FIG. 2: (a) Wide field image, where the sample was simultaneously illuminated by a white-light source and a laser. Individual molecules are clearly visible within the crystalline domains. (b) Fluorescence lifetime measurement on a DBT molecule. The red curve represents an exponential fit to the experimental data, yielding a decay time of 4 . 8 ns. Inset: fluorescence spectrum of a single DBT molecule. (c) Photon-correlation measurement under CW excitation. Strong anti- bunching is observable. The red curve is a fit to the experimental data. (d) Histogram of the inter-photon arrival times. The obtained decay time yields a 1 . 5 µ s lifetime of the triplet state. (e) Saturation measurement: number of detected photons per second depending on the pump power. The red curve is a two-level model fit to the saturation behavior [5]. (f) Photostability of DBT molecules: the insets show wide field images at the beginning of the measurement and after 10 hours of continuous illumination. Ten molecules out of 43 could not be photobleached. Twiss (HBT) photon correlator. Figure 2 (a) shows a wide-field CCD camera image of DBT molecules in an AC film. Individual molecules can be clearly distinguished. By switching to confocal excitation, we selected individual molecules for further investigations. The inset in Fig, 2 (b) displays the fluorescence spectrum of a DBT molecule. It has its maximum at 790 nm and a width of about 50 nm. However, because the detection efficiency of the spectrometer drops between 850 nm and 900 nm by more than a factor of two, the spectrum is slightly distorted in this wavelength range. To gain further information on the molecule’s properties, we directed the photons generated by pulsed excitation on an APD and applied a time correlated single- 4 GLYPH<c116> = 4.8 ns Intensity [a.u.] g ( ) (2) GLYPH<c116> Time delay [ns] GLYPH<c116> Time [ns] b) -2 0 2 4 6 8 10 0.5 1.0 1.5 2.0 2.5 3.0 -20 0 0.0 0.2 0.4 0.6 0.8 1.0 1.2 T = 3.9 ns 5 m GLYPH<c109> 800 850 900 0 d) 0 5 10 15 20 10 0 10 1 10 2 10 3 10 4 10 5 10 6 Time delay [ s] GLYPH<c109> t =1.5 s GLYPH<c109> Counts/Bin 0 1000 2000 3000 4000 5 6 7 8 9 10 detected photons [10 s ] 5 -1 Excitation intensity [KW/cm ] 2 0 2 4 1 10 100 Time [hours] t = 0 h # of fluorescent molecules e) a) Anthracene crystal Wavelength [nm] 0.2 0.4 0.6 0.8 Intensity [a.u.] 1.0 750 20 40 60 6 8 10 t = 10 h f) c)