Single-Emitter Spectra from an Ensemble

Abstract

The heterogeneity in nanoscale emitters hinders efforts to understand their basic photophysics and limits their use in practical applications. Existing methods have difficulty accurately characterizing single-emitter spectra and optical heterogeneity on a statistical scale. Here, we introduce SPICEE (SPectrally Imbalanced Correlations from Ensemble Emission), a spectrally filtered photon-correlation technique that recovers single-particle emission lineshapes from an ensemble sample. Analytical derivations, numerical modeling, and experiments on a solution ensemble of emitters validate the technique. We apply SPICEE to blue-emitting ZnSeTe semiconductor nanocrystals relevant to display applications and find that the low color purity in the ensemble spectrum is primarily caused by a small subpopulation of nanocrystals with a distinct emission mechanism. This work demonstrates that SPICEE is a powerful high-throughput tool for accurately characterizing the single-emitter properties of nanoscale systems.

Figures (4)

• Figure 1: Illustration of the SPICEE technique. (a) Emission from a solution ensemble of emitters is collected by a confocal microscope and passed through a 50:50 beamsplitter (BS) and two independently tunable spectral bandpass filters before being collected at two single photon avalanche diodes (SPADs) Abbreviations: Obj., microscope objective. (b) Single-emitter (black) and ensemble (gray) spectra with select filter transmission profiles (red, blue). (c) Intensity cross-correlations $g^{\times}(\tau)$ of the detected emission (gray) for the corresponding filter positions in the previous panel. (d) Broad (orange) and narrow (black) single-emitter spectra with same ensemble (gray) are scanned over with the same filters. (e) The magnitude of the filtered $g^{\times}(\tau)$ at short $\tau$ is shown for filter positions from the scans in the previous panel.
• Figure 2: SPICEE resolves asymmetric lineshapes and lineshape evolution in model systems. (a) Ensemble spectrum (gray) overlaid with the two sets of 10 spectral filters (red, blue) with offset for clarity. The single-emitter spectrum (inset, gray) shows a clear asymmetry. (b) $g^\times(\tau)$ for select filter positions where $\langle N\rangle =1$. (c) $g^\times(\tau\to0)$ for each set of filter positions, creating a 10 x 10 grid. (d) Fitted SPICEE grid recovers the single emitter and ensemble spectrum with high fidelity. (e) SPICEE grid for a system where the spectral linewidth evolves as a function of peak position. (f) Fitting the SPICEE grid from the previous panel recovers the single-emitter spectral evolution, as well as the ensemble spectrum (inset).
• Figure 3: SPICEE experiment on a solution ensemble of InP/ZnSe/ZnS NCs. (a) Ensemble absorption and PL spectra for InP/ZnSe/ZnS. (b) Normalized spectral filter transmission profiles (red, blue, 10 each, vertical offset for clarity) relative to the ensemble spectrum (gray, shaded). (c) $g^{\times}(\tau)$ as one filter is moved from closely overlapped with the other (light red) to far apart (dark red) with fits (black). (d) The normalized SPICEE grid for all combinations of filters with the corresponding $g^\times$ positions in the previous panel marked with asterisks (*). (e) Fitted single-NC spectrum (red) and its components (dashed) with centers and relative weights of the spectral components as black bars. The ensemble spectrum is shown in gray. (f) Comparison to sPCFS spectral correlation shows excellent agreement between the two measurements.
• Figure 4: SPICEE applied to a solution ensemble of ZnSeTe/ZnSe/ZnS NCs. (a) Ensemble absorption (gray, dotted) and PL (blue, shaded). (b) Normalized filter transmission profiles relative to the ensemble spectrum. (c) Normalized SPICEE grid. (d) Spectral decomposition of the ZnSeTe sample. The ensemble spectrum (blue) is composed of two subpopulations of NCs (relative weights in black) where the dominant population has a narrow 80 meV linewidth and the smaller population has a broader 140 meV linewidth. (e) Overlay of the broad, low-energy single-NC spectrum and the narrow, high-energy single-NC spectrum.