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

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.