Letter Cite This: Nano Lett. XXXX, XXX, XXX−XXX
pubs.acs.org/NanoLett
Efficient Emission Enhancement of Single CdSe/CdS/PMMA Quantum Dots through Controlled Near-Field Coupling to Plasmonic Bullseye Resonators F. Werschler,† B. Lindner,† C. Hinz,† F. Conradt,† P. Gumbsheimer,† Y. Behovits,† C. Negele,‡ T. de Roo,‡ O. Tzang,§ S. Mecking,‡ A. Leitenstorfer,† and D. V. Seletskiy*,†,∥ †
Department Department § Department ∥ Department
Nano Lett. Downloaded from pubs.acs.org by UNIV OF CAMBRIDGE on 08/09/18. For personal use only.
‡
of of of of
Physics and Center for Applied Photonics, University of Konstanz, D-78457 Konstanz, Germany Chemistry, Chair of Chemical Materials Science, University of Konstanz, D-78457 Konstanz, Germany Electrical, Computer, and Energy Engineering, University of Colorado, Boulder, Colorado 80309, United States Engineering Physics, Polytechnique Montréal, Montréal, H3T 1J4, Canada
S Supporting Information *
ABSTRACT: A strong increase of spontaneous radiative emission from colloidally synthesized CdSe/CdS/PMMA hybrid particles is achieved when manipulated into plasmonic bullseye resonators with the tip of an atomic force microscope (AFM). This type of antenna provides a broadband resonance, which may be precisely matched to the exciton ground state energy in the inorganic cores. Statistically analyzing the spectral photoluminescence (PL) of a large number of individual coupled and uncoupled CdSe/CdS/PMMA quantum dots, we find an order of magnitude of intensity enhancement due to the Purcell effect. Time-resolved PL shows a commensurate increase of the spontaneous emission rate with radiative lifetimes below 230 ps for the bright exciton transition. The combination of AFM and PL imaging allows for sub-200 nm localization of the particle position inside the plasmonic antenna. This capability unveils a different coupling behavior of dark excitonic states: even stronger PL enhancement occurs at positions with maximum spatial gradient of the nearfield, effectively adding a dipolar component to original quadrupole transitions. The broadband maximization of lightmatter interaction provided by our nanoengineered compound systems enables an attractive class of future experiments in ultrafast quantum optics. KEYWORDS: Colloidal quantum dots, near-field enhancement, light-matter coupling, Purcell factor, time-domain quantum physics
S
Aside from the combination of quantum emitters with highquality photonic crystals,15,16 a practical advantage of coupling colloidal QDs to plasmonic nanoantennas is in well-established fabrication methods and straightforward access to geometrically defined polarization properties and eigen-frequencies of the resonators.17−20 The broadband character of the plasmonic resonance, sustaining field enhancement even on few femtosecond time scales,21 represents a distinct advantage over photonic cavities, which are more suitable for frequencydomain applications. Thus, QD-antenna systems form an encouraging route toward ultrafast quantum photonics. Despite the advanced capabilities to produce the nanoantennas and the semiconductor quantum emitters separately, merging the two is an ongoing challenge. In a first approach, the optical antenna is built around the emitter by using electron beam lithography22,23 or chemical etching.24 Modest enhancements of the spontaneous radiative decay rates have been achieved,17,25 possibly associated with degradation of the optical quality of the QD due to harsh processing steps.
emiconductor quantum dots (QDs) have long been considered promising candidates for quantum information processing, as well-defined and optically addressable spin configurations of isolated few-charge carrier systems form natural qubits.1−3 In addition to applications in single-mode quantum optics and perhaps against common intuition, sharp optical transitions in QDs can also be used for generation of nonclassical photonic wave packets in the time domain.4 QDs composed of II−VI semiconductors are particularly attractive, as strong Coulomb interactions and deep confinement potentials support well-separated energy eigenstates.5−7 These QDs excel in bright, nonblinking, and photostable photoluminescence (PL) emission at cryogenic temperatures.8 Over the past few years, numerous time-resolved studies of II−VI-based single quantum emitters4,9−11 have been performed to gain insight into ultrafast dynamics of charge and spin degrees of freedom. Toward quantum functionality, deterministic manipulation of the photon number12,13 of a femtosecond pulse through single-photon amplification has been recently demonstrated in single epitaxial II−VI QDs on subpicosecond time scales and with high photon-to-photon efficiencies.14 To further enhance the coupling to the radiation field, semiconductor QDs are often embedded into nanoresonators. © XXXX American Chemical Society
Received: April 17, 2018 Revised: July 27, 2018 Published: August 3, 2018 A
DOI: 10.1021/acs.nanolett.8b01533 Nano Lett. XXXX, XXX, XXX−XXX
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Figure 1. Characterization of concentric Bullseye resonators. (a) SEM micrograph of a typical bullseye antenna milled in a thin gold film by a focused Ga2+ ion beam. r indicates the radius of the central hole, a is the distance of the first groove to the center of the resonator, p is the distance between two grooves, and w is the width of a groove. (b) Sketch of a bullseye cross-section. t describes the thickness of the gold layer thermally evaporated onto a SiO2 substrate and s is the groove depth. Color coded: finite difference time domain simulation of the absolute value of the electric field distribution
E E0
. The sketch illustrates the manipulation of the CdSe/CdS/PMMA hybrid particles (upper right inset) via an AFM tip.
(c) Normalized transmission through the plasmonic resonator as a function of energy. The measured white-light transmission (green line) reveals the plasmonic resonance of an antenna with the nominal parameters r = 125 nm, t = 280 nm, s = 110 nm, p = 530 nm, a = 560 nm, and w = 220 nm. A pronounced maximum at an energy of Ec = 2.01 eV possesses a full width at half maximum of ΔE = 240 meV, resulting in a quality factor of Q ≈ 8. The red solid line shows the calculated resonance properties of a bullseye with similar nominal parameters. Inset: Measured resonance energy Ec as a function of parameter a, showing the extensive tunability of the plasmon resonance.
over a wide frequency range while the number of grooves controls the quality factor of the resonator. The bullseye resonators are milled using a focused Ga2+ ion beam into a thermally evaporated gold layer with a thickness of t ≈ 300 nm deposited on a monocrystalline SiO2 substrate (Figure 1a). A central hole of radius r in the range of 125 nm−175 nm is surrounded by three concentric grooves of a width of w = 220 nm and a depth of s = 110 nm. To match the resonance of the plasmonic antenna to the emission energy of our QDs, the distance a between the center of the bullseye and the first groove is typically varied in the range of a = 530 nm−600 nm. Slightly influencing the resonance shape, the spacing between two adjacent grooves is adjusted within p = 520 nm−570 nm. Figure 1b shows the absolute value of the electric field profile |E/E0|, normalized to the magnitude of the incident field E0 and calculated for a bullseye with parameters r = 150 nm, a = 540 nm, p = 530 nm, w = 220 nm, t = 280 nm, and s = 110 nm. We use a commercial computational electrodynamics solver36 based on the finite-difference time-domain (FDTD) method (see Supporting Information). The calculated field enhancement reaches |Emax/E0| ≈ 17 at the top inner edges of the central opening. The antenna design supports |E/E0| ≈ 5 when averaged over a relatively large volume bounded by a transverse area of radius 309 nm and a longitudinal depth of approximately 60 nm. Thus, our three-groove bullseye geometry achieves a desired optimum in the trade-off between strong field enhancement and a relatively large central opening, into which the quantum dots can be conveniently manipulated via an AFM tip. We design the resonance of our bullseye resonators around the mean photoluminescence emission energy of the colloidal nanocrystals around 2 eV. The normalized transmission spectrum of a typical antenna (green line) with the nominal parameters a = 560 nm, p = 530 nm, w = 220 nm, s = 110 nm, and t = 280 nm is depicted in Figure 1c (complete measurement details are contained in Supporting Information). A well-defined resonance at an energy of Ec = 2.01 eV exhibits a full width at half maximum of ΔE = 240 meV, corresponding to a quality factor of Q ≈ 8. Numerical simulations demonstrate that the small peak at an energy of
An alternative approach relies on a targeted manipulation of colloidal quantum emitters into a gap of an existing plasmonic antenna, or vice versa, by using a tip of an atomic force microscope (AFM).26−30 While alleviating the exposure of a delicate quantum emitter to high-energy electrons or chemical etchants, the challenge of utilizing high field enhancements is in the precise positioning of the QD into a small volume of the plasmonic mode.18 This challenge is further amplified by the fact that the placement of a colloidal QD within few nanometers of a metal antenna can result in a detrimental quenching of the photoluminescence. In this Letter, we demonstrate targeted AFM manipulation and efficient coupling of single colloidally synthesized CdSe/ CdS/PMMA hybrid particles31,32 to plasmonic bullseye antennas33,34 resulting in more than an order-of-magnitude speedup of the observed radiative lifetimes and a commensurate increase of the collected photoluminescence intensity of the emitter. This feat is made possible by careful engineering of the coupled antenna-QD system. First, we utilize a specialized design of the nanocrystals, consisting of a CdSe/CdS center encapsulated into a polymer shell exhibiting a well-defined radius of 30 nm (see Supporting Information for additional details). The organic shell guarantees high mechanical stability of the hybrid particles for the AFM manipulation, while at the same time providing a controlled separation of the QD from the metal surface of the antenna. The size of the PMMA shell is chosen such as to avoid inadvertent quenching of the emission,35 thus ensuring efficient radiative light-matter coupling. Second, the plasmonic resonance of the bullseye antenna can be tailored to the QD emission with the broadband character being ideal for targeted time-domain applications. Finally, the cylindrical symmetry preserves the helicity of scattered photons, which is a necessity for future tasks of quantum manipulations using our engineered spinphoton interfaces. Our plasmonic structures are intrinsically prepared for experiments in transmission geometry. A central hole in an optically dense gold layer is surrounded by three concentric grooves, establishing the so-called bullseye geometry. By changing the groove radii we can tune the plasmonic resonance B
DOI: 10.1021/acs.nanolett.8b01533 Nano Lett. XXXX, XXX, XXX−XXX
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the quantum dot ground state |GS⟩ in the framework of an envelope function approach (inset Figure 2a).38,40 The finestructure states |F, mF⟩ possess a total angular momentum of F = 2 with the projections mF = ±1 and mF = ±2, respectively. In the proximity of the zero-phonon lines acoustic phonon replica (Ac) manifest efficient vibrational isolation of the inorganic core by the PMMA shell.41−43 The sidebands resulting from discrete exciton-LO phonon (LO) coupling can also be observed. Finally, we note that our extensive characterization of the QDs, both in terms of second order photon correlation g(2) via Hanbury-Brown and Twiss interferometry and the spectral shape of the emission, consistently reveals PMMA shells to be either empty or to possess single emitters (see e.g. Supporting Information) with negligible signs of QD agglomeration.31 Once individual emitters are characterized, a diluted aqueous solution containing the CdSe/CdS/PMMA particles is spin-coated onto the bullseye antenna sample. Nanoparticles which are in the proximity or within a single concentric gold resonator are individually manipulated via an AFM tip toward the region of interest if needed (Figure 2b,c). Because not every PMMA shell contains a CdSe/CdS quantum dot, a precise localization of the quantum emitters after the nanomanipulation is required. Our approach is schematically represented in Figure 2d (see Supporting Information for details of the experimental setup). First, the bullseye center is located by imaging white light which is transmitted through the antenna (Figure 2e). Next, we image the QD luminescence when excited by a pulsed laser (Figure 2f). The comparison of both images enables our first method of estimation of the QD position relative to the bullseye center. Second, analysis of the AFM micrographs allows determination of the mean particle positions (see Supporting Information). Following the successful step of mechanical manipulation and confirmation of localization of a QD in the center of the bullseye, an optical characterization of the emitter-antenna system is performed. Figure 3a shows the normalized PL
2.50 eV is due to an out-of-plane resonance in the cylindrical central hole. The red line in Figure 1c shows the calculated normalized transmission (see Supporting Information) through a bullseye resonator with comparable geometric parameters, which is in good agreement with the experimental results. The wide tunability of the bullseye resonance (inset Figure 1c) is more than adequate for tailoring the concentric antennas to the emission properties of our nanoparticle quantum emitters. In order to quantify the efficiency of the emitter-antenna coupling, we first describe characteristics of the uncoupled nanoparticles. They consist of CdSe/CdS core−shell quantum dots which are fabricated via a hot injection approach.37,31 The inorganic center possesses a mean radius of RCdSe/CdS ≈ 4.5 nm and is embedded into a dense poly(methyl methacrylate) (PMMA) shell. The organic outer layer serves for improvement of the mechanical stability, ensuring a nondestructive positioning via an AFM tip (inset Figure 1b). Furthermore, the PMMA shell provides decoupling of bound electronic states38,39,1 from the environment, thus ensuring a highly stable, nonblinking emission of the semiconductor QDs. Finally, the miniemulsion polymerization31 allows for adjusting the radius of the polymer shell. From FDTD calculations of the near-field distribution (Figure 1b) and taking into account the requirements for the AFM manipulations, we find that RPMMA ≈ 30 nm is an optimal radius to ensure efficient overlap between the semiconductor core and the plasmonic field, in turn minimizing quenching and maximizing the coupling. Figure 2a depicts a typical normalized PL spectrum of a single hybrid particle at a temperature of T = 5 K deposited on a crystalline SiO2 substrate. Two fine-structure emission lines (A and F) can be observed, corresponding to the transitions of the two lowest exciton fine-structure states |2, ±1⟩ and |2, ±2⟩ to
Figure 3. Photoluminescence intensity enhancement and lifetime shortening. (a) Photoluminescence spectrum of a single hybrid particle located close to the inner edge of a bullseye resonator (red line), and a typical quantum dot located on a SiO2 substrate (blue line, centered to the maximum emission energy of the red line). Inset: representative resonance spectrum of a bullseye resonator (green line), tuned to the exciton ground state transition energy of 1.99 eV. An intensity enhancement of 5.5 compared to an uncoupled hybrid particle is observed. (b) Normalized spectrally integrated photoluminescence decay of the coupled quantum dot (red line) and an uncoupled nanocrystal (inset). The black line depicts the IRF. Two photoluminescence decay constants τA−BE ≤ 230 ps and τF−BE = 2.2 ns can be extracted in the case of the coupled hybrid particle, showing a tremendous speedup compared to an uncoupled nanocrystal exhibiting typical time constants of τA = 2.6 ns and τF = 55.6 ns (inset).
Figure 2. Photoluminescence spectrum of a nanocrystal, nanopositioning and localization. (a) Normalized photoluminescence spectrum of a single CdSe/CdS/PMMA hybrid particle on a SiO2 substrate. Two zero-phonon lines (ZPLs) originate from a dipoleallowed (A) and a nominally dipole-forbidden (F) exciton finestructure transition at an energy of 2.005 eV and 2.004 eV, respectively. Ac and LO are observed. Inset: three-level model to visualize the exciton decay pathways of the two lowest fine-structure states |2, ±1⟩ and |2, ±2⟩ to the quantum dot |GS⟩. (b) Hybrid particle located on a plasmonic bullseye resonator before nanopositioning via an AFM tip and (c) after manipulation. (d) Schematic depicting the nanocrystal localization process by overlapping the exemplary images of transmitted light through the concentric plasmon resonator (e) and of the photoluminescence emission of a quantum dot (f). C
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Nano Letters intensity (IQD−SiO2, blue line) of a typical uncoupled CdSe/ CdS/PMMA particle, placed on a SiO2 substrate at T = 5 K. In contrast, the emission of a coupled QD-bullseye system (IQD−BE) is shown in red. An impressive factor of (5.5 ± 3.0) in overall enhancement of the emission is observed. The enhancement factor is derived based on the mean value of photoluminescence counts collected from approximately 40 individual average QDs positioned on a monocrystalline SiO2 substrate. Excellent coupling of the QD is ensured by a position close to the center of a concentric plasmon resonator as well as a good match of the resonances in the coupled system at 1.99 eV (see inset Figure 3a). If the emission enhancement is due to the Purcell effect,44 the radiative lifetimes of the quantum emitter should also be strongly affected. Figure 3b depicts the normalized spectrally integrated time-resolved PL signal IQD−BE (red line), which follows after ultrafast resonant excitation of the same QDbullseye system. Differentiated from the instrument response function (IRF, black line), two distinct decay constants τA−BE ≤ 230 ps and τF−BE = 2.2 ns are extracted via multiexponential least-squares fitting. The identified components correspond to the decays of the A and F transition, respectively. For comparison, the inset of Figure 3b shows a representative photoluminescence decay of a typical uncoupled nanocrystal (IQD−SiO2). To quantify the speedup of the spontaneous emission of the coupled system, we compare it to the radiative lifetimes extracted from more than 30 radiative decay signals of uncoupled QDs (IQD−SiO2), revealing average time constants for relaxation of the |2, ±1⟩ state τA = (1.63 ± 0.95) ns and τF = (60.5 ± 10.4) ns of the |2, ±2⟩ state (see Supporting Information). Thus, a dramatic shortening of photoluminesτ cence lifetimes by a factor of - A = τ A ≥ (7 ± 5) and -F =
τF τF − BE
Figure 4. (a) Decomposition of the electric field distribution of a quadrupole located at the inner edge of a bullseye antenna. (b) Purcell enhancement versus radial distance. Purcell factor -F of different quantum dots as a function of the radial distance to the bullseye center (red dots). The blue line depicts FDTD simulation of the absolute value of the normalized intensity gradient
∂I(r ) ∂r
versus
the radial coordinate. The experimentally determined enhancement factors qualitatively follow the calculated intensity gradient.
spatial gradient
∂I(r ) ∂r
, where I(r) ∼ |E(r)|2, shows a
distribution (blue line in Figure 4b) which is in very good agreement with the measurement. Our results suggest that with increased precision in nanopositioning, selective enhancement of either dark, bright or both transitions can be achieved, depending on the nanoscale distribution of the concentrated electric field. In summary, we designed plasmonic bullseye resonators to enhance the radiative properties of single CdSe/CdS/PMMA hybrid particles. By proper selection of the organic-shell radius and through a careful matching of the plasmonic resonance of the antenna to the exciton ground state emission energy of the nanocrystals, we observe an order-of-magnitude increase in the photoluminescence signal of the quantum emitters. This enhancement is corroborated in time-resolved measurements of the photoluminescence decay. The recombination dynamics of the |2, ±1⟩ and |2, ±2⟩ exciton fine-structure states reveal a speedup in the radiative decay time constants by a factor of - A ≥ 7 and -F = 28, respectively. An enhancement of the photoluminescence yield together with an increase in the radiative rate provide unequivocal proof of efficient lightmatter coupling and therefore strong Purcell enhancement in our coupled QD-antenna system. Our engineered broadband and polarization-preserving spinphoton interface forms a promising system for high-fidelity qubit operations. Moreover, control over a spin system via ultrashort initialization and readout pulses may enable the possibility for efficient generation of novel states of quantum light at the few-cycle scale.
A − BE
= (28 ± 6) is observed. It is worth noting that
with τA−BE ≤ 230 ps, the coupled QD-bullseye system already exceeds the radiative decay constants of typical singly charged epitaxially grown semiconductor quantum dots45,14 which have been employed in femtosecond nonlinear experiments so far. While the increase of the spontaneous emission rate of the A line appears to be commensurate with the enhancement of photoluminescence intensity (Figure 2a), it is interesting to note that -F is instead larger than - A by a factor of 4. To understand the different behavior of the dark exciton transition, connecting the excitonic |2, ±2⟩ state and the QD |GS⟩, we consider its symmetry in more detail. Indeed, the selection rules for the electric quadrupole transition can enable this coupling confining the resulting emission predominantly to the nearfield. However, an effective dipole can be induced if the symmetry of the quadrupole is broken along one direction (Figure 4a) in turn enabling the coupling to the far-field.46 In fact, strong gradients associated with the distribution of the localized electric field of the plasmonic mode in the center hole of the bullseye can provide the required symmetry break. To check this assumption, we investigated the dependence of the enhancement factors on the radial distance from the bullseye center. Figure 4a depicts the quantity -F (red dots), extracted from the PL decays of different coupled quantum dots, as a function of the radial distance from the bullseye center, as determined from the AFM localization data. The strongest enhancement is evident around the radial position of 200 nm, corresponding to the inner-edge of the smallest metallic ring of the bullseye structure. Indeed, comparison with the calculated
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.nanolett.8b01533. Synthesis of CdSe/CdS/PMMA hybrid particles, experimental setup, optical characterization of CdSe/CdS/ D
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PMMA hybrid particles, AFM manipulation, nanocrystal localization, verification of single photon emitters, finite difference time domain simulations, polarization-dependent bullseye transmission, enhancement of F-line emission via intensity gradients (PDF)
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
S. Mecking: 0000-0002-6618-6659 D. V. Seletskiy: 0000-0003-3480-4595 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS Support by the Deutsche Forschungsgemeinschaft (DFG) via collaborative research center SFB767 is gratefully acknowledged. D.V.S. acknowledges support by the DFG project 283908774.
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