dye has a maximum absorption at 739 nm, and shows 1 order of magnitude and 2-fold improved chemical- and photostability, compared to the commonly used cyanine-based dye IR-806, respectively. When SQ-739 is used to sensitize UCNPs, the resulting SQ-739-UCNPs exhibit excellent photostability and reduced ACQ in the presence of polar solvents. Moreover, at the single particle level, the SQ-739-UCNPs exhibit a 97-fold increase in UCL emission compared to bare UCNPs. This squaraine dye-based system represents a new design strategy for developing highly stable and efficient NIR upconversion probes.

Plasmonic Metasurface Integrated Black Phosphorus‐Based Mid‐Infrared Photodetector with High Responsivity and Speed

AbstractBlack Phosphorus (BP), a van der Waals (vdW) semiconducting material, is studied intensively due to its unique structural and anisotropic optical properties. BP exhibits optical anisotropy with thickness tunable direct bandgap from 0.3–2.0 eV. Many BP‐based photodetectors operating in the mid‐infrared (MIR) band are proposed, but their performance is still limited due to the small optical absorption cross section at room temperature. In this work, a plasmonic metasurface that has localized surface plasmon resonance (LSPR) at 3.7 µm is designed, matching well with the band edge of a BP flake and field confinement around the edge of Au‐disk as confirmed by finite difference time domain simulation and Fourier‐transform infrared spectroscopy measurement results. In addition, a BP flake is integrated on the plasmonic metasurface and a significant quenching (twelve‐fold) is observed in the photoluminescence owing to the Förster resonance energy transfer effect induced by dipole–dipole coupling. Following that, a BP‐based MIR photodetector is fabricated on the plasmonic metasurface. Here, the demonstrated BP/plasmonic metasurface‐based photodetector achieves the peak responsivity of 495.85 mAW−1 and ultrahigh operation speed (>10 MHz) at the power of 8.55 µW under 3.7 µm incident wavelength. This demonstrated photodetector opens a new opportunity for optoelectronic applications in the MIR region.

Achieving two-photon fluorescence bioimaging and photodynamic therapy for D–A conjugated polymers through manipulating twisted intramolecular charge transfer

Two-photon fluorescence bioimaging holds immense promise for deep tissue imaging.

Research on energy transfer mechanism and ∼1 μm broadband luminescence properties in Nd3+/Yb3+ co-doped Bi2O3–B2O3–BaF2 glass

Through the traditional molten method, we designed and fabricated Nd3+/Yb3+ co-doped bismuth borate glasses, and completed the study of the luminescence mechanism of rare earth ions in 1 μm band as well as the doping concentration on the luminescence performance of the glass. Firstly, the thermodynamic properties of the glass sample was evaluated by utilizing DSC curve. Subsequently, the spectral properties of BBFNY-2 glass sample was revealed by analyzing the absorption spectra, fluorescence spectra, fluorescence lifetimes and Judd-Ofelt (J-O) theory. Finally, the energy transfer mechanism between rare earth ions Nd3+→Yb3+ was analyzed in detail. The increase of fluorescence intensity at 1008 nm in the glass was calculated corresponding to the broadening of Δλeff from 103 nm to 107 nm, and the obtainable OCT (optical coherence tomography) axial resolution increased from 4.4 μm to 4.1 μm. In this work, the small absorption cross section of Yb3+ at 980 nm in 55Bi2O3–35B2O3–10BaF2 (mol%) glass is effectively solved by Nd3+→Yb3+ energy transfer. Meanwhile, the relevant parameters of the prepared BBFNY-2 glass sample were calculated, and the results suggested that the fabricated glass samples have large microscopic parameters, high transfer efficiency and long fluorescence lifetime. In the results, it was revealed that the via non-radiative electric dipole-dipole process was responsible for the energy transfer and was proportional to the concentration of the Nd3+ donor ions. With all the results, the prepared BBFNY-2 glass sample is a high-potential candidate material for ∼1 μm laser applications.

Near-Infrared in and out: Observation of Autophagy during Stroke via a Lysosome-Targeting Two-Photon Viscosity-Dependent Probe.

Fluorescence imaging using probes with two-photon excitation and near-infrared emission is currently the most popular in situ method for monitoring biological species or events, with a large imaging depth, low background fluorescence, low optical damage, and high spatial and temporal resolution. Nevertheless, current fluorescent dyes with near-infrared emission still have some disadvantages such as poor water solubility, low fluorescence quantum yield, and small two-photon absorption cross sections. These drawbacks are mainly caused by the structural characteristics of dyes with large conjugation surfaces but lacking strong and rigid structures. Herein, a lysosome-targeted and viscosity-sensitive probe (NCIC-VIS) is designed and synthesized. The protonation of morpholine not only helps anchor NCIC-VIS to the lysosome but also significantly enhances its water solubility. More importantly, its viscosity can increase the rigid structure of NCIC-VIS, which will improve the fluorescence quantum yield and the two-photon absorption cross section due to the imposed restrictions on molecular torsion. Based on the abovementioned characteristics, the real-time imaging of cellular autophagy (could increase the viscosity of lysosomes) was realized using NCIC-VIS. The results demonstrated that the level of autophagy was significantly enhanced in mice during stroke, while the inhibition of oxidative stress significantly reduced the degree of autophagy. The study corroborates that oxidative stress induced by stroke can lead to the development of autophagy.

Excellent Multiphoton Excitation Fluorescence with Large Multiphoton Absorption Cross Sections of Arginine-Modified Gold Nanoclusters for Bioimaging.

Fluorescent gold nanoclusters (Au NCs) with excellent one-photon and multiphoton properties have been demonstrated as promising candidates in many application fields. However, small multiphoton absorption (MPA) cross sections and weak multiphoton excitation (MPE) fluorescence impede their practical applications under near-infrared (NIR) excitation for biological imaging. Here, we report the regulated one-photon and multiphoton properties and mechanisms of arginine-stabilized 6-aza-2-thiothymine Au NCs (Arg/ATT-Au NCs) and the applications for MPE fluorescence imaging. The introduction of arginine into the capping layer of ATT-Au NCs significantly modifies the electronic structure, the absorption cross sections, and the relaxation dynamics of the lowest excited state, drastically reducing the nonradiative relaxation, suppressing the blinking, and greatly enhancing the fluorescence. Besides the improved one-photon properties, Arg/ATT-Au NCs demonstrate remarkable MPE fluorescence with a large MPA cross section. The two-photon (λex = 850 nm), three-photon (λex = 1400 nm), and four-photon (λex = 1700 nm) absorption cross sections have been determined to be 6.1 × 10-47 cm4 s1 photon-1, 1.5 × 10-78 cm6 s2 photon-2, and 5.5 × 10-108 cm8 s3 photon-3, respectively, much higher than those of conventional organic compounds and previously reported Au NCs. Moreover, Arg/ATT-Au NCs have been successfully applied in two-photon and three-photon excitation fluorescence imaging of living cells with NIR excitation. The manifold advantages of small size, high quantum yield, suppressed blinking, good photostability and cytocompatibility, large MPA cross sections, and excellent MPE fluorescence imaging performances make fluorescent Arg/ATT-Au NCs a great candidate of imaging probes with vis-NIR excitation.

Giant magneto field effect in up-conversion amplified spontaneous emission via spatially extended states in organic-inorganic hybrid perovskites

Up-conversion lasing actions are normally difficult to realize in light-emitting materials due to small multi-photon absorption cross section and fast dephasing of excited states during multi-photon excitation. This paper reports an easily accessible up-conversion amplified spontaneous emission (ASE) in organic-inorganic hybrid perovskites (MAPbBr₃) films by optically exciting broad gap states with sub-bandgap laser excitation. The broad absorption was optimized by adjusting the grain sizes in the MAPbBr₃ films. At low sub-bandgap pumping intensities, directly exciting the gap states leads to 2-photon, 3-photon, and 4-photon up-conversion spontaneous emission, revealing a large optical cross section of multi-photon excitation occurring in such hybrid perovskite films. At moderate pumping intensity (1.19 mJ/cm²) of 700 nm laser excitation, a significant spectral narrowing phenomenon was observed with the full width at half maximum (FWHM) decreasing from 18 nm to 4 nm at the peak wavelength of 550 nm, simultaneously with a nonlinear increase on spectral peak intensity, showing an up-conversion ASE realized at low threshold pumping fluence. More interestingly, the up-conversion ASE demonstrated a giant magnetic field effect, leading to a magneto-ASE reaching 120%. In contrast, the up-conversion photoluminescence (PL) showed a negligible magnetic field effect (< 1%). This observation provides an evidence to indicate that the light-emitting states responsible for up-conversion ASE are essentially formed as spatially extended states. The angular dependent spectrum results further verify the existence of spatially extended states which are polarized to develop coherent in-phase interaction. Clearly, using broad gap states with spatially extended light-emitting states presents a new approach to develop up-conversion ASE in organic-inorganic hybrid perovskites.

Enhancing Dye-Triplet-Sensitized Upconversion Emission Through the Heavy-Atom Effect in CsLu2 F7 :Yb/Er Nanoprobes.

Lanthanide (Ln3+ )-doped upconversion (UC) nanoprobes, which have drawn extensive attention for various bioapplications, usually suffer from small absorption cross-sections and weak luminescence intensity of Ln3+ ions. Herein, we report the controlled synthesis of a new class of Ln3+ -doped UC nanoprobes based on CsLu2 F7 :Yb/Er nanocrystals (NCs), which can effectively increase the intersystem crossing (ISC) efficiency from singlet excited state to triplet excited state of IR808 up to 99.3 % through the heavy atom effect. By virtue of the efficient triplet sensitization of IR808, the optimal UC luminescence (UCL) intensity of IR808-modified CsLu2 F7 :Yb/Er NCs is enhanced by 1309 times upon excitation at 808 nm. Benefiting from the intense dye-triplet-sensitized UCL, the nanoprobes are demonstrated for sensitive assay of extracellular and intracellular hypochlorite with an 808-nm/980-nm dual excited ratiometric strategy.

A measurement of the Ly β forest power spectrum and its cross with the Ly α forest in X-Shooter XQ-100

ABSTRACT The Ly α forest is the large-scale structure probe for which we appear to have modelling control to the highest wavenumbers. This makes the Ly α forest of great interest for constraining the warmness/fuzziness of dark matter and the timing of reionization processes. However, the standard statistic, the Ly α forest power spectrum, is unable to strongly constrain the intergalactic medium (IGM) temperature–density relation, and this inability further limits how well other high-wavenumber-sensitive parameters can be constrained. With the aim of breaking these degeneracies, we measure the power spectrum of the Ly β forest and its cross-correlation with the coeval Ly α forest using the 100 spectra of z = 3.5–4.5 quasars in the VLT/X-Shooter XQ-100 Legacy Survey, motivated by the Ly β transition’s smaller absorption cross-section that makes it sensitive to somewhat higher densities relative to the Ly α transition. Our inferences from this measurement for the IGM temperature–density relation appear to latch consistently on to the recent tight lower redshift Ly α forest constraints. The z = 3.4–4.7 trends we find using the Ly α–Ly β cross-correlation show a flattening of the slope of the temperature–density relation with decreasing redshift. This is the trend anticipated from ongoing He ii reionization and there being sufficient time to reach the asymptotic temperature–density slope after hydrogen reionization completes. Furthermore, our measurements provide a consistency check on IGM models that explain the Ly α forest, with the cross-correlation being immune to systematics that are uncorrelated between the two forests, such as metal line contamination.

Research of ring RPT method on spherical and cylindrical Double-Heterogeneous systems

Due to the double heterogeneity of the dispersed-particle system, the traditional Volume Homogenization Method (VHM) may have a large calculation deviation. The traditional Reactivity-Equivalent Physical Transformation (RPT) method can be used to deal with the Double-Heterogeneous (DH) system of dispersed fuel particles with relatively small absorption cross section. The deviation is still large for the DH system with dispersed burnable poison particles which has large absorption cross section. The Ring RPT (RRPT) method is an improved RPT method that we have proposed not only to handle the DH system of dispersed fuel particles but also the DH system of dispersed burnable poison particles, and it has been verified in cylindrical DH systems. In this paper the RRPT method is applied to spherical DH systems, and its application range is further analyzed. It is found that the RRPT method has corresponding applicable range when dealing with cylindrical and spherical DH systems. A simple method has been proposed to determine whether a DH system can be processed by the RRPT method, and it provides guidance for the specific application of the RRPT method. And it’s also proved that the RRPT method can treat equivalently the DH systems with the particle types and particle sizes commonly used in engineering. At the same time, through theoretical analysis and calculation verification, and comparison with the calculation results of the Monte Carlo particle model, it is found that the RRPT method is particularly suitable for processing cylindrical DH systems with dispersed-particle burnable poison that the traditional RPT method can’t treat and it is also suitable for spherical DH systems.

Enhanced photothermal conversion performances with ultra‐broad plasmon absorption of Au in Au/Sm2O3 composites

AbstractLanthanide oxides are ideal candidates as photothermal conversion agents owing to their larger photon energy and advantages in biomedical applications. However, the small absorption cross section of rare earth is not conducive of absorbing near infrared light, which will affect the photothermal conversion efficiency of lanthanide oxides. Herein, Au particles are successfully introduced into Sm2O3 to form Au/Sm2O3 composites. The investigation of broadband emission and thermal performances in Sm2O3 and Au/Sm2O3 composites confirm the ultra‐broad plasmon absorption of Au induced thermal effect is in favor of the formation of broadband emission, meanwhile, enhances the photothermal conversion capability of Au/Sm2O3 composites. The temperature increases of the Au/Sm2O3 composites are 5.5°C and 19.6°C compared to Sm2O3 particles under the irradiation of near infrared laser with power density of 11.5 and 29.0 mW/mm2, respectively. In additional, the enhanced photothermal conversion effect is confirmed by the alcohol volatilization experiment and visual infrared thermal images. We present here an idea for enhancing the photothermal conversion capabilities of lanthanide oxides and highlight the promise of using this kind of materials for photothermal therapy.

Safety features of fast reactor with heavy atomic weight weakly neutron absorbing reflector

The purpose of the present study is the justification of the possibility of improving fast reactor safety by surrounding reactor cores with reflectors made of material with special neutron physics properties. Such properties of 208Pb lead isotope as heavy atomic weight, small neutron absorption cross section, and high inelastic scattering threshold result in certain peculiarities in neutron kinetics of the fast reactor equipped with 208Pb reflector, which can significantly enhance reactor safety. The reflector will also make possible generation of additional delayed neutrons characterized by the “dead” time. This will improve the resistibility of the fission chain reaction to stepwise reactivity excursions and exclude prompt supercriticality. Let us note that generation of additional delayed neutrons can be shaped by reactor designers. The relevance of the study amounts to the fact that generation of additional delayed neutrons in the reflector will make it possible mitigating the consequences of a reactivity accident even if the introduced reactivity exceeds the effective fraction of delayed neutrons. At the same time, the role of the fraction of delayed neutrons as the maximum permissible reactivity for reactor safety is depreciated. Scientific originality of the study pertains to the fact that the problem of yield of additional neutrons with properties close to normal delayed neutrons, has not been posed before. The authors suggest a new method for enhancing safety of fast reactors by increasing the fraction of delayed neutrons due to the time delay of prompt neutrons during their transfer in the reflector. In order to benefit from the expected advantages, the following combination is acceptable: lead enriched by 208Pb is used as a neutron reflector while natural lead or other material (sodium, etc.) is used as a coolant in the reactor core.

Ultrasensitive detection of saccharides using terahertz sensor based on metallic nano-slits

Unambiguous identification of trace amounts of biochemical molecules in a complex background using terahertz spectroscopy is extremely challenging owing to the extremely small absorption cross sections of these molecules in the terahertz regime. Herein, we numerically propose a terahertz nonresonant nano-slits structure that serves as a powerful sensor. The structure exhibits strongly enhanced electric field in the slits (five orders of magnitude), as well as high transmittance over an extra-wide frequency range that covers the characteristic frequencies of most molecules. Fingerprint features of lactose and maltose are clearly detected using this slits structure, indicating that this structure can be used to identify different saccharides without changing its geometrical parameters. The absorption signal strengths of lactose and maltose with a thickness of 200 nm are strongly enhanced by factors of 52.5 and 33.4, respectively. This structure is very sensitive to thin thickness and is suitable for the detection of trace sample, and the lactose thickness can be predicted on the basis of absorption signal strength when the thickness is less than 250 nm. The detection of a mixture of lactose and maltose indicates that this structure can also achieve multi-sensing which is very difficult to realize by using the resonant structures.

Strongly enhanced upconversion in trivalent erbium ions by tailored gold nanostructures: Toward high-efficient silicon-based photovoltaics

Upconversion of sub-band-gap photons constitutes a promising way for improving the efficiency of silicon-based solar cells beyond the Shockley-Queisser limit. 1500nm to 980nm upconversion by trivalent erbium ions is well-suited for this purpose, but the small absorption cross section hinders real-world applications. We employ tailored gold nanostructures to vastly improve the upconversion efficiency in erbium-doped TiO2 thin films. The nanostructures are found using topology optimization and parameter optimization and fabricated by electron beam lithography. In qualitative agreement with a theoretical model, the samples show substantial electric-field enhancements inside the upconverting films for excitation at 1500nm for both s- and p-polarization under a wide range of incidence angles and excitation intensities. An unprecedented upconversion enhancement of 913±51 is observed at 1.7Wcm−2. We derive a semi-empirical expression for the photonically enhanced upconversion efficiency, valid for all excitation intensities. This allows us to determine the upconversion properties needed to achieve significant improvements in real-world solar-cell devices through photonic-enhanced upconversion.