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The optically transparent biological window in the near-infrared (NIR) spectral range allows deep-tissue excitation and the detection of fluorescence signals^,. Spectrum-domain discrimination of NIR contrast agents via an upconversion or downshifting scheme requires sufficient (anti-) Stokes shift to separate excitation and fluorescence emission. Here, we report a time-domain (τ) scheme in which about 5,000 ytterbium signal transducers are condensed within an optically inert and biocompatible CaF₂ shell (2.3 nm), which forms a 14.5 nm τ-dot. Because of the long-lived and spectrally narrowly defined excited state of pure ytterbium ions, the NIR τ-dot can convert the NIR pulsed excitation into long-decaying luminescence with an efficiency approaching 100%. Within a safe injection dosage of 13 μg g⁻¹, an excitation power density of 1.1 mW cm⁻² was sufficient to image organs with a signal-to-noise ratio of …