A 2016 study demonstrated that a two-color laser field combining 850 nm and 980 nm radiation cooperatively pumps population directly into the radiative levels of Er3+^{3+} in NaYF4_4 nanocrystals, improving the green and red upconversion luminescence efficiency. Two-color excitation was already a well-established strategy for improving upconversion efficiency when that study appeared — its own reference list includes earlier multi-wavelength enhancement in other hosts and a companion two-color study of Er3+^{3+}/Yb3+^{3+}-codoped glass by the same group — so what the demonstration left open is the transition from case-by-case demonstrations to a predictive strategy: whether the cooperative two-color direct-pumping mechanism extends, in a predictable form rather than one demonstration at a time, to other lanthanide ions, host materials, nanocrystal sizes, and emission bands; and whether one can predict in advance which auxiliary wavelength and intensity combination will pump population directly into a desired radiative level and improve the corresponding upconversion emission. Later literature — a 2019 Progress in Materials Science review of two-/multi-wavelength excitation and a series of dual-wavelength demonstrations in nanothermometry, frequency-encoded multiplexed imaging, optical logic, and depletion nanoscopy — confirms the strategy is not specific to the original system, but no general predictive framework for auxiliary-wavelength selection or systematic laser-field design has been published. The problem of establishing a predictive design principle for two-color cooperative excitation of lanthanide upconversion remains open.

Background

Photon upconversion (UC) in lanthanide-doped nanocrystals is the emission of higher-energy, visible photons under near-infrared excitation, produced when one ion sequentially absorbs two or more low-energy photons. It underpins applications in bioimaging, nanothermometry, anti-counterfeiting, photovoltaics, and optical logic. In Er3+^{3+}-doped materials the dominant green and red UC emissions originate from the closely spaced 2H11/2^2H_{11/2} and 4S3/2^4S_{3/2} levels and from the 4F9/2^4F_{9/2} level of Er3+^{3+}. The standard host NaYF4_4 is a low-phonon-energy fluoride that suppresses multiphonon relaxation — nonradiative loss of excitation through lattice vibrations — and NaYF4_4 nanocrystals are among the brightest known UC materials. Conventional excitation uses a single 980 nm laser, resonant with the Er3+^{3+} 4I15/2^4I_{15/2} to 4I11/2^4I_{11/2} ground-state absorption (often reinforced by Yb3+^{3+} co-doping, where Yb3+^{3+} acts as a sensitizing antenna absorbing 980 nm light and transferring energy to Er3+^{3+}). UC then proceeds via excited-state absorption (ESA, absorption of a second photon from a long-lived intermediate, metastable level) or energy transfer upconversion (ETU, energy delivered by a neighboring excited ion). Efficiency is intrinsically low: population typically reaches an emitting level only after cascading down from higher-lying states, losing energy at each multiphonon step, and cross-relaxation (pairwise energy exchange between neighboring lanthanide ions) and surface-related quenching further divert population. The standard theoretical description is a rate-equation model tracking the populations of all relevant levels with measurable transition rates.

A complementary route to higher efficiency is to shape the excitation field itself. Multi-wavelength excitation had earlier been used both destructively — stimulated-emission-depletion-style suppression of an emitting level, as in UC nanoscopy driven by cross-relaxation (Zhan et al., Nature Communications 2017) — and constructively, by adding a second pump color that populates states the first color cannot reach. In this context Yao et al. (Journal of Materials Science, 2016) showed that a two-color laser field combining 980 nm and 850 nm improves both the green and red UC luminescence efficiency of Er3+^{3+}:NaYF4_4 nanocrystals. Their central mechanistic claim is that in a cooperative-excitation process the two colors jointly pump population directly into the radiative level (for the green band, 4S3/2^4S_{3/2}) rather than into higher levels whose spontaneous decay merely feeds the emitting level, so excitation is not lost in intermediate relaxation steps. A comprehensive 2019 review of two-/multi-wavelength light excitation in optical materials (Chen et al., Progress in Materials Science) consolidated this rapidly growing field — mechanisms, emitting centers, hosts, applications — and closed with remaining questions and future perspectives, confirming that the generality and design of cooperative two-color pumping is an active open program rather than a settled result.

Problem Statement

The 2016 demonstration of two-color cooperative direct-pumping in Er3+^{3+}:NaYF4_4 leaves open the transition from case-by-case demonstrations to a predictive strategy. The problem has two linked strands sharing one scientific objective — making two-color cooperative direct-pumping of upconversion a systematically designable tool:

  1. Generality. Does the cooperative two-color direct-pumping mechanism established for Er3+^{3+} in NaYF4_4 nanocrystals at 850 + 980 nm extend, in a predictable form beyond the case-by-case demonstrations already available, to other lanthanide ions, other host materials, other nanocrystal sizes, and other emission bands, improving the upconversion luminescence efficiency of the targeted emission bands relative to conventional excitation?

  2. Prediction. For a specified lanthanide ion in a specified host and nanocrystal size, and a chosen target radiative level, can one predict in advance which auxiliary wavelength (paired with which primary wavelength) and which intensity combination will best pump population directly into that level, improving the upconversion luminescence efficiency of that band relative to conventional excitation?

The objective shared by both strands is the one the source itself points to: a predictive capability to properly design the laser fields for upconversion luminescence generation and control — that is, for a chosen radiative level of a lanthanide-doped nanocrystal system, the combination of excitation wavelengths and the intensity conditions that pump population directly into that level and improve the corresponding upconversion emission.

The two strands are two formulations of this single objective rather than independent requirements. The four axes listed under Generality — other lanthanide ions, host materials, nanocrystal sizes, and emission bands — describe the class of systems the predictive capability must cover, not four separate demonstrations that must each be supplied; a selection criterion that predicts the auxiliary wavelength and intensity combination for a specified system answers both strands, whereas a criterion confined to the original Er3+^{3+}:NaYF4_4 case at 850 + 980 nm answers neither. Resolving the task therefore requires a predictive selection criterion together with at least one worked validation in which the predicted combination pumps the chosen radiative level and improves that emission band relative to conventional single-color excitation; validation may be theoretical, numerical, or experimental, but a system-specific retrospective fit does not by itself answer the objective.

References

  1. Yunhua Yao, Cheng Xu, Ye Zheng, Chengshuai Yang, Pei Liu, Jingxin Ding, Tianqing Jia, Jianrong Qiu, Shian Zhang, Zhenrong Sun (2016), 'Improving upconversion luminescence efficiency in Er3+-doped NaYF4 nanocrystals by two-color laser field', Journal of Materials Science 51(11):5460-5468, DOI 10.1007/s10853-016-9849-z, https://doi.org/10.1007/s10853-016-9849-z
  2. Zhi Chen, Guoping Dong, Hanwei Gao, Jianrong Qiu (2019), 'Two-/multi-wavelength light excitation effects in optical materials: From fundamentals to applications', Progress in Materials Science 105:100568, DOI 10.1016/j.pmatsci.2019.05.001, https://doi.org/10.1016/j.pmatsci.2019.05.001
  3. X. F. Wang, Y. Wang, Y. Q. Lu, L. Meng, Y. Y. Bu (2018), 'Dual-mode infrared photon-excited synergistic effect in Er3+-doped NaYF4 glass ceramics', Applied Physics A 124:521, DOI 10.1007/s00339-018-1944-3, https://doi.org/10.1007/s00339-018-1944-3
  4. Qiuqiang Zhan, Haichun Liu, Baoju Wang, Qiusheng Wu, Rui Pu, Chao Zhou, Bingru Huang, Xingyun Peng, Hans Ågren, Sailing He (2017), 'Achieving high-efficiency emission depletion nanoscopy by employing cross relaxation in upconversion nanoparticles', Nature Communications 8:1058, DOI 10.1038/s41467-017-01141-y, https://doi.org/10.1038/s41467-017-01141-y
  5. Jiaomei Wang, Hang Lin, Yao Cheng, Xiangshui Cui, Yan Gao, Zeliang Ji, Ju Xu, Yuansheng Wang (2019), 'A novel high-sensitive upconversion thermometry strategy: Utilizing synergistic effect of dual-wavelength lasers excitation to manipulate electron thermal distribution', Sensors and Actuators B: Chemical 278:165-171, DOI 10.1016/j.snb.2018.09.086, https://doi.org/10.1016/j.snb.2018.09.086
  6. Jingheng Nie, Weitao Ying, Jianmin Gu, Fei Wang, Shiqing Xu, Shimin Liu (2020), 'Upconversion logic gates based on dual-wavelength excitation', Journal of Physics D: Applied Physics 53:285103, DOI 10.1088/1361-6463/ab84a8, https://doi.org/10.1088/1361-6463/ab84a8
  7. Mikkel Baldtzer Liisberg, Satu Lahtinen, Ane Beth Sloth, Tero Soukka, Tom Vosch (2021), 'Frequency Encoding of Upconversion Nanoparticle Emission for Multiplexed Imaging of Spectrally and Spatially Overlapping Lanthanide Ions', Journal of the American Chemical Society 143:19399-19405, DOI 10.1021/jacs.1c07691, https://doi.org/10.1021/jacs.1c07691
  8. Xiaoyu Meng, Yu Xie, Tao Shen, Jinsong Wang, Kai Ning, Jiang Wang, Cong Cao (2024), 'Rare earth fluoride with dye-sensitized upconversion luminescence under dual-excitation wavelength', Inorganic Chemistry Communications 167:112672, DOI 10.1016/j.inoche.2024.112672, https://doi.org/10.1016/j.inoche.2024.112672
  9. Shuwu Xu, Huajie Chen, Lingxiao Wang, Yunxia Huang (2025), 'Zeeman-effect-induced luminescence suppression in Er3+-doped GdF3 nanoparticles under dual-wavelength excitation', Applied Physics Letters 126:153301, DOI 10.1063/5.0267611, https://doi.org/10.1063/5.0267611
  10. Ping Chen, Shaoliang Yu, Beibei Xu, Juechen Wang, Xiangwen Sang, Xiaofeng Liu, Jianrong Qiu (2014), 'Enhanced upconversion luminescence in NaYF4:Er nanoparticles with multi-wavelength excitation', Materials Letters 128:299-302, DOI 10.1016/j.matlet.2014.04.179, https://doi.org/10.1016/j.matlet.2014.04.179
  11. Zhi Chen, Xiaowen Zhang, Shengfeng Zeng, Zhengjun Liu, Zhijun Ma, Guoping Dong, Shifeng Zhou, Xiaofeng Liu, Jianrong Qiu (2015), 'Highly efficient up-conversion luminescence in BaCl2:Er3+ phosphors via simultaneous multiwavelength excitation', Applied Physics Express 8:032301, DOI 10.7567/APEX.8.032301, https://doi.org/10.7567/APEX.8.032301
  12. Yunhua Yao, Cheng Xu, Ye Zheng, Chengshuai Yang, Pei Liu, Jingxin Ding, Tianqing Jia, Jianrong Qiu, Shian Zhang, Zhenrong Sun (2016), 'Enhancing up-conversion luminescence of Er3+/Yb3+-codoped glass by two-color laser field excitation', RSC Advances 6:3440-3445, DOI 10.1039/C5RA23464F, https://doi.org/10.1039/C5RA23464F