Excitation of Hybrid Plasmonic Waveguide Modes by Colloidal

Letter pubs.acs.org/journal/apchd5

Cite This: ACS Photonics 2019, 6, 1587−1593

Excitation of Hybrid Plasmonic Waveguide Modes by Colloidal Quantum Dots Shailesh Kumar* and Sergey I. Bozhevolnyi Centre for Nano Optics, University of Southern Denmark, Campusvej 55, Odense M, DK-5230, Denmark

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S Supporting Information *

ABSTRACT: Quantum information technologies will greatly benefit from efficiently coupled quantum emitter−waveguide systems. Hybrid plasmonic waveguide modes are relatively low-loss confined modes, which provide an opportunity for coupling to quantum emitters with the rate of emission being significantly enhanced and channeled into the waveguide. Here, we report on the excitation of hybrid plasmonic waveguide modes supported by titanium dioxide nanowires placed on monocrystalline silver flakes with a low-index polymer gap. Quantum dots emitting at 630 nm are located in the gap, where the mode field strength is maximum. This results in a decay-rate enhancement of ∼42 and an outstanding figureof-merit of ∼361, defined as a product of decay-rate enhancement, excitation efficiency of the waveguide mode, and propagation length normalized by the emission wavelength in a vacuum. We have studied this configuration numerically as well, and the results of numerical simulations support our experimental findings. KEYWORDS: quantum plasmonics, quantum optics, hybrid plasmonic mode, quantum dots, titanium oxide nanowires, silver flakes

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Here, we utilize colloidal quantum dots of mean diameter 6 nm to excite hybrid plasmonic waveguide modes supported by structures obtained by placing titanium oxide nanowires on monocrystalline silver flakes with a low-index gap. In Figure 1, we present the schematics of the experiment. The sample consists of silver flakes coated with 5 nm of a polymer onto which colloidal quantum dots with a mean diameter of 6 nm are distributed randomly. Titanium oxide nanowires with mean diameters of 100 nm are put on top by spin-coating, which results in structures supporting a hybrid plasmonic mode.

hotonic quantum systems can be built with an efficiently coupled quantum emitter−waveguide system as a building block.1−3 Efficient coupling between a quantum emitter and a waveguide can be achieved if the waveguide supports a very confined mode.4,5 Waveguide modes confined down to few nanometers in cross-section are supported by some plasmonic waveguides.6 Due to the confinement, these modes can enhance the emission rate and efficiently channel photons from a single emitter.7 Various quantum emitter−plasmonic waveguide systems have been experimentally studied.8−18 However, plasmonic waveguides utilized so far have supported either poorly confined modes,12,15,17 or modes with high propagation losses.8,11,18 Hybrid plasmonic waveguide modes supported by a high-index nanowire placed on a metallic surface with a low-index gap are sufficiently confined modes with relatively long propagation lengths.19,20 Such waveguides have been utilized for a demonstration of nanoscale lasers, for example.21,22 Pulses shorter than 800 fs have been observed with plasmonic lasers due to enhanced light−matter interaction in the hybrid mode.22 Moreover, these hybrid plasmonic waveguides can be integrated with dielectric waveguides.23,24 Similarly to other single-photon emitters, colloidal quantum dots (QDs) have been coupled to various plasmonic structures, as it changes the emission properties of quantum dots, opening new avenues of their application.8,13,25−29 Colloidal QDs have been utilized as a gain medium in lasers and for probing the field enhancements as well.30,31 Colloidal QDs due to their small size can be put in small gaps, where mode can be highly confined, and therefore strong enhancements in decay-rate can be obtained. © 2019 American Chemical Society

Figure 1. Schematic of the experiment. The inset shows cross-sections of two possible coupled structure configurations. QDs: quantum dots, TiO2: titanium oxide nanowire. Received: March 11, 2019 Published: July 2, 2019 1587

DOI: 10.1021/acsphotonics.9b00379 ACS Photonics 2019, 6, 1587−1593

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nanowires varies around 100 nm, we present the real part of the effective index and propagation lengths as a function of titanium oxide wire diameter in Figure 2(b). When an emitter is placed in a propagating plasmonic mode, there are three decay channels for the emitter.4,5 The three channels are decay into the free space (Γr), decay into the propagating plasmonic mode (Γpl), and decay into the loss channels (Γnr). We have calculated the decay-rate of an emitter into the propagating plasmonic mode normalized by its decay-rate when placed in a vacuum (Γ0) with 2D modeling of the gap mode.5 The decayrate into the plasmonic mode for a dipole emitter emitting at 630 nm, placed with optimal orientation (aligned with the electric field) at different positions, is presented in Figure 2(c) for the configuration presented in Figure 2(a). It can be observed that Γpl is largest when the dipole is in the gap and nearest to the titanium oxide nanowire. To calculate the βfactor, total decay-rate was obtained utilizing a finite element model in COMSOL Multiphysics. A 3D model was used, where QDs were modeled as single dipole emitters along the yaxis emitting at 630 nm, and total decay-rate enhancement compared to its decay-rate in a vacuum is obtained. β = Γpl/ Γtotal is plotted as a function of position, along the x-axis at y = 8 nm, that is, 8 nm above the silver flake, in Figure 2(d). For the optimum position and orientation of the emitter, the βfactor can reach a value of 0.83. As can be observed from Figure 2(c) and (d), when a quantum emitter is placed in the gap between the titanium oxide nanowire and the silver flake, emission into the waveguide mode can be more than 80× larger than it is when the emitter is placed in a vacuum. This is a promising system where propagation lengths can be more than 20 μm, together with a total decay-rate enhancement larger than 100 and a β-factor of 0.83. For distances, along the x-axis, larger than 300 nm, coupling into the hybrid mode is negligible and the total decay-rate enhancement for QDs would be 4.4 due to coupling of QDs to surface plasmons and nonradiative decay channels. We also note that including QDs with a 6 nm diameter, with a refractive index of 2.4, does not significantly change the decay-rate enhancements, as only the relative decay-rates are considered. However, it does reduce the propagation lengths (see the SI). For configuration 2, the simulations are presented in the SI. Even though the maximum Γpl/Γ0 is 200, due to the constraints of the dimensions, the QDs can only be sitting far from the field maximum and the maximum Γpl/Γ0 for QDs is 32 with a β-factor of 0.87, that is, a maximum total decayrate enhancement of 37. We have experimentally obtained higher decay-rate enhancements; therefore configuration 2 is less likely to be the configuration formed for the coupled system presented in this Letter. Also, the density of QDs on the surface is high enough for titanium oxide nanowires to be sitting on QDs (see the AFM image in the SI). Next, we present our experiments and results obtained in the experiments. First of all, monocrystalline silver flakes were chemically prepared. We have followed a procedure based on platinum (Pt)-nanoparticle-catalyzed and ammonium hydroxide (NH4OH)-controlled polyol reduction of silver nitrate.34 More details of the fabrication procedure can be found in the SI. Silver flakes, thus obtained, have been shown to support surface plasmon polaritons with the lowest propagation loss in the visible wavelength region.17,18,34−36 Silver flakes are spincoated on a silicon sample. Subsequently, alternate layers of poly(allylamine hydrochloride) (PAH) and polystyrenesulfonate (PSS) are put on the sample following a procedure

There are two configurations possible for the waveguide as presented in the inset of Figure 1. The waveguide mode shown in configuration 1 is very confined with mode area A = 197 nm2, defined as the ratio of a mode’s total energy density per unit length and its peak energy density (A1) in ref 32, at a wavelength of 630 nm, and couples well with QDs (see Supporting Information (SI) for more details about calculation of mode area.) Similarly, for configuration 2, the mode area A = 79 nm2, at a wavelength of 630 nm, is obtained. In the experiment, quantum dots are excited by a green, 532 nm continuous wave (CW) or pulsed laser. Quantum dots coupled to the waveguide emit plasmons into the waveguide mode. The plasmons propagate to the end of the waveguide, and get scattered to the far field. A fraction of emission from QDs is emitted directly into the far field, as shown in the schematics. We observe a total decay-rate enhancement of ∼42 and a βfactor, defined as the fraction of photons channeled into the waveguide mode, up to 73%. This results in a figure-of-merit (FOM), defined for a quantum emitter−plasmonic waveguide system as the product of total decay rate enhancement, βfactor, and propagation length normalized by vacuum wavelength, of ∼361 for the coupled system, which is the highest FOM reported so far.8,11−13,15,17,18 We present a comparison of FOMs and propagation lengths for different quantum emitter−plasmonic waveguide systems in the SI. We now present results of numerical simulations for configuration 1 presented in Figure 1. In Figure 2(a), we

Figure 2. (a) Plot of the normalized electric field. Arrows show the direction of electric field at its tail, and lengths of arrows are proportional to the magnitude. (b) Effective mode index and propagation lengths as a function of titanium oxide wire diameter. (c) Decay rate into the gap plasmonic mode normalized to the decay rate of the emitter in a vacuum. (d) β-Factor for the plasmonic mode, for y = 8 nm along the x-axis in (c).

present the electric field distribution for the hybrid mode, which is calculated with a finite element method (FEM) using a commercial software (COMSOL Multiphysics). We have simulated the mode for the experimental conditions, that is, titanium oxide (refractive index −2.4) nanowire diameter of 100 and 5 nm layer of polymer (refractive index −1.5) and a gap of 6 nm on top of the polymer (corresponding to the QD diameter), at a vacuum wavelength of 630 nm, and used corresponding silver refractive indices.33 The mode presented in Figure 2 is without QDs. As diameter of the titanium oxide 1588


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reported before.25 Briefly, the sample was immersed in a 3 mM solution of PAH for 5 min and 3 mM PSS for 5 min and terminated with a final step in PAH for 5 min. Each PAH/PSS step deposits a ∼1 nm polymeric film with a surface positive/ negative charge. After each step (PAH or PSS), the sample was rinsed with ultrapure deionized (DI) water and 1 min in 1 M NaCl. In total, a 5 nm layer of polymers is put on top of the silver flakes. Subsequently, colloidal quantum dots, CdSe/ZnS core/shell quantum dots, emitting at 630 nm (Sigma-Aldrich) dissolved in toluene are spin-coated so that they are welldispersed on the sample. As a final step in the sample fabrication, titanium oxide nanowires dissolved in DI water are spin-coated. Titanium oxide wires have mean diameters of 100 nm and mean lengths of 10 μm (Sigma-Aldrich). In Figure 3(a), we present an atomic force microscope (AFM) image of

the titanium oxide nanowire axis and is a signature of the structure supporting a propagating plasmonic waveguide mode. We next characterize emission properties of our colloidal quantum dots. A schematic of the optical setup for characterizing QDs and coupled systems can be found in the SI. Colloidal quantum dots are spin-coated on glass following the same process as on the silver flakes. A confocal fluorescence image is presented in Figure 4(a). In the image, it can be observed that each spot has pixels that have high and low counts next to each other, which is due to blinking of the quantum dots. The image was taken by dwelling at each point for 20 ms. An emission spectrum of a single quantum dot is presented in Figure 4(b). An autocorrelation measurement taken for a spot is shown in Figure 4(c). g2(τ = 0) is 0.16, which clearly suggests that the autocorrelation measured corresponds to a single-photon emitter. The quantum dots used in this experiment have a distribution in their lifetimes, which we present in Figure 4(d). The average lifetime obtained is 8.3 ± 2.2 ns, where 2.2 ns is one standard deviation. Similarly, the average lifetime obtained for QDs sitting on polymer-coated silver flakes is 2.3 ± 0.4 ns, where 0.4 ns is one standard deviation (see histogram of lifetimes in the SI). This suggest a decay-rate enhancement of ∼3.7 for QDs on flakes compared to QDs on glass substrates. After characterization of quantum dots as well as the waveguides, we now present characterization of a coupled system. A fluorescence image of a coupled system is shown in Figure 5(a), where QDs are excited with a 532 nm CW laser and a fluorescence camera image is taken. One can see two more spots, other than the excitation spot where the QDs lie, corresponding to the ends of the titanium oxide nanowire. This suggests that the QD emission is channeled into the waveguide mode, propagates to the end of the waveguide, and scatters into the far field from the ends of the waveguide (spots A and B in Figure 5(a)). Due to the coupling of QDs to the confined mode, the decay rate of the QDs is greatly enhanced. The lifetime measurement data and a single-exponential fit (A exp(−t/τ) + C, where A and C are constants) for the three spots are presented in Figure 5(c), (d), and (e). The instrument response function (IRF) is also presented for our experimental setup, and it does not affect the measured lifetime estimations. The lifetime obtained for the QDs is 41.5 times smaller than the mean lifetime of QDs measured when they are situated on a glass substrate. Also, the lifetimes obtained for spots A and B are close to the lifetime obtained for the spot QDs. This suggests that spots A and B and QDs arise from the same source.37 In Figure 5(b), we present an AFM image of the coupled system. We indicate a particle in the AFM image that can also be seen in the fluorescence image in Figure 5(a) (indicated by arrows in both figures). The fluorescence from the particle is observed due to coupling of hybrid modes to surface plasmons at the end of the waveguide and subsequent scattering of surface plasmons by the particle (simulation results supporting coupling between hybrid mode and surface plasmons can be found in the SI). Next, we estimate the figure-of-merit for the coupled system. FOM as defined before is a product of propagation length, total decay rate enhancement, and β-factor normalized by the vacuum wavelength. The QDs emit with the same probability in two propagation directions, that is, toward ends A and B. Also, the scattering from the ends can be assumed to have the same efficiency. This can be utilized to estimate the propagation length for the hybrid plasmonic mode as Lp =

Figure 3. (a) AFM topography image of titanium oxide wires. (b and c) Optical microscope images when the waveguides are excited with a supercontinuum laser with polarizations along and perpendicular to the titanium oxide wire, respectively. Double-sided arrows represent the excitation polarizations.

a couple of titanium oxide wires on the fabricated sample. The two wires have diameters of 130 and 98 nm and lengths of 7.6 and 7.1 μm, respectively. We subsequently characterized the sample optically to test whether fabricated structures support a waveguide mode. We excite the waveguide mode by shining one end of a titanium oxide nanowire with a supercontinuum laser (SuperK, NKT Photonics) filtered with transmission between wavelengths 600 and 650 nm. When the polarization of the laser is along the length of the titanium oxide nanowire, we observe emission from the distal end (Figure 3(b)), whereas when the polarization of the laser is perpendicular to the length of the titanium oxide nanowire, we see only weak emission from the distal end (Figure 3(c)). This means that the waveguide mode can only be excited when the excitation polarization is along 1589


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Figure 4. (a) Confocal fluorescence image of quantum dots when they are spin coated on a fused silica glass substrate. (b) Emission spectrum of the spot indicated in (a). (c) Autocorrelation measurement and a model fit to the data for the spot indicated in (a). (d) Histogram showing measured lifetimes for 24 QDs.

Figure 5. (a) Fluorescence image of coupled system captured with QDs excited continuously by a 532 nm CW laser. (b) AFM image of the coupled system. (c, d, and e) Lifetime measurement data and a single-exponential fit for spots QDs, A, and B in (a), respectively.

(LB − LA)/ln(IA/IB), where, LA and LB are distances to ends A and B from QDs, respectively. IA and IB are the intensities measured at ends A and B, respectively.12,15 The propagation length for the system presented in Figure 5(a) is calculated to be 7.5 μm. This is smaller than the estimated propagation length of 20.0 μm with silver refractive indices from Johnson and Christy.33 This difference could be due to the presence of QDs in the cross-section, which decreases the propagation lengths from 20.0 μm to 8.4, 5.8, and 4.5 μm depending on the number of QDs (1, 2, and 3, respectively) in the cross-section (see SI for simulation results). Furthermore, scattering from the rougher surface due to polymer layer and QDs on which

the titanium oxide wire rests can add to the propagation losses. We estimate the β = (I′A + I′B)/(I′A + I′B + I′QDs), with I′A = (IA exp(LA/Lp))/(0.28), I′B = (IB exp(LB/Lp))/0.28, and I′QDs = (IQDs)/0.42 where IQDs is the intensity at spot QDs, to be 0.73 for this system. Here, 0.42 is the collection efficiency of QDs’ radiation to the far field and 0.28 is the collection efficiency of photons coupled to the hybrid mode. Forty-six percent of photons coupled to the hybrid mode are further coupled to surface plasmons, and 52% of scattered photons are collected from waveguide ends, resulting in 28% of photons coupled to hybrid modes being collected by the objective (see the SI for more details). In the above calculation of the β1590


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by the index contrast. For all other quantum emitter− waveguide systems, the decay-rate enhancements are lower than 10.8−15,17 Therefore, we conclude that hybrid plasmonic waveguides presented in this Letter are attractive because of allowing one to combine high decay-rate enhancements and relatively long propagation lengths. Due to relatively large propagation lengths of the waveguide mode supported by the structure presented above, in the visible region, we can excite the QDs remotely as well. In Figure 6(a), we present a camera image, where the green CW laser (532 nm) with polarization as indicated is shone on one of the ends of a titanium oxide wire. The laser coupled to the waveguide, and emission from the distal end can be observed as laser light outcoupled from the far end. In addition, a red spot can be seen in the middle of the waveguide, which is due to the fluorescence from the QDs that lie in the waveguide mode and get excited by the green laser light. The QDs excited in this way get coupled to the waveguide as well, and emission from the distal ends could be observed. A fluorescence image for the system is presented in Figure 6(b). The image suggests that QDs2 couple to the waveguide, as we observed for the direct excitation of QDs. In the fluorescence image, spot C has less intensity compared to spot D due to QDs present near end D getting excited directly, which can couple to the waveguide mode as well, but its fraction compared to that coupled from QDs2 is negligible. This is indirectly seen in fluorescence spectra collected from the three spots QDs2, C, and D in Figure 6(c), where emission from spot D is slightly shifted, and the emission spectra from spots C and QDs2 match closely and originate from QDs2. In conclusion, we have presented a waveguide system that supports a very confined mode in combination with relatively long propagation lengths in the visible wavelength region. For quantum dots coupled to such a waveguide, we have obtained a very high FOM, which is a measure of how well such a system would support a quantum network. Moreover, we have also shown remote excitation of QDs, which shows their potential for integration of excitation source together with a quantum emitter. This paves the way for such coupled systems to be integrated into a device where excitation source, quantum emitter, and detectors can all be on a chip. As an outlook, this kind of waveguide can be coupled to other promising quantum emitters for quantum information processing, such as defect centers in diamonds or molecules

factor, nonradiative decay was neglected. Nonradiative decay may not be negligible for this system, and there is no direct way to measure the nonradiative decay rate in our experiment. Therefore, to obtain a lower bound of the experimental βfactor, we assume the fraction of nonradiative decay to be 1 − βsim, where βsim is the β-factor obtained in simulations. This way, we obtain a lower bound of the experimental β-factor, that is, a βexp of 0.60. The FOM for this system, (LpβΓtotal)/(λ0Γ0), is in the range 296 to 361, due to the range for the β-factor being 0.60−0.73, where λ0 = 630 nm, and Γtotal/Γ0 is the total decay-rate enhancement. This FOM is the highest FOM reported in the literature so far and is achieved due to a combination of large decay-rate enhancement and relatively long propagation length.8,11−13,15,17,18 We have observed coupled systems, decay-rate enhancements, and emission from the distal ends for more systems. In Table 1, we summarize properties of five coupled systems. Table 1. Properties of Five Coupled Systems system no. lifetime of coupled QDs (ns) total decay rate enhancement propagation length (μm) β-factor FOM

1

2

3

4

5

0.20 41.5 7.5 0.73 361

0.23 36.1 6.6 0.66 250

0.34 24.4 8.2 0.78 248

0.21 39.5 5.5 0.71 245

0.28 29.6 6.8 0.69 220

System 1 is presented in the main text of this Letter. Fluorescence images of the four other systems can be found in the SI. The highest decay-rate enhancement observed among coupled systems is 41.5, whereas the calculated total decay-rate enhancement is ∼100. This is due to the random orientation of the dipole moment of a colloidal QD in the plane perpendicular to its crystalline c-axis, and experimentally a Purcell enhancement of at most {(Γtotal/Γ0)max}/2 is expected under an optimal condition where the QD c-axis is horizontal.38,39 For an all-dielectric system utilized in ref 38, the decay-rate enhancement was lower (31) with a β-factor of 0.78, whereas the decay-rate enhancements obtained in ref 18 for metal−dielectric−metal (MIM) is higher (50) with a βfactor of 0.82. For MIM waveguides, the propagation losses are much higher, whereas for the all-dielectric (high−low−high refractive index) system, the decay-rate enhancement is limited

Figure 6. (a) Excitation of waveguide mode with a 532 nm CW laser. Double-sided arrow represents the excitation polarization. The red spot (QDs2) in the middle appears due to emission from QDs excited by the plasmonic mode. (b) Fluorescence image obtained for the remotely excited QDs presented in (a). (c) Spectra for three spotsQDs2, C, and Dindicated in (b). 1591


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in nanocrystals.40,41 Moreover, such structures can be made lithographically, which will allow for fabrication of complex quantum plasmonic circuits.

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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/acsphotonics.9b00379. Optical setup, fabrication of silver flakes, silver surface properties at different stages of fabrication, simulations, lifetime of QDs on silver flakes, coupled systems, and figure-of-merit (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Shailesh Kumar: 0000-0001-5795-0910 Sergey I. Bozhevolnyi: 0000-0002-0393-4859 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS All authors gratefully acknowledge the financial support from the European Research Council, grant no. 341054 (PLAQNAP).

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REFERENCES

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ACS Photonics

Letter

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Excitation of Hybrid Plasmonic Waveguide Modes by Colloidal

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