Near-infrared single-photons from aligned molecules in ultrathin crystalline films at room temperature

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 one million 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.

ing 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 [3] 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 [6], which protects the molecule against quencher agents such as oxygen [7].Doping single 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][9][10][11].Here we report on the fabrication of ultrathin 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 20 s at 1500 RPM) on a commercial spin coater we obtained areas with crystalline islands that covered several mm 2 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 20 nm.
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) 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-  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 [12].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 [13].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 [10], we can neglect the effect of the triplet state on the efficiency of a DBT single-photon source.Another important property is the brightness, which can be extracted from saturation measurements.Fig. 2 (e) shows that we can detect almost to one million photons per second at pump intensities close to saturation.Such count rates are among the highest ever reported [14].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 10 4 − 10 7 photon emissions [15].We investigated the stability of DBT by irradiating a sample continuously with an intensity of 30 kW/cm 2 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 10 12 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.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 easy-to-use active emitter in nanophotonics and quantum optics.

10 FIG. 1 :
FIG. 1: (a,b) Polarization microscope images of a thin AC film, spin coated on a glass cover slip.The analyzer is oriented perpendicular to the polarizer.In (a) one of the main crystal axes is aligned with the incoming polarized light.The contrast between cover glass and crystalline features is therefore low.In (b) the sample is rotated by 45 • .This leads to a birefringence of the crystalline AC film which rotates the polarization of the incoming light.The contrast is therefore maximal.(c) AFM topography image of the sample.The well defined growth angles give further evidence for the crystalline nature of the film.(d) Cross section as indicated in (c).The sample is typically flat with a height of a few tens of nanometers.

FIG. 2 :
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 antibunching 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 µs 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.

Figure 2 (
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-

FIG. 3 :
FIG. 3: (a) Inset: Back focal plane image of a single molecule.Angular distribution of the emitted photons for two cross sections which correspond to s and p polarization.(b) Emission pattern of p-polarized light from an ensemble of molecules, fitted with the angular distribution of a single dipole one degree out of plane (red curve).(c) Dependency of the detected fluorescence intensity of a single molecule on the orientation of a polarizer in the detection path.
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).