Researchers have achieved over three orders of magnitude enhancement in upconversion emission by integrating lanthanide nanoparticles with a metasurface, potentially reshaping infrared imaging sensitivity. The work demonstrates a new approach to infrared-to-visible light conversion, where infrared information is transformed into visible photons using upconverting nanoparticles. The metasurface design maintains uniform enhancement across all relevant viewing angles, preserving image clarity. This hybrid screen offers a path toward “compact, detector-free, and scalable infrared imaging technologies” based on optical upconversion, according to the study published in Light: Science & Applications.
Metasurface-UCNP Hybrid Enables Infrared-to-Visible Conversion
The titanium dioxide metasurface employed in this work supports optical resonances at excitation and emission bands critical for upconversion, a strategy designed to amplify infrared signal strength. Researchers integrated this metasurface with alloyed NaYb₀. ₈Er₀. ₂F₄@NaYF₄ upconverting nanoparticles (UCNPs) to create a hybrid screen capable of significantly boosting infrared-to-visible light conversion efficiency. These UCNPs, unlike those with lower lanthanide concentrations, are fully composed of ytterbium and erbium ions, functioning as both sensitizers and emitters to maximize brightness, particularly under weak infrared illumination, a necessity for practical imaging applications.
The core-shell alloyed structure of the UCNPs is not merely architectural; it directly addresses a limitation of conventional upconversion materials. The higher overall lanthanide content within the alloyed UCNPs enables brighter emission, an important factor when detecting faint infrared signals.
Detailed characterization via high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) revealed the UCNP size distribution and confirmed the core-shell structure, providing visual evidence of the material’s composition. This detailed analysis supports the claim that the alloyed composition is key to achieving enhanced upconversion performance, as the energy diagram of the Yb³⁺/Er³⁺ alloyed UCNPs illustrates the efficient multi-photon absorption process.
A uniform layer of these UCNPs, with a mean nanoparticle size of 21.4 nm, was deposited onto the metasurface via spin-coating, creating a functional infrared-to-visible conversion screen. The design of the metasurface itself relies on an asymmetric architecture and a band-folding strategy, a technique used to manipulate light at the nanoscale.
This approach allows the metasurface to concentrate infrared light onto the UCNPs, further enhancing the upconversion process and ultimately improving the signal-to-noise ratio. “Flat-band angular dispersion of this resonance enables uniform enhancement across incident angles relevant to imaging, thereby preserving spatial frequency content and yielding sharp, high-contrast images,” the researchers write. The significance of this flat-band angular dispersion cannot be overstated; it directly addresses a longstanding challenge in infrared imaging.
Conventional upconversion systems often suffer from angular dependence, where the conversion efficiency varies depending on the angle of the incoming infrared light. This variation leads to distorted images and reduced spatial resolution. This enhancement translates directly into improved imaging capabilities.
The researchers report that the platform provides a pathway toward lightweight and scalable infrared imaging systems based on optical upconversion, with potential applications spanning infrared vision to remote sensing. The ability to convert infrared light into visible photons bypasses the need for traditional, bulky infrared sensors like InGaAs and HgCdTe, which are costly, noisy, and often require active cooling. This simplification opens the door to more compact, energy-efficient, and affordable infrared imaging solutions.
The implications extend beyond simply reducing the size and cost of infrared cameras. This approach could lead to new applications in areas such as environmental monitoring, industrial inspection, security, and biomedical diagnostics, where the ability to detect and visualize infrared radiation is critical. The study’s findings, published in Light: Science & Applications, represent a step toward realizing the full potential of upconversion imaging.
Flat-Band Metasurface Design Preserves Image Spatial Frequencies
Preserving spatial frequencies, a critical factor previously limiting the resolution of upconversion-based systems, is the design principle behind a new infrared imaging screen that maintains image clarity. This preservation stems from a flat-band angular dispersion within the metasurface, ensuring consistent enhancement of infrared signals regardless of their angle of incidence. This uniform enhancement allows for the reconstruction of detailed images when converting infrared light into the visible spectrum, a feat previously hampered by signal distortion and loss of fine features.
The researchers demonstrated this capability by integrating a symmetry-broken dielectric metasurface with a coating of upconverting nanoparticles (UCNPs), creating a functional infrared-to-visible imaging screen. Angle-resolved transmission measurements confirmed the flat-band resonance, revealing minimal variation in performance across a range of incident angles.
Quantitative analysis using an edge-based modulation transfer function (MTF) revealed a reduction in high-spatial-frequency contrast by approximately a factor of two after upconversion, but features as small as 20 micrometers remained clearly discernible. This suggests the metasurface design effectively mitigates resolution loss within a practical range for many imaging applications. The observed reduction in MTF50, while present, does not stem from limitations of the imaging system itself; the diffraction limit of the objective and detector sampling rate are substantially larger.
Instead, the researchers attribute the contrast reduction to a combination of relay optics degradation and the finite angular bandwidth of the metasurface. “The metasurface angular bandwidth (±6°) theoretically supports spatial frequencies up to ~0.16 cycles µm −1 at the emission wavelength, which is comparable to the measured upconverted bandwidth,” the study reports, indicating the design is performing close to its theoretical limits.
Three-Order Magnitude Upconversion Enhancement Achieved
The fabricated titanium dioxide metasurface achieved an upconversion enhancement of 1100-fold at a wavelength of 660 nanometers under an excitation intensity of 15.9 watts per square centimeter, a performance comparable to the highest enhancement factors previously reported for both metasurface-UCNP and plasmonic-UCNP hybrid structures. This substantial gain in signal strength stems from careful design of the metasurface to support multiple optical resonances, aligning them with both the excitation and emission bands of the alloyed ytterbium/erbium upconverting nanoparticles.
Detailed mode analysis revealed the presence of bright, high-Q, and spatially overlapping resonances at these critical bands, facilitating efficient light-matter interaction and maximizing upconversion efficiency. The design prioritizes coupling to linearly polarized free-space light, an essential requirement for both efficient experimental excitation and collection, and a key criterion for achieving significant upconversion enhancement.
Researchers decomposed eigenmodes with respect to the C4v E irreducible representation to identify these bright modes, ensuring optimal performance under realistic illumination conditions. This enhancement is attributed to the strong resonance established at the excitation band, demonstrating the robustness of the approach. The team employed a figure of merit (FOM) to quantify the upconversion enhancement provided by the metasurface, placing equal emphasis on excitation and emission processes while intentionally disregarding the nonlinear, multiphoton nature of the excitation.
This approach, they explain, promotes the development of multi-resonant structures capable of maximizing upconversion efficiency. Light absorption was used as a proxy for light-matter interaction, establishing a direct correlation between absorption at 660 nanometers and emission intensity at the same wavelength.
Normalization by the square of the UCNP film thickness accounted for the approximate linear scaling of absorption with film thickness in the absence of the metasurface, ensuring accurate comparison of enhancement factors. The presented metasurface-based strategy offers a pathway toward substantially enhancing the upconversion efficiency of lanthanide-based nanoparticles while simultaneously enabling high-resolution infrared-to-visible imaging. The researchers have demonstrated a functional infrared-to-visible imaging system.
Lanthanide Upconversion Circumvents Optoelectronic Limitations
The fabrication process carefully controls nanoparticle size, yielding a mean diameter of 21.4 nm. This precise control minimizes energy loss during upconversion, a critical factor in maximizing signal clarity and imaging efficiency. Encapsulating each lanthanide-based upconverting nanoparticle (UCNP) core with a four-nanometer shell of undoped NaYF4 prevents energy migration to surface quenchers, further enhancing luminescence. The energy-level diagram demonstrates that 980-nanometer excitation results in both red and green emission through sequential multiphoton absorption and energy transfer, a process confirmed by prominent emission peaks at 654 and 542 nanometers.
Analysis of the red emission’s nonlinear excitation dependence revealed a power-law relationship, with intensity proportional to the excitation power raised to approximately 1. 9. This indicates a predominantly two-photon upconversion process under the tested conditions, a finding that validates the efficiency of the energy transfer mechanisms within the nanoparticles.
The researchers observed that the upconversion screen successfully imaged an object under infrared excitation, demonstrating its capacity to convert otherwise invisible light into a visible representation. Comparing images captured with and without the screen under broadband illumination clearly showed the upconversion process in action, with the patterned metasurface area distinctly visible in the upconverted infrared image. The team’s approach addresses a fundamental limitation of conventional infrared imaging, which relies on narrow-bandgap optoelectronic sensors like InGaAs and HgCdTe.
These sensors are not only expensive but also susceptible to noise and often require active cooling, adding to system complexity and cost. The observed upconversion process offers a potential alternative, converting infrared information into the visible spectrum where it can be detected by more readily available and affordable silicon-based detectors. This enhancement is critical for achieving high-sensitivity infrared imaging without the need for complex and costly detection systems.
The study’s findings build on previous work exploring the potential of upconverting nanoparticles for low-energy super-resolution applications and broadband photodetectors. Earlier research demonstrated the feasibility of using these nanoparticles to amplify photon upconversion in alloyed materials and create graphene-based hybrid photodetectors with improved sensitivity. The current work extends these efforts by integrating the UCNPs with a metasurface, a carefully engineered structure designed to manipulate light at the nanoscale.
This combination allows for precise control over the upconversion process, maximizing the conversion efficiency and preserving image clarity. The researchers also investigated the dynamics of energy transfer within the UCNPs, finding that these networks are complex systems with collective, robust, and history-dependent behavior. Understanding these dynamics is crucial for optimizing the performance of upconversion-based imaging systems and developing new strategies for controlling the flow of energy within the nanoparticles.
The ability to create visible images from infrared light without the need for traditional sensors could lead to more affordable, energy-efficient, and versatile imaging solutions. The study’s methodology involved detailed characterization of the UCNPs and the metasurface, including HAADF-STEM imaging, energy-level diagrams, and emission spectroscopy. These techniques allowed the researchers to confirm the size, morphology, and upconversion properties of the nanoparticles, as well as the resonant behavior of the metasurface.
The results demonstrate a clear correlation between the metasurface design and the enhanced upconversion efficiency, validating the effectiveness of the approach. The team’s findings are published in Light: Science & Applications and are dated September 9, 2026.
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