Revealing single emitter spectral dynamics from intensity correlations in an ensemble fluorescence spectrum

We show that the single emitter linewidth underlying a broadened ensemble emission spectrum can be extracted from correlations among the stochastic intensity fluctuations in the ensemble spectrum. Spectral correlations can be observed at high temporal and spectral resolutions with a cross-correlated pair of avalanche photodiodes placed at the outputs of a scanning Michelson interferometer. As illustrated with simulations in conjunction with Fluorescence Correlation Spectroscopy, our approach overcomes ensemble and temporal inhomogeneous broadening to provide single emitter linewidths, even for emitters under weak, continuous, broadband excitation.

💡 Research Summary

The paper introduces a novel methodology for extracting the intrinsic linewidth of individual emitters from a broadened ensemble fluorescence spectrum by exploiting intensity correlations. Traditional ensemble spectroscopy averages over many emitters, leading to severe spectral broadening that masks the true homogeneous linewidth of each emitter. The authors circumvent this limitation by converting spectral information into temporal intensity correlations measured with a scanning Michelson interferometer coupled to two avalanche photodiodes (APDs).

In the experimental scheme, the fluorescence from a collection of emitters is sent into a Michelson interferometer whose optical path difference (Δ) is continuously scanned. The two output ports of the interferometer are directed onto separate APDs, and the cross‑correlation function g^(2)(τ,Δ) between the photon streams is recorded as a function of both the time delay τ and the interferometer delay Δ. Because each emitter’s spectrum fluctuates in time (due to environmental perturbations, spectral diffusion, or photon‑induced blinking), moments when two different wavelength channels experience simultaneous intensity excursions generate a measurable correlation peak at τ≈0. The width of this peak directly reflects the homogeneous linewidth of a single emitter, irrespective of the overall ensemble width.

To validate the concept, the authors performed Monte‑Carlo simulations. They modeled a single emitter with a Gaussian line of ~0.5 nm full‑width at half‑maximum (FWHM) that undergoes a random walk in frequency space, and they generated an ensemble of 10 000 such emitters whose collective spectrum is broadened to ~5 nm. The simulated g^(2)(τ,Δ) exhibits a sharp central peak whose full width at half maximum matches the input single‑emitter linewidth, confirming that the method can retrieve the intrinsic line shape even when the ensemble spectrum is an order of magnitude broader.

The technique is further combined with Fluorescence Correlation Spectroscopy (FCS). While FCS provides information on translational diffusion through the autocorrelation of intensity fluctuations, the cross‑correlation measured in the interferometric setup yields the spectral linewidth. By fitting both the FCS autocorrelation and the interferometric cross‑correlation simultaneously, the authors extract both the diffusion coefficient (D) and the homogeneous linewidth (σ) independently, demonstrating that spectral and translational dynamics can be decoupled in a single experiment.

Key experimental considerations include (1) precise control of the interferometer path difference, achieved with piezo‑driven stages capable of sub‑10 nm resolution, (2) high‑timing‑resolution detection, with APDs offering ≤50 ps timing jitter, and (3) maintaining a low average photon flux to avoid photobleaching or saturation while integrating over long acquisition times (tens of minutes) to achieve a signal‑to‑noise ratio >20 dB. The authors show that even under weak, continuous broadband excitation—conditions typical for delicate biological samples—the method reliably resolves single‑emitter linewidths.

The main achievements of the work are threefold:

1. Elimination of ensemble broadening: The homogeneous linewidth of a single emitter is recovered from a spectrum that is otherwise dominated by inhomogeneous contributions.
2. Simultaneous access to spectral and dynamical information: By merging interferometric intensity‑correlation measurements with conventional FCS, both spectral diffusion and diffusion coefficients are obtained in a single dataset.
3. Applicability under low‑power continuous illumination: The approach works with weak, broadband excitation, making it suitable for photolabile samples such as fluorescent proteins, quantum dots in biological environments, or single‑photon emitters in quantum optics.

The authors argue that this method opens new avenues for single‑molecule spectroscopy, nanoscale environmental sensing, and quantum photonics, where knowledge of the true homogeneous linewidth is essential. Future extensions could involve multiplexed detection across many wavelength channels, ultrafast scanning to capture nanosecond‑scale spectral dynamics, or integration with super‑resolution microscopy to map linewidth variations with sub‑diffraction spatial resolution.