Spontaneous emission modulation of colloidal quantum dots via efficient coupling with hybrid plasmonic photonic crystal X. W. Yuan,1 L. Shi,2 Qi Wang,3 C. Q.Chen,3 X. H. Liu,2 L.X. Sun,1,* Bo Zhang,1 J. Zi,2,4 and Wei Lu1,5 1
National Laboratory for Infrared Physics, Shanghai Institute of Technical Physics, Chinese Academy of Sciences, Shanghai 200083, China 2 Department of Physics, Key Laboratory of Micro & Nano Photonic Structures (MOE) and Key Laboratory of Surface Physics, Fudan University, Shanghai 200433, China 3 Wuhan National Laboratory for Optoelectronics, College of optical and electronic information, Huazhong University of Science and Technology, Wuhan 430074, China 4 [email protected] 5 [email protected] * [email protected]
Abstract: The spontaneous emission of colloidal CdSe/ZnS quantum dots (CQDs) modified by the hybrid plasmonic-photonic crystal is reported in this paper. By using a spin coater, the spatial overlap between CQDs and the surface resonance modes in this quasi-2D crystal slab is achieved. In this case, the coupling efficiency of them is enhanced greatly and most excited CQDs radiate through the surface modes. Consequently, despite the low refractive index contrast of our hybrid structure, the directionality of spontaneous emission, increased radiative probability and narrowed full width at half maximum of emission peak are all clearly observed by our home-made microscopic angle-resolved spectroscopy and time-resolved photoluminescence system. Our results manifest that the quasi-2D hybrid plasmonic-photonic crystal is an ideal candidate to tailor the radiative properties of CdSe/ZnS CQDs, which might be significant for the applications of light emitting devices. ©2014 Optical Society of America OCIS codes: (300.6500) Spectroscopy, time-resolved; (300.6280) Spectroscopy, fluorescence and luminescence; (350.4238) Nanophotonics and photonic crystals.
References and links 1.
M. Bruchez, Jr., M. Moronne, P. Gin, S. Weiss, and A. P. Alivisatos, “Semiconductor nanocrystals as fluorescent biological labels,” Science 281(5385), 2013–2016 (1998). 2. V. L. Colvin, M. C. Schlamp, and A. P. Alivisatos, “Light-emitting-diodes made from cadmium selenide nanocrystals and a semiconducting polymer,” Nature 370(6488), 354–357 (1994). 3. Z. N. Tan, F. Zhang, T. Zhu, J. Xu, A. Y. Wang, J. D. Dixon, L. S. Li, Q. Zhang, S. E. Mohney, and J. Ruzyllo, “Bright and color-saturated emission from blue light-emitting diodes based on solution-processed colloidal nanocrystal quantum dots,” Nano Lett. 7(12), 3803–3807 (2007). 4. D. C. Oertel, M. G. Bawendi, A. C. Arango, and V. Bulovic, “Photodetectors based on treated CdSe quantumdot films,” Appl. Phys. Lett. 87(21), 213505 (2005). 5. G. Konstantatos, I. Howard, A. Fischer, S. Hoogland, J. Clifford, E. Klem, L. Levina, and E. H. Sargent, “Ultrasensitive solution-cast quantum dot photodetectors,” Nature 442(7099), 180–183 (2006). 6. I. L. Medintz, A. R. Clapp, H. Mattoussi, E. R. Goldman, B. Fisher, and J. M. Mauro, “Self-assembled nanoscale biosensors based on quantum dot FRET donors,” Nat. Mater. 2(9), 630–638 (2003). 7. D. Geißler, L. J. Charbonnière, R. F. Ziessel, N. G. Butlin, H.-G. Löhmannsröben, and N. Hildebrandt, “Quantum Dot Biosensors for Ultrasensitive Multiplexed Diagnostics,” Angew. Chem. Int. Ed. Engl. 49(8), 1396–1401 (2010). 8. J. F. Xu and M. Xiao, “Lasing action in colloidal CdS/CdSe/CdS quantum wells,” Appl. Phys. Lett. 87(17), 173117 (2005). 9. J. Renner, L. Worschech, A. Forchel, S. Mahapatra, and K. Brunner, “CdSe quantum dot microdisk laser,” Appl. Phys. Lett. 89(23), 231104 (2006). 10. E. M. Purcell, “Spontaneous emission probabilities at radio frequencies,” Phys. Rev. 69, 681 (1946).
#211384 - $15.00 USD Received 5 May 2014; revised 24 Jul 2014; accepted 11 Aug 2014; published 18 Sep 2014 (C) 2014 OSA 22 September 2014 | Vol. 22, No. 19 | DOI:10.1364/OE.22.023473 | OPTICS EXPRESS 23473
11. E. Yablonovitch, “Inhibited spontaneous emission in solid-state physics and electronics,” Phys. Rev. Lett. 58(20), 2059–2062 (1987). 12. S. P. Ogawa, M. Imada, S. Yoshimoto, M. Okano, and S. Noda, “Control of light emission by 3D photonic crystals,” Science 305(5681), 227–229 (2004). 13. G. R. Maskaly, M. A. Petruska, J. Nanda, I. V. Bezel, R. D. Schaller, H. Htoon, J. M. Pietryga, and V. I. Klimov, “Amplified spontaneous emission in semiconductor-nanocrystal/synthetic-opal composites: Optical-gain enhancement via a photonic crystal pseudogap,” Adv. Mater. 18(3), 343–347 (2006). 14. K. Y. Yao and Y. C. Shi, “High-Q width modulated photonic crystal stack mode-gap cavity and its application to refractive index sensing,” Opt. Express 20(24), 27039–27044 (2012). 15. L. Shi, T. K. Hakala, H. T. Rekola, J.-P. Martikainen, R. J. Moerland, and P. Törmä, “Spatial Coherence Properties of Organic Molecules Coupled to Plasmonic Surface Lattice Resonances in the Weak and Strong Coupling Regimes,” Phys. Rev. Lett. 112(15), 153002 (2014). 16. S. Laxon, N. Peacock, and D. Smith, “High interannual variability of sea ice thickness in the Arctic region,” Nature 425(6961), 947–950 (2003). 17. Y. Akahane, T. Asano, B. S. Song, and S. Noda, “Fine-tuned high-Q photonic-crystal nanocavity,” Opt. Express 13(4), 1202–1214 (2005). 18. M. Fujita, S. Takahashi, Y. Tanaka, T. Asano, and S. Noda, “Simultaneous inhibition and redistribution of spontaneous light emission in photonic crystals,” Science 308(5726), 1296–1298 (2005). 19. B. S. Song, S. Noda, T. Asano, and Y. Akahane, “Ultra-high-Q photonic double-heterostructure nanocavity,” Nat. Mater. 4(3), 207–210 (2005). 20. L. Ferrier, O. El Daif, X. Letartre, P. Rojo Romeo, C. Seassal, R. Mazurczyk, and P. Viktorovitch, “Surface emitting microlaser based on 2D photonic crystal rod lattices,” Opt. Express 17(12), 9780–9788 (2009). 21. Z. Q. Liu, J. T. Hang, J. Chen, Z. D. Yan, C. J. Tang, Z. Chen, and P. Zhan, “Optical transmission of corrugated metal films on a two-dimensional hetero-colloidal crystal,” Opt. Express 20(8), 9215–9225 (2012). 22. R. F. Oulton, V. J. Sorger, D. A. Genov, D. F. P. Pile, and X. Zhang, “A hybrid plasmonic waveguide for subwavelength confinement and long-range propagation,” Nat. Photonics 2(8), 496–500 (2008). 23. M. A. Schmidt, D. Y. Lei, L. Wondraczek, V. Nazabal, and S. A. Maier, “Hybrid nanoparticle-microcavitybased plasmonic nanosensors with improved detection resolution and extended remote-sensing ability,” Nat. Commun. 3, 1108 (2012). 24. X. L. Zhu, F. X. Xie, L. Shi, X. H. Liu, N. A. Mortensen, S. S. Xiao, J. Zi, and W. Choy, “Broadband enhancement of spontaneous emission in a photonic-plasmonic structure,” Opt. Lett. 37(11), 2037–2039 (2012). 25. L. Shi, X. H. Liu, H. W. Yin, and J. Zi, “Optical response of a flat metallic surface coated with a monolayer array of latex spheres,” Phys. Lett. A 374(8), 1059–1062 (2010). 26. L. Shi, H. W. Yin, X. L. Zhu, X. H. Liu, and J. Zi, “Direct observation of iso-frequency contour of surface modes in defective photonic crystals in real space,” Appl. Phys. Lett. 97(25), 251111 (2010). 27. X. D. Yu, L. Shi, D. Z. Han, J. Zi, and P. V. Braun, “High quality factor metallodielectric hybrid plasmonicphotonic crystals,” Adv. Funct. Mater. 20(12), 1910–1916 (2010). 28. X. W. Yuan, Q. Wang, X. F. Zhu, L. Shi, Q. Zhao, L. Sun, C. Chen, and B. Zhang, “High Q factor propagating plasmon modes based on low-cost metals,” J. Phys. D Appl. Phys. 47(8), 085109 (2014). 29. M. López-García, J. F. Galisteo-López, A. Blanco, J. Sánchez-Marcos, C. López, and A. García-Martín, “Enhancement and Directionality of Spontaneous Emission in Hybrid Self-Assembled Photonic-Plasmonic Crystals,” Small 6(16), 1757–1761 (2010). 30. Y. Chen, J. Herrnsdorf, B. Guilhabert, Y. Zhang, I. M. Watson, E. Gu, N. Laurand, and M. D. Dawson, “Colloidal quantum dot random laser,” Opt. Express 19(4), 2996–3003 (2011). 31. X. H. Wu, Y. G. Sun, and M. Pelton, “Recombination rates for single colloidal quantum dots near a smooth metal film,” Phys. Chem. Chem. Phys. 11(28), 5867–5870 (2009). 32. K. T. Shimizu, W. K. Woo, B. R. Fisher, H. J. Eisler, and M. G. Bawendi, “Surface-Enhanced Emission from Single Semiconductor Nanocrystals,” Phys. Rev. Lett. 89(11), 117401 (2002). 33. M. Scheibner, T. Schmidt, L. Worschech, A. Forchel, G. Bacher, T. Passow, and D. Hommel, “Superradiance of quantum dots,” Nat. Phys. 3(2), 106–110 (2007). 34. S. Malik, E. C. Le Ru, D. Childs, and R. Murray, “Time-resolved studies of annealed InAs/GaAs self-assembled quantum dots,” Phys. Rev. B 63(15), 155313 (2001). 35. H. L. Ning, A. Mihi, J. B. Geddes III, M. Miyake, and P. V. 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1. Introduction Colloidal quantum dots (CQDs), the emitters much superior to conventional organic fluorescence materials [1], have drawn a great deal of interest during the past decades for their unique advantages. Besides strong photochemical stability and high emission efficiency, CQDs have unique size-dependent optical properties. Especially, the emission wavelengths covering from visible to infrared region could be simply tuned by varying the size of CQDs. So far, CQDs have shown significant superiority in application of light-emitting diodes (LED) [2, 3], photo detectors [4, 5], biosensors [6, 7], laser and even quantum optics [8, 9].
#211384 - $15.00 USD Received 5 May 2014; revised 24 Jul 2014; accepted 11 Aug 2014; published 18 Sep 2014 (C) 2014 OSA 22 September 2014 | Vol. 22, No. 19 | DOI:10.1364/OE.22.023473 | OPTICS EXPRESS 23474
In the diversity applications of CQDs, control of spontaneous emission (SE) rate plays a key role. Photonic crystals (Phcs) have been proved to be a effective way to modulate the SE based on Purcell effect [10, 11], specially for 3D periodic structures due to the photonic band gap (PBG) in all directions [12, 13]. However, the complex fabrication processes hinder the development and real applications of 3D photonic crystals. Recently, the 2D Phcs with high refractive index contrast [14, 15] and some special plasmonic structures have been proven to be good choices to modify SE for high field confinement [16–21]. In this letter, a 2D hybrid plasmonic–photonic crystal slab, with low refractive index contrast, is introduced. Similar with some other hybrid systems [22, 23], which support the surface plasmons modes and dielectric resonance modes (guide modes or FP resonance modes) simultaneously, this hybrid structure combines the high field localization of surface plasmons system and the long propagation length of dielectric photonic crystal together [24]. As a result, Q-factors of the surface modes are one order of magnitude higher than that of typical all-metal plasmonic system and might cause much stronger modulation of electromagnetic fields [25–29]. So the SE modulation of CQDs by this hybrid crystal slab is studied. In our work, the spatial overlap between the distribution of CQDs and the surface resonance modes in the hybrid crystal is achieved, which is the experiment challenge here. Thus, the coupling between the emission and the surface modes is enhanced efficiently, resulting in that the probability of radiative recombination is modified. As a result, besides the directional emission, the radiative lifetime, full width at half maximum (FWHM) and center wavelength are both modified simultaneously. 2. Experiments
Fig. 1. (a) Schematic of the crystal structure under study. (b) SEM of the hybrid crystal. (c) (d) The side-view and top-view field distribution of the surface mode we are interested in. (e) An emision map which corresponds to the spatial distribution of CQDs on the structure.
Through the self-assembly techniques, the hybrid plasmonic photonic crystal slab is prepared. It consists of a monolayer array of polystyrene (PS) microspheres (d = 510nm) on a flat Ag film, which is deposited on a quartz substrate. The schematic and the SEM of our sample are shown in Figs. 1(a) and 1(b), respectively. Calculated by Rigorous coupled-wave analysis (RCWA) method, the hybrid structure supports several surface resonance modes [25] and the side-view and top-view electric field distribution of the surface mode we are interested in is shown in Figs. 1(c) and 1(d) respectively. The results demonstrate that the maximum position of the electric field intensity is in the center of the PS microspheres while the sub-maximal position is around the equator. However, it is difficult to guarantee the spatial overlap
#211384 - $15.00 USD Received 5 May 2014; revised 24 Jul 2014; accepted 11 Aug 2014; published 18 Sep 2014 (C) 2014 OSA 22 September 2014 | Vol. 22, No. 19 | DOI:10.1364/OE.22.023473 | OPTICS EXPRESS 23475
between the emitters and the maximum position of electric field, which requires the emitters to locate in the center of the spheres precisely. Oppositely, the sub-maximal space morphology is exposed outside, making the spatial overlap easy to achieve. In our work, the colloidal CdSe/ZnS quantum dots are chosen as light sources for the highly efficiency, narrow line-width and quasi two-level systems [8, 9, 20, 30]. Meanwhile, the spectral overlap between the emission and the surface modes is easy to implement by choosing the appropriate size of CQDs. Based on these, the colloidal quantum dots whose spontaneous emission wavelength is consistent with that of the surface mode, are coated on the hybrid crystal by using spin coater. The spatial distribution of the CQDs on the crystal is obtained by the fluorescence microscopy and the result is shown in Fig. 1 (e). So one can address that the CQDs are periodically distributed and the amount round the equator of the PS microspheres is much greater than that on the top. By comparison, the spatial overlap between the distribution of CQDs and the surface modes can be verified.
Fig. 2. Schematic of experiment setup for angle-resolved reflectance and emission technique.
To make it clear how the surface resonance modes in the hybrid crystal modify the spontaneous emission of CQDs, microscopic angle-resolved reflectance and emission technique are used. The experimental setup is shown in Fig. 2. As known, the reflected lights from the structure, if with the same direction, will be focused on the rear focal plane (also named Fourier plane) of the microscope objective. So the points on Fourier plane correspond to the information of reflectance at specific angles. With the help of image lens, Fourier plane is imaged on the slit of the spectrometer that is equipped with matrix CCD. Here, different from the conventional imaging, the 2D CCD obtains the information of both angle and spectrum with a snapshot, from which the photonic band structure can be extracted. 3. Results and discussion The Γ-X high-symmetric direction of the hexagonal lattice (shown in Fig. 1(b)) is selected and the reflectance spectra at different angles are measured for the crystal with CQDs coated. The results with different polarizations are shown in Figs. 3(a) and 3(b). The black regions characterize the reflection dips, meaning strong absorption of the incident light. The normal reflectance is extracted and shown as the red curve on the left side of the figure, implying the existence of a surface mode. When departing from normal incidence, the reflectance dips shift in spectrum and show obvious dispersion behavior in the hybrid crystal. Figures 3(c) and 3(d) depict the results of emission spectrum with different polarizations. It is obvious that the
#211384 - $15.00 USD Received 5 May 2014; revised 24 Jul 2014; accepted 11 Aug 2014; published 18 Sep 2014 (C) 2014 OSA 22 September 2014 | Vol. 22, No. 19 | DOI:10.1364/OE.22.023473 | OPTICS EXPRESS 23476
emission is channeled by the surface modes. As a result, the distribution of the modified spontaneous emission agrees well with dispersion behavior. The phenomenon that the spontaneous emission is limited in a specific direction for a given wavelength is called as the directionality of SE [29].
Fig. 3. The results of microscopic angle-resolved reflectance (shown in (a) (b)) and emission (shown in (c) (d)) spectra with different polarized configurations for the structure with CQDs coated. The red line represents the normal reflectance spectra of the hybrid structure.
In order to further understand how the surface mode modifies spontaneous emission, timeresolved photoluminescence measurements are performed on the hybrid structure coated with CQDs. Meanwhile, Ag film with CQDs painted on is chosen as a reference sample for the following two reasons. Firstly, for the lack of periodical dielectric spheres, which introduce reciprocal lattice vector for the hybrid structure, the incident electromagnetic waves cannot couple with the reference sample. Secondly, when the metal film is present, the intrinsic recombination rate of CQDs includes not only decay to the environment but also energy transfer to the metal [31,32]. In Fig. 4(a) time-resolved photoluminescence at center wavelength is shown and it is evident that the photoluminescence intensity of CQDs on the hybrid structure decays much faster than that of CQDs on the Ag film. The average lifetimes of the two samples are 1.47 ns and 3.75 ns, respectively. The former is nearly three times faster than the latter. Two main contributions are present in the data as the radiative and nonradiative decay, so the PL decay curves are usually fitted with dual-exponential functions [31, 32]. In Fig. 4(b), the measured lifetimes as a function of wavelength are shown. The red square represents the radiative lifetime and the black cycle represents the non-radiative lifetime, respectively [31, 32]. By increasing the wavelength from 600 nm to 620 nm, the lifetime of non-radiative changes slightly and the variation between the maximum and minimum values is about 0.25 ns. In strong contrast, for the radiative lifetime, the variation reaches up to 3.65 ns in the same wavelength range. Furthermore, the radiative lifetime is getting longer when deviating from the center wavelength of the surface mode, which is different from the non-radiative decay. Here, one can address that such efficient modulation of photoluminescence lifetime is mainly due to the variations of the radiative decay rather than the non-radiative. To get a more intuitive observation how the surface mode affects the radiative decay, radiative lifetime at different wavelengths for the two substrates are summarized in Fig. 4(c), and the deviation is roughly 0.2 ns. Different from the CQDs painted on the hybrid crystal, the lifetime increases with the wavelength increasing when the emitters are painted on Ag. This is mainly due to the variation of the colloidal quantum dots’ size [33, 34]. The variation #211384 - $15.00 USD Received 5 May 2014; revised 24 Jul 2014; accepted 11 Aug 2014; published 18 Sep 2014 (C) 2014 OSA 22 September 2014 | Vol. 22, No. 19 | DOI:10.1364/OE.22.023473 | OPTICS EXPRESS 23477
tendencies of lifetime curves for the two different structures are in stark contrast. This phenomenon can be interpreted in terms of the optical resonance cavity. In Fig. 4(d), the reflectance and density of state (DOS) at Γ point (corresponding to the normal incidence) are calculated and the small oscillations of calculated reflectance are due to the convergence problem. The result for DOS is the integration of different position and the spectral overlap between DOS and the emission of CQDs is obvious. Meanwhile, the spatial distribution can be extracted from the electric field distribution shown in Fig. 1(d) and the location of strong field means high DOS. So in this case, the spatial and spectral agreements between the emitters with DOS are both achieved. According to Fermi’s golden rule that the decay rate is proportional to the local density of state (LDOS), the radiative decay is significantly enhanced by the surface mode. Notably, the CQDs are not considered in our calculation, resulting in the deviation of center wavelength from the experimental measurement shown in Fig. 4(d).
Fig. 4. (a) The results of the time-resolved photoluminescence at 610nm. (b) The radiative lifetime and the non-radiative lifetime at different wavelength as the CQDs are pained on the hybrid crystal. (c) The radiative lifetime of PL decay on the two substrates. (d) The reflectance and DOS at Γ point for our structure.
Extracted from Fig. 3(c), SE of p polarization at different angles when the CQDs are painted on our hybrid crystal is shown Fig. 5. It is evident that, besides the significant change of decay rate, the FWHM and center wavelength of SE are also modified. Through the variation of FWHM and emission intensity, one can also address that the modulation
#211384 - $15.00 USD Received 5 May 2014; revised 24 Jul 2014; accepted 11 Aug 2014; published 18 Sep 2014 (C) 2014 OSA 22 September 2014 | Vol. 22, No. 19 | DOI:10.1364/OE.22.023473 | OPTICS EXPRESS 23478
efficiency is getting higher when the emission angle is close to zero angle (corresponding to Γ point). In the inset, the normalized SE spectrums around zero angle on different substrates are compared since it is difficult to control the quantity of CQDs strictly equal for them. The violet and pink regions correspond to the spontaneous emission of CQDs painted on the Ag film and hybrid crystal, respectively. The experimental results suggest that the FWHM of SE is compressed to 9 nm in the hybrid structure, much narrower than 27 nm on the Ag film. The center wavelength of SE also shifts, consistent with the reflectance dip in Fig. 4(d). All of these verify that most excited CQDs radiate via efficient coupling with the same mode, leading to the directionality of SE [31, 35, 36].
Fig. 5. The spontaneous emission at different angles when the CQDs are painted on hybrid crystal. The inset shows the normalized SE spectrums around zero angle on different substrates.
4. Conclusions In conclusion, remarkable enhancement of the coupling efficiency is achieved through the spatial and spectral overlap between colloidal CdSe/ZnS quantum dots and the surface resonance mode in the hybrid crystal. Consequently, the spontaneous emission of CQDs is strongly modified by the hybrid crystal. By measuring the decay rate of the excited CQDs and the FWHM of SE in the 2D hybrid plasmonic–photonic crystal slab, it is confirmed that most of the excited CQDs radiate via coupling with the same surface modes efficiently, which is the essential reason of the directionality of the SE. Finally, we believe that the economy and ease of fabrication of high-quality samples, together with the ability of tuning emission characteristics via the dielectric periodicity, endows this hybrid system with the potential to be exploited in future efficient light-emitting devices. Acknowledgments This research reported in this publication was supported by the National Natural Science Foundation of China (Grant Nos: 11274330, 10990103, 11104302, 11404064), the Shanghai Committee of Science and Technology of China (Grant No.11ZR1442300), Shanghai Pujiang Program (14PJ1401100, 14PJ1409600), 973 Program (2013CB632701 and 2011CB922004) and Fudan University Start-Up Research Fund, XWY and LS contributed equally to this work.
#211384 - $15.00 USD Received 5 May 2014; revised 24 Jul 2014; accepted 11 Aug 2014; published 18 Sep 2014 (C) 2014 OSA 22 September 2014 | Vol. 22, No. 19 | DOI:10.1364/OE.22.023473 | OPTICS EXPRESS 23479
National Laboratory for Infrared Physics, Shanghai Institute of Technical Physics, Chinese Academy of Sciences, Shanghai 200083, China 2 Department of Physics, Key Laboratory of Micro & Nano Photonic Structures (MOE) and Key Laboratory of Surface Physics, Fudan University, Shanghai 200433, China 3 Wuhan National Laboratory for Optoelectronics, College of optical and electronic information, Huazhong University of Science and Technology, Wuhan 430074, China 4 [email protected] 5 [email protected] * [email protected]
Abstract: The spontaneous emission of colloidal CdSe/ZnS quantum dots (CQDs) modified by the hybrid plasmonic-photonic crystal is reported in this paper. By using a spin coater, the spatial overlap between CQDs and the surface resonance modes in this quasi-2D crystal slab is achieved. In this case, the coupling efficiency of them is enhanced greatly and most excited CQDs radiate through the surface modes. Consequently, despite the low refractive index contrast of our hybrid structure, the directionality of spontaneous emission, increased radiative probability and narrowed full width at half maximum of emission peak are all clearly observed by our home-made microscopic angle-resolved spectroscopy and time-resolved photoluminescence system. Our results manifest that the quasi-2D hybrid plasmonic-photonic crystal is an ideal candidate to tailor the radiative properties of CdSe/ZnS CQDs, which might be significant for the applications of light emitting devices. ©2014 Optical Society of America OCIS codes: (300.6500) Spectroscopy, time-resolved; (300.6280) Spectroscopy, fluorescence and luminescence; (350.4238) Nanophotonics and photonic crystals.
References and links 1.
M. Bruchez, Jr., M. Moronne, P. Gin, S. Weiss, and A. P. Alivisatos, “Semiconductor nanocrystals as fluorescent biological labels,” Science 281(5385), 2013–2016 (1998). 2. V. L. Colvin, M. C. Schlamp, and A. P. Alivisatos, “Light-emitting-diodes made from cadmium selenide nanocrystals and a semiconducting polymer,” Nature 370(6488), 354–357 (1994). 3. Z. N. Tan, F. Zhang, T. Zhu, J. Xu, A. Y. Wang, J. D. Dixon, L. S. Li, Q. Zhang, S. E. Mohney, and J. Ruzyllo, “Bright and color-saturated emission from blue light-emitting diodes based on solution-processed colloidal nanocrystal quantum dots,” Nano Lett. 7(12), 3803–3807 (2007). 4. D. C. Oertel, M. G. Bawendi, A. C. Arango, and V. Bulovic, “Photodetectors based on treated CdSe quantumdot films,” Appl. Phys. Lett. 87(21), 213505 (2005). 5. G. Konstantatos, I. Howard, A. Fischer, S. Hoogland, J. Clifford, E. Klem, L. Levina, and E. H. Sargent, “Ultrasensitive solution-cast quantum dot photodetectors,” Nature 442(7099), 180–183 (2006). 6. I. L. Medintz, A. R. Clapp, H. Mattoussi, E. R. Goldman, B. Fisher, and J. M. Mauro, “Self-assembled nanoscale biosensors based on quantum dot FRET donors,” Nat. Mater. 2(9), 630–638 (2003). 7. D. Geißler, L. J. Charbonnière, R. F. Ziessel, N. G. Butlin, H.-G. Löhmannsröben, and N. Hildebrandt, “Quantum Dot Biosensors for Ultrasensitive Multiplexed Diagnostics,” Angew. Chem. Int. Ed. Engl. 49(8), 1396–1401 (2010). 8. J. F. Xu and M. Xiao, “Lasing action in colloidal CdS/CdSe/CdS quantum wells,” Appl. Phys. Lett. 87(17), 173117 (2005). 9. J. Renner, L. Worschech, A. Forchel, S. Mahapatra, and K. Brunner, “CdSe quantum dot microdisk laser,” Appl. Phys. Lett. 89(23), 231104 (2006). 10. E. M. Purcell, “Spontaneous emission probabilities at radio frequencies,” Phys. Rev. 69, 681 (1946).
#211384 - $15.00 USD Received 5 May 2014; revised 24 Jul 2014; accepted 11 Aug 2014; published 18 Sep 2014 (C) 2014 OSA 22 September 2014 | Vol. 22, No. 19 | DOI:10.1364/OE.22.023473 | OPTICS EXPRESS 23473
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1. Introduction Colloidal quantum dots (CQDs), the emitters much superior to conventional organic fluorescence materials [1], have drawn a great deal of interest during the past decades for their unique advantages. Besides strong photochemical stability and high emission efficiency, CQDs have unique size-dependent optical properties. Especially, the emission wavelengths covering from visible to infrared region could be simply tuned by varying the size of CQDs. So far, CQDs have shown significant superiority in application of light-emitting diodes (LED) [2, 3], photo detectors [4, 5], biosensors [6, 7], laser and even quantum optics [8, 9].
#211384 - $15.00 USD Received 5 May 2014; revised 24 Jul 2014; accepted 11 Aug 2014; published 18 Sep 2014 (C) 2014 OSA 22 September 2014 | Vol. 22, No. 19 | DOI:10.1364/OE.22.023473 | OPTICS EXPRESS 23474
In the diversity applications of CQDs, control of spontaneous emission (SE) rate plays a key role. Photonic crystals (Phcs) have been proved to be a effective way to modulate the SE based on Purcell effect [10, 11], specially for 3D periodic structures due to the photonic band gap (PBG) in all directions [12, 13]. However, the complex fabrication processes hinder the development and real applications of 3D photonic crystals. Recently, the 2D Phcs with high refractive index contrast [14, 15] and some special plasmonic structures have been proven to be good choices to modify SE for high field confinement [16–21]. In this letter, a 2D hybrid plasmonic–photonic crystal slab, with low refractive index contrast, is introduced. Similar with some other hybrid systems [22, 23], which support the surface plasmons modes and dielectric resonance modes (guide modes or FP resonance modes) simultaneously, this hybrid structure combines the high field localization of surface plasmons system and the long propagation length of dielectric photonic crystal together [24]. As a result, Q-factors of the surface modes are one order of magnitude higher than that of typical all-metal plasmonic system and might cause much stronger modulation of electromagnetic fields [25–29]. So the SE modulation of CQDs by this hybrid crystal slab is studied. In our work, the spatial overlap between the distribution of CQDs and the surface resonance modes in the hybrid crystal is achieved, which is the experiment challenge here. Thus, the coupling between the emission and the surface modes is enhanced efficiently, resulting in that the probability of radiative recombination is modified. As a result, besides the directional emission, the radiative lifetime, full width at half maximum (FWHM) and center wavelength are both modified simultaneously. 2. Experiments
Fig. 1. (a) Schematic of the crystal structure under study. (b) SEM of the hybrid crystal. (c) (d) The side-view and top-view field distribution of the surface mode we are interested in. (e) An emision map which corresponds to the spatial distribution of CQDs on the structure.
Through the self-assembly techniques, the hybrid plasmonic photonic crystal slab is prepared. It consists of a monolayer array of polystyrene (PS) microspheres (d = 510nm) on a flat Ag film, which is deposited on a quartz substrate. The schematic and the SEM of our sample are shown in Figs. 1(a) and 1(b), respectively. Calculated by Rigorous coupled-wave analysis (RCWA) method, the hybrid structure supports several surface resonance modes [25] and the side-view and top-view electric field distribution of the surface mode we are interested in is shown in Figs. 1(c) and 1(d) respectively. The results demonstrate that the maximum position of the electric field intensity is in the center of the PS microspheres while the sub-maximal position is around the equator. However, it is difficult to guarantee the spatial overlap
#211384 - $15.00 USD Received 5 May 2014; revised 24 Jul 2014; accepted 11 Aug 2014; published 18 Sep 2014 (C) 2014 OSA 22 September 2014 | Vol. 22, No. 19 | DOI:10.1364/OE.22.023473 | OPTICS EXPRESS 23475
between the emitters and the maximum position of electric field, which requires the emitters to locate in the center of the spheres precisely. Oppositely, the sub-maximal space morphology is exposed outside, making the spatial overlap easy to achieve. In our work, the colloidal CdSe/ZnS quantum dots are chosen as light sources for the highly efficiency, narrow line-width and quasi two-level systems [8, 9, 20, 30]. Meanwhile, the spectral overlap between the emission and the surface modes is easy to implement by choosing the appropriate size of CQDs. Based on these, the colloidal quantum dots whose spontaneous emission wavelength is consistent with that of the surface mode, are coated on the hybrid crystal by using spin coater. The spatial distribution of the CQDs on the crystal is obtained by the fluorescence microscopy and the result is shown in Fig. 1 (e). So one can address that the CQDs are periodically distributed and the amount round the equator of the PS microspheres is much greater than that on the top. By comparison, the spatial overlap between the distribution of CQDs and the surface modes can be verified.
Fig. 2. Schematic of experiment setup for angle-resolved reflectance and emission technique.
To make it clear how the surface resonance modes in the hybrid crystal modify the spontaneous emission of CQDs, microscopic angle-resolved reflectance and emission technique are used. The experimental setup is shown in Fig. 2. As known, the reflected lights from the structure, if with the same direction, will be focused on the rear focal plane (also named Fourier plane) of the microscope objective. So the points on Fourier plane correspond to the information of reflectance at specific angles. With the help of image lens, Fourier plane is imaged on the slit of the spectrometer that is equipped with matrix CCD. Here, different from the conventional imaging, the 2D CCD obtains the information of both angle and spectrum with a snapshot, from which the photonic band structure can be extracted. 3. Results and discussion The Γ-X high-symmetric direction of the hexagonal lattice (shown in Fig. 1(b)) is selected and the reflectance spectra at different angles are measured for the crystal with CQDs coated. The results with different polarizations are shown in Figs. 3(a) and 3(b). The black regions characterize the reflection dips, meaning strong absorption of the incident light. The normal reflectance is extracted and shown as the red curve on the left side of the figure, implying the existence of a surface mode. When departing from normal incidence, the reflectance dips shift in spectrum and show obvious dispersion behavior in the hybrid crystal. Figures 3(c) and 3(d) depict the results of emission spectrum with different polarizations. It is obvious that the
#211384 - $15.00 USD Received 5 May 2014; revised 24 Jul 2014; accepted 11 Aug 2014; published 18 Sep 2014 (C) 2014 OSA 22 September 2014 | Vol. 22, No. 19 | DOI:10.1364/OE.22.023473 | OPTICS EXPRESS 23476
emission is channeled by the surface modes. As a result, the distribution of the modified spontaneous emission agrees well with dispersion behavior. The phenomenon that the spontaneous emission is limited in a specific direction for a given wavelength is called as the directionality of SE [29].
Fig. 3. The results of microscopic angle-resolved reflectance (shown in (a) (b)) and emission (shown in (c) (d)) spectra with different polarized configurations for the structure with CQDs coated. The red line represents the normal reflectance spectra of the hybrid structure.
In order to further understand how the surface mode modifies spontaneous emission, timeresolved photoluminescence measurements are performed on the hybrid structure coated with CQDs. Meanwhile, Ag film with CQDs painted on is chosen as a reference sample for the following two reasons. Firstly, for the lack of periodical dielectric spheres, which introduce reciprocal lattice vector for the hybrid structure, the incident electromagnetic waves cannot couple with the reference sample. Secondly, when the metal film is present, the intrinsic recombination rate of CQDs includes not only decay to the environment but also energy transfer to the metal [31,32]. In Fig. 4(a) time-resolved photoluminescence at center wavelength is shown and it is evident that the photoluminescence intensity of CQDs on the hybrid structure decays much faster than that of CQDs on the Ag film. The average lifetimes of the two samples are 1.47 ns and 3.75 ns, respectively. The former is nearly three times faster than the latter. Two main contributions are present in the data as the radiative and nonradiative decay, so the PL decay curves are usually fitted with dual-exponential functions [31, 32]. In Fig. 4(b), the measured lifetimes as a function of wavelength are shown. The red square represents the radiative lifetime and the black cycle represents the non-radiative lifetime, respectively [31, 32]. By increasing the wavelength from 600 nm to 620 nm, the lifetime of non-radiative changes slightly and the variation between the maximum and minimum values is about 0.25 ns. In strong contrast, for the radiative lifetime, the variation reaches up to 3.65 ns in the same wavelength range. Furthermore, the radiative lifetime is getting longer when deviating from the center wavelength of the surface mode, which is different from the non-radiative decay. Here, one can address that such efficient modulation of photoluminescence lifetime is mainly due to the variations of the radiative decay rather than the non-radiative. To get a more intuitive observation how the surface mode affects the radiative decay, radiative lifetime at different wavelengths for the two substrates are summarized in Fig. 4(c), and the deviation is roughly 0.2 ns. Different from the CQDs painted on the hybrid crystal, the lifetime increases with the wavelength increasing when the emitters are painted on Ag. This is mainly due to the variation of the colloidal quantum dots’ size [33, 34]. The variation #211384 - $15.00 USD Received 5 May 2014; revised 24 Jul 2014; accepted 11 Aug 2014; published 18 Sep 2014 (C) 2014 OSA 22 September 2014 | Vol. 22, No. 19 | DOI:10.1364/OE.22.023473 | OPTICS EXPRESS 23477
tendencies of lifetime curves for the two different structures are in stark contrast. This phenomenon can be interpreted in terms of the optical resonance cavity. In Fig. 4(d), the reflectance and density of state (DOS) at Γ point (corresponding to the normal incidence) are calculated and the small oscillations of calculated reflectance are due to the convergence problem. The result for DOS is the integration of different position and the spectral overlap between DOS and the emission of CQDs is obvious. Meanwhile, the spatial distribution can be extracted from the electric field distribution shown in Fig. 1(d) and the location of strong field means high DOS. So in this case, the spatial and spectral agreements between the emitters with DOS are both achieved. According to Fermi’s golden rule that the decay rate is proportional to the local density of state (LDOS), the radiative decay is significantly enhanced by the surface mode. Notably, the CQDs are not considered in our calculation, resulting in the deviation of center wavelength from the experimental measurement shown in Fig. 4(d).
Fig. 4. (a) The results of the time-resolved photoluminescence at 610nm. (b) The radiative lifetime and the non-radiative lifetime at different wavelength as the CQDs are pained on the hybrid crystal. (c) The radiative lifetime of PL decay on the two substrates. (d) The reflectance and DOS at Γ point for our structure.
Extracted from Fig. 3(c), SE of p polarization at different angles when the CQDs are painted on our hybrid crystal is shown Fig. 5. It is evident that, besides the significant change of decay rate, the FWHM and center wavelength of SE are also modified. Through the variation of FWHM and emission intensity, one can also address that the modulation
#211384 - $15.00 USD Received 5 May 2014; revised 24 Jul 2014; accepted 11 Aug 2014; published 18 Sep 2014 (C) 2014 OSA 22 September 2014 | Vol. 22, No. 19 | DOI:10.1364/OE.22.023473 | OPTICS EXPRESS 23478
efficiency is getting higher when the emission angle is close to zero angle (corresponding to Γ point). In the inset, the normalized SE spectrums around zero angle on different substrates are compared since it is difficult to control the quantity of CQDs strictly equal for them. The violet and pink regions correspond to the spontaneous emission of CQDs painted on the Ag film and hybrid crystal, respectively. The experimental results suggest that the FWHM of SE is compressed to 9 nm in the hybrid structure, much narrower than 27 nm on the Ag film. The center wavelength of SE also shifts, consistent with the reflectance dip in Fig. 4(d). All of these verify that most excited CQDs radiate via efficient coupling with the same mode, leading to the directionality of SE [31, 35, 36].
Fig. 5. The spontaneous emission at different angles when the CQDs are painted on hybrid crystal. The inset shows the normalized SE spectrums around zero angle on different substrates.
4. Conclusions In conclusion, remarkable enhancement of the coupling efficiency is achieved through the spatial and spectral overlap between colloidal CdSe/ZnS quantum dots and the surface resonance mode in the hybrid crystal. Consequently, the spontaneous emission of CQDs is strongly modified by the hybrid crystal. By measuring the decay rate of the excited CQDs and the FWHM of SE in the 2D hybrid plasmonic–photonic crystal slab, it is confirmed that most of the excited CQDs radiate via coupling with the same surface modes efficiently, which is the essential reason of the directionality of the SE. Finally, we believe that the economy and ease of fabrication of high-quality samples, together with the ability of tuning emission characteristics via the dielectric periodicity, endows this hybrid system with the potential to be exploited in future efficient light-emitting devices. Acknowledgments This research reported in this publication was supported by the National Natural Science Foundation of China (Grant Nos: 11274330, 10990103, 11104302, 11404064), the Shanghai Committee of Science and Technology of China (Grant No.11ZR1442300), Shanghai Pujiang Program (14PJ1401100, 14PJ1409600), 973 Program (2013CB632701 and 2011CB922004) and Fudan University Start-Up Research Fund, XWY and LS contributed equally to this work.
#211384 - $15.00 USD Received 5 May 2014; revised 24 Jul 2014; accepted 11 Aug 2014; published 18 Sep 2014 (C) 2014 OSA 22 September 2014 | Vol. 22, No. 19 | DOI:10.1364/OE.22.023473 | OPTICS EXPRESS 23479
