Spectral diffusion time scale measurements. (a) Simplified explanation of the spectral diffusion time scale measurement, whereby a filter is used to block half of the emission that fluctuates in time. The narrow measurement window leads to an additional bunching in the autocorrelation data due to the fluctuation of the emission in and out of the measurement window. (b) Spectral diffusion measurement at an excitation power of 0.2 μW, revealing a bunching with a characteristic time scale of 897 ± 43 ns. (c) Power dependence of the measured spectral diffusion time scale (the figure insets show the fitting used to extract the spectral diffusion time scales).