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Optics Express

Optics Express

  • Editor: C. Martijn de Sterke
  • Vol. 17, Iss. 25 — Dec. 7, 2009
  • pp: 22474–22483
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Cryogenic spectroscopy of ultra-low density colloidal lead chalcogenide quantum dots on chip-scale optical cavities towards single quantum dot near-infrared cavity QED

Ranojoy Bose, Jie Gao, James F. McMillan, Alex D. Williams, and Chee Wei Wong  »View Author Affiliations


Optics Express, Vol. 17, Issue 25, pp. 22474-22483 (2009)
http://dx.doi.org/10.1364/OE.17.022474


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Abstract

We present evidence of cavity quantum electrodynamics from a sparse density of strongly quantum-confined Pb-chalcogenide nanocrystals (between 1 and 10) approaching single-dot levels on moderately high-Q mesoscopic silicon optical cavities. Operating at important near-infrared (1500-nm) wavelengths, large enhancements are observed from devices and strong modifications of the QD emission are achieved. Saturation spectroscopy of coupled QDs is observed at 77K, highlighting the modified nanocrystal dynamics for quantum information processing.

© 2009 OSA

1. Introduction

Cavity quantum electrodynamics (cQED) experiments have been strongly motivated by the need for an efficient on-demand single-photon source that is relevant in many quantum cryptography and quantum information processing applications [1

1. H. Mabuchi and A. C. Doherty, “Cavity quantum electrodynamics: coherence in context,” Science 298(5597), 1372–1377 ( 2002). [CrossRef] [PubMed]

4

4. Y.-F. Xiao, J. Gao, X.-B. Zou, J. F. McMillan, X. Yang, Y.-L. Chen, Z.-F. Han, G.-C. Guo, and C. W. Wong, “Coupled quantum electrodynamics in photonic crystal cavities towards controlled phase gate operations,” N. J. Phys. 10(12), 123013 ( 2008). [CrossRef]

]. Charged carriers, or excitons, in single quantum dots (QD) can be excited in a controllable way to inhibit multi-photon emission, enabling a sub-Poissonian single-photon source [5

5. M. Pelton, C. Santori, J. Vucković, B. Zhang, G. S. Solomon, J. Plant, and Y. Yamamoto, “Efficient source of single photons: a single quantum dot in a micropost microcavity,” Phys. Rev. Lett. 89(23), 233602 ( 2002). [CrossRef] [PubMed]

,6

6. 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(12), 704–708 ( 2007). [CrossRef]

]. Coupling of QDs to photonic structures result in a faster recombination rate for the excitons—overcoming problems due to decoherence in single photon source applications [7

7. C. Santori, D. Fattal, J. Vucković, G. S. Solomon, and Y. Yamamoto, “Indistinguishable photons from a single-photon device,” Nature 419(6907), 594–597 ( 2002). [CrossRef] [PubMed]

,8

8. F. W. Sun and C. W. Wong, “Indistinguishability of independent single photons,” Phys. Rev. A 79(1), 013824 ( 2009). [CrossRef]

].

As an alternative to the highly mature domain of self-assembled QDs [9

9. A. Faraon, I. Fushman, D. Englund, N. Stoltz, P. Petroff, and J. Vučković, “Coherent generation of nonclassical light on a chip via photon-induced tunneling and blockade,” Nat. Phys. 4(11), 859–863 ( 2008). [CrossRef]

11

11. J. P. Reithmaier, G. Sęk, A. Löffler, C. Hofmann, S. Kuhn, S. Reitzenstein, L. V. Keldysh, V. D. Kulakovskii, T. L. Reinecke, and A. Forchel, “Strong coupling in a single quantum dot-semiconductor microcavity system,” Nature 432(7014), 197–200 ( 2004). [CrossRef] [PubMed]

] to study cQED in photonic structures, several novel systems have been proposed—such as nitrogen-vacancy centers in diamond [12

12. M. V. Dutt, L. Childress, L. Jiang, E. Togan, J. Maze, F. Jelezko, A. S. Zibrov, P. R. Hemmer, and M. D. Lukin, “Quantum register based on individual electronic and nuclear spin qubits in diamond,” Science 316(5829), 1312–1316 ( 2007). [CrossRef] [PubMed]

,13

13. Y. Shen, T. M. Sweeney, and H. Wang, “Zero-phonon linewidth of single nitrogen vacancy centers in diamond nanocrystals,” Phys. Rev. B 77(3), 033201 ( 2008). [CrossRef]

] and strongly quantum-confined nanomaterials such as III-nitrides binary materials with large optical phonon energies to operate at 200K temperatures [14

14. S. Kako, C. Santori, K. Hoshino, S. Götzinger, Y. Yamamoto, and Y. Arakawa, “A gallium nitride single-photon source operating at 200 K,” Nat. Mater. 5(11), 887–892 ( 2006). [CrossRef] [PubMed]

]. Nanocrystals formed through synthetic routes, such as CdSe and PbS nanocrystals, have also been used in successfully demonstrating coupled QD-cavity interactions in photonic structures [15

15. C. B. Poitras, M. Lipson, M. A. Hahn, H. Du, and T. D. Krauss, “Photoluminescence enhancement of colloidal quantum dots embedded in a monolithic microcavity,” Appl. Phys. Lett. 82(23), 4032 ( 2003). [CrossRef]

,16

16. I. Fushman, D. Englund, and J. Vučković, “Coupling of PbS quantum dots to photonic crystal cavities at room temperature,” Appl. Phys. Lett. 87(24), 241102 ( 2005). [CrossRef]

], and offer important possibilities towards scalable quantum computation that are presently unprecedented in self-assembled technologies. Particularly promising candidates for QED applications are the Pb-chacolgenide nanocrystals, that can be post-integrated with the vast silicon processing infrastructure [17

17. R. Bose, X. Yang, R. Chatterjee, J. Gao, and C. W. Wong, “Weak coupling interactions of colloidal lead sulphide nanocrystals with silicon photonic crystal nanocavities near 1.55 μm at room temperature,” Appl. Phys. Lett. 90(11), 111117 ( 2007). [CrossRef]

,18

18. Z. Wu, Z. Mi, P. Bhattacharya, T. Zhu, and J. Xu, “Enhanced spontaneous emission at 1.55 μm from colloidal PbSe quantum dots in a Si photonic crystal microcavity,” Appl. Phys. Lett. 90(17), 171105 ( 2007). [CrossRef]

], spatially positioned through electron-beam lithography in a host resist matrix [16

16. I. Fushman, D. Englund, and J. Vučković, “Coupling of PbS quantum dots to photonic crystal cavities at room temperature,” Appl. Phys. Lett. 87(24), 241102 ( 2005). [CrossRef]

,17

17. R. Bose, X. Yang, R. Chatterjee, J. Gao, and C. W. Wong, “Weak coupling interactions of colloidal lead sulphide nanocrystals with silicon photonic crystal nanocavities near 1.55 μm at room temperature,” Appl. Phys. Lett. 90(11), 111117 ( 2007). [CrossRef]

,19

19. R. Bose, D. V. Talapin, X. Yang, R. J. Harniman, P. T. Nguyen, and C. W. Wong, “Interaction of infilitrated colloidal PbS nanocrystals with high Q/V silicon photonic bandgap nanocavities for near-infrared enhanced spontaneous emissions,” Proc. SPIE 6005, 600509 ( 2005). [CrossRef]

] or other novel techniques [20

20. A. G. Pattantyus-Abraham, H. Qiao, J. Shan, K. A. Abel, T.-S. Wang, F. C. J. M. van Veggel, and J. F. Young, “Site-selective optical coupling of PbSe nanocrystals to Si-based photonic crystal microcavities,” Nano Lett. 9(8), 2849–2854 ( 2009). [CrossRef] [PubMed]

,21

21. S. Vignolini, F. Riboli, F. Intonti, M. Belotti, M. Gurioli, Y. Chen, M. Colocci, L. C. Andreani, and D. S. Wiersma, “Local nanofluidic light sources in silicon photonic crystal microcavities,” Phys. Rev. E Stat. Nonlin. Soft Matter Phys. 78(4), 045603 ( 2008). [CrossRef] [PubMed]

], and have exciton ground state transitions in the near-infrared (1.55-µm) for direct compatibility with the embedded fiber communications network. PbS (Se) QDs also exhibit large exciton Bohr radii compared to the physical dot sizes, resulting in strong quantum confinement [22

22. A. I. Akimov, “Al. L. Efros, A. A. Onushchenko, “Quantum size effect in semiconductor nanocrystals,” Solid State Commun. 56, 921 ( 1985).

24

24. F. W. Wise, “Lead salt quantum dots: the limit of strong quantum confinement,” Acc. Chem. Res. 33(11), 773–780 ( 2000). [CrossRef] [PubMed]

], and devices incorporating these QDs are capable of room temperature operation [16

16. I. Fushman, D. Englund, and J. Vučković, “Coupling of PbS quantum dots to photonic crystal cavities at room temperature,” Appl. Phys. Lett. 87(24), 241102 ( 2005). [CrossRef]

21

21. S. Vignolini, F. Riboli, F. Intonti, M. Belotti, M. Gurioli, Y. Chen, M. Colocci, L. C. Andreani, and D. S. Wiersma, “Local nanofluidic light sources in silicon photonic crystal microcavities,” Phys. Rev. E Stat. Nonlin. Soft Matter Phys. 78(4), 045603 ( 2008). [CrossRef] [PubMed]

] allowing remarkable possibilities for integrated photon sources. Prior work in silicon cavities has been limited to large ensembles of QDs (>10,000) [16

16. I. Fushman, D. Englund, and J. Vučković, “Coupling of PbS quantum dots to photonic crystal cavities at room temperature,” Appl. Phys. Lett. 87(24), 241102 ( 2005). [CrossRef]

21

21. S. Vignolini, F. Riboli, F. Intonti, M. Belotti, M. Gurioli, Y. Chen, M. Colocci, L. C. Andreani, and D. S. Wiersma, “Local nanofluidic light sources in silicon photonic crystal microcavities,” Phys. Rev. E Stat. Nonlin. Soft Matter Phys. 78(4), 045603 ( 2008). [CrossRef] [PubMed]

] due to poorer detection efficiency of near-infrared photon detectors as well as the long lifetimes of the PbS QDs, reported to be around 1-2 μs in solution [25

25. J. Warner, E. Thomsen, A. R. Watt, N. R. Heckenberg, and H. Rubinsztein-Dunlop, “Time-resolved photoluminescence spectroscopy of ligand-capped PbS nanocrystals,” Nanotechology 16(2), 175–179 ( 2005). [CrossRef]

28

28. R. Bose, R. J. F. McMillan, J. Gao, C. J. Chen, D. V. Talapin, C. B. Murray, K. M. Rickey, and C. W. Wong, “Temperature-tuning of near-infrared monodisperse quantum dots at 1.5 μm for controllable Förster energy transfer,” Nano Lett. 8(7), 2006–2011 ( 2008). [CrossRef] [PubMed]

] and between 100 ns (at 1500 nm) [27

27. L. Cademartiri, J. Bertolotti, R. Sapienza, D. S. Wiersma, G. von Freymann, and G. A. Ozin, “Multigram scale, solventless, and diffusion-controlled route to highly monodisperse PbS nanocrystals,” J. Phys. Chem. B 110(2), 671–673 ( 2006). [CrossRef] [PubMed]

] and 2 μs (at 900 nm) [26

26. S. W. Clark, J. M. Harbold, and F. W. Wise, “Resonant energy transfer in PbS quantum dots,” J. Phys. Chem. C 111(20), 7302–7305 ( 2007). [CrossRef]

] in films. Here we examine a few Pb-chalcogenide QDs coupled to moderately high-Q heterostructured photonic crystal cavities approaching the single quantum dot limit, enabled through coupling interactions of the QDs with the cavities. Through non-resonant photoluminescence spectroscopy, excited state saturation is observed at 77 K as an effort to determine the Purcell factor [29

29. E. M. Purcell, “Spontaneous emission probabilities at radio frequencies,” Phys. Rev. 69, 681 ( 1946).

], with large (15 × ) emission enhancements observed for the few QDs on-resonance with the cavity mode.

2. Cavity system

Our mesoscopic optical cavity (Fig. 1a
Fig. 1 (a) Scanning electron micrograph (SEM) of a multistep heterostructure cavity with few quantum dots at the cavity, on the device surface. (b) Schematic of the mode-gap cavity confinement. The white regions show photonic bandgaps for in-plane wavevectors. The curve represents 1-D field intensity (|E x|2) computed using 3D FDTD (c) Top- and side views of the cavity field-mode (|E x|2) computed with the 3D FDTD method. r = 0.3024a1, where the other definitions are the same as in Fig. a.
) consists of a silicon-on-insulator heterostructured photonic crystal lattice [30

30. T. Tanabe, M. Notomi, E. Kuramochi, A. Shinya, and H. Taniyama, “Trapping and delaying photons for one nanosecond in an ultrasmall high-Q photonic-crystal nanocavity,” Nat. Photonics 1(1), 49–52 ( 2007). [CrossRef]

,31

31. S. Noda, M. Fujita, and T. Asano, “Spontaneous-emission control by photonic crystals and nanocavities,” Nat. Photonics 1(8), 449–458 ( 2007). [CrossRef]

], which confines modes exhibiting wavelength-scale [~1.2(λ/n)3] volumes. Our design has heterostructured lattices a1, a2, and a3 of 410, 415 and 420 nm respectively to achieve mode-gap-type confinement. These designed optical cavities offer a smoother electric field envelope function at the heterostructure boundaries for higher Qs, differing from earlier QD-cavity studies involving point-defect cavities. The optical cavity has designed air hole radii r of (118-, 124-, and 130-nm) with a thickness t of 0.61 a1 (250 nm) on a SiO2 insulator substrate. The designed cavity field profiles (Ex which are the dominant modes) (Fig. 1c) are calculated from complete 3D finite-difference time-domain (FDTD) that solves the time-dependent Maxwell’s equations with subpixel accuracy [32

32. A. Farjadpour, D. Roundy, A. Rodriguez, M. Ibanescu, P. Bermel, J. D. Joannopoulos, S. G. Johnson, and G. W. Burr, “Improving accuracy by subpixel smoothing in the finite-difference time domain,” Opt. Lett. 31(20), 2972–2974 ( 2006). [CrossRef] [PubMed]

]. The side profiles of the fields emphasize that while the surface QDs do not see the field maximum, they are still able to couple effectively into the mode due to the evanescent field at the silicon/air interface, with 44% of the field amplitude at the silicon/air interface instead of the cavity maxima. The sparse density of surface QDs (Fig. 1a) does not significantly change the cavity mode field distribution.

3. Device fabrication and experiments

Our samples are fabricated in a 248 nm lithography CMOS foundry, with low (sub-20 Å) statistically quantified disorder [33

33. S. Kocaman, R. Chatterjee, N. C. Panoiu, J. F. McMillan, M. B. Yu, R. M. Osgood, D. L. Kwong, and C. W. Wong, “Observations of zero-order bandgaps in negative-index photonic crystal superlattices at the near-infrared,” Phys. Rev. Lett. 102, 203905 ( 2009). [CrossRef] [PubMed]

]. This fabrication technique allows for on-chip devices with near-identical device performances and quality factors for the same design, and validates the mature silicon CMOS infrastructure for fabricating near-infrared optical components. The cavities consist of two primary designs (6- or 40-linearly missing holes), with varying waveguide-cavity coupling for planar characterizations. We use resonant cross-polarization spectroscopy [34

34. M. W. McCutcheon, G. W. Rieger, I. W. Cheung, J. F. Young, D. Dalacu, S. Frederick, P. J. Poole, G. C. Aers, and R. L. Williams, “Resonant scattering and second-harmonic spectroscopy of planar photonic crystal nanocavities,” Appl. Phys. Lett. 87(22), 221110 ( 2005). [CrossRef]

,35

35. P. B. Deotare, M. W. McCutcheon, I. W. Frank, M. Khan, and M. Loncar, “High quality factor photonic crystal nanobeam cavities,” Appl. Phys. Lett. 94(12), 121106 ( 2009). [CrossRef]

] for characterization of the cavity mode and find this technique to be more reliable than traditional waveguide based measurements for passively characterizing the system, due to the inherent device architecture. The cavities typically exhibit Qs between 200 and 500, although our 3D FDTD calculations predict higher Qs (theoretical Q 5240). The discrepancy is primarily due to fabrication for a cavity mode that admits low tolerance in error. The colloidal Pb-chalcogenide (PbS) QDs are synthesized using standard methods [36

36. C. B. Murray, S. Sun, W. Gaschler, H. Doyle, T. A. Betley, and C. R. Kagan, “Colloidal synthesis of nanocrystals and nanocrystal superlattices,” IBM J. Res. Dev. 45, 47 ( 2001). [CrossRef]

38

38. D. V. Talapin and C. B. Murray, “PbSe nanocrystal solids for n- and p-channel thin film field-effect transistors,” Science 310(5745), 86–89 ( 2005). [CrossRef] [PubMed]

], dispersed in chloroform, and exhibit a photoluminescence spectrum centered at 1460 nm with a large spectral width Δλ of ~150 nm at room temperature with a <10% size dispersion. The QDs are obtained from Evident Technologies, and are carefully integrated through a spin-coating procedure that results in a sparse distribution of single dots on the device surface (Fig. 2a
Fig. 2 (a) SEM of less than 50 dots in the heterostructure cavity region. Circles are used to highlight regions of random QD localization after spin-coating. Scale bar: 500 nm (b) |E x|2 for the calculated field profile (r = 124 nm) at silicon slab surface. (c) Schematic of the vertical pump/collection experiment for QD coupling measurements as well as the cross-polarization measurements with the blue region being the region of the confined cavity mode. (d) PL spectra of dots at the cavity region for design radii of 130, 124- 118 nm dots (left to right, blue), at room temperature. The device shown in (a) corresponds to the second mode. Additionally a high resolution scan (1 nm) of the QD photoluminescence (gray) is shown for a large QD density at a cavity region for a device with design radius of 124 nm. The green curve is the passive cross-polarization characterization for the cavity shown at 1560 nm.
). Through passive cross-polarization characterization measurements of the cavities with and without quantum dots, we can confirm that the surface QD do not induce any degradation in the cavity Q.

PbS QDs are reported to possess a barrierless rapid Auger recombination that is controlled by the QD size [39

39. J. M. Pietryga, K. K. Zhuravlev, M. Whitehead, V. I. Klimov, and R. D. Schaller, “Evidence for barrierless auger recombination in PbSe nanocrystals: a pressure-dependent study of transient optical absorption,” Phys. Rev. Lett. 101(21), 217401 ( 2008). [CrossRef] [PubMed]

,40

40. J. C. Johnson, K. A. Gerth, Q. Song, J. E. Murphy, A. J. Nozik, and G. D. Scholes, “Ultrafast exciton fine structure relaxation dynamics in lead chalcogenide nanocrystals,” Nano Lett. 8(5), 1374–1381 ( 2008). [CrossRef] [PubMed]

], allowing multi-exciton suppression and antibunching for single-photon applications [41

41. P. Michler, A. Imamoğlu, 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]

,42

42. X. Brokmann, G. Messin, P. Desbiolles, E. Giaocobino, M. Dahan, and J. P. Hermier, “Colloidal CdSe/ZnS quantum dots as single-photon sources,” N. J. Phys. 6, 99 ( 2004). [CrossRef]

]. The Pb-chalcogenide system also has order-of-magnitude comparable oscillator strengths to CdSe colloidal QDs, supporting efforts in strong light-matter coupling [43

43. N. Le Thomas, U. Woggon, O. Schöps, M. V. Artemyev, M. Kazes, and U. Banin, “Cavity QED with semiconductor nanocrystals,” Nano Lett. 6(3), 557–561 ( 2006). [CrossRef] [PubMed]

45

45. J. M. An, A. Franceschetti, and A. Zunger, “The excitonic exchange splitting and radiative lifetime in PbSe quantum dots,” Nano Lett. 7(7), 2129–2135 ( 2007). [CrossRef]

]. At sub-1-um wavelengths with silicon detectors, recent measurements have remarkably isolated single Pb-chalcogenide QDs [46

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

], although the linewidths are unusually large, which may be a result of spectral diffusion and power broadening. At the longer 1.55 µm near-infrared communication wavelengths, detection of a few or a single Pb-chalcogenide QD is even more challenging, even with our state-of-the-art non-silicon detectors and avalanche photodiodes, and must rely on cavity-enhanced spontaneous emission dynamics to shorten the radiative lifetime for enhanced photon counts. As we show below, our cavity-enhanced experiments allow for the detection of approximately less than 10 QDs at 1.55 µm wavelengths.

The strongest intensity contrast is typically observed at the 124-nm hole radii samples, possibly due to QD availability at that wavelength. Due to the strong correlation between the QD-coupled and passive cavity spectra (which emphasizes TE polarization of the observed mode), we are confident that coupling occurs to the heterostructure cavity mode.

We note that AFM topography of the devices confirm that our PbS QD-induced surface roughness over the cavity is less than 4-nm (root-mean-squared), on parity with state-of-the-art InAs cavity-QD systems [48

48. A. Badolato, K. Hennessy, M. Atatüre, J. Dreiser, E. Hu, P. M. Petroff, and A. Imamoğlu, “Deterministic coupling of single quantum dots to single nanocavity modes,” Science 308(5725), 1158–1161 ( 2005). [CrossRef] [PubMed]

], and confirmed the very few QDs on the devices. We also do not observe any localization of QDs on the hole-sidewalls or on the bottom (Si/SiO2) interface.

4. Cryogenic measurements

We find in the QD spectroscopy measurements for this device that the cavity mode remains visible at all scanned temperatures, with intensity enhancements (2 to 4 × ) over the background QDs as shown in Fig. 4b. The observed enhancements are higher at low temperature. The cavity line has a slight blue-shift (60 pm/K) when cooled, and the blue-shift is attributed to the well-known silicon refractive index-dependence of 1.86 × 10−4 K −1 [51

51. G. T. Reed, and A. P. Knights, Silicon Photonics: An introduction (Wiley, 2004)

]. However the results are not typical of every device. At room temperature, the QDs at this density in all devices can couple effectively to the cavity modes through phonon-mediated interactions (room-temperature dephasing). At cryogenic temperatures, QD coupling to the cavity mode is observed to be less efficient with the mode and is only observed in 20% of devices which might arise from a reduced single dot linewidth at 4K, although this effect has not been demonstrated for single PbS QDs [46

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

] in literature. The effectiveness of the cavity in redirecting incoherent QD radiation at room-temperature (where the QD linewidths are broad) to the cavity mode is promising for many quantum information systems applications [52

52. A. Auffèves, J.-M. Gérard, and J.-P. Poizat, “Pure emitter dephasing: a resource for advanced solid-state single-photon sources,” Phys. Rev. A 79(5), 053838 ( 2009). [CrossRef]

].

Due to the moderate Q of the devices, the maximum Purcell factors [27

27. L. Cademartiri, J. Bertolotti, R. Sapienza, D. S. Wiersma, G. von Freymann, and G. A. Ozin, “Multigram scale, solventless, and diffusion-controlled route to highly monodisperse PbS nanocrystals,” J. Phys. Chem. B 110(2), 671–673 ( 2006). [CrossRef] [PubMed]

],
FP=34π2(λn)3(QV)
(1)
are calculated to be between 12 and 20. The Purcell factor expression is ideally suited for a simple two-level system, and it remains to be verified whether the complex exciton structure of PbS QDs [45

45. J. M. An, A. Franceschetti, and A. Zunger, “The excitonic exchange splitting and radiative lifetime in PbSe quantum dots,” Nano Lett. 7(7), 2129–2135 ( 2007). [CrossRef]

,53

53. I. Kang and F. W. Wise, “Electron structure and optical properties of PbS and PbSe quantum dots,” J. Opt. Soc. Am. B 14(7), 1632 ( 1997). [CrossRef]

,54

54. G. Allan and C. Delerue, “Confinement effects in PbSe quantum wells and nanocrystals,” Phys. Rev. B 70(24), 245321 ( 2004). [CrossRef]

] presents a different domain for cavity QED. The high visibility (10-15 × ) of the very low density of QDs far above the uncoupled QDs is not purely through enhanced collection efficiencies for the cavity-emitted photons (<8%), and strongly suggest spontaneous emission enhancements.

5. Saturation Spectroscopy

In order to further support this spontaneous emission enhancement,, a delayed onset of emission saturation for QDs coupled to the cavity mode can be used to demonstrate the Purcell effect [48

48. A. Badolato, K. Hennessy, M. Atatüre, J. Dreiser, E. Hu, P. M. Petroff, and A. Imamoğlu, “Deterministic coupling of single quantum dots to single nanocavity modes,” Science 308(5725), 1158–1161 ( 2005). [CrossRef] [PubMed]

,55

55. J. M. Gérard and B. Gayral, “Strong Purcell effect for InAs quantum boxes in three-dimensional solid-state microcavities,” J. Lightwave Technol. 17(11), 2089–2095 ( 1999). [CrossRef]

] based on the idea that a QD coupled well with the cavity mode exhibits a faster radiative recombination rate through the Purcell effect. On resonance with the cavity, mode, it should therefore take more photons to saturate the QD (ground state) emission. This approach was chosen instead of a direct QD lifetime measurement due to the low photon counts and higher dark counts in the near-infrared. We perform saturation spectroscopy of few QDs coupled to the cavity at 1.55-um wavelengths at 77K, comparing between dots at the resonance peak and away from the resonance peak as the only experimental possibilities. Figure 5a
Fig. 5 (a). SEM of approximately 50 QD per µm2 at the cavity mode (scale bar: 500 nm) for the device used for power-saturation measurements (different from device shown in Fig. 3). (b) PL spectra of QDs at the cavity showing the cavity mode with an intensity contrast of >15 over the background. (c) Power saturation measurements for dots at 1505 and 1515 nm showing a delayed onset of saturation (shown by the arrows) for dots at the peak of emission. This is performed at 77K. The value of Psat is estimated around 5 mW.
shows the sample used in this study, with measured QD coverage of approximately 50 dots per µm2 derived from the SEM. Figure 5b shows the PL spectrum of dots, showing a peak at the cavity resonance at 1505 nm with an intensity contrast of 15 at 77K. The estimated number of QDs with emission in the observed spectrum in this experiment is between 5 and 10. The power-saturation of QDs at the resonance peak (1505 nm) and off-peak (1515 nm) is shown in Fig. 5c. The photoluminescence intensity at 1515 nm shows a linear increase with pump power at low excitation, and then a saturation of the signal at high pumping rate.

We use Psat to denote the power at which saturation occurs for these dots that are slightly detuned from the peak of the cavity mode, meaning that the rate of exciton occupation exceeds the rate of radiative recombination in these dots. We then examine the signal nearly exactly on-resonance with the cavity mode and observe a slightly delayed onset of saturation where a ~10% increase in pumping rate (Pon / Poff ) is observed as shown by arrows in Fig. 5c. A comparison with QDs completely off-resonance with the cavity line would show further differences in saturation, but due to the very low (less than 50 QD per µm2) dot densities, the photon counts are below the dark counts in the near-infrared. We note that, since the QDs are off-resonantly pumped, the pumping rate is matched for both cases. Moreover, to ascertain the excited saturation against other artifacts such as QD photobleaching, measurement drift or nonlinear absorption, we also vary the pump powers non-monotonically in the measurements. For even lower QD densities such as at a single QD per µm2, the delayed onset of saturation can offer an opportunity to experimentally observe QD radiation above the noise floor using high excitation powers. While the results do not allow for a direct estimation of the Purcell factor, the observed delayed saturation onset (10%) for the dots emitting at the peak of the cavity emission qualitatively shows a spontaneous emission enhancement on resonance with the cavity mode.

We note that we did not observe blinking for our few QDs. Although blinking of colloidal QDs can be perceived as a drawback for single photon source applications, recent remarkable efforts have significantly suppressed blinking in nanocrystal QDs, through growth of a thick shell or modification of surface environment [56

56. B. Mahler, P. Spinicelli, S. Buil, X. Quelin, J.-P. Hermier, and B. Dubertret, “Towards non-blinking colloidal quantum dots,” Nat. Mater. 7(8), 659–664 ( 2008). [CrossRef] [PubMed]

,57

57. V. Fomenko and D. J. Nesbitt, “Solution control of radiative and nonradiative lifetimes: a novel contribution to quantum dot blinking suppression,” Nano Lett. 8(1), 287–293 ( 2008). [CrossRef] [PubMed]

]. The several coupled QDs estimated here are based on SEM imaging, and it is further likely that some QDs are not active. The devices are therefore expected to be already approaching single dot operation. Compared to smaller PbS QDs with lower photostability, we find these dots to be highly stable under laser excitation, with similar emission levels from the cavity region. We emphasize that the ability to isolate a few quantum dots at the cavity region based on solvent-dilution and post-fabrication integration of QD is an extremely powerful technique. For a single-QD device, the ability to exchange dots based on whether the dot and cavity emission match is not possible in self-assembled semiconductor systems, but is possible here, through selective e-beam lithography techniques that we have proposed earlier. Q values in excess of 1000 are easily achievable in the silicon photonic crystal cavity system, and may allow larger photon counts for time-resolved lifetime measurements for a sparse strongly-enhanced sample through an enhanced Purcell effect, and for photon coincidence measurements of the single Pb-chalcogenide quantum dot at 1.55-um, and for further elucidation of the radiative dynamics of single quantum dots.

Acknowledgments

The authors acknowledge helpful discussions with K. Srinivasan, M. Rakher, F. Sun, S. Jockusch, N. Turro, and R. L. Williams, and lithography fabrication from M. Yu and D.-L. Kwong, at the Institute of Microelectronics in Singapore. The authors acknowledge funding support from the NSF CAREER program, DARPA MTO, and the New York State Office of Science, Technology and Academic Research. C. W. W. is supported as part of the Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences under award number DE-SC0001085.

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M. Pelton, C. Santori, J. Vucković, B. Zhang, G. S. Solomon, J. Plant, and Y. Yamamoto, “Efficient source of single photons: a single quantum dot in a micropost microcavity,” Phys. Rev. Lett. 89(23), 233602 ( 2002). [CrossRef] [PubMed]

6.

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(12), 704–708 ( 2007). [CrossRef]

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C. Santori, D. Fattal, J. Vucković, G. S. Solomon, and Y. Yamamoto, “Indistinguishable photons from a single-photon device,” Nature 419(6907), 594–597 ( 2002). [CrossRef] [PubMed]

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K. Hennessy, A. Badolato, M. Winger, D. Gerace, M. Atatüre, S. Gulde, S. Fält, E. L. Hu, and A. Imamoğlu, “Quantum nature of a strongly coupled single quantum dot-cavity system,” Nature 445(7130), 896–899 ( 2007). [CrossRef] [PubMed]

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R. Bose, X. Yang, R. Chatterjee, J. Gao, and C. W. Wong, “Weak coupling interactions of colloidal lead sulphide nanocrystals with silicon photonic crystal nanocavities near 1.55 μm at room temperature,” Appl. Phys. Lett. 90(11), 111117 ( 2007). [CrossRef]

18.

Z. Wu, Z. Mi, P. Bhattacharya, T. Zhu, and J. Xu, “Enhanced spontaneous emission at 1.55 μm from colloidal PbSe quantum dots in a Si photonic crystal microcavity,” Appl. Phys. Lett. 90(17), 171105 ( 2007). [CrossRef]

19.

R. Bose, D. V. Talapin, X. Yang, R. J. Harniman, P. T. Nguyen, and C. W. Wong, “Interaction of infilitrated colloidal PbS nanocrystals with high Q/V silicon photonic bandgap nanocavities for near-infrared enhanced spontaneous emissions,” Proc. SPIE 6005, 600509 ( 2005). [CrossRef]

20.

A. G. Pattantyus-Abraham, H. Qiao, J. Shan, K. A. Abel, T.-S. Wang, F. C. J. M. van Veggel, and J. F. Young, “Site-selective optical coupling of PbSe nanocrystals to Si-based photonic crystal microcavities,” Nano Lett. 9(8), 2849–2854 ( 2009). [CrossRef] [PubMed]

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S. Vignolini, F. Riboli, F. Intonti, M. Belotti, M. Gurioli, Y. Chen, M. Colocci, L. C. Andreani, and D. S. Wiersma, “Local nanofluidic light sources in silicon photonic crystal microcavities,” Phys. Rev. E Stat. Nonlin. Soft Matter Phys. 78(4), 045603 ( 2008). [CrossRef] [PubMed]

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A. I. Akimov, “Al. L. Efros, A. A. Onushchenko, “Quantum size effect in semiconductor nanocrystals,” Solid State Commun. 56, 921 ( 1985).

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L. E. Brus, “Electron-electron and electron-hole interactions in small semiconductor crystallites: The size dependence of the lowest excited excitonic state,” J. Chem. Phys. 80(9), 4403 ( 1984). [CrossRef]

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F. W. Wise, “Lead salt quantum dots: the limit of strong quantum confinement,” Acc. Chem. Res. 33(11), 773–780 ( 2000). [CrossRef] [PubMed]

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J. Warner, E. Thomsen, A. R. Watt, N. R. Heckenberg, and H. Rubinsztein-Dunlop, “Time-resolved photoluminescence spectroscopy of ligand-capped PbS nanocrystals,” Nanotechology 16(2), 175–179 ( 2005). [CrossRef]

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L. Cademartiri, J. Bertolotti, R. Sapienza, D. S. Wiersma, G. von Freymann, and G. A. Ozin, “Multigram scale, solventless, and diffusion-controlled route to highly monodisperse PbS nanocrystals,” J. Phys. Chem. B 110(2), 671–673 ( 2006). [CrossRef] [PubMed]

28.

R. Bose, R. J. F. McMillan, J. Gao, C. J. Chen, D. V. Talapin, C. B. Murray, K. M. Rickey, and C. W. Wong, “Temperature-tuning of near-infrared monodisperse quantum dots at 1.5 μm for controllable Förster energy transfer,” Nano Lett. 8(7), 2006–2011 ( 2008). [CrossRef] [PubMed]

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34.

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37.

M. A. Hines and G. D. Scholes, “Colloidal PbS nanocrystals with size-tunable near-infrared emission: observation of post-synthesis self-narrowing of the particle size distribution,” Adv. Mater. 15(21), 1844–1849 ( 2003). [CrossRef]

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OCIS Codes
(000.0000) General : General
(000.2700) General : General science

ToC Category:
Quantum Optics

History
Original Manuscript: August 19, 2009
Revised Manuscript: October 16, 2009
Manuscript Accepted: November 4, 2009
Published: November 23, 2009

Citation
Ranojoy Bose, Jie Gao, James F. McMillan, Alex D. Williams, and Chee Wei Wong, "Cryogenic spectroscopy of ultra-low density colloidal lead chalcogenide quantum dots on chip-scale optical cavities towards single quantum dot near-infrared cavity QED," Opt. Express 17, 22474-22483 (2009)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-17-25-22474


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References

  1. H. Mabuchi and A. C. Doherty, “Cavity quantum electrodynamics: coherence in context,” Science 298(5597), 1372–1377 (2002). [CrossRef] [PubMed]
  2. G. Khitrova, H. M. Gibbs, M. Kira, S. W. Koch, and A. Scherer, “Vacuum Rabi splitting in semiconductors,” Nat. Phys. 2(2), 81–90 (2006). [CrossRef]
  3. K. Srinivasan and O. Painter, “Linear and nonlinear optical spectroscopy of a strongly coupled microdisk-quantum dot system,” Nature 450(7171), 862–865 (2007). [CrossRef] [PubMed]
  4. Y.-F. Xiao, J. Gao, X.-B. Zou, J. F. McMillan, X. Yang, Y.-L. Chen, Z.-F. Han, G.-C. Guo, and C. W. Wong, “Coupled quantum electrodynamics in photonic crystal cavities towards controlled phase gate operations,” N. J. Phys. 10(12), 123013 (2008). [CrossRef]
  5. M. Pelton, C. Santori, J. Vucković, B. Zhang, G. S. Solomon, J. Plant, and Y. Yamamoto, “Efficient source of single photons: a single quantum dot in a micropost microcavity,” Phys. Rev. Lett. 89(23), 233602 (2002). [CrossRef] [PubMed]
  6. 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(12), 704–708 (2007). [CrossRef]
  7. C. Santori, D. Fattal, J. Vucković, G. S. Solomon, and Y. Yamamoto, “Indistinguishable photons from a single-photon device,” Nature 419(6907), 594–597 (2002). [CrossRef] [PubMed]
  8. F. W. Sun and C. W. Wong, “Indistinguishability of independent single photons,” Phys. Rev. A 79(1), 013824 (2009). [CrossRef]
  9. A. Faraon, I. Fushman, D. Englund, N. Stoltz, P. Petroff, and J. Vučković, “Coherent generation of nonclassical light on a chip via photon-induced tunneling and blockade,” Nat. Phys. 4(11), 859–863 (2008). [CrossRef]
  10. K. Hennessy, A. Badolato, M. Winger, D. Gerace, M. Atatüre, S. Gulde, S. Fält, E. L. Hu, and A. Imamoğlu, “Quantum nature of a strongly coupled single quantum dot-cavity system,” Nature 445(7130), 896–899 (2007). [CrossRef] [PubMed]
  11. J. P. Reithmaier, G. Sęk, A. Löffler, C. Hofmann, S. Kuhn, S. Reitzenstein, L. V. Keldysh, V. D. Kulakovskii, T. L. Reinecke, and A. Forchel, “Strong coupling in a single quantum dot-semiconductor microcavity system,” Nature 432(7014), 197–200 (2004). [CrossRef] [PubMed]
  12. M. V. Dutt, L. Childress, L. Jiang, E. Togan, J. Maze, F. Jelezko, A. S. Zibrov, P. R. Hemmer, and M. D. Lukin, “Quantum register based on individual electronic and nuclear spin qubits in diamond,” Science 316(5829), 1312–1316 (2007). [CrossRef] [PubMed]
  13. Y. Shen, T. M. Sweeney, and H. Wang, “Zero-phonon linewidth of single nitrogen vacancy centers in diamond nanocrystals,” Phys. Rev. B 77(3), 033201 (2008). [CrossRef]
  14. S. Kako, C. Santori, K. Hoshino, S. Götzinger, Y. Yamamoto, and Y. Arakawa, “A gallium nitride single-photon source operating at 200 K,” Nat. Mater. 5(11), 887–892 (2006). [CrossRef] [PubMed]
  15. C. B. Poitras, M. Lipson, M. A. Hahn, H. Du, and T. D. Krauss, “Photoluminescence enhancement of colloidal quantum dots embedded in a monolithic microcavity,” Appl. Phys. Lett. 82(23), 4032 (2003). [CrossRef]
  16. I. Fushman, D. Englund, and J. Vučković, “Coupling of PbS quantum dots to photonic crystal cavities at room temperature,” Appl. Phys. Lett. 87(24), 241102 (2005). [CrossRef]
  17. R. Bose, X. Yang, R. Chatterjee, J. Gao, and C. W. Wong, “Weak coupling interactions of colloidal lead sulphide nanocrystals with silicon photonic crystal nanocavities near 1.55 μm at room temperature,” Appl. Phys. Lett. 90(11), 111117 (2007). [CrossRef]
  18. Z. Wu, Z. Mi, P. Bhattacharya, T. Zhu, and J. Xu, “Enhanced spontaneous emission at 1.55 μm from colloidal PbSe quantum dots in a Si photonic crystal microcavity,” Appl. Phys. Lett. 90(17), 171105 (2007). [CrossRef]
  19. R. Bose, D. V. Talapin, X. Yang, R. J. Harniman, P. T. Nguyen, and C. W. Wong, “Interaction of infilitrated colloidal PbS nanocrystals with high Q/V silicon photonic bandgap nanocavities for near-infrared enhanced spontaneous emissions,” Proc. SPIE 6005, 600509 (2005). [CrossRef]
  20. A. G. Pattantyus-Abraham, H. Qiao, J. Shan, K. A. Abel, T.-S. Wang, F. C. J. M. van Veggel, and J. F. Young, “Site-selective optical coupling of PbSe nanocrystals to Si-based photonic crystal microcavities,” Nano Lett. 9(8), 2849–2854 (2009). [CrossRef] [PubMed]
  21. S. Vignolini, F. Riboli, F. Intonti, M. Belotti, M. Gurioli, Y. Chen, M. Colocci, L. C. Andreani, and D. S. Wiersma, “Local nanofluidic light sources in silicon photonic crystal microcavities,” Phys. Rev. E Stat. Nonlin. Soft Matter Phys. 78(4), 045603 (2008). [CrossRef] [PubMed]
  22. A. I. Akimov, “Al. L. Efros, A. A. Onushchenko, “Quantum size effect in semiconductor nanocrystals,” Solid State Commun. 56, 921 (1985).
  23. L. E. Brus, “Electron-electron and electron-hole interactions in small semiconductor crystallites: The size dependence of the lowest excited excitonic state,” J. Chem. Phys. 80(9), 4403 (1984). [CrossRef]
  24. F. W. Wise, “Lead salt quantum dots: the limit of strong quantum confinement,” Acc. Chem. Res. 33(11), 773–780 (2000). [CrossRef] [PubMed]
  25. J. Warner, E. Thomsen, A. R. Watt, N. R. Heckenberg, and H. Rubinsztein-Dunlop, “Time-resolved photoluminescence spectroscopy of ligand-capped PbS nanocrystals,” Nanotechology 16(2), 175–179 (2005). [CrossRef]
  26. S. W. Clark, J. M. Harbold, and F. W. Wise, “Resonant energy transfer in PbS quantum dots,” J. Phys. Chem. C 111(20), 7302–7305 (2007). [CrossRef]
  27. L. Cademartiri, J. Bertolotti, R. Sapienza, D. S. Wiersma, G. von Freymann, and G. A. Ozin, “Multigram scale, solventless, and diffusion-controlled route to highly monodisperse PbS nanocrystals,” J. Phys. Chem. B 110(2), 671–673 (2006). [CrossRef] [PubMed]
  28. R. Bose, R. J. F. McMillan, J. Gao, C. J. Chen, D. V. Talapin, C. B. Murray, K. M. Rickey, and C. W. Wong, “Temperature-tuning of near-infrared monodisperse quantum dots at 1.5 μm for controllable Förster energy transfer,” Nano Lett. 8(7), 2006–2011 (2008). [CrossRef] [PubMed]
  29. E. M. Purcell, “Spontaneous emission probabilities at radio frequencies,” Phys. Rev. 69, 681 (1946).
  30. T. Tanabe, M. Notomi, E. Kuramochi, A. Shinya, and H. Taniyama, “Trapping and delaying photons for one nanosecond in an ultrasmall high-Q photonic-crystal nanocavity,” Nat. Photonics 1(1), 49–52 (2007). [CrossRef]
  31. S. Noda, M. Fujita, and T. Asano, “Spontaneous-emission control by photonic crystals and nanocavities,” Nat. Photonics 1(8), 449–458 (2007). [CrossRef]
  32. A. Farjadpour, D. Roundy, A. Rodriguez, M. Ibanescu, P. Bermel, J. D. Joannopoulos, S. G. Johnson, and G. W. Burr, “Improving accuracy by subpixel smoothing in the finite-difference time domain,” Opt. Lett. 31(20), 2972–2974 (2006). [CrossRef] [PubMed]
  33. S. Kocaman, R. Chatterjee, N. C. Panoiu, J. F. McMillan, M. B. Yu, R. M. Osgood, D. L. Kwong, and C. W. Wong, “Observations of zero-order bandgaps in negative-index photonic crystal superlattices at the near-infrared,” Phys. Rev. Lett. 102, 203905 (2009). [CrossRef] [PubMed]
  34. M. W. McCutcheon, G. W. Rieger, I. W. Cheung, J. F. Young, D. Dalacu, S. Frederick, P. J. Poole, G. C. Aers, and R. L. Williams, “Resonant scattering and second-harmonic spectroscopy of planar photonic crystal nanocavities,” Appl. Phys. Lett. 87(22), 221110 (2005). [CrossRef]
  35. P. B. Deotare, M. W. McCutcheon, I. W. Frank, M. Khan, and M. Loncar, “High quality factor photonic crystal nanobeam cavities,” Appl. Phys. Lett. 94(12), 121106 (2009). [CrossRef]
  36. C. B. Murray, S. Sun, W. Gaschler, H. Doyle, T. A. Betley, and C. R. Kagan, “Colloidal synthesis of nanocrystals and nanocrystal superlattices,” IBM J. Res. Dev. 45, 47 (2001). [CrossRef]
  37. M. A. Hines and G. D. Scholes, “Colloidal PbS nanocrystals with size-tunable near-infrared emission: observation of post-synthesis self-narrowing of the particle size distribution,” Adv. Mater. 15(21), 1844–1849 (2003). [CrossRef]
  38. D. V. Talapin and C. B. Murray, “PbSe nanocrystal solids for n- and p-channel thin film field-effect transistors,” Science 310(5745), 86–89 (2005). [CrossRef] [PubMed]
  39. J. M. Pietryga, K. K. Zhuravlev, M. Whitehead, V. I. Klimov, and R. D. Schaller, “Evidence for barrierless auger recombination in PbSe nanocrystals: a pressure-dependent study of transient optical absorption,” Phys. Rev. Lett. 101(21), 217401 (2008). [CrossRef] [PubMed]
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