Excitation Wavelength‐Dependent Upconversion Luminescence Enhancement in Tm³⁺‐Doped LiErF₄@LiYF₄ System Under High Pressure

Abstract Local structural engineering is an endogenous approach to modulate upconversion luminescence (UCL) from upstream to meet the needs of specific application scenarios. Herein, high pressure is utilized as a means to modulate the local structure, and the designed LiErF₄:0.5%Tm³⁺@LiYF₄ (Er:Tm@Y) nanoparticles with fast energy transfer rates, abundant cross‐relaxation processes, and multiple near‐infrared wavelengths (808, 980, 1530 nm) excitation properties are tailored as local structure‐sensitive hosts. A unique excitation wavelength‐dependent UCL enhancement of Er:Tm@Y upconversion nanoparticles is observed by pressure‐induced local structure distortions. When the pressure of ≈6 GPa is applied, the UCL is enhanced by a factor of 2.6 at 980 nm excitation only. After pressure release, the luminescence diminishes and recovers. Density functional theory calculations show that the symmetry distortion of the LiErF₄ crystal reaches a maximum at pressurization to 6 GPa, while a new Er‐4f state emerges, greatly reducing the bandgap from 8.3 to 5.7 eV. Comparative experiments demonstrate that the local symmetry distortion caused by 0.5%Tm³⁺ doping and the different energy transfer patterns of Er³⁺ to Tm³⁺ at different excitations are responsible for this wavelength‐dependent luminescence enhancement.