Upconverting nanocrystals as luminescent temperature probes for local-heating imaging during direct laser writing 3D nanolithography

Luminescence measurements of upconverting nanocrystals (UCNCs) dispersed in SZ2080 prepolymer being pro- cessed by direct laser writing (DLW) nanopolymerization technique are presented. Er³⁺ ions doped β-NaYbF₄ and Er³⁺,Yb³⁺ co-doped β-NaGdF₄ core and core-shell UCNCs were prepared by a thermal decomposition method. The ratio of the ²H_11/2 → ⁴I_15/2 and ⁴S_3/2 → ⁴I_15/2 emission intensities under λ = 975 nm excitation was confirmed to follow Boltzmann-type distribution in the temperature range from 20 °C to 200 °C and enabled a self-referenced optical readout of the sample temperature changes. Variation of thermally-coupled spectral bands fluorescence intensity ratio (FIR) was observed while prepolymer being processed under typical DLW conditions (1030 nm, 300 fs, 200 kHz, NA = 0.8) and E_pulse varying from below modification threshold to the optical breakdown. Average fitted temperature changes around polymerized voxel measured ∆T₁ < 30 °C within polymerization window and increases up to ∆T₂~100 °C in overexposing regime.