NANO LETTERS
Two-Level Ultrabright Single Photon Emission from Diamond Nanocrystals
2009 Vol. 9, No. 9 3191-3195
Igor Aharonovich,*,† Stefania Castelletto,† David A. Simpson,‡ Alastair Stacey,† Jeff McCallum,† Andrew D. Greentree,† and Steven Prawer†,‡ School of Physics, UniVersity of Melbourne, ParkVille, Victoria, 3010, Australia, and Quantum Communications Victoria, ParkVille, Victoria, 3010, Australia Received May 4, 2009; Revised Manuscript Received July 7, 2009
ABSTRACT The fabrication of stable ultrabright single photon sources operating at room temperature is reported. The emitter is based on a color center within a diamond nanocrystal grown on a sapphire substrate by chemical vapor deposition method and exhibits a two-level electronic behavior with a maximum measured count rate of 3.2 × 106 counts/s at saturation. The emission is centered at ∼756 nm with a full width at halfmaximum ∼11 nm and an excited state lifetime of 3.7 ns. These unique properties make it a leading candidate for quantum photonics and communication applications as well as for cellular biomarking.
Reliable true single photon sources (SPSs) are highly sought after1,2 for testing of fundamental quantum optics experiments,3 as well as for the emerging interdisciplinary field of quantum information processing (QIP), including optical quantum computing, quantum metrology4 and in more recent times quantum cryptography employing quantum key distribution (QKD).5 The implementation of practical QKD requires a stable SPS with high emission rate and narrow emission bandwidth to allow effective signal filtering. To date, SPSs have been demonstrated from single molecules,6,7 quantum dots,8,9 and even carbon nanotubes.10 Color centers in diamond promise robust, photostable SPSs operating at room temperature.11 Recently, diamond nanocrystals have been employed in another emerging application, as fluorescent cellular biomarkers in biological systems due to their biocompatibility and room-temperature photo stability.12 Bright luminescent centers in the near-infrared (IR) are highly desirable for biological applications as they can avoid interference with the cell autofluorescence.13 The negatively charged nitrogen vacancy (N-V) optical color center in diamond, which consists of substitutional nitrogen coupled to a neighboring vacancy in a carbon lattice, is the most studied of all the diamond optical centers, due in part to the available spin manipulation and its optical readout.14 However, its broad spectral emission and relatively long lifetime (20 ns) limits its use in QKD protocols. Other diamond-based SPSs originating from nickel-nitrogen complexes15,16 and silicon impurities17 have been demonstrated * To whom correspondence should
[email protected]. Ph: +61-3-9347-4783. † University of Melbourne. ‡ Quantum Communications Victoria. 10.1021/nl9014167 CCC: $40.75 Published on Web 08/11/2009
be addressed. E-mail: +61-3-8344-0429. Fax:
2009 American Chemical Society
with observed narrow spectral lines in the near IR. However, spectroscopic studies of these centers reveal the presence of a third metastable16,17 state with a long lifetime, which significantly reduces the count rate, and quenches the total luminescence of the center. Just recently, an unknown, efficient two-level single photon emitter was reported in chemical vapor deposition (CVD) diamond.18 The center exhibits promising characteristics for QKD applications, but with its origin unknown, reproducing such sources remains difficult. Hence, a major effort has been made to improve the fabrication techniques of various diamond based color centers and an exploration of alternative, more efficient diamond based SPSs is in progress.19 Nevertheless, so far the highest reported emission rate from a known diamond related SPS is ∼75 × 103 counts/s,16 which might not be sufficient for a realization of a quantum optical device competitive with attenuated lasers.20 The count rate is primarily limited by the presence of nonradiative long lifetime dark states of the center.16 In this letter, we demonstrate the controlled fabrication and characterization of a SPS originating from a color center in CVD diamond nanocrystals. The SPS exhibits a two-level system behavior, has a zero phonon line (ZPL) centered around 756 nm and an emission rate in the MHz regime that surpasses the current limitations of diamond color centers in terms of brightness. These properties mean that the center is competitive with other solid state SPSs (e.g., quantum dots) in terms of efficiency and count rate but has the tremendous advantage of a photo stable operation at room temperature.1 The CVD diamond crystals employed in this work were grown to an average size of few hundreds nanometers from
Figure 1. (a) A typical photoluminescence intensity confocal map (20 × 20 µm2) showing fluorescent nanodiamonds grown on a sapphire substrate recorded using a cw laser diode emitting at 682 nm with an excitation power of 60 µW. (b) Typical SEM image (20 × 20 µm2) of the nanodiamonds grown on a sapphire substrate. Inset, a high magnification SEM image of a typical diamond nanocrystal. The scale bar is 500 nm.
diamond seeds (4-6 nm, Nanoamor Inc., Houston, TX) on a sapphire substrate using a microwave plasma enhanced CVD technique (900 W, 150 Torr).21 A confocal microscope with a spatial resolution ∼400 nm (100× objective, 0.95 N.A.) and a Hanbury Brown and Twiss (HBT) interferometer were used to identify the emitting centers and measure the time correlation of photoluminescence (PL) intensity. A fibercoupled CW diode laser emitting at 682 nm was used for excitation and its polarization was controlled by a Glan Taylor polarizer and a half-wave plate. The diamond sample was mounted on a piezo XYZ stage with 0.2 nm resolution, allowing 100 × 100 µm2 scans. The unwanted residual laser line was eliminated by a dichroic beam splitter and a 794 ( 80 band-pass filter. The PL from the emitting centers was then coupled into a 62.5 µm core multimode fiber, which acts as an aperture. A 1 × 2 50:50 fiber coupler guided the photons to two single photon counting detectors (SPCMs) and their outputs were sent to the start and stop inputs of the time correlator card. Figure 1a shows a 20 × 20 µm PL confocal map of the diamond nanocrystals grown on sapphire recorded at room temperature using an excitation power of 60 µW at 682 nm. Figure 1b shows a representative scanning electron microscope (SEM) image of nanodiamonds grown on a sapphire substrate. The inset is the image of a typical diamond nanocrystal shown at high magnification. Roughly, one out of ten crystals shows a very bright emission of more than 200 kcounts/s. Figure 2a-c shows PL spectra recorded from the bright nanodiamonds. At least three typical emission lines centered at 746, 756, and 764 nm were observed from the inspected crystals. These lines exhibit a full width at half-maximum (fwhm) of a few nanometers and do not have significant phonon sidebands. These lines are most likely associated with a Cr impurity within the diamond crystal, incorporated from the sapphire substrate during growth.22,23 This will be discussed in details in the following paragraphs. Figure 2d-f shows the corresponding normalized secondorder time autocorrelation function, g(2)(τ) ) / 2, for the respective PL lines, recorded at low excitation power (Figure 2d-f, red circles) and at saturation excitation 3192
powers (Figure 2d-f, black squares) using the HBT setup. A 760 ( 12 nm band-pass filter was used in addition to 794 ( 80 nm broadband-pass filter in the coincidence measurements to maximize the signal-to-noise ratio from the center. The g(2)(τ) function, which represents the photon statistics of the emitted light, has a dip at zero delay time which represents nonclassical emission. Single photon behavior from these centers was verified first by recording the g(2)(τ) function at low excitation powers. With higher excitation powers, the g(2)(0) increases due to the finite time response jitter of the photo detectors and correlation electronics. Remarkably, all of the studied crystals that hosted the typical emission line in the range of 740-770 nm revealed nonclassical photon emission. However, only crystals with a ZPL centered at 756 nm showed a clear absence of bunching of the g(2)(τ) function at saturation excitation powers, which is described well by the two-level energy model indicating the absence of a third metastable state.16,17,24 All other single photon emitters in the range of (740-770 nm) showed bunching at high excitation powers similar to a well-known N-V and NE8 centers.16,24 We now continue to characterize only the crystals which exhibit a ZPL centered at 756 nm. This emission has a fwhm of ∼11 nm with no significant phonon sidebands. During all the measurements, the center was stable and no photo bleaching or blinking behavior was observed; the stable count rate was measured over a period of weeks. The g(2)(τ) function was measured at several different excitation powers ranging from 60 µW to 1.3 mW without observing the typical bunching behavior ascribed generally to three level systems with a dark long-lived shelving state. To date, all the known diamond-based single photon emitters (Si-V, N-V, and the NE8) reveal a third longlived energy level in the electronic structure that is responsible for the bunching behavior16,17,24 with an exception of a recently reported unknown diamond related color center.18 The lack of bunching behavior of the g(2)(τ) function is a confirmation that this center can be described using a two-level energy model. The population dynamics of the center, assuming twolevel model, can be described by the following set of equations p˙1 ) -r12p1(t) + r21p2(t)
(1)
p˙2 ) r12p1(t) - r21p2(t)
(2)
where p1 and p2 are the ground- and the excited-state populations, respectively. r12 denotes the transition rates from the ground state to an excited state, and r21 denotes the transition from an excited state to the ground state. Solving this set of equations, assuming that the population at t ) 0 is concentrated in the ground state only (i.e., p1(0) ) 1, p2(0) ) 0), the normalized second order autocorrelation function g(2)(τ) is given by g(2)(τ) ) 1 - e-λτ
(3)
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Figure 2. Photoluminescence spectra (a-c) and corresponding second order normalized autocorrelation functions (d-f), g(2)(τ), recorded at room temperature using a 682 nm excitation laser from individual diamond nanocrystals. Only the 794 ( 80 nm band-pass filter was used to for the PL measurements while an additional 760 ( 12 nm band-pass filter was used for coincidence measurements. The g(2)(τ) functions were recorded at low powers (red circles) and at saturation powers (black squares). An absence of bunching of the g(2)(τ) function at saturation was observed only for the emission centered at 756 nm. For other emission lines, clear bunching at saturation is observed. The integration time in all cases was 300 s with a coincidence time bin of 154 ps. The normalization of the time g(2)(τ) functions was done following the procedure described in ref 24.
where λ ) r12 + r21 is the decay rate of the center. By fitting the parameter λ with different excitation powers and extrapolating the fit to zero power, the lifetime of the center is obtained as τl ) 1/λ. The fit to the experimental data shown in Figure 2d-f was obtained from the convolution of eq 3 with the temporal response function of the detectors. The equation is given by (2) gmeas (τ′) )
1 √2πσ
∫
∞
-∞
g(2)(τ)·e-(τ
- τ′)2/2σ2
dτ
(4)
where σ is the total jitter of the two detectors and the electronics. The resulting lifetime of the 756 nm center is therefore τ1 ) 3.7 ns. This lifetime is slightly longer than the lifetime of the nickel or silicon related defects16,17 and much shorter than the lifetime of the (N-V) center.5,14 To estimate the overall efficiency of the 756 nm center, the emission rate was recorded as a function of excitation power. The results are depicted in Figure 3. The saturation count rates for the two centers identified in Figure 2a,b were 2.2 × 106 and 5.7 × 105 counts/s, respectively. The reduction in count rate and efficiency in these centers, as compared with the two state emitter, is attributed to the presence of a third long-lived state as evidenced by the bunching observed in the anticorrelation histogram at high excitation powers. The background counts were measured by changing the polarization of the excitation beam and monitoring the vanishing of the dip in the g(2)(τ) function to test that the residual light has Poissonian statistics and an antibunched signal was obtained for every data point in Figure 3. This Nano Lett., Vol. 9, No. 9, 2009
Figure 3. Fluorescence intensity saturation curve recorded from the 756 nm single photon emitter (Figure 2c) as a function of excitation power. The black filled circles represent the background noise, the blue squares represent the raw data, the red triangles represent the background corrected data and the solid line is a fit using eq 4. The saturation optical power (Psat) is reached at ∼0.5 mW while the emission rate at the limit of saturation power (Isat) is 3.2 × 106 counts/s.
technique provides an alternative method to the one of measuring the background independently next to the crystal24 and it appears more appropriate for the detection of parasitic light emitted by other impurities and defects within the same diamond crystal. The data was fit to the following two level saturation expression I(P) ) IsatP/(Psat + P)
(5) 3193
Figure 4. Photoluminescence intensity from the 756 nm single photon emitter as a function of excitation polarization angle. The dots are the experimental data while the solid line is a sinusoidal fit to the experimental data. The extinction ration is greater than 90%.
where Isat,Psat is the count rate at saturation and the saturation power, respectively and I(P),P represent the measured count rate and incident excitation pump power, respectively. At saturation, extremely high count rate of 3.2 × 106 counts/s was deduced from the fit. To the best of our knowledge, this is the brightest SPS observed at room temperature. The measured count rate is comparable with the ultra fast cavity based SPS based on QD;1 however, unlike the QD, which requires cryogenic temperatures, the fabricated diamondbased SPS is operating at room temperature. Moreover, the measured count rate was achieved without the enhancement of the collection efficiency by integration of the SPS in cavities or with plasmonic structures.25 Such a high count rate can be explained by the proposed two-level energy scheme, which allows the center to be efficiently pumped and eliminates the possibility of population leaking into any longer lived dark states. From the saturation count rate the total effective collection efficiency of our system can be directly deduced as follows: assuming the total count rate, from two SPCMs at saturation is ∼3.2 × 106 counts/s and the lifetime, τ1 ) 3.7 ns, the total collection efficiency is estimated ∼1.2% and comparable with previous works.15,18 Nevertheless, further improvement in terms of collection efficiency and background reduction should be achieved before this SPS can be used in a real quantum optical device. Because of the high efficiency, narrow emission, high count rate, and a controllable fabrication of the source, both cavity mode26 and plasmonic resonance coupling will be facilitated.25 The PL intensity of the 756 nm center was also measured as a function of the excitation polarization angle. It can be seen from Figure 4 that the center is sensitive to the polarization of the excitation beam with an extinction ratio of more than 90%. Since the relationship between the laser beam field and the nanocrystal orientation is unknown, it is not possible at this stage to infer the crystallographic alignment of the 756 nm center within the diamond lattice. However, a variation of 90 degrees in the polarization angle results in a PL evolution from minimum to maximum intensity, which agrees with characteristic dipole behavior on the emission transition. Finally we report on a series of experiments designed to understand the origin of the center. In order to ascertain 3194
whether the 756 nm center was associated with Cr and to check for the effects of ion induced damage that might accompany the implantation, three different batches of samples were prepared. (i) CVD grown diamond nanocrystals on a sapphire substrate implanted with Ni ions with acceleration energy of 30 keV and a dose of 5 × 1010 Ni/cm2. (ii) CVD grown diamond nanocrystals on a sapphire substrate implanted with Ar ions with acceleration energy of 14 keV and a dose of 1 × 1011 Ar/cm2. (iii) CVD grown but unimplanted diamond nanocrystals. All the samples were then annealed at 1000 °C in 95%Ar-5%H2 ambient for 2 h. According to SRIM simulations,27 the stopping range of the implanted ions is 10-20 of nm beneath the diamond surface. The damage created by the implanted Ar and Ni is 1.17 × 1013 and 1.38 × 1013 vacancies/cm3, respectively. The confocal scans revealed that significantly more optical centers were formed at the implanted areas (from both Ni and Ar implantations) than at the unimplanted ones. The PL signals in both Ni and Ar implanted sets were similar to the ones shown in Figure 2a-c. Statistically, roughly 10% of the implanted and annealed crystals showed bright PL features and single photon behavior. Out of those, approximately 10% showed a single photon emission with a ZPL centered at 756 nm. We emphasize that only those crystals with a specific ZPL around 756 showed a clear two-level system antibunching behavior. All other PL features originating from the implanted crystals were associated with bunched autocorrelation functions at high excitation powers, as shown in Figure 2d,e. Therefore, only a narrow window of formation probability is available to fabricate the efficient optical center with a ZPL centered at 756 nm. Nevertheless, this center was repeatedly observed in the implanted regions. On the other hand, in the unimplanted region only one crystal was found to fluoresce at 756 nm. This implies that the implantation of Ni or Ar plays a crucial role in the creation of color centers by introducing vacancies within the crystals but the observed emitters are not related to either Ar or Ni ions. Recent work has shown that during CVD growth impurities present in the substrate can be incorporated into the diamond nanocrystals and that these impurities can lead to stable single photon emitters.19 In the present case, the diamond nanocrystals were grown on sapphire substrates, which contain a significant amount of Cr atoms (∼ppm). This was confirmed by exciting the sapphire with 514 nm excitation laser and the observation of strong luminescence at ∼ 693/695 nm, attributed to Cr3+ atoms in the sapphire lattice.28 Furthermore, it is known that Cr-related centers in diamond exhibit narrow PL lines in the region of 740-770 nm22,23 and it was demonstrated that natural diamond can trap a crystalline Al2O3:Cr3+ during its growth.29 Our growth conditions of high pressure plasma (150 Torr) lead to considerable etching of the substrate making it more likely that the substrate material will be incorporated into the growing crystals. To test this hypothesis, the same growth/ implantation/annealing route was repeated, but this time into diamond nanocrystals grown on a silica substrate, which does not contain Cr atoms. The PL studies of the diamond nanocrystals grown on silica did not reveal any of the PL Nano Lett., Vol. 9, No. 9, 2009
lines shown in Figure 2a-c. Therefore, one can conclude that the incorporation of an impurity from the sapphire substrate, most likely Cr, into the growing diamond nanocrystal gives rise to the PL lines shown in Figure 2a-c. Our measurements, however, do not allow unambiguous correlation of Cr to the 756 nm center at this point of time. One must bear in mind that the incorporation of oxygen or aluminum from the sapphire substrate is also possible, since these are the main elements of the sapphire structure. The incorporation of silicon from the quartz bell jar chamber is also possible, although given that we do not observe the 756 nm center in samples grown on silica substrates, it would appear that silicon is not strongly implicated in the 756 nm center. Nevertheless, an interaction between various codopants to form a specific center within the diamond crystal should be considered. A post growth implantation of Cr into nanocrystals grown on silica or into a single crystal diamond would involve substantially different formation kinetics and pathways. Hence it is not obvious that the same set of centers will be found, nor is it obvious that they will share the same properties.23 During the growth, the impurities from the substrate are randomly dispersed within the crystal. Assuming that Cr atoms do indeed play a crucial role, but need to be paired with another impurity (such as Al, O, Si), there is only a finite chance for such an event to occur. Note that the relatively low annealing temperature of 1000 °C is not sufficient to cause a migration of heavy atoms like Cr, Si, or even O; however, the diffusion of vacancies toward the specific atom to stabilize the optical properties of the center is expected. This may explain why only some of the crystals exhibit the desired luminescence, while others do not. To summarize, we have controllably fabricated and characterized novel single photon emitters, likely based on incorporation of Cr atoms into CVD diamond nanocrystals, operating at room temperature. The most promising emitter has a ZPL centered at ∼756 nm with a fwhm of ∼11 nm and exhibits photon statistics which are described well by a two-level model, confirmed by the absence of “bunching” in the g(2)(τ) function at saturation. An excited state lifetime of 3.7 ns and a high emission rate of 3.2 × 106 counts/s make this source ideal for QIP, metrology, and cellular bio markers. This is the most efficient true single photon source reported to date and holds a great potential to realize future fundamental quantum optics experiments. Acknowledgment. The authors would like to thank A. M. Zaitsev for useful discussions regarding Cr defects in single crystal diamond. This work was supported by the Australian
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Research Council, The International Science Linkages Program of the Australian Department of Innovation, Industry, Science and Research (Project No. CG110039) and by the European Union Sixth Framework Program under the EQUIND IST-034368. A.D.G. is the recipient of an Australian Research Council Queen Elizabeth II Fellowship (Project No. DP0880466). References (1) Strauf, S.; Stoltz, N. G.; Rakher, M. T.; Coldren, L. A.; Petroff, P. M.; Bouwmeester, D. Nat. Photonics 2007, 704, 1. (2) Santori, C. Nat. Photonics 2007, 686, 1. (3) Jacques, V.; Wu, E.; Grosshans, F.; Treussart, F.; Grangier, P.; Aspect, A.; Roch, J. F. Science 2007, 315, 966. (4) Giovannetti, V.; Lloyd, S.; Maccone, L. Phys. ReV. Lett. 2006, 96, 010401. (5) Beveratos, A.; Brouri, R.; Gacoin, T.; Villing, A.; Poizat, J. P.; Grangier, P. Phys. ReV. Lett. 2002, 89, 187901. (6) Brunel, C.; Lounis, B.; Tamarat, P.; Orrit, M. Phys. ReV. Lett. 1999, 83, 2722. (7) Lounis, B.; Moerner, W. E. Science 2000, 407, 491. (8) Santori, C.; Pelton, M.; Solomon, G.; Dale, Y.; Yamamoto, Y. Phys. ReV. Lett. 2001, 86, 1502. (9) Tribu, A.; Sallen, G.; Aichele, T.; Andre, R.; Poizat, J. P.; Bougerol, C.; Tatarenko, S.; Kheng, K. Nano Lett. 2008, 8, 4326. (10) Hogele, A.; Galland, C.; Winger, M.; Imamoglu, A. Phys. ReV. Lett. 2008, 100, 217401. (11) Greentree, A. D.; Fairchild, B. A.; Hossain, F. M.; Prawer, S. Mater. Today 2008, 11, 22. (12) Krueger, A. AdV. Mater. 2008, 20, 2445. (13) Fu, C. C.; Lee, H. Y.; Chen, K.; Lim, T. S.; Wu, H. Y.; Lin, P. K.; Wei, P. K.; Tsao, P. H.; Chang, H. C.; Fann, W. Proc. Natl. Acad. Sci. U.S.A. 2007, 104, 727. (14) Jelezko, F.; Gaebel, T.; Popa, I.; Domhan, M.; Gruber, A.; Wrachtrup, J. Phys. ReV. Lett. 2004, 93, 130501. (15) Wu, E; Rabeau, J. R.; Roger, G.; Treussart, F.; Zeng, H.; Grangier, P.; Prawer, S.; Roch, J. F. New J. Phys. 2007, 9, 434. (16) Gaebel, T.; Popa, I.; Gruber, A.; Domhan, M.; Jelezko, F.; Wrachtrup, J. New J. Phys. 2004, 6, 98. (17) Wang, C. L.; Kurtsiefer, C.; Weinfurter, H.; Burchard, B. J. Phys. B 2006, 39, 37. (18) Simpson, D. A.; Ampem-Lassen, E.; Gibson, B. C.; Trpkovski, S.; Hossain, F. M.; Huntington, S. T.; Greentree, A. D.; Hollenberg, L. C. L.; Prawer, S. Appl. Phys. Lett. 2009, 94, 203107. (19) Aharonovich, I.; Zhou, C.; Stacey, A.; Treussart, F.; Roch, J. F.; Prawer, S. Appl. Phys. Lett. 2008, 93, 243112. (20) Su, C. H.; Greentree A. D.; Hollenberg, L. C. L. arXiV:0904.2267V1. (21) Stacey, A.; Aharonovich, I.; Prawer, S.; Butler, J. E. Diamond Relat. Mater. 2009, 18, 51. (22) Zaitsev, A. M. Phys. ReV. B 2000, 61, 12909. (23) Zaitsev, A. M. private communication . (24) Beveratos, A.; Kuhn, S.; Brouri, R.; Gacoin, T.; Poizat, J. P.; Grangier, P. Eur. Phys. J. D 2002, 18, 191. (25) Schietinger, S.; Barth, M.; Aichele, T; Benson, O. Nano Lett. 2009, 9, 1694. (26) McCutcheon, M.; Loncar, M. Opt. Exp. 2008, 16, 19136. (27) The Stopping and Range of Ions in Matter; www.srim.org. (28) Jheeta, K. S; Jain, D. C.; Kumar, R.; Garg, K. B. Solid State Commun. 2007, 144, 460. (29) Iakoubovskii, K.; G. J. Adriaenssens, G. J. J. Phys.: Cond. Matter 2002, 14, 5459.
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