Fig. 1

Nanoparticles for low irradiance imaging. a Multiphoton energy absorption, transfer, surface loss, and emission in Yb³⁺/Er³⁺ UCNPs and aUCNPs. Energy transfer pathways were calculated with a kinetic model as described^{18, 33}. Comparison of alloyed (b) and doped (c) UCNPs, showing energy transfer in matrix Y³⁺ (gray), sensitizer Yb³⁺ (blue), and emitter Er³⁺ (green) ions. d Emission spectra of single core/shell NaYb_0.8Er_0.2F₄ aUCNPs and NaYF₄: 20% Yb³⁺, 20% Er³⁺ UCNPs under 2 × 10⁶ W cm⁻² 980 nm laser excitation. e High-resolution TEM image of an 8-nm NaYb_0.4Er_0.6F₄ with 4-nm NaY_0.8Gd_0.2F₄ shell (scale bar is 5 nm) and f high-angle annular dark-field STEM of 8-nm NaYb_0.4Er_0.6F₄ with 8-nm NaY_0.8Gd_0.2F₄ shells (scale bar is 25 nm)