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Optics Express

Optics Express

  • Editor: C. Martijn de Sterke
  • Vol. 18, Iss. 7 — Mar. 29, 2010
  • pp: 6577–6582
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Near-infrared single-photons from aligned molecules in ultrathin crystalline films at room temperature

C. Toninelli, K. Early, J. Bremi, A. Renn, S. Götzinger, and V. Sandoghdar  »View Author Affiliations


Optics Express, Vol. 18, Issue 7, pp. 6577-6582 (2010)
http://dx.doi.org/10.1364/OE.18.006577


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Abstract

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

1. B. Lounis and M. Orrit, “Single-photon sources,” Rep. Prog. Phys. 68, 1129–1179 (2005). [CrossRef]

, 2

2. S. Scheel, “Single-photon sources: an introduction,” J. Mod. Opt. 56, 141–160 (2009). [CrossRef]

]. 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

3. C. Kurtsiefer, S. Mayer, P. Zarda, and H. Weinfurter, “Stable solid-state source of single photons,” Phys. Rev. Lett. 85, 290–293 (2000). [CrossRef] [PubMed]

] and terrylene molecules in a para-terphenyl host [4

4. L. Fleury, B. Sick, G. Zumofen, B. Hecht, and U. P. Wild, “High photo-stability of single molecules in an organic crystal at room temperature observed by scanning confocal optical microscopy,” Mol. Phys. 95, 1333–1338 (1998). [CrossRef]

, 5

5. B. Lounis and W. E. Moerner, “Single photons on demand from a single molecule at room temperature,” Nature 407, 491–493 (2000). [CrossRef] [PubMed]

] have shown at room temperature stable single-photon emission over extended periods of time.

In this paper we investigate Dibenzoterrylene (DBT) molecules, which have been previously studied in crystals at cryogenic temperatures [8–11

8. F. Jelezko, P. Tamarat, B. Lounis, and M. Orrit, “Dibenzoterrylene in naphthalene: A new crystalline system for single molecule spectroscopy in the near infrared,” J. Phys. Chem. 100, 13892–13894 (1996). [CrossRef]

]. 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.

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.

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

12. M. Kreiter, M. Prummer, B. Hecht, and U. P. Wild, “Orientation dependence of fluorescence lifetimes near an interface,” J. Chem. Phys. 117, 9430–9433 (2002). [CrossRef]

, 13

13. L. Rogobete and C. Henkel, “Spontaneous emission in a subwavelength environment characterized by boundary integral equations,” Phys. Rev. A 70, 63815 (2004). [CrossRef]

]. 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.

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. The small but finite fluorescence signal at the beginning of the decay curve is reminiscent of the excitation by the previous laser pulse. 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 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.

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 [14

14. R. Verberk, A. M. van Oijen, and M. Orrit, “Simple model for the power-law blinking of single semiconductor nanocrystals,” Phys. Rev. B 66, 233202 (2002). [CrossRef]

]. 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 [15

15. J. Bernard, L. Fleury, H. Talon, and M. Orrit, “Photon bunching in the fluorescence from single molecules: A probe for intersystem crossing,” J. Chem. Phys. 98, 850–859 (1993). [CrossRef]

]. 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

10. A. A. L. Nicolet, P. Bordat, C. Hofmann, M. A. Kol’chenko, B. Kozankiewicz, R. Brown, and M. Orrit, “Single dibenzoterrylene molecules in an anthracene crystal: Main insertion sites,” ChemPhysChem 8, 1929–1936 (2007). [CrossRef] [PubMed]

], 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 [10

10. A. A. L. Nicolet, P. Bordat, C. Hofmann, M. A. Kol’chenko, B. Kozankiewicz, R. Brown, and M. Orrit, “Single dibenzoterrylene molecules in an anthracene crystal: Main insertion sites,” ChemPhysChem 8, 1929–1936 (2007). [CrossRef] [PubMed]

]. 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 [16

16. S. Strauf, N. G. Stoltz, M. T. Rakher, L. A. Coldren, P. M. Petroff, and D. Bouwmeester, “High-frequency single-photon source with polarization control,” Nat. Photonics 1, 704–708 (2007). [CrossRef]

]. By considering the maximum count rate together with the determined lifetime, we can deduce a total detection efficiency of 0.5%.

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.

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 [17

17. C. Eggeling, J. Widengren, R. Rigler, and C. A. M. Seidel, “Photobleaching of fluorescent dyes under conditions used for single-molecule detection: Evidence of two-step photolysis,” Anal. Chem. 70, 2651–2659 (1998). [CrossRef] [PubMed]

]. 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 [18

18. M. A. Lieb, J. M. Zavislan, and L. Novotny, “Single-molecule orientations determined by direct emission pattern imaging,” J. Opt. Soc. Am. B 21, 1210–1215 (2004). [CrossRef]

]. 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.

Acknowledgements

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).

References and links

1.

B. Lounis and M. Orrit, “Single-photon sources,” Rep. Prog. Phys. 68, 1129–1179 (2005). [CrossRef]

2.

S. Scheel, “Single-photon sources: an introduction,” J. Mod. Opt. 56, 141–160 (2009). [CrossRef]

3.

C. Kurtsiefer, S. Mayer, P. Zarda, and H. Weinfurter, “Stable solid-state source of single photons,” Phys. Rev. Lett. 85, 290–293 (2000). [CrossRef] [PubMed]

4.

L. Fleury, B. Sick, G. Zumofen, B. Hecht, and U. P. Wild, “High photo-stability of single molecules in an organic crystal at room temperature observed by scanning confocal optical microscopy,” Mol. Phys. 95, 1333–1338 (1998). [CrossRef]

5.

B. Lounis and W. E. Moerner, “Single photons on demand from a single molecule at room temperature,” Nature 407, 491–493 (2000). [CrossRef] [PubMed]

6.

R. J. Pfab, J. Zimmermann, C. Hettich, I. Gerhardt, A. Renn, and V. Sandoghdar, “Aligned terrylene molecules in a spin-coated ultrathin crystalline film of p-terphenyl,” Chem. Phys. Lett. 387, 490–495 (2004). [CrossRef]

7.

A. Renn, J. Seelig, and V. Sandoghdar, “Oxygen-dependent photochemistry of fluorescent dyes studied at the single molecule level,” Mol. Phys. 104, 409–414 (2006). [CrossRef]

8.

F. Jelezko, P. Tamarat, B. Lounis, and M. Orrit, “Dibenzoterrylene in naphthalene: A new crystalline system for single molecule spectroscopy in the near infrared,” J. Phys. Chem. 100, 13892–13894 (1996). [CrossRef]

9.

C. Hofmann, A. Nicolet, M. A. Kol’chenko, and M. Orrit, “Towards nanoprobes for conduction in molecular crystals: Dibenzoterrylene in anthracene crystals,” Chem. Phys. 318, 1–6 (2005). [CrossRef]

10.

A. A. L. Nicolet, P. Bordat, C. Hofmann, M. A. Kol’chenko, B. Kozankiewicz, R. Brown, and M. Orrit, “Single dibenzoterrylene molecules in an anthracene crystal: Main insertion sites,” ChemPhysChem 8, 1929–1936 (2007). [CrossRef] [PubMed]

11.

J. B. Trebbia, H. Ruf, P. Tamarat, and B. Lounis, “Efficient generation of near infra-red single photons from the zero-phonon line of a single molecule,” Opt. Express 17, 23986–23991 (2009). [CrossRef]

12.

M. Kreiter, M. Prummer, B. Hecht, and U. P. Wild, “Orientation dependence of fluorescence lifetimes near an interface,” J. Chem. Phys. 117, 9430–9433 (2002). [CrossRef]

13.

L. Rogobete and C. Henkel, “Spontaneous emission in a subwavelength environment characterized by boundary integral equations,” Phys. Rev. A 70, 63815 (2004). [CrossRef]

14.

R. Verberk, A. M. van Oijen, and M. Orrit, “Simple model for the power-law blinking of single semiconductor nanocrystals,” Phys. Rev. B 66, 233202 (2002). [CrossRef]

15.

J. Bernard, L. Fleury, H. Talon, and M. Orrit, “Photon bunching in the fluorescence from single molecules: A probe for intersystem crossing,” J. Chem. Phys. 98, 850–859 (1993). [CrossRef]

16.

S. Strauf, N. G. Stoltz, M. T. Rakher, L. A. Coldren, P. M. Petroff, and D. Bouwmeester, “High-frequency single-photon source with polarization control,” Nat. Photonics 1, 704–708 (2007). [CrossRef]

17.

C. Eggeling, J. Widengren, R. Rigler, and C. A. M. Seidel, “Photobleaching of fluorescent dyes under conditions used for single-molecule detection: Evidence of two-step photolysis,” Anal. Chem. 70, 2651–2659 (1998). [CrossRef] [PubMed]

18.

M. A. Lieb, J. M. Zavislan, and L. Novotny, “Single-molecule orientations determined by direct emission pattern imaging,” J. Opt. Soc. Am. B 21, 1210–1215 (2004). [CrossRef]

OCIS Codes
(030.5290) Coherence and statistical optics : Photon statistics
(270.0270) Quantum optics : Quantum optics
(300.0300) Spectroscopy : Spectroscopy

ToC Category:
Quantum Optics

History
Original Manuscript: January 26, 2010
Revised Manuscript: March 3, 2010
Manuscript Accepted: March 3, 2010
Published: March 15, 2010

Citation
C. Toninelli, K. Early, J. Bremi, A. Renn, S. Götzinger, and V. Sandoghdar, "Near-infrared single-photons from aligned molecules in ultrathin crystalline films at room temperature," Opt. Express 18, 6577-6582 (2010)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-18-7-6577


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References

  1. B. Lounis and M. Orrit, "Single-photon sources," Rep. Prog. Phys. 68, 1129-1179 (2005). [CrossRef]
  2. S. Scheel, "Single-photon sources: an introduction," J. Mod. Opt. 56, 141-160 (2009). [CrossRef]
  3. C. Kurtsiefer, S. Mayer, P. Zarda, and H. Weinfurter, "Stable solid-state source of single photons," Phys. Rev. Lett. 85, 290-293 (2000). [CrossRef] [PubMed]
  4. L. Fleury, B. Sick, G. Zumofen, B. Hecht, and U. P. Wild, "High photo-stability of single molecules in an organic crystal at room temperature observed by scanning confocal optical microscopy," Mol. Phys. 95, 1333-1338 (1998). [CrossRef]
  5. B. Lounis and W. E. Moerner, "Single photons on demand from a single molecule at room temperature," Nature 407, 491-493 (2000). [CrossRef] [PubMed]
  6. R. J. Pfab, J. Zimmermann, C. Hettich, I. Gerhardt, A. Renn, and V. Sandoghdar, "Aligned terrylene molecules in a spin-coated ultrathin crystalline film of p-terphenyl," Chem. Phys. Lett. 387, 490-495 (2004). [CrossRef]
  7. A. Renn, J. Seelig, and V. Sandoghdar, "Oxygen-dependent photochemistry of fluorescent dyes studied at the single molecule level," Mol. Phys. 104, 409-414 (2006). [CrossRef]
  8. F. Jelezko, P. Tamarat, B. Lounis, and M. Orrit, "Dibenzoterrylene in naphthalene: A new crystalline system for single molecule spectroscopy in the near infrared," J. Phys. Chem. 100, 13892-13894 (1996). [CrossRef]
  9. C. Hofmann, A. Nicolet, M. A. Kol’chenko, and M. Orrit, "Towards nanoprobes for conduction in molecular crystals: Dibenzoterrylene in anthracene crystals," Chem. Phys. 318, 1-6 (2005). [CrossRef]
  10. A. A. L. Nicolet, P. Bordat, C. Hofmann, M. A. Kol’chenko, B. Kozankiewicz, R. Brown, and M. Orrit, "Single dibenzoterrylene molecules in an anthracene crystal: Main insertion sites," ChemPhysChem 8, 1929-1936 (2007). [CrossRef] [PubMed]
  11. J. B. Trebbia, H. Ruf, P. Tamarat, and B. Lounis, "Efficient generation of near infra-red single photons from the zero-phonon line of a single molecule," Opt. Express 17, 23986-23991 (2009). [CrossRef]
  12. M. Kreiter and M. Prummer and B. Hecht and U. P. Wild, "Orientation dependence of fluorescence lifetimes near an interface," J. Chem. Phys. 117, 9430-9433 (2002). [CrossRef]
  13. L. Rogobete and C. Henkel, "Spontaneous emission in a subwavelength environment characterized by boundary integral equations," Phys. Rev. A 70, 63815 (2004). [CrossRef]
  14. R. Verberk, A. M. van Oijen, and M. Orrit, "Simple model for the power-law blinking of single semiconductor nanocrystals," Phys. Rev. B 66, 233202 (2002). [CrossRef]
  15. J. Bernard, L. Fleury, H. Talon, and M. Orrit, "Photon bunching in the fluorescence from single molecules: A probe for intersystem crossing," J. Chem. Phys. 98, 850-859 (1993). [CrossRef]
  16. S. Strauf, N. G. Stoltz, M. T. Rakher, L. A. Coldren, P. M. Petroff, and D. Bouwmeester, "High-frequency singlephoton source with polarization control," Nat. Photonics 1, 704-708 (2007). [CrossRef]
  17. C. Eggeling, J. Widengren, R. Rigler, and C. A. M. Seidel, "Photobleaching of fluorescent dyes under conditions used for single-molecule detection: Evidence of two-step photolysis," Anal. Chem. 70, 2651-2659 (1998). [CrossRef] [PubMed]
  18. M. A. Lieb, J. M. Zavislan, and L. Novotny, "Single-molecule orientations determined by direct emission pattern imaging," J. Opt. Soc. Am. B 21, 1210-1215 (2004). [CrossRef]

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