Energy Back Transfer Turns Green Upconversion Yellow in Yb-Rich Erbium Crystals

Under a 980 nm laser, crystals and phosphors doped with erbium and rich in ytterbium do something visually striking: as the pump power density climbs, their bright green upconversion glow gradually turns yellow. A research team led by Miaomiao Wang, Mengyu Zhang, Shoujun Ding, Haitang Hu, Chuancheng Zhang and Yong Zou has now traced this color change to a specific microscopic process, energy back transfer, in which excited erbium ions hand their energy back to the ytterbium ions that originally sensitized them. The work, published in Frontiers of Optoelectronics, examines three related materials and shows exactly when and why the back transfer switches on.

The materials at the center of the study are molybdate hosts, a family of compounds widely used as luminescent matrices. The team synthesized a 5 at% Er3+ doped NaYb(MoO4)2 phosphor by a high temperature solid state method, and grew two single crystals by the Czochralski method using a JGD-400 top mounted furnace: a 5 at% Er3+:NaYb(MoO4)2 crystal and a NaBi(MoO4)2 crystal codoped with 10 at% Yb3+ and 1 at% Er3+. The key structural difference between the phosphor and the crystal lies in ordering. The phosphor is polycrystalline, with only local order, while the crystal is monocrystalline, with long range order, a distinction that turns out to matter for how the materials behave under intense illumination.

X-ray diffraction confirmed that all prepared samples matched the standard reference patterns for NaYb(MoO4)2 and NaBi(MoO4)2, demonstrating pure phase formation and showing that dopant ions did not significantly alter the host crystal structure. Diffuse reflectance spectra in the 200 to 1000 nm range revealed the expected absorption bands of Er3+, with peaks at 379, 407, 451, 489, 523, 545, 655, 802 and 970 nm corresponding to transitions from the 4I15/2 ground state to various excited states. The band near 970 nm appeared unusually wide, which the authors attribute to the overlap of the Er3+ 4I15/2 to 4I11/2 transition with the 2F7/2 to 2F5/2 absorption of the high concentration of Yb3+ ions in the self activated host.

Using Tauc analysis of the diffuse reflectance data, the team estimated optical band gaps for the samples. The Er3+/Yb3+ codoped NaBi(MoO4)2 single crystal showed a band gap of 3.20 eV, while the Er3+ doped NaYb(MoO4)2 phosphor showed 3.68 eV, a difference ascribed to the larger ionic radius of Bi3+ compared with Yb3+. Interestingly, the band gap of the Er3+ doped NaYb(MoO4)2 single crystal was slightly larger than that of the phosphor, which the authors connect to the segregation behavior of Er3+ during crystal growth, meaning the dopant does not fully replace Yb3+ in the grown crystal.

When excited by the 980 nm semiconductor laser, all three samples displayed the same basic emission pattern: two prominent green bands and one weak red band in the visible spectrum. The green emissions arise from the 2H11/2 to 4I15/2 transition centered at 530 nm and the 4S3/2 to 4I15/2 transition centered at 554 nm of Er3+, while the weak red band comes from the 4F9/2 to 4I15/2 transition centered at 660 nm. Notably, the concentration of Yb3+ ions in the matrix did not significantly shift the emission peak positions, meaning the color changes observed later in the study come from changing intensity ratios, not from spectral shifts of the individual lines.

The crucial experiments involved recording spectra across a broad range of pump power densities and presenting them as contour plots. For the codoped NaBi(MoO4)2 crystal, which contains a comparatively low Yb3+ concentration of 10 at% in a host that also supplies Bi3+ rather than Yb3+ as the dominant lattice cation, the spectrum remained constant across the whole power density range, always dominated by green emission. In contrast, the spectra of both self activated NaYb(MoO4)2 samples, where the host lattice itself is built from Yb3+, became distorted at higher power densities, a distortion the researchers attribute to the onset of energy back transfer between Yb3+ and Er3+ ions.

Luminescence photographs made the transition directly visible. Both the NaYb(MoO4)2 phosphor and crystal showed yellow emission at high power densities, but the phosphor turned yellow at a lower power threshold than the crystal. The authors propose an explanation rooted in thermal properties: crystals with long range ordered structures conduct heat better and are more thermally stable than phosphors with locally ordered structures, so the power point at which back transfer becomes visible is shifted higher in the crystal. Meanwhile, the codoped NaBi(MoO4)2 crystal glowed bright green throughout the measured range, confirming the absence of a back transfer process in that material.

To probe the mechanism more directly, the team monitored the near infrared emission of Yb3+ ions in the 5 at% Er3+:NaYb(MoO4)2 crystal at different power densities. At low power density, the Yb3+ emission gradually weakened as power increased, consistent with the normal forward energy transfer from Yb3+ to Er3+ that drives upconversion. At high power density, however, the broad Yb3+ emission band centered around 1048 nm weakened while a signal around 1010 nm grew stronger, which the authors interpret as evidence of energy flowing from Er3+ back to Yb3+, the energy back transfer process, ultimately producing the yellow luminescence seen in the images.

The microscopic picture the authors assemble is detailed. At low pump power, Yb3+ ions absorb 980 nm photons and move from the 2F7/2 ground state to the 2F5/2 excited state, then transfer energy to neighboring Er3+ ions, lifting them from 4I15/2 to 4I11/2. A second energy transfer, or excited state absorption within Er3+ itself, raises Er3+ to 4F7/2, which relaxes nonradiatively to 2H11/2 and 4S3/2, the levels responsible for green emission. Red emission follows through additional pathways populating 4F9/2. At high Yb3+ concentration and high pump power, this conventional scheme breaks down. The back transfer process 4S3/2 plus 2F7/2 yielding 4I13/2 plus 2F5/2 repopulates Yb3+ excited states while draining the green emitting 4S3/2 level of Er3+, suppressing green output. A cross relaxation process between two Er3+ ions could also diminish green emission, but the authors argue it only dominates at high Er3+ concentration, so back transfer is the more efficient and dominant route at high Yb3+ loading, enhancing red emission.

Beyond clarifying the mechanism, the findings carry practical weight for optical temperature sensing based on the fluorescence intensity ratio technique, which relies on the intensity ratio of two thermally coupled Er3+ emission bands and enables noncontact temperature measurement. Because back transfer distorts those emission bands in a power dependent way, the excitation power used in such measurements matters enormously, yet published temperature sensing studies have employed widely different powers, from 150 mW to several watts per square centimeter. By mapping how back transfer onset depends on Yb3+ concentration, sample form and pump power density, this work offers a rational basis for choosing an optimal excitation power density, a step the authors describe as a reliable direction for future optical thermometry design in ytterbium rich upconversion materials.

Subject of Research: Energy back transfer between Er3+ and Yb3+ ions in upconversion luminescent molybdate phosphors and crystals

Article Title: Effect of energy back transfer from Er3+ to Yb3+ ions on the upconversion luminescence of Er:NaYb(MoO4)2 and Yb,Er:NaBi(MoO4)2

Article References: Wang, M., Zhang, M., Ding, S., Hu, H., Zhang, C., & Zou, Y. (2025). Effect of energy back transfer from Er3+ to Yb3+ ions on the upconversion luminescence of Er:NaYb(MoO4)2 and Yb,Er:NaBi(MoO4)2. Frontiers of Optoelectronics, 18(1), Article 12. https://doi.org/10.1007/s12200-025-00155-5

Image Credits: AI Generated

DOI: 10.1007/s12200-025-00155-5

Keywords: upconversion luminescence, energy back transfer, erbium, ytterbium, NaYb(MoO4)2, NaBi(MoO4)2, phosphors, single crystals, optical temperature sensing, 980 nm excitation, green emission, power density