Fig. 5: Properties of resonantly excited molecules.

a Excitation spectrum of the 0-0 ZPL of a single molecule that did not jump out of resonance with the laser, even at higher excitation intensities. The linewidth is about 4.7 ± 0.3 GHz and is measured with a scan rate of 5 s per row. The line plot underneath the figure shows the asymmetric profile of the 0-0 ZPL. A resonance fluorescence time trace of the fluorescence signal of this molecule in (b) shows characteristic quantum jumps due to intersystem crossing to the triplet state. However, in some cases the molecule remained dark for much longer times than the triplet lifetime, as is also visible in (a). These long dark times may be attributed to a relatively short spectral jump back and forth from and to another (out-of-range) spectral position. The purple line represents the threshold set between the ON and OFF state of the fluorescence. c Distribution of 0-0 ZPLs linewidths found for molecules on different flakes. The red dashed vertical line represents the lower limit set by the fluorescence lifetime. Due to the large distribution of linewidths, the horizontal axis has been set to a logarithmic scale. d Saturation curve of the fluorescence of the molecule in (a), fitted with \(F\left(I\right)={F}_{\infty }\frac{I/{I}_{{sat}}}{1+I/{I}_{{sat}}}\), where \({I}_{{sat}}\) is the saturation intensity of the laser and \({F}_{\infty }\) is the maximum fluorescence rate. The saturation intensity is 105 ± 22 W/cm², 2 orders of magnitude larger than typically obtained with our setup for near-lifetime-limited emitters. This is explained by the 2 orders of magnitude broader linewidth, as the saturation intensity scales linearly with the linewidth of the transition. e Distribution of dark periods in the resonance fluorescence signal, recorded over 60 s with 100 μs time bins (as in (b)). The characteristic timescales of the fit (red curve) are shown on the top right and bottom left.