We investigate the optical properties of Dibenzoterrylene (DBT) molecules in a spin-coated crystalline film of anthracence. By performing single molecule studies, we show that the dipole moments of the DBT molecules are oriented parallel to the plane of the film. Despite a film thickness of only 20 nm, we observe an exceptional photostability at room temperature and photon count rates around 106 per second from a single molecule. These properties together with an emission wavelength around 800 nm make this system attractive for applications in nanophotonics and quantum optics.
© 2010 Optical Society of America
Room temperature single-photon sources are desirable for a variety of applications, ranging from quantum key distribution to building a standard to measure the luminous intensity of a light source [1, 2]. Solid state systems are especially appealing, not only because they are easy-to-use but also because of their potential for integration and scalability. However, most solid state emitters suffer from a limited photostability. So far only nitrogen-vacancy centers in diamond  and terrylene molecules in a para-terphenyl host [4, 5] have shown at room temperature stable single-photon emission over extended periods of time.
Several years ago we discovered that terrylene is also extremely photostable in a crystalline p-terphenyl film as thin as 20 molecular layers , which protects the molecule against quencher agents such as oxygen . Doping emitters into thin films has several advantages, especially in the context of single emitter experiments. First, background fluorescence is strongly suppressed due to the optimized ratio between emitter and matrix. Second, thin films are inherently compatible with nanostructures such as plasmonic waveguides, since the emitter is by definition in the near-field. Furthermore they can easily be integrated into microcavities, either as part of a layered structure in a linear cavity or via evanescent coupling to a whispering-gallery resonator or a photonic crystal defect cavity.
In this paper we investigate Dibenzoterrylene (DBT) molecules, which have been previously studied in crystals at cryogenic temperatures [8–11]. Here we report on the fabrication of ultra-thin crystalline anthracene (AC) by a simple spin coating procedure on glass cover slides. To produce the desired films, we prepared a solution of AC in diethyl ether with a concentration of 2.5 mg/ml and added 10 μl/ml of benzene. The latter serves to improve the quality of the crystals obtained from the spin-coating process. DBT was then dissolved in toluene to obtain a 10 μM solution, which was further diluted by a factor of 100 with the AC/diethyl ether mixture. Then we spin casted 20 μl of the solution containing AC and DBT onto a glass cover slide. With a two-step process (30 s at 3000 RPM followed by 20s at 1500 RPM) on a commercial spin coater we obtained areas with crystalline islands that covered several mm2 of the substrate. Figure 1(a) shows an optical polarization microscope image of a typical sample area, containing both film and bare glass regions. The contrast between glass and crystalline film is very low since one of the main axes of the anthracene crystal is aligned with the polarization vector of the incoming light. In Fig. 1(b) the same portion of the crystal is rotated by 45°. In contrast to the amorphous glass the crystal shows some birefringence. As a result, the polarization vector of the transmitted light is rotated and the contrast to glass is increased. We thus conclude that the AC film is crystalline with the same optical axis over hundreds of square microns. To obtain information about the topography of the host matrix we performed atomic force microscopy (AFM). A typical measurement is displayed in Fig. 1(c), where well defined crystalline structures are visible. In Fig. 1(d) a cross section is plotted, showing a fairly constant thickness of about 20nm.
The optical investigations of DBT were carried out on single molecules in thin films by means of fluorescence microscopy. Our fluorescence microscopy setup was equipped with a continuous wave (CW) and a pulsed Ti:Sapphire laser (120 fs pulse width, 76 MHz repetition rate) to efficiently excite the molecules at a wavelength of 725 nm using an oil immersion objective (N.A. 1.4). A lens could be inserted in the excitation path to switch between confocal and wide-field illumination. Fluorescence was then collected by the same objective and separated from the excitation light with a longpass filter. Several detection paths allowed access to a CCD camera, a fiber-coupled avalanche photodiode (APD), a spectrometer or a Hanbury-Brown-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 790nm and a width of about 50nm. 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-photon counting technique to determine the lifetime of the excited state. Figure 2(b) shows an example of a time-resolved intensity measurement, which could be fitted with a single exponential decay. Repeating the measurement on several molecules, we obtained lifetimes between 3.3 ns and 5.7 ns. The spread in these values is most probably due to the interface and edge effects in the thin film [12, 13]. Then we employed photon correlation measurements using the HBT setup to verify the identification of isolated single DBT molecules. The CW photon autocorrelation measurement in Fig. 2(c) shows a dip in the second order correlation function at delay τ = 0, which corresponds to a reduction of the coincidence probability to 0.28, limited by the detector time resolution and a small amount of background fluorescence. This result motivates the use of DBT as a near-infrared single-photon source.
The performance of a single-photon source is in many cases compromised by fluorescence intermittency. The blinking of semiconductor nanocrystals is a well-known example of this phenomenon . In the case of molecules one has to worry about a long-lived triplet state, populated by intersystem crossing, which can interrupt the continuous stream of photons. To determine the lifetime of the triplet state, we recorded a histogram of the inter-photon arrival times with a pump rate higher than the triplet decay rate . Under this condition the dark intervals in the fluorescence are limited by the triplet lifetime, which can then be extracted from the slow decay in 2(d). The initial fast decay is a measure for the pump rate. Considering the obtained triplet lifetime of 1.5 μs together with an extremely low intersystem crossing yield of 10-7 , we can neglect the effect of the triplet state on the efficiency of a DBT single-photon source. We point out in passing that the observed triplet lifetime is about 25 times shorter than that reported by low temperature measurements . Another important property is the brightness, which can be extracted from saturation measurements. Fig.2(e) shows that we can detect almost one million photons per second at pump intensities close to saturation. Such count rates are among the highest ever reported . By considering the maximum count rate together with the determined lifetime, we can deduce a total detection efficiency of 0.5%.
The photostability of DBT molecules embedded in thin crystalline AC films is especially noteworthy, when considering that other molecular emitters typically photobleach after 104 – 107 photon emissions . We investigated the stability of DBT by irradiating a sample continuously with an intensity of 30kW/cm2 and recorded a wide-field image every 20 min over a time period of more than 10 hours [see Fig. 2(f)]. For about 30 out of 40 molecules we could attribute a ‘half-life’ of 4 h by fitting an exponential decay to the experimental data. These molecules emitted more than 1012 photons before photobleaching, assuming the above-calculated detection efficiency of 0.5%. The remaining ten molecules, however, did not suffer from any photobeaching, even after more than 10 hours of constant illumination.
A further feature of our sample is the fixed orientation of the molecular dipole moment parallel to the cover glass. To investigate the dipole orientation we utilized a back focal plane imaging technique, which allows us to study the angular emission pattern of single molecules . Cross sections through the back focal plane of an exemplary image along the two orthogonal polarization axes are plotted in Fig. 3(a). The symmetry of the obtained shape indicates a horizontally aligned molecule. We found that all molecules within an area of a few tens of microns showed similar emission patterns and were out of plane at most by a few degrees. The symbols in Fig. 3(b) show that the sum of the signals from many molecules was also centered around zero. The solid curve in this figure displays the theoretically expected pattern for a single dipole at the interface. The central part of the angular pattern shows a very good agreement with the experimental results and was fitted. The experimental side lobes miss the fast modulations due to the finite angular resolution of the experiment. They also fall short of the theoretical prediction because the latter did not take into account the exact distance of the molecule from the AC-air interface, which sensitively determines how much light is emitted at angles beyond the critical angle. As a second check for the alignment of the molecules, we performed measurements where the orientation of a polarizer in the detection path was varied. Figure 3(c) shows that the fluorescence signal of a single DBT molecule could be varied with a visibility of 97 %. We note in passing that the maximum detected fluorescence occurred at similar polarizer positions for molecules in the same field of view, supporting the fact that large crystalline domains exist.
In conclusion, we have prepared by a simple spin coating procedure ultrathin crystalline AC films doped with DBT. An analysis of single molecule fluorescence reveals that DBT is horizontally aligned, exceptionally photostable and bright. The near-infrared emission wavelength of 800 nm is in many cases advantageous. Microcavities are easier to fabricate for longer operation wavelengths and the losses in gold or silver plasmonic structures are significantly reduced. Furthermore, the orientation of the molecules can be exploited to efficiently couple the emitted photons to any of the above mentioned photonic structures, which makes this molecule extremely attractive as an easy-to-use active emitter in nanophotonics and quantum optics.
This work was supported by the ETH Zurich via the INIT program Quantum Systems for Information Technology (QSIT) and the Swiss National Science Foundation. K.E. acknowledges support from the NSF IGERT Program (DGE-0504485).
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