Fig. 5: Observations of radiative spin-exchange Mn–PbSe-core coupling.

a, The PL spectrum of Mn-doped QDs (sample Mn-1) spanning from a NIR to a visible spectral range. The lower-energy part of the spectrum (~0.5 eV to ~1.4 eV) is instantaneous PL measured using a superconducting single-photon detector (SSPD) at 10 ns after excitation with 1.55 eV, 50 fs pump pulses. Using a time-resolved SSPD technique, we are able to observe simultaneously a long-lived hv_PbSe band and a short-lived hv_SE1 feature. The higher-spectral-energy PL (>1.5 eV), which comprises short-lived hv_CdSe and hv_SE2 features, is time-integrated emission measured with a standard Si detector using 3.1 eV excitation. The observed PL features are fitted to Gaussian bands whose widths (defined as a full-width at half-maximum) are indicated in the figure. b, An excitonic representation of radiative channels leading to the hv_SE1 and hv_SE2 emission features. Steps 1′ and 1 are excitation of the ⁴T₁ and ⁴T₂ states of the Mn ion via exciton transfer from the CdSe shell. Step 2 or 2′ is Mn* decay to produce a photon and a PbSe-core exciton. c, A more rigorous spin-exchange depiction of the emission pathway leading to the hv_SE1 PL features. Due to spin conservation, decay of the ⁴T₁ state produces a spin-1 dark PbSe core exciton. d, Spectral energies hv_PbSe (black squares) and hv_SE1 (green circles), and their sum (hv_PbSe + hv_SE1; orange triangles) as a function of hv_PbSe (based on the measurements of samples Mn-1 to Mn-4). e, Streak-camera measurements of the dynamics of the hv_SE1 PL band (black trace) reveal a fast, resolution-limited decay component. Based on the deconvolution using the measured 8 ps IRF (the black line in the inset; the red line is a Gaussian fit; the grey arrow shows the IRF full width at half maximum), we determine that the relaxation time constant of the hv_SE1 PL feature (τ_SE1) is 3 ps (blue) or shorter (red; τ_SE1 = 1 ps). The use of longer time constants (5 ps and 20 ps; green and orange traces, respectively) leads to an appreciable deviation of the modelling from the measurement. The deconvolution procedure also indicates that the fast decay component is responsible for more than 90% of the overall PL signal. The dashed and the solid lines are the model traces before and after convolution with the IRF, respectively.

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