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Optics Express

Optics Express

  • Editor: C. Martijn de Sterke
  • Vol. 20, Iss. 10 — May. 7, 2012
  • pp: 11536–11547
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Incoherent photon conversion in selectively infiltrated hollow-core photonic crystal fibers for single photon generation in the near infrared

Ping Jiang, Tim Schroeder, Michael Bath, Vladimir Lesnyak, Nikolai Gaponik, Alexander Eychmüller, and Oliver Benson  »View Author Affiliations


Optics Express, Vol. 20, Issue 10, pp. 11536-11547 (2012)
http://dx.doi.org/10.1364/OE.20.011536


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Abstract

At present, there exist a number of on-demand single photon sources with high emission rates and stability even at room temperature. However, their emission wavelength is restricted to specific transitions in single quantum emitters. Single photon generation in the near infrared, possibly within the telecom band, though most urgently needed, is particularly crucial. In this paper, we suggest an experimental method to convert visible single photons from a defect center in diamond to the near infrared. The conversion relies on efficient absorption by colloidal quantum dots and subsequent Stokes-shifted emission. The desired target wavelength can be chosen almost arbitrarily by selecting quantum dots with a suitable emission spectrum. A hollow core photonic crystal fiber selectively filled with a solution of quantum dots was used to achieve at the same time a single photon absorption probability of near unity and a very high re-collection efficiency of Stokes-shifted fluorescence (theoretically estimated to be 26%). A total conversion efficiency of light of 0.1% is achieved. Experimental strategies to significantly enhance this number are presented.

© 2012 OSA

1. Introduction

On-demand single photon sources rely on spontaneous emission from single quantum systems. Such sources have been realized with single ions [1

1. A. Kuhn, M. Hennrich, and G. Rempe, “Deterministic single-photon source for distributed quantum networking,” Phys. Rev. Lett. 89(6), 067901 (2002). [CrossRef] [PubMed]

] molecules [2

2. K. G. Lee, X. W. Chen, H. Eghlidi, P. Kukura, R. Lettow, A. Renn, V. Sandoghdar, and S. Gotzinger, “A planar dielectric antenna for directional single-photon emission and near-unity collection efficiency,” Nat. Photonics 5(3), 166–169 (2011). [CrossRef]

] and quantum dots [3

3. C. Santori, M. Pelton, G. Solomon, Y. Dale, and Y. Yamamoto, “Triggered single photons from a quantum dot,” Phys. Rev. Lett. 86(8), 1502–1505 (2001). [CrossRef] [PubMed]

,4

4. A. Imamoğlu, P. Michler, M. D. Mason, P. J. Carson, G. F. Strouse, and S. K. Buratto, “Quantum correlation among photons from a single quantum dot at room temperature,” Nature 406(6799), 968–970 (2000). [CrossRef] [PubMed]

]. More recently, defect centers in diamond, such as the nitrogen-vacancy center (NV) or the silicon-vacancy center (SiV) in diamond [5

5. C. Kurtsiefer, S. Mayer, P. Zarda, and H. Weinfurter, “Stable solid-state source of single photons,” Phys. Rev. Lett. 85(2), 290–293 (2000). [CrossRef] [PubMed]

7

7. E. Neu, D. Steinmetz, J. Riedrich-Möller, S. Gsell, M. Fischer, M. Schreck, and C. Becher, “Single photon emission from silicon-vacancy colour centres in chemical vapour deposition nano-diamonds on iridium,” New J. Phys. 13(2), 025012 (2011). [CrossRef]

] have been utilized as stable single photon sources operating even at room temperature. Single photons in the near infrared, particularly the telecom band, are highly desirable for long-distance fiber-based quantum key distribution [8

8. P. M. Intallura, M. B. Ward, O. Z. Karimov, Z. L. Yuan, P. See, A. J. Shields, P. Atkinson, and D. A. Ritchie, “Quantum key distribution using a triggered quantum dot source emitting near 1.3 μm,” Appl. Phys. Lett. 91(16), 161103 (2007). [CrossRef]

] or entanglement transfer [9

9. N. Sangouard, C. Simon, H. de Riedmatten, and N. Gisin, “Quantum repeaters based on atomic ensembles and linear optics,” Rev. Mod. Phys. 83(1), 33–80 (2011). [CrossRef]

]. Unfortunately, there are only few quantum systems with strong transitions in this wavelength regime. Self-organized quantum dots are in principle capable to emit in the infrared, e.g. at 1.3 μm [10

10. M. B. Ward, T. Farrow, P. See, Z. L. Yuan, O. Z. Karimov, A. J. Bennett, A. J. Shields, P. Atkinson, K. Cooper, and D. A. Ritchie, “Electrically driven telecommunication wavelength single-photon source,” Appl. Phys. Lett. 90(6), 063512 (2007). [CrossRef]

], but they require low-temperature operation and their growth with low density is challenging. Individual lead chalcogenide colloidal quantum dots (such as PbS) or CdHgTe quantum dots (QDs) can provide single photons in the near-infrared wavelength range, but they are not photo-stable over longer periods of laser excitation [11

11. J. J. Peterson and T. D. Krauss, “Fluorescence spectroscopy of single lead sulfide quantum dots,” Nano Lett. 6(3), 510–514 (2006). [CrossRef] [PubMed]

].

A closer look at the generation of on-demand photons using spontaneous emission reveals that the key requirement is to generate a single excitation in an optically active system. A single quantum system fulfills this requirement automatically, since it can only absorb a single quantum of light (see Fig. 1(a)
Fig. 1 Two complementary approaches for on-demand single photon generation. (a) Excitation of a single quantum system with a classical source and subsequent spontaneous emission of a single photon. (b) Excitation of an arbitrary ensemble with a single photon. Also in this case only a single excitation is present, hence decay of the ensemble also leads to emission of a single photon only.
). A complementary approach is to utilize an arbitrary ensemble of emitters, but to excite it with only a single quantum of light (see Fig. 1(b)). In both cases single photons are emitted. Here we present such an approach (as in Fig. 1(b)) to realize a stable, non-blinking, room temperature infrared single photon source. The key idea of wavelength conversion is then to use efficient absorption of single photons in the visible and subsequent Stokes-shifted emission at a longer wavelength.

There are two requirements for photon conversion by single photon absorption and subsequent re-emission: (1) a bright single photon source and (2) an efficient absorber with an appropriate Stokes-shifted emission spectrum in the targeted optical spectrum. As photon source we utilize a single nitrogen-vacancy (NV) center in diamond. Such sources have photon emission rates up to 2.4 Mcts/s [12

12. T. Schröder, F. Gädeke, M. J. Banholzer, and O. Benson, “Ultrabright and efficient single-photon generation based on nitrogen-vacancy centres in nanodiamonds on a solid immersion lens,” New J. Phys. 13(5), 055017 (2011). [CrossRef]

]. A suitable absorber is an ensemble of colloidal QDs, since they have large above-band absorption and their emission spectrum can be tailored by choosing the right size and material [13

13. D. K. Harris, P. M. Allen, H. S. Han, B. J. Walker, J. Lee, and M. G. Bawendi, “Synthesis of cadmium arsenide quantum dots luminescent in the infrared,” J. Am. Chem. Soc. 133(13), 4676–4679 (2011). [CrossRef] [PubMed]

]. In order to achieve both a large single photon absorption probability and a very high re-collection efficiency of Stokes-shifted fluorescence we utilize hollow-core photonic crystal fibers (HCPCFs) infiltrated with a solution of colloidal QDs [14

14. M. Barth, H. Bartelt, and O. Benson, “Fluid-filled optical fibers” in Handbook of Optofluidics, edts. A. R. Hawkins, H. Schmidt, CRC Press Tylor & Francis (2010).

]. By adopting the concentration of QDs and fiber length, the absorption can be up to unity on cost of re-absorption which will be discussed in the experimental section. At the same time, a selectively filled HCPCF provides a large numerical aperture so that a large fraction of fluorescence can be collected and guided. Utilizing specific QDs, a bright single photon source, and a HCPCF, a novel kind of fiber-coupled single photon source in the telecom band can be realized. In the following we present experiments towards such a system.

2. Hollow-core photonic crystal fibers

Micro-structured optical fibers allow for light confinement in microscopic dimensions and guiding over macroscopic distances. Therefore, these fibers are ideally suited for enhanced interaction between light and matter. Many of these micro-structured fibers feature a hollow center hole that can be filled with various substances. Such filled HCPCFs allow strong and well-controlled interactions with solid state, fluidic and gas systems. In recent years, such micro-structured optical fibers have been used for absorption [15

15. F. M. Cox, A. Argyros, and M. C. J. Large, “Liquid-filled hollow core microstructured polymer optical fiber,” Opt. Express 14(9), 4135–4140 (2006). [CrossRef] [PubMed]

], fluorescence [16

16. S. Smolka, M. Barth, and O. Benson, “Highly efficient fluorescence sensing with hollow core photonic crystal fibers,” Opt. Express 15(20), 12783–12791 (2007). [CrossRef] [PubMed]

], Raman [17

17. S. O. Konorov, C. J. Addison, H. G. Schulze, R. F. B. Turner, and M. W. Blades, “Hollow-core photonic crystal fiber-optic probes for Raman spectroscopy,” Opt. Lett. 31(12), 1911–1913 (2006). [CrossRef] [PubMed]

], and surface enhanced Raman scattering (SERS) [18

18. Y. Zhang, C. Shi, C. Gu, L. Seballos, and J. Z. Zhang, “Liquid core photonic crystal fiber sensor based on surface enhanced Raman scattering,” Appl. Phys. Lett. 90(19), 193504 (2007). [CrossRef]

,19

19. X. Yang, C. Shi, D. Wheeler, R. Newhouse, B. Chen, J. Z. Zhang, and C. Gu, “High-sensitivity molecular sensing using hollow-core photonic crystal fiber and surface-enhanced Raman scattering,” J. Opt. Soc. Am. A 27(5), 977–984 (2010). [CrossRef] [PubMed]

] experiments and have shown great potential for chemical, biological, and environmental sensing applications. Due to the much longer interaction length between the guided light and the sample (e.g., gases or liquids), accomplished sensitivities and detection signals are orders of magnitudes higher than in unguided beam light – matter interaction experiments.

Generally, HCPCFs confine and guide light within the lower dielectric material in the central core via the photonic band-gap effect [20

20. R. F. Cregan, B. J. Mangan, J. C. Knight, T. A. Birks, P. S. J. Russell, P. J. Roberts, and D. C. Allan, “Single-mode photonic band gap guidance of light in air,” Science 285(5433), 1537–1539 (1999). [CrossRef] [PubMed]

]. This allows infiltration of gases in the hollow core without substantially changing the band-gap properties. When the hole is filled with liquid, the band-gap effect and therefore band-gap light-guiding are destroyed due to the high refractive index of liquids. Yet, another optical process enables light guiding within the filled hole of the fiber. The refractive index of liquids of about n = 1.4 to n = 1.6 is higher than the surrounding microstructure which consists mainly of hollow regions. The filled region forms a dielectric rod and total internal reflection happens at the interface of the filled core and the holey cladding structure if the incident angle of radiation is higher than the angle of total internal reflection.

The collection efficiency of fluorescence for a selectively filled HCPCF, as used in our experiment, can be estimated as follows. We infiltrate a solution of QDs and polystyrene dissolved in toluene (ntol.=1.49,npoly.=1.57 with 1:1 proportion). This results in a refractive index of the liquid in the fiber of n = 1.53. For the structured, hollow part surrounding the center hole (see Fig. 2(b)
Fig. 2 (a). Optical microscope image of a hollow-core fiber (side view) after treatment in the fusion splicer. The arrows indicate the region of the open core hole, while the cladding holes are sealed. (b) Optical microscope image of a hollow-core photonic crystal fiber selectively filled with liquid.
) the index of refraction is estimated to be about n = 1.05. The emission from the quantum dots is totally reflected inside the fiber when the radiation angle is greater than the angle of total internal reflection of 43° (arcsin(1.05/1.53)). Via this reflection mechanism, 26% of the total emission is guided inside the fiber core to the end of the fiber to be detected or further guided in air. A similar portion of the emission is guided to the front facet of the fiber. Both fractions could be easily sent to one end of the fiber by adding a reflective filter on the facet of the other fiber end. In this way a re-collection efficiency of 52%, collected at both ends of the fiber, can be achieved when neglecting re-absorption.

3. Experimental methods

3.1. Synthesis of CdHgTe QDs

Aqueous mercaptopropionic acid (MPA)-capped CdHgTe QDs have been synthesized according to the method reported in ref [21

21. V. Lesnyak, A. Lutich, N. Gaponik, M. Grabolle, A. Plotnikov, U. Resch-Genger, and A. Eychmüller, “One-pot aqueous synthesis of high quality near infrared emitting Cd1_xHgxTe nanocrystals,” J. Mater. Chem. 19(48), 9147–9152 (2009). [CrossRef]

]. In a typical synthesis 0.562 g of Cd(ClO4)2 × 6H2O, 0.014 g of Hg(ClO4)2 × 6H2O and 0.189 g of MPA were dissolved in 60 mL of Milli-Q (Millipore) water. The pH of the resulting mixture was adjusted to 12 using 1 M NaOH solution and subsequently was deaerated by bubbling with Ar for 30 min. Then, under stirring, H2Te gas (generated by the reaction of 0.1 g of Al2Te3 lumps with an excess of 0.5M H2SO4 solution) was bubbled into the solution together with a slow argon flow. The molar ratio of Cd2+/Hg2+/Te2–/MPA was 0.98/0.02/0.5/1.3. Further nucleation and growth of the nanocrystals proceeded by refluxing the solution at 100°C for 30–35 min under open-air conditions. The QD colloid obtained was purified by reprecipitation of the particles by addition of 2-propanol to the concentrated (on a rotor evaporator) crude solution with subsequent dissolving of the QD precipitate in ~2 mL of Milli-Q water.

Thus obtained CdHgTe QDs were transferred from water into toluene utilizing octadecyl-p-vinylbenzyldimetylammonium chloride (OVDAC) as a transfer agent according to a procedure described in ref [22

22. H. Zhang, Z. Cui, Y. Wang, K. Zhang, X. Ji, C. Lü, B. Yang, and M. Gao, “From water-soluble CdTe nanocrystals to fluorescent nanocrystal–polymer transparent composites using polymerizable Surfactants,” Adv. Mater. (Deerfield Beach Fla.) 15(10), 777–780 (2003). [CrossRef]

]. Briefly, ~2 mL of the reprecipitated QD water solution, 70 mg of OVDAC and 2 mL of toluene were vigorously stirred for 30 min. Toluene layer containing QDs was separated and mixed with 6 mL of acetone with subsequent centrifugation. The QD precipitate was dissolved in ~3 mL of toluene.

UV-Vis absorption spectrum was recorded using a Cary 5000 spectrophotometer (Varian Inc.). Fluorescence measurement was performed with a Fluorolog-3 spectrofluorometer (HORIBA Jobin Yvon Inc.)

3.2. Preparation of functionalized hollow core photonic crystal fibers

The hollow-core photonic crystal fiber (HC-800-01, NKT Photonics) employed in our experiment is made of fused silica (refractive index n2 = 1.45) and produced from a preform by a heating and stretching process in a drawing tower (so-called stack-and-draw method) [23

23. E. F. Chillcce, C. M. B. Cordeiro, L. C. Barbosa, and C. H. Brito Cruz, “Tellurite photonic crystal fiber made by a stack-and-draw technique,” J. Non-Cryst. Solids 352(32-35), 3423–3428 (2006). [CrossRef]

]. The fiber is designed for the operation in air via the photonic band-gap effect at a central wavelength of 830 nm with a transmission bandwidth larger than 70 nm. Its hollow cladding structure exhibits an air filling fraction f > 90% and has a diameter of 40 µm while the central core diameter is about 9.5 µm.

When preparing the hollow-core fiber, liquid should be selectively injected into the core hole. Therefore additional preparation steps prior to the infiltration are needed. We utilize a selective melting method for the holes in the facet [24

24. L. Xiao, W. Jin, M. S. Demokan, H. L. Ho, Y. L. Hoo, and C. L. Zhao, “Fabrication of selective injection microstructured optical fibers with a conventional fusion splicer,” Opt. Express 13(22), 9014–9022 (2005). [CrossRef] [PubMed]

] in order to selectively seal the cladding holes, while the central core hole remains open. An optical microscopy image of a melted hollow-core fiber after the treatment in the fusion splicer is shown in Fig. 2(a).

After the cladding holes are sealed, the core hole is equipped with colloidal quantum dots, which are dissolved in toluene. In order to suppress evaporation of toluene, a polymer (polystyrene) is dissolved in the solution containing the quantum dots. It dries fast and blocks the end of the core hole, stopping the evaporation of toluene. Furthermore, the polymer has a higher refractive index than toluene, resulting in improved collection efficiency due to a higher angle of total internal reflection. To equip the core hole, the fiber is glued to a syringe that is filled with the polymer QD solution. Pressure is applied until the core is entirely filled [16

16. S. Smolka, M. Barth, and O. Benson, “Highly efficient fluorescence sensing with hollow core photonic crystal fibers,” Opt. Express 15(20), 12783–12791 (2007). [CrossRef] [PubMed]

]. As shown in Fig. 2(b), after selective infiltration and cleaving, only the core hole is filled with liquid while other cladding holes are empty.

3.3. Description of the experimental setup

A schematic of the experimental setup used in our measurement is shown in Fig. 3
Fig. 3 Experimental setup. BC, BS, DBS, RM, F, PH, SILs, HBT refer to beam control, beam splitter, dichroic beam splitter, removable mirror, filter, pinhole, solid immersion lens, Hanbury Brown and Twiss setup, respectively.
. Single photons are generated by single NV-centers that emit in the visible at 690 nm with a FWHM of the fluorescence spectrum of approximately 100 nm (see spectrum in Sec. 4). For efficient photon collection a home-built solid immersion lens (SIL) microscope is applied [12

12. T. Schröder, F. Gädeke, M. J. Banholzer, and O. Benson, “Ultrabright and efficient single-photon generation based on nitrogen-vacancy centres in nanodiamonds on a solid immersion lens,” New J. Phys. 13(5), 055017 (2011). [CrossRef]

]. For filtering the green 532 nm laser excitation and guiding the red single photons to the functionalized fiber core, a dichroic beam splitter with a 625 nm edge is used. The transmission rate of the red photons through the dichroic beam splitter is about 89%. The other 11% of single photons are reflected back to single photon counting modules in order to monitor the total single photon generation rate, align the setup, and estimate the number of visible photons directed to the HCFCF.

The HCPCF that is functionalized with QDs is mounted on a mechanical 3-D translation stage and the red single photons are coupled in and out of the fiber via two microscope objectives (Olympus, 20 × /0.4 NA), which are also mounted on a mechanical translation stage. Before coupling into the filled fiber, a 540 nm long pass filter is used to block the remaining green laser light. After the absorption and re-emission process of photons by the quantum dots inside the fiber, the photoluminescence of the QDs is detected by a spectrograph (SpectraPro 500i, Princeton Instruments) or with a CCD camera (SPEC-10, Roper Scientific). Detection is carried out either in transmission or backward direction (see Fig. 3). Therefore the fluorescence light is collected by the microscope objective at the end of the fiber and guided to the detection systems after passing a 540 nm long-pass filter and a 790 nm long-pass filter. In this setup, removable mirrors are put on magnetic stages to change the direction of the signal path. In backward direction a dichroic plate beam splitter (Semrock, BrightLine, 801 nm single-edge) is used, which has a high transmission for the red photons and a high reflectivity for near-IR photons (above 801nm). Thereby, converted near-IR emission from the front facet of the fiber can be detected. To verify single photon character of the converted photons, autocorrelation measurements are performed in a Hanbury Brown and Twiss (HBT) setup [25

25. H. J. Kimble, M. Dagenais, and L. Mandel, “Photon antibunching in resonance fluorescence,” Phys. Rev. Lett. 39(11), 691–695 (1977). [CrossRef]

]. The 640nm laser is used for preliminary analysis of the fiber system to determine the optimum concentration of QDs inserted into the fiber. It was also used to determine the conversion efficiency.

4. Experimental results

For preparing the conversion experiment, at first, single photon generation from a single NV-center inside nanodiamond on a SIL is optimized [12

12. T. Schröder, F. Gädeke, M. J. Banholzer, and O. Benson, “Ultrabright and efficient single-photon generation based on nitrogen-vacancy centres in nanodiamonds on a solid immersion lens,” New J. Phys. 13(5), 055017 (2011). [CrossRef]

]. After scanning the laser focus of the confocal microscope over a region of about 10 µm x 10 µm, a single NV-center with emission showing single photon character is identified. With 532 nm CW laser excitation at an excitation power of 230 μW, 31 kcts/s are detected. 280 kcts/s of single photons are guided to the fiber as was estimated from the transmission of the dichroic beam splitter of 89%. The measured and normalized autocorrelation function g(2)(τ) (collected with the HBT setup) is shown in Fig. 4
Fig. 4 Normalized second-order correlation function of the fluorescence of a single NV-center in a nano-diamond located on a solid immersion lens. The data shows a deep dip at zero, indicating single photon statistics. The red curve is a fit to the data according to a three level model [33].
. For zero time delay g(2)(0)=0.2 indicates strong single photon character stemming from a single NV-center.

To evaluate the fiber system, the transmission properties of a HCPCF filled only with liquid (toluene with dissolved polymer, but without QDs) is analyzed. Figure 5
Fig. 5 Measured spectral properties of the liquid-filled HCPCF, the NV-center, and the solid immersion lens (SIL). The black curve shows the fluorescence from the bare ZrO2 SIL. The red and blue curves are spectra of NV-centers on a SIL and the total fluorescence from the end of the filled fiber, respectively. The blue curve is normalized to the red one. The edges in the spectrum are caused by a 590 nm long pass and a 795 short pass filter. The excitation wavelength was 532 nm.
shows the measured fluorescence spectrum of light under 532 nm excitation coming directly from the SIL as black curve. A pronounced fluorescence is observed above 795 nm which stems from the ZrO2. This fluorescence can be blocked by a 795 nm short pass filter. The red and blue curves in Fig. 5 indicate the spectrum of the NV-center on the SIL and the total fluorescence from the end of the filled fiber, respectively. The blue curve is normalized to the red one by means of the zero phonon line emission of the NV center at 637 nm. Comparing both spectra reveals that the spectrum of the fluorescence light that has passed through the polymer filled fiber is qualitatively the same as that of the NV-center in diamond. Only below the wavelength of 625 nm a strong suppression of fluorescence is visible for the blue curve, caused by the dichroic beam splitter in the setup. The measurement proves that the single photons from the NV center are transmitted in the fiber without spectral changes. Yet, a high transmission loss of 86% is observed. This is mainly due to the scattering out of the core mode induced by tiny bubbles in the polymer toluene solution in the fiber. Moreover, the coupling efficiency from free space to the filled fiber is reduced by partial sputtering of polymer around the core region when the fiber is cleaved.

The concentration of the QDs is a crucial parameter for preparing the QD-HCPCF system. Absorption of single photons in the visible should be maximized while re-absorption of infrared photons should be minimized. Generally, absorption and re-absorption depend on the concentration of emitters and can be estimated theoretically by the absorption cross-section. As the exact concentration of CdHgTe QDs after the transfer from aqueous solution into toluene is not accessible a priori, the optimum concentration is determined directly from spectral measurements.

This is done by utilizing a diode laser emitting at a wavelength of 640 nm. The laser is coupled into a HCPCF selectively filled with QD-polymer solution (see Fig. 3) where it excites the QDs. Fluorescence from the QDs centered at around 850 nm (Fig. 6(b)
Fig. 6 (a) Schematics of the HCPCF as in (b). The fiber has a diameter of 135 µm, the core a diameter of 9.5 µm. The cladding consists of a hexagonal glass structure with a pitch of 2.3 µm and an air fraction of 0.9. (b) CCD image of the photoluminescence from the CdHgTe QD-solution in the HCPCF collected at the end of the fiber. The white dashed lines indicate the fiber and the cladding region, the dotted line shows the core. The largest fraction of light is guided in the central core region.
) is filtered out with the help of a 790 nm long pass filter. As can be seen in Fig. 6(b), most emission from the CdHgTe QDs is confined inside the filled core region and can be collected selectively. The dotted circle line in Fig. 6(b) indicates the region from where fluorescence is collected in the measurements. Although the highest intensity (red and yellow color encoding) is collected at the core region of the fiber, a significant fraction of fluorescence is found also in the cladding region. This lost fraction is fluorescence light that does not satisfy the total reflection condition or that is scattered into the cladding by small air bubbles in the liquid. An additional loss channel is scattering due to the polymer sputtering around the core region close to the facets caused by cleaving the fiber.

Photoluminescence spectra under 640 nm laser excitation (see Fig. 3) collected at the end of the fiber with different CdHgTe QDs concentrations are shown in Fig. 7(b)
Fig. 7 (a) Absorption (red) and PL (black) spectra of CdHgTe QDs in toluene. (b) PL spectra collected at the end of the HCPCF fiber (see Fig. 6(b)) with different CdHgTe QDs concentrations (increasing from the black to the blue curve). The black curve represents the appropriate concentration used for our experiment, where re-absorption is absent. The red and the blue curve correspond to a two times and five times higher concentration than in the black curve, respectively.
. The PL intensity increases with increasing concentration, which represents higher conversion efficiency from visible light at 640 nm to the near infrared. However, at higher concentration, re-absorption appears. It can be observed in the near infrared spectrum as a re-distribution of light towards longer wavelengths. In Fig. 7(b) (red and blue curves) this effect is already quite pronounced and is observed as the onset of additional peaks at around 900 nm.

Re-absorption has to be avoided in the incoherent photon emission via absorption and Stokes-shifted emission. In order to maintain single photon statistics the excitation rate of the QDs has to be smaller than the emission rate of the converted photons. Otherwise, the probability to emit more than one photon at a time increases. In our experiment we kept the generations rate of visible photons far below the reciprocal of the lifetime τlife = 150 ns of the CdHgTe QDs. A re-absorption of converted photons would double the effective lifetime which enhances the probability of two-photon events. To avoid this, a concentration corresponding to that of the black curve in Fig. 7(b) with negligible re-absorption is used for the conversion experiments.

The onset of re-absorption limits the maximum concentration of QDs which leads to a lower conversion efficiency of visible to infrared light. To measure the absolute conversion efficiency we utilized again the red diode laser (640 nm) as classical photon source which we coupled into the HCPCF. We compared the intensity, i.e. the photon generation rate, of the red laser to the photon counts of near-infrared photons collected at the end of the fiber. The ratio is the total conversion efficiency which we found as ηtot = 0.1%. This number includes the coupling efficiency of laser light from free space to the fiber, the transmission of the fluorescence light inside the fiber and the collection efficiency of the photoluminescence from the QDs at the end of the fiber, all together having a total efficiency ηtrans = 14%. From this we can derive the probability that a single visible photon at 640 nm is absorbed by a QD in the HCPCF and re-emitted as infrared photon at 850 nm to be ηconvert = 3%. This number is quite large compared to simply focusing and collecting on a QD solution with high-NA objectives [16

16. S. Smolka, M. Barth, and O. Benson, “Highly efficient fluorescence sensing with hollow core photonic crystal fibers,” Opt. Express 15(20), 12783–12791 (2007). [CrossRef] [PubMed]

].

5. Towards single photon generation in the infrared

Since re-absorption of infrared photons limits the maximum concentration of QDs, a large Stokes shift between absorption and emission spectrum (see properties of our QDs in Fig. 7(a)) would be desirable. Colloidal chemistry holds promising prospects to develop QDs with a large Stokes-shift and high quantum efficiency at the same time [26

26. D. Dorfs, T. Franzl, R. Osovsky, M. Brumer, E. Lifshitz, T. Klar, and A. Eychmüller, “Type-I and Type-II nanoheterostructures based on CdTe nanocrystals – a comparative study,” Small 4, 1148–1153 (2008).

31

31. D. Dorfs, A. Salant, I. Popov, and U. Banin, “ZnSe quantum dots within CdS nanorods: a seeded-growth type-II system,” Small 4(9), 1319–1323 (2008). [CrossRef] [PubMed]

].

Another improvement of the conversion efficiency concerns infiltration. It is also possible to use infiltration only for coating the inner wall of the hollow core in the HCPCF [32

32. S. Smolka, M. Barth, and O. Benson, “Selectively coated photonic crystal fiber for highly sensitive fluorescence detection,” Appl. Phys. Lett. 90(11), 111101 (2007). [CrossRef]

]. In this case light guiding and a large NA are maintained by the photonic band-gap effect, yet scattering by air bubbles in the liquid is absent.

Finally, QDs with shorter lifetime would allow for a higher single photon excitation rate and at the same time would also reduce the problem of re-absorption. The following Fig. 8
Fig. 8 Overview of single photon sources realized so far and expected single photon count rates from incoherent photon conversion to the near infrared using our HCPCF system. The red cross marks the achieved conversion rate of 0.1 (blue line) and the resulting single photon counts of 280 cts/s for the conversion of NV-centers in diamond to CdHgTe QDs emitting at 850 nm. The black lines show the expected infrared single photon counts for ηtot = 1% and ηtot = 10%. These conversion efficiencies can be achieved with experimental improvements. The black vertical dotted lines represent the employed single photon source as well as other sources in the visible that have been realized [7,12], also indicated by an asterisk. The grey vertical dotted line gives an outlook of a possible implementation of NV-centers in a dielectric layer system with a collection efficiency of 99% as has been realized with single molecules [34]. Conversion with the present efficiency of such high flux would generate 80 kcts/s infrared single photons.
compares the state of the art of stable single photon sources in the visible with the source used in our experiment.

6. Conclusions

In conclusion, we suggested a method to convert single photons from the visible to the infrared by incoherent absorption and Stokes-shifted re-emission. We performed experimental studies to demonstrate feasibility of the method. Selectively infiltrated hollow-core photonic crystal fibers equipped with colloidal quantum dots are very promising systems for an implementation in an integrated platform. With this system we could realize a total conversion efficiency of 0.1%. Although, this number was yet too small to verify conversion of single photons from a true single photon source based on nitrogen-vacancy defect centers in diamond, it is still quite large compared to simply focusing and collecting with high-NA objectives. An estimation based on the absorption cross section of similar QDs, the maximum density of QDs in a spin-coated or drop-casted sample, and an NA of 0.95 reveals at least an order of magnitude better conversion efficiency with our approach.

With improvements by two orders of magnitude which are experimentally feasible as well as visible photon sources approaching 10 Mcts/s, the generation rate of single photons in the infrared could be as high as several 100 kcts/s which is better than existing true single photon sources in the infrared [8

8. P. M. Intallura, M. B. Ward, O. Z. Karimov, Z. L. Yuan, P. See, A. J. Shields, P. Atkinson, and D. A. Ritchie, “Quantum key distribution using a triggered quantum dot source emitting near 1.3 μm,” Appl. Phys. Lett. 91(16), 161103 (2007). [CrossRef]

]. With our suggested method single photon generation in the near infrared, e.g. the telecom band at around 1550nm, is accessible without the need to have quantum emitters in that wavelength regime.

Finally, we also would like to point out that the HCPCF can be filled with arbitrary material. Also cooling of the fiber is straightforward. In this way the re-emitted photons could have much better properties, e.g., in terms of linewidth, coherence, or indistinguishability than the photons that are converted. In this respect our experiments show that ultra-bright single photon sources even of poor optical quality may be very useful as pump sources to generate single quanta of excitation.

Acknowledgments

The authors acknowledge financial support by the Alexander von Humboldt-Stiftung (P.J.) and by the Deutsche Forschungsgemeinschaft through the Sfb 787.

References and links

1.

A. Kuhn, M. Hennrich, and G. Rempe, “Deterministic single-photon source for distributed quantum networking,” Phys. Rev. Lett. 89(6), 067901 (2002). [CrossRef] [PubMed]

2.

K. G. Lee, X. W. Chen, H. Eghlidi, P. Kukura, R. Lettow, A. Renn, V. Sandoghdar, and S. Gotzinger, “A planar dielectric antenna for directional single-photon emission and near-unity collection efficiency,” Nat. Photonics 5(3), 166–169 (2011). [CrossRef]

3.

C. Santori, M. Pelton, G. Solomon, Y. Dale, and Y. Yamamoto, “Triggered single photons from a quantum dot,” Phys. Rev. Lett. 86(8), 1502–1505 (2001). [CrossRef] [PubMed]

4.

A. Imamoğlu, P. Michler, M. D. Mason, P. J. Carson, G. F. Strouse, and S. K. Buratto, “Quantum correlation among photons from a single quantum dot at room temperature,” Nature 406(6799), 968–970 (2000). [CrossRef] [PubMed]

5.

C. Kurtsiefer, S. Mayer, P. Zarda, and H. Weinfurter, “Stable solid-state source of single photons,” Phys. Rev. Lett. 85(2), 290–293 (2000). [CrossRef] [PubMed]

6.

A. Beveratos, S. Kühn, R. Brouri, T. Gacoin, J. P. Poizat, and P. Grangier, “Room temperature stable single-photon source,” Eur. Phys. J. D 18(2), 191–196 (2002). [CrossRef]

7.

E. Neu, D. Steinmetz, J. Riedrich-Möller, S. Gsell, M. Fischer, M. Schreck, and C. Becher, “Single photon emission from silicon-vacancy colour centres in chemical vapour deposition nano-diamonds on iridium,” New J. Phys. 13(2), 025012 (2011). [CrossRef]

8.

P. M. Intallura, M. B. Ward, O. Z. Karimov, Z. L. Yuan, P. See, A. J. Shields, P. Atkinson, and D. A. Ritchie, “Quantum key distribution using a triggered quantum dot source emitting near 1.3 μm,” Appl. Phys. Lett. 91(16), 161103 (2007). [CrossRef]

9.

N. Sangouard, C. Simon, H. de Riedmatten, and N. Gisin, “Quantum repeaters based on atomic ensembles and linear optics,” Rev. Mod. Phys. 83(1), 33–80 (2011). [CrossRef]

10.

M. B. Ward, T. Farrow, P. See, Z. L. Yuan, O. Z. Karimov, A. J. Bennett, A. J. Shields, P. Atkinson, K. Cooper, and D. A. Ritchie, “Electrically driven telecommunication wavelength single-photon source,” Appl. Phys. Lett. 90(6), 063512 (2007). [CrossRef]

11.

J. J. Peterson and T. D. Krauss, “Fluorescence spectroscopy of single lead sulfide quantum dots,” Nano Lett. 6(3), 510–514 (2006). [CrossRef] [PubMed]

12.

T. Schröder, F. Gädeke, M. J. Banholzer, and O. Benson, “Ultrabright and efficient single-photon generation based on nitrogen-vacancy centres in nanodiamonds on a solid immersion lens,” New J. Phys. 13(5), 055017 (2011). [CrossRef]

13.

D. K. Harris, P. M. Allen, H. S. Han, B. J. Walker, J. Lee, and M. G. Bawendi, “Synthesis of cadmium arsenide quantum dots luminescent in the infrared,” J. Am. Chem. Soc. 133(13), 4676–4679 (2011). [CrossRef] [PubMed]

14.

M. Barth, H. Bartelt, and O. Benson, “Fluid-filled optical fibers” in Handbook of Optofluidics, edts. A. R. Hawkins, H. Schmidt, CRC Press Tylor & Francis (2010).

15.

F. M. Cox, A. Argyros, and M. C. J. Large, “Liquid-filled hollow core microstructured polymer optical fiber,” Opt. Express 14(9), 4135–4140 (2006). [CrossRef] [PubMed]

16.

S. Smolka, M. Barth, and O. Benson, “Highly efficient fluorescence sensing with hollow core photonic crystal fibers,” Opt. Express 15(20), 12783–12791 (2007). [CrossRef] [PubMed]

17.

S. O. Konorov, C. J. Addison, H. G. Schulze, R. F. B. Turner, and M. W. Blades, “Hollow-core photonic crystal fiber-optic probes for Raman spectroscopy,” Opt. Lett. 31(12), 1911–1913 (2006). [CrossRef] [PubMed]

18.

Y. Zhang, C. Shi, C. Gu, L. Seballos, and J. Z. Zhang, “Liquid core photonic crystal fiber sensor based on surface enhanced Raman scattering,” Appl. Phys. Lett. 90(19), 193504 (2007). [CrossRef]

19.

X. Yang, C. Shi, D. Wheeler, R. Newhouse, B. Chen, J. Z. Zhang, and C. Gu, “High-sensitivity molecular sensing using hollow-core photonic crystal fiber and surface-enhanced Raman scattering,” J. Opt. Soc. Am. A 27(5), 977–984 (2010). [CrossRef] [PubMed]

20.

R. F. Cregan, B. J. Mangan, J. C. Knight, T. A. Birks, P. S. J. Russell, P. J. Roberts, and D. C. Allan, “Single-mode photonic band gap guidance of light in air,” Science 285(5433), 1537–1539 (1999). [CrossRef] [PubMed]

21.

V. Lesnyak, A. Lutich, N. Gaponik, M. Grabolle, A. Plotnikov, U. Resch-Genger, and A. Eychmüller, “One-pot aqueous synthesis of high quality near infrared emitting Cd1_xHgxTe nanocrystals,” J. Mater. Chem. 19(48), 9147–9152 (2009). [CrossRef]

22.

H. Zhang, Z. Cui, Y. Wang, K. Zhang, X. Ji, C. Lü, B. Yang, and M. Gao, “From water-soluble CdTe nanocrystals to fluorescent nanocrystal–polymer transparent composites using polymerizable Surfactants,” Adv. Mater. (Deerfield Beach Fla.) 15(10), 777–780 (2003). [CrossRef]

23.

E. F. Chillcce, C. M. B. Cordeiro, L. C. Barbosa, and C. H. Brito Cruz, “Tellurite photonic crystal fiber made by a stack-and-draw technique,” J. Non-Cryst. Solids 352(32-35), 3423–3428 (2006). [CrossRef]

24.

L. Xiao, W. Jin, M. S. Demokan, H. L. Ho, Y. L. Hoo, and C. L. Zhao, “Fabrication of selective injection microstructured optical fibers with a conventional fusion splicer,” Opt. Express 13(22), 9014–9022 (2005). [CrossRef] [PubMed]

25.

H. J. Kimble, M. Dagenais, and L. Mandel, “Photon antibunching in resonance fluorescence,” Phys. Rev. Lett. 39(11), 691–695 (1977). [CrossRef]

26.

D. Dorfs, T. Franzl, R. Osovsky, M. Brumer, E. Lifshitz, T. Klar, and A. Eychmüller, “Type-I and Type-II nanoheterostructures based on CdTe nanocrystals – a comparative study,” Small 4, 1148–1153 (2008).

27.

S. Kim, B. Fisher, H.-J. Eisler, and M. Bawendi, “Type-II quantum dots: CdTe/CdSe(core/shell) and CdSe/ZnTe(core/shell) heterostructures,” J. Am. Chem. Soc. 125(38), 11466–11467 (2003). [CrossRef] [PubMed]

28.

V. I. Klimov, S. A. Ivanov, J. Nanda, M. Achermann, I. Bezel, J. A. McGuire, and A. Piryatinski, “Single-exciton optical gain in semiconductor nanocrystals,” Nature 447(7143), 441–446 (2007). [CrossRef] [PubMed]

29.

D. Oron, M. Kazes, and U. Banin, “Multiexcitons in type-II colloidal semiconductor quantum dots,” Phys. Rev. B 75(3), 035330 (2007). [CrossRef]

30.

S. Kumar, M. Jones, S. S. Lo, and G. D. Scholes, “Nanorod heterostructures showing photoinduced charge separation,” Small 3(9), 1633–1639 (2007). [CrossRef] [PubMed]

31.

D. Dorfs, A. Salant, I. Popov, and U. Banin, “ZnSe quantum dots within CdS nanorods: a seeded-growth type-II system,” Small 4(9), 1319–1323 (2008). [CrossRef] [PubMed]

32.

S. Smolka, M. Barth, and O. Benson, “Selectively coated photonic crystal fiber for highly sensitive fluorescence detection,” Appl. Phys. Lett. 90(11), 111101 (2007). [CrossRef]

33.

P. Neumann, R. Kolesov, V. Jacques, J. Beck, J. Tisler, A. Batalov, L. Rogers, N. B. Manson, G. Balasubramanian, F. Jelezko, and J. Wrachtrup, “Excited-state spectroscopy of single NV defects in diamond using optically detected magnetic resonance,” New J. Phys. 11(1), 013017 (2009). [CrossRef]

34.

X. W. Chen, S. Götzinger, and V. Sandoghdar, “99% efficiency in collecting photons from a single emitter,” Opt. Lett. 36(18), 3545–3547 (2011). [CrossRef] [PubMed]

OCIS Codes
(230.5590) Optical devices : Quantum-well, -wire and -dot devices
(230.6080) Optical devices : Sources
(300.2140) Spectroscopy : Emission
(300.6340) Spectroscopy : Spectroscopy, infrared
(060.5295) Fiber optics and optical communications : Photonic crystal fibers
(230.7405) Optical devices : Wavelength conversion devices

ToC Category:
Fiber Optics and Optical Communications

History
Original Manuscript: March 23, 2012
Revised Manuscript: April 17, 2012
Manuscript Accepted: April 17, 2012
Published: May 4, 2012

Citation
Ping Jiang, Tim Schroeder, Michael Bath, Vladimir Lesnyak, Nikolai Gaponik, Alexander Eychmüller, and Oliver Benson, "Incoherent photon conversion in selectively infiltrated hollow-core photonic crystal fibers for single photon generation in the near infrared," Opt. Express 20, 11536-11547 (2012)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-20-10-11536


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References

  1. A. Kuhn, M. Hennrich, and G. Rempe, “Deterministic single-photon source for distributed quantum networking,” Phys. Rev. Lett.89(6), 067901 (2002). [CrossRef] [PubMed]
  2. K. G. Lee, X. W. Chen, H. Eghlidi, P. Kukura, R. Lettow, A. Renn, V. Sandoghdar, and S. Gotzinger, “A planar dielectric antenna for directional single-photon emission and near-unity collection efficiency,” Nat. Photonics5(3), 166–169 (2011). [CrossRef]
  3. C. Santori, M. Pelton, G. Solomon, Y. Dale, and Y. Yamamoto, “Triggered single photons from a quantum dot,” Phys. Rev. Lett.86(8), 1502–1505 (2001). [CrossRef] [PubMed]
  4. A. Imamoğlu, P. Michler, M. D. Mason, P. J. Carson, G. F. Strouse, and S. K. Buratto, “Quantum correlation among photons from a single quantum dot at room temperature,” Nature406(6799), 968–970 (2000). [CrossRef] [PubMed]
  5. C. Kurtsiefer, S. Mayer, P. Zarda, and H. Weinfurter, “Stable solid-state source of single photons,” Phys. Rev. Lett.85(2), 290–293 (2000). [CrossRef] [PubMed]
  6. A. Beveratos, S. Kühn, R. Brouri, T. Gacoin, J. P. Poizat, and P. Grangier, “Room temperature stable single-photon source,” Eur. Phys. J. D18(2), 191–196 (2002). [CrossRef]
  7. E. Neu, D. Steinmetz, J. Riedrich-Möller, S. Gsell, M. Fischer, M. Schreck, and C. Becher, “Single photon emission from silicon-vacancy colour centres in chemical vapour deposition nano-diamonds on iridium,” New J. Phys.13(2), 025012 (2011). [CrossRef]
  8. P. M. Intallura, M. B. Ward, O. Z. Karimov, Z. L. Yuan, P. See, A. J. Shields, P. Atkinson, and D. A. Ritchie, “Quantum key distribution using a triggered quantum dot source emitting near 1.3 μm,” Appl. Phys. Lett.91(16), 161103 (2007). [CrossRef]
  9. N. Sangouard, C. Simon, H. de Riedmatten, and N. Gisin, “Quantum repeaters based on atomic ensembles and linear optics,” Rev. Mod. Phys.83(1), 33–80 (2011). [CrossRef]
  10. M. B. Ward, T. Farrow, P. See, Z. L. Yuan, O. Z. Karimov, A. J. Bennett, A. J. Shields, P. Atkinson, K. Cooper, and D. A. Ritchie, “Electrically driven telecommunication wavelength single-photon source,” Appl. Phys. Lett.90(6), 063512 (2007). [CrossRef]
  11. J. J. Peterson and T. D. Krauss, “Fluorescence spectroscopy of single lead sulfide quantum dots,” Nano Lett.6(3), 510–514 (2006). [CrossRef] [PubMed]
  12. T. Schröder, F. Gädeke, M. J. Banholzer, and O. Benson, “Ultrabright and efficient single-photon generation based on nitrogen-vacancy centres in nanodiamonds on a solid immersion lens,” New J. Phys.13(5), 055017 (2011). [CrossRef]
  13. D. K. Harris, P. M. Allen, H. S. Han, B. J. Walker, J. Lee, and M. G. Bawendi, “Synthesis of cadmium arsenide quantum dots luminescent in the infrared,” J. Am. Chem. Soc.133(13), 4676–4679 (2011). [CrossRef] [PubMed]
  14. M. Barth, H. Bartelt, and O. Benson, “Fluid-filled optical fibers” in Handbook of Optofluidics, edts. A. R. Hawkins, H. Schmidt, CRC Press Tylor & Francis (2010).
  15. F. M. Cox, A. Argyros, and M. C. J. Large, “Liquid-filled hollow core microstructured polymer optical fiber,” Opt. Express14(9), 4135–4140 (2006). [CrossRef] [PubMed]
  16. S. Smolka, M. Barth, and O. Benson, “Highly efficient fluorescence sensing with hollow core photonic crystal fibers,” Opt. Express15(20), 12783–12791 (2007). [CrossRef] [PubMed]
  17. S. O. Konorov, C. J. Addison, H. G. Schulze, R. F. B. Turner, and M. W. Blades, “Hollow-core photonic crystal fiber-optic probes for Raman spectroscopy,” Opt. Lett.31(12), 1911–1913 (2006). [CrossRef] [PubMed]
  18. Y. Zhang, C. Shi, C. Gu, L. Seballos, and J. Z. Zhang, “Liquid core photonic crystal fiber sensor based on surface enhanced Raman scattering,” Appl. Phys. Lett.90(19), 193504 (2007). [CrossRef]
  19. X. Yang, C. Shi, D. Wheeler, R. Newhouse, B. Chen, J. Z. Zhang, and C. Gu, “High-sensitivity molecular sensing using hollow-core photonic crystal fiber and surface-enhanced Raman scattering,” J. Opt. Soc. Am. A27(5), 977–984 (2010). [CrossRef] [PubMed]
  20. R. F. Cregan, B. J. Mangan, J. C. Knight, T. A. Birks, P. S. J. Russell, P. J. Roberts, and D. C. Allan, “Single-mode photonic band gap guidance of light in air,” Science285(5433), 1537–1539 (1999). [CrossRef] [PubMed]
  21. V. Lesnyak, A. Lutich, N. Gaponik, M. Grabolle, A. Plotnikov, U. Resch-Genger, and A. Eychmüller, “One-pot aqueous synthesis of high quality near infrared emitting Cd1_xHgxTe nanocrystals,” J. Mater. Chem.19(48), 9147–9152 (2009). [CrossRef]
  22. H. Zhang, Z. Cui, Y. Wang, K. Zhang, X. Ji, C. Lü, B. Yang, and M. Gao, “From water-soluble CdTe nanocrystals to fluorescent nanocrystal–polymer transparent composites using polymerizable Surfactants,” Adv. Mater. (Deerfield Beach Fla.)15(10), 777–780 (2003). [CrossRef]
  23. E. F. Chillcce, C. M. B. Cordeiro, L. C. Barbosa, and C. H. Brito Cruz, “Tellurite photonic crystal fiber made by a stack-and-draw technique,” J. Non-Cryst. Solids352(32-35), 3423–3428 (2006). [CrossRef]
  24. L. Xiao, W. Jin, M. S. Demokan, H. L. Ho, Y. L. Hoo, and C. L. Zhao, “Fabrication of selective injection microstructured optical fibers with a conventional fusion splicer,” Opt. Express13(22), 9014–9022 (2005). [CrossRef] [PubMed]
  25. H. J. Kimble, M. Dagenais, and L. Mandel, “Photon antibunching in resonance fluorescence,” Phys. Rev. Lett.39(11), 691–695 (1977). [CrossRef]
  26. D. Dorfs, T. Franzl, R. Osovsky, M. Brumer, E. Lifshitz, T. Klar, and A. Eychmüller, “Type-I and Type-II nanoheterostructures based on CdTe nanocrystals – a comparative study,” Small4, 1148–1153 (2008).
  27. S. Kim, B. Fisher, H.-J. Eisler, and M. Bawendi, “Type-II quantum dots: CdTe/CdSe(core/shell) and CdSe/ZnTe(core/shell) heterostructures,” J. Am. Chem. Soc.125(38), 11466–11467 (2003). [CrossRef] [PubMed]
  28. V. I. Klimov, S. A. Ivanov, J. Nanda, M. Achermann, I. Bezel, J. A. McGuire, and A. Piryatinski, “Single-exciton optical gain in semiconductor nanocrystals,” Nature447(7143), 441–446 (2007). [CrossRef] [PubMed]
  29. D. Oron, M. Kazes, and U. Banin, “Multiexcitons in type-II colloidal semiconductor quantum dots,” Phys. Rev. B75(3), 035330 (2007). [CrossRef]
  30. S. Kumar, M. Jones, S. S. Lo, and G. D. Scholes, “Nanorod heterostructures showing photoinduced charge separation,” Small3(9), 1633–1639 (2007). [CrossRef] [PubMed]
  31. D. Dorfs, A. Salant, I. Popov, and U. Banin, “ZnSe quantum dots within CdS nanorods: a seeded-growth type-II system,” Small4(9), 1319–1323 (2008). [CrossRef] [PubMed]
  32. S. Smolka, M. Barth, and O. Benson, “Selectively coated photonic crystal fiber for highly sensitive fluorescence detection,” Appl. Phys. Lett.90(11), 111101 (2007). [CrossRef]
  33. P. Neumann, R. Kolesov, V. Jacques, J. Beck, J. Tisler, A. Batalov, L. Rogers, N. B. Manson, G. Balasubramanian, F. Jelezko, and J. Wrachtrup, “Excited-state spectroscopy of single NV defects in diamond using optically detected magnetic resonance,” New J. Phys.11(1), 013017 (2009). [CrossRef]
  34. X. W. Chen, S. Götzinger, and V. Sandoghdar, “99% efficiency in collecting photons from a single emitter,” Opt. Lett.36(18), 3545–3547 (2011). [CrossRef] [PubMed]

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