Conventional luminescence intensity ratio (LIR) thermometry based on two thermally coupled levels (TCLs) suffers from an intrinsic trade-off between a large energy gap (ΔE), which is required for high relative sensitivity ($S_r$), and the strong thermal coupling needed to maintain Boltzmann equilibrium. Nonthermally coupled thermometers can also provide high sensitivity and robust performance after appropriate calibration, but their temperature response is generally more system specific because it is not directly constrained by a simple Boltzmann relation. Here, we implement a three-level thermal-coupling scheme in ${\mathrm{Nd}}^{3+}$-doped ${\mathrm{NaYF}}_4$ microparticles to mitigate this sensitivity-thermalization trade-off. The intermediate ${}^4\mathrm{F}_{5/2}$ manifold partitions the ∼1700 ${\mathrm{cm}}^{-1}$ separation between ${}^4\mathrm{F}_{3/2}$ and ${}^4\mathrm{F}_{7/2}$ into two moderate adjacent subgaps of about 800–900 ${\mathrm{cm}}^{-1}$, which is expected to facilitate thermally assisted population redistribution while retaining a large effective $\mathrm{\Delta }E$. At the same time, a thermally enhanced anti-Stokes emission at 753 nm (${}^4\mathrm{F}_{7/2}\to {}^4\mathrm{I}_{9/2}$) and a thermally quenched Stokes emission at 1061 nm (${}^4\mathrm{F}_{3/2}\to {}^4\mathrm{I}_{11/2}$) are combined to construct the LIR, making the intensity ratio strongly temperature dependent and producing a steep absolute LIR response, as reflected by absolute sensitivity ($S_a$). Notably, the 753 nm anti-Stokes emission lies in the NIR-I region, whereas the 1061 nm Stokes emission is located in the NIR-II region. This NIR-I/NIR-II-separated configuration reduces spectral overlap and cross-talk between the two thermometric channels, allowing the thermally enhanced and thermally quenched emissions to be tracked independently in the anti-Stokes/Stokes ratiometric readout. Experimentally, the resulting near-infrared thermometer operates reliably from 298 to 548 K, reaching a maximum $S_r$ of 2.73% ${\mathrm{K}}^{-1}$ at 298 K, while the selected fixed anti-Stokes/Stokes readout exhibits an $S_a$ value of 29.81% ${\mathrm{K}}^{-1}$ at 548 K, with consistent performance under both 864 and 808 nm excitation. Compared with a simple two-level TCL description, this three-level strategy offers a practical way to use a larger overall energy separation while retaining a Boltzmann-type interpretation of the temperature-dependent LIR. This work provides an energy-level engineering approach for designing high-sensitivity luminescent thermometers based on sequential thermal coupling.

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• Light-matter interaction
• Lanthanides
• Rare-earth doped crystals
• Boltzmann theory
• Infrared spectroscopy
• Luminescence
• Thermometry