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Two Photon NIR-to-Red Upconversion Mechanism in CaF₂:Er³⁺, Yb³.

Zhifeng Deng^1,2, Hao Wu¹, Huajun Wu¹

¹State Key Laboratory of Luminescence Science and Technology, Changchun Institute of Optics, Fine Mechanics and Physics, Chinese Academy of Sciences, Changchun 130033, China.

The Journal of Physical Chemistry Letters

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Summary

Erbium-ytterbium upconversion in CaF2 generates intense red light via an unusual two-photon mechanism. This process, driven by efficient round-trip energy transfer, explains unique luminescence properties in these materials.

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Area of Science:

Materials Science
Photonics
Spectroscopy

Background:

Erbium (Er3+) and Ytterbium (Yb3+) co-doped materials are known for upconversion (UC) luminescence under near-infrared (NIR) excitation.
Typically, red emission in these systems involves two- or three-photon processes, influenced by host material properties like phonon energy.
Fluoride hosts usually favor three-photon processes due to suppressed multiphonon relaxation (MPR).

Purpose of the Study:

To investigate the mechanism behind the intense two-photon red upconversion emission in CaF2:Er3+/Yb3+.
To elucidate the role of round-trip energy transfer (RTET) in the red UC emission process.
To explain the concentration-dependent changes in UC emission color and intensity ratios.

Main Methods:

Analysis of spectral and temporal luminescence behavior.
Investigating Er3+ and Yb3+ concentration effects on UC emission.
Utilizing a round-trip energy transfer (RTET) model.

Main Results:

CaF2:Er3+/Yb3+ exhibits intense red UC emission primarily through an unusual two-photon mechanism.
Efficient RTET between Er3+ and Yb3+ facilitates relaxation from the green-emitting state to the red-emitting state.
RTET was found to dominate red UC emission across a wide range of dopant concentrations.
The RTET model successfully explains the decrease in red-to-green intensity ratio and color shift with increasing Yb3+ concentration.

Conclusions:

The unique clustering of rare earth ions in CaF2 enables efficient RTET, driving the dominant two-photon red UC emission.
Understanding RTET is crucial for controlling UC emission properties in Er3+-Yb3+ doped materials.
This study provides insights into optimizing UC materials for applications requiring specific emission colors and intensities.