Excitation of V-groove plasmonic waveguides by single molecules

We report on the excitation of V-groove plasmonic waveguides, fabricated using focussed ion beam milling of monocrystalline gold flakes, by single dibenzoterrylene (DBT) molecules embedded in anthracene nanocrystals.
Efficiently coupled waveguide-quantum emitter systems have many applications in quantum information technologies. For efficient coupling, the waveguide mode should be very confined. V-groove plasmonic waveguides support confined modes with reasonably long propagation lengths. Among quantum emitters, DBT molecules in anthracene nanocrystals have been shown to be suitable for quantum optics applications due to their lifetime limited and stable emission. Here, we present the results of excitation of V-groove plasmonic waveguides by single DBT molecules embedded in anthracene nanocrystals. We fabricate V-grooves by milling monocrystalline gold flakes using focussed gallium (Ga) ion beam and polishing with helium (He) ion beam. In Fig. 1(a), we show a scanning electron microscope (SEM) image of a cross-section of a V-groove. These V- grooves support guided modes with an extraordinary propagation length of ~14μm, measured at a vacuum wavelength of 800nm. Anthracene nanocrystals doped with DBT molecules are grown with a reprecipitation method. The size of nanocrystals is controlled, by choosing the concentration of anthracene in the growth process, so that the crystals can fit in the V-grooves. DBT concentration is chosen so that single DBT molecules can be found in the nanocrystals. An atomic force microscope (AFM) image of nanocrystals is presented in Fig. 1(b). To obtain the DBT molecule – waveguide coupled systems, the nanocrystal solution is drop-cast on the sample and dried. In Fig. 1(c), an SEM image of a V-groove is shown with a fluorescence image of a DBT molecule coupled to a V-groove waveguide overlaid on it. The DBT molecule coupled to the V-groove emits into the guided mode, which propagates to the two ends, A and B in Fig. 1(c), of the waveguide and is out- coupled to the far-field. We have observed up to 50% emission of a single DBT molecule coupled into the V- groove mode. In Fig. 1(d), a typical emission spectrum of a DBT molecule at room temperature is shown. Fig. 1(e) shows measured auto-correlation for a DBT molecule coupled to a V-groove, with g2(τ=0) < 0.5, clearly indicating that the DBT molecules emit single photons.