Extremely confined gap-plasmon waveguide modes excited by

Dec 10, 2018 - Here, we present experimental results on the GPW mode excitation in a gap between a monocrystalline silver nanowire and a monocrystalli...
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Extremely confined gap-plasmon waveguide modes excited by nitrogen vacancy centers in diamonds Shailesh Kumar, Sebastian K.H. Andersen, and Sergey I Bozhevolnyi ACS Photonics, Just Accepted Manuscript • DOI: 10.1021/acsphotonics.8b01225 • Publication Date (Web): 10 Dec 2018 Downloaded from http://pubs.acs.org on December 11, 2018

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Extremely confined gap-plasmon waveguide modes excited by nitrogen-vacancy centers in diamonds Shailesh Kumar,∗ Sebastian K. H. Andersen, and Sergey I. Bozhevolnyi Centre for Nano Optics, University of Southern Denmark, Campusvej 55, Odense M, DK-5230, Denmark E-mail: [email protected] Abstract Quantum emitters with high emission rates and efficiently coupled to optical waveguides are in demand for various applications in quantum information technologies. Accurate positioning of a quantum emitter within a strongly confined gap-plasmon waveguide (GPW) mode would allow one to significantly enhance the decay rate and efficiency of channeling of emitted photons into the waveguide mode. Here, we present experimental results on the GPW mode excitation in a gap between a monocrystalline silver nanowire and a monocrystalline silver flake by using a single nitrogen-vacancy center in a nanodiamond. The coupled systems containing a nanodiamond and the structure supporting the GPW mode are created by a combination of electron beam lithography and nano manipulation with an atomic force microscope (AFM). In these systems, we find the decay rate enhancements of up to ~50, and the efficiency of channeling of photons into the GPW mode of up to 82%, resulting in an exceptionally high figure-of-merit of 212 for the emitter-plasmonic waveguide coupled system. The

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results obtained suggest new avenues for practical realization of integrated solid-state quantum systems.

Keywords Quantum Plasmonics, Quantum Optics, Gap Plasmons, Nitrogen-Vacancy Center, Silver Nanowires, Silver Flakes Efficiently coupled emitter-waveguide system can be utilized for making efficient on-chip single photon sources, or a quantum network based on single photon emitters interacting via photons in the waveguide. 1–4 The phenomenon of emitter waveguide coupling is being studied extensively, and is coined the waveguide quantum electrodynamics (wQED). For wQED to be useful for applications, such as for making a quantum network, it is essential that emitters emit with very high efficiency and high rate into the waveguide. 5–11 This can be realized by a high emitter-waveguide coupling strength, achievable for a waveguide that supports an extremely confined mode. Confined modes down to nanometer-scale are supported by some plasmonic waveguides. Gap-plasmon waveguide (GPW) modes can confine light to extremely small mode areas, and therefore, these modes can enhance the emission rate as well as efficiently channel photons from a single emitter. 12 Among quantum emitters, nitrogen-vacancy (NV) centers have been used extensively, due to suitable properties for quantum information processing, such as stable single photon emission at room temperature, long spin coherence time and optical read-out of spin states. 13 NV centers have been utilized for certain quantum information related tasks, such as quantum interference of photons, entanglement of electronic spin to single photons, entanglement between two distant electronic spins, teleportation between distant qubits, loophole free violation of Bell’s inequality and, recently, deterministic delivery of remote entanglement. 14–20 All these quantum information protocols have limited rates of operation due to low rate of emission and poor collection from the emitters. Therefore, an on-chip integrated emitter with high rate of

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emission and efficient channeling of photons into a waveguide will pave the way for a practical quantum network. In this direction, quantum emitters have been coupled to various plasmonic waveguides. 21–28 However, the decay rate enhancements obtained for quantum emitters coupled to waveguides have been below 10, either due to poor confinement of the mode or due to poor placement of the quantum emitter in the waveguide. For example, for NV centers coupled to single silver nanowire, V-groove or dielectric-loaded surface plasmon polariton waveguide (DLSPPW), 22,23,25,28 total decay rate enhancements reported is below 10 due to poor confinement of modes. For NV centers coupled to gap mode supported by two parallel silver nanowires, 24 total decay rate enhancement remained below 10 due to poor placement of nanodiamonds. Here, we use a single NV center to excite a propagating GPW mode supported by an all monocrystalline silver nanowire and silver flake separated by a gap of ~20nm. The coupled system of a nanodiamond containing a single NV center and the gap mode structure is fabricated using a combination of electron beam (e-beam) lithography and atomic force microscope (AFM) manipulation of nanoparticles. This combination of techniques allows us to deterministically put the nanodiamonds in the well-controlled gap between a silver nanowire and a silver flake. We obtain a total decay rate enhancement of ~50 and a β-factor, defined as the fraction of photons channeled into the gap mode, up to 82%. This results in a figure-of-merit (FOM), defined as the product of total decay rate enhancement, β-factor and propagation length normalized by vacuum wavelength, of 212 for one of the coupled system, which is the highest FOM reported so far. 25,28 Figure 1(a) schematically illustrates the experiment. The gap between a silver nanowire and a silver flake, both chemically synthesized, is created by a spacer layer of ~20nm thick hydrogen silsesquioxane (HSQ) pads. In the slot between two pads, a nanodiamond containing a single NV center is placed in the nanowire-flake air gap, which is excited by a green 532nm laser. The NV center emits with high efficiency into the GPW mode, single plasmons propagate to ends of the waveguide and get scattered into the far field. In Figure 1(b), we present the electric field distribution in the GPW mode, which is calculated with a finite

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Ag nanowire

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Ag flake max Ag Air

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Figure 1: (a) Schematic of the experiment. (b) Electric field distribution for the gap mode, where length and direction of green arrows are proportional to the magnitude and direction,respectively, of the electric field. (c) Real part of effective index and propagation length is plotted as a function of ’Gap’. element method (FEM) using a commercial software (COMSOL Multiphysics). We have simulated the mode for the experimental conditions, with a gap of 20nm and silver nanowire diameter of 200nm, at a vacuum wavelength of 700nm, and used corresponding silver refractive indices. 29 For GPW modes, the effective index and the propagation length depends a lot on the gap size, which is presented in Figure 1(c). Figure 2 shows the steps taken to obtain a coupled system as well as its optical characterization. In Figure 2(a), we show a scanning electron microscope (SEM) image of a single crystalline silver flake. We have synthesized colloidal Ag crystal flakes following a recently reported procedure based on platinum (P t)-nanoparticle-catalyzed and ammonium hydroxide (N H4 OH)-controlled polyol reduction of silver nitrate. 30 Details of the fabrication method can be found in the supplementary information (SI). Monocrystalline silver flakes have been utilized for fabrication of plasmonic waveguides before. 30–32 They do not produce much background fluorescence and are the lowest ohmic loss plasmonic material in the visible region. As a next step, HSQ pads were created using e-beam lithography. HSQ (Dow 4

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(b)

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Figure 2: (a) SEM image of a silver flake. (b) AFM image of HSQ pads on a silver flake. The height presented in the upper panel is along the dashed line shown in the lower panel. (c) AFM image of a nanodiamond containing a single NV center (indicated by the arrow) lying in the slot between pads close to a silver nanowire. (d) AFM image of a coupled system obtained by pushing a silver nanowire on top of the nanodiamond indicated in (c), so that the nanodiamond, indicated by the arrow, lies in the gap between the silver nanowire and the silver flake. HSQ pads act as support for the nanowire. (e) Fluorescence image when the NV center is excited. Spots A and B correspond to the ends of the nanowire. (f) Change in the lifetime of the NV center before and after silver nanowire is moved on top of the nanodiamond. Dots are measurements, whereas the continuous curves show tail fitting to the data with a single exponential. (g) Lifetimes of Spots A and B as denoted in the graphs.

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Corning XR-1541-006) is diluted five times with methyl isobutyl ketone (M IBK) (Sigma Aldrich), and spin coated (5000 rpm, 1 min) to make a ~20 nm thick film on silver flakes. With e-beam lithography and development in tetramethylammonium hydroxide (TMAH), pads of 5µm x 5µm with separations of 1µm are created, as presented in Figure 2(b) with an AFM topography image. The cured HSQ is hard enough to allow for AFM manipulation on pads, while its relatively low refractive index(1.41) provides for reasonable propagation length at the chosen gap size of 20nm (See SI for effective indices and propagation lengths for a range of nanowire diameters and gaps). Nanodiamonds (microdiamant 0-0.05µm) followed by silver nanowires are spin-coated on the sample. Thereafter, fluorescence images of silver flakes are obtained and various spots in the image are characterized for their spectrum, lifetimes and auto-correlation measurements (A schematic of our optical set-up can be found in SI). The nanodiamond which is found to contain a single NV center, i.e. auto-correlation g 2 (0) < 0.5, is moved in-between HSQ pads using the cantilever of an AFM, in a similar fashion as has been utilized in the past by the authors. 24,33 A silver nanowire is subsequently moved with a similar technique. 24 AFM images of a system created in this way are presented in Figure 2(c) and (d). The nanodiamond can be seen in the slot, before the silver nanowire is moved on top (Figure 2(c)).

(a)

(c)

(b)

Figure 3: (a) Spectra of the NV center before (red) and after (blue) coupling to the gap mode for the system presented in Figure 2. (b) and (c) Spectra obtained from spots A and B, respectively, indicated in Figure 2(e). Having realized the coupled system, we subsequently characterize the system optically. The NV center is excited continuously with a 532 nm continuous wave (CW) laser, and a

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fluorescence camera image is taken. The camera image, in Figure 2(e), shows three spots, one at the excitation point corresponding to emission from NV center directly into the free space, and two more spots corresponding to ends of the silver nanowire. The emission from ends of the silver nanowire suggests that the NV center emission is channeled into the waveguide mode, propagates to the end of the waveguide and scatters into the far field from ends of the waveguide (Spots A and B in Figure 2(e)). Due to coupling to the confined mode, the decay rate of the NV center is greatly enhanced. In Figure 2(f), we compare the total decay rate of the NV center before the silver wire was moved on top of the NV center and after the movement of the silver nanowire. The decay-rate enhancement, in this case, is ~14.7. In our experiment, nanodiamonds before coupling to GPW are situated on a silver surface. The silver surface does change the lifetime of the emitter, compared to when the emitter lies on a fused silica substrate. This lifetime change also depends strongly on the emitter position in the nanocrystal and its quantum efficiency. We do not have any direct way of estimating the decay-rate enhancement due to the silver surface. Therefore, we have chosen the statistical method of estimating the average decay-rate enhancement due to the silver surface. This approach has previously been employed for estimating decay rate enhancements of quantum emitters coupled to plasmonic waveguides, 21,22 due to a metallic surface, 28,34,35 due to plasmonic nano-particles 36 and due to hyperbolic metamaterials. 37 When compared to decay rates of NV centers situated on a glass substrate, the decay rate of NV centers on a silver surface is enhanced by a factor of ~2, on average (See SI for more details). Therefore, we estimate the total decay rate enhancement to be ~29.4 for this structure. In Figure 2(g), we present the lifetimes measured for spots A and B, when NV center is excited. The lifetimes observed for these spots are within error margins of the lifetime measured for the NV center spot, confirming the observed spots arise from the same NV center. The NV centers emit with equal probability towards nanowire ends, A and B. Also, the scattering from the ends can be assumed to have the same efficiency. This symmetry can be utilized to estimate the propagation length for the gap mode as Lp = (LB − LA )/ln(IA /IB ),

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g2(0)=0.21 (a)

g2(0)=0.48 (b) Figure 4: (a) and (b) Autocorrelation measured and a fit to the data before and after coupling, respectively, of NV center to the gap mode.

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where, LA and LB are distances from the NV center to ends A and B, respectively. IA and IB are the intensities measured at ends A and B, respectively. 25,28 The propagation length for the system presented in Figure 2 is calculated to be 3.7µm. This propagation length is close to the estimated propagation length of 5.0µm with silver refractive indices from Johnson and Christy 29 and the gap being filled with HSQ. Effective refractive indices and propagation lengths for various configurations with and without HSQ between silver nanowire and silver flakes can be found in SI (Fig. S6). In Figure 3(a), we present the NV center emission spectrum before and after its incorporation into the waveguide. All spectra shown in Figure 3, are measured with the same excitation power and spectrometer settings, 20µW 532nm CW excitation and 300s of acquisition time. Due to the large Purcell enhancement (14.7), the emission into the far field from NV center is also enhanced for the coupled system at the same excitation power, although a large part (82%) of the NV center emission does couple to the GPW mode. In Figure 3(b), we present the emission from Spot A in Figure 2(e), where we can see fringes in the spectrum. The fringes for spot A are observed due to interference between the plasmons that propagate directly from the NV center towards end A, and plasmons that propagate towards end B, get reflected and subsequently propagate towards end A. Similarly, fringes in the spectrum for spot B (Figure 3(c))appears due to the interference between NV center emission that is directly emitted towards end B and NV center emission towards end A, that gets reflected from end A and propagates towards end B. As end A is closer to NV center than end B, spectrum at end A has more fringes and the fringe visibility is smaller (due to larger contrast in propagation losses for two interfering contributions) compared to the spectrum at end B. Fringe visibilities for spots A and B are 0.09 and 0.16, respectively, which suggests an average reflection of 3% from the nanowire ends. We estimate β= 1.03 {IA exp(LA /Lp ) + IB exp(LB /Lp )} /[1.03 {IA exp(LA /Lp ) + IB exp(LB /Lp )}+IN V ] to be 0.82 for this system, with IN V being the intensity at spot NV. We note that the collection efficiency for spots NV, A and B are assumed to be the same, and non-radiative decay is ignored for this calculation. The FOM for this system, Lp βΓtotal /λ0 Γ0 = 127, where

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λ0 = 700nm, and Γtotal /Γ0 is the total decay rate enhancement. Γpl/ Γ0

Ag Air ND

Ag

(a)

(b) Figure 5: Numerical modelling of experimental system 1. (a) Decay rate into the gap plasmonic mode normalized to the decay rate of the emitter in vacuum. Estimated experimental position of the nanodiamond is indicated by a circle. ND: nanodiamond. (b) β - factor for the plasmonic mode. We have repeated this experiment seven times, and have obtained decay rate enhancements ranging from ~8 to ~50, β-factor ranging from ~0.63 to ~0.82, and FOM ranging from ~21 to ~212. This variation is attributed to geometrial variation in the size of nanodiamonds, gap and nanowire diameter, the spatial position of the nanodiamond in gap and NV center in the diamond as well as the quantum efficiency of individual NV centers. The parameters of the coupled system are put in table 1 for different coupled systems that we created. The highest β-factor 0.82 is obtained for system 1, highest decay rate enhancement ~50 and highest FOM ~212 is obtained for system 2, whereas the lowest lifetime 0.3ns is obtained for system 4. In SI, fluorescence images overlaid on AFM images of coupled systems 2-7 can be

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found. We note that the propagation lengths, β-factor and FOM could not be estimated for system 6, as the NV center was placed very close to one of the nanowire ends. In general, we demonstrate efficient coupling in combination with fast decay rates are experimentally attainable in GPW systems. The lifetimes of coupled NV centers are much smaller than the average lifetime of NV centers in nanodiamonds situated on fused silica glass. To obtain lower lifetimes, smaller gaps together with smaller nanodiamonds can be utilized. Table 1: Properties of seven coupled systems. Sl. No. 1 Gap (nm) 16 Nanodiamond height (nm) 18 Ag Wire diameter (nm) 210 Lifetime before coupling (ns) 22.1 Lifetime after coupling (ns) 1.5 Total decay rate enhancement 29.4 Propagation length (µm) 3.7 β-factor 0.82 FOM 127

2 3 4 5 6 21 17 14 24 15 19 21 15 24 14 128 105 154 202 125 29.9 9.6 3.2 11.2 7.1 1.2 2.4 0.3 2.0 0.8 49.8 8 21.5 11.2 17.7 3.9 3.2 3.5 1.8 0.76 0.65 0.78 0.71 212 24 84 21 -

7 24 28 122 28.2 6.6 8.5 2.9 0.63 27

In figure 4(a), we present auto-correlation measurement for an NV center, measured before it was incorporated in the coupled system 7. A correlation dip at g 2 (0) down to 0.19 is obtained, which indicates that the NV center emits single photons, as g 2 (0) is well below 0.5. The auto-correlation measurement after the NV center was incorporated in the gap mode was 0.48, as presented in figure 4(b). The dots are measurements, whereas the continuous curves are fit according to a model. 38 The main reason, we suspect, for the increase of the g 2 (0) is fluorescence from small silver particles that might form when nanodiamonds are moved on top of the silver flake, or when silver wires are moved on top of the nanodiamonds. Small particles of silver are known to fluoresce in the visible region. 39 We have studied system 1 with numerical simulations as well, which we present below. When an emitter is placed in a propagating plasmonic mode, there are three decay channels for the emitter. One is decay into the free space channels, called the radiative mode (Γr ), the second is decay into the propagating plasmonic mode (Γpl ), and the third is decay into the 11

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loss channels (Γnr ). We have calculated the decay rate of an emitter into the propagating plasmonic mode normalized by its decay when placed in the vacuum(Γ0 ) with 2d modeling of the gap mode. 40 Decay rate into the plasmonic mode for a dipole emitter emitting at 700nm, placed with optimal orientation at different positions, is presented in Figure 5(a) for a silver nanowire diameter of 210nm and a gap of 16nm, that is, dimensions corresponding to system 1. It can be observed that Γpl is largest when the dipole is in the gap and nearest to the silver nanowire. However, the decay into the lossy channels increases near the metal surfaces. Therefore, the fraction of emission into the plasmonic mode, namely the β-factor, decreases. To calculate the β-factor, total decay rate was obtained utilizing a finite element model in COMSOL Multiphysics. A 3D model was used, where NV center was modeled as a single dipole emitter along y-axis emitting at 700nm, and total decay rate enhancement compared to its decay in the vacuum is obtained. β = Γpl /Γtotal is plotted as a function of position in Figure 5(b). For the optimum position of the emitter, the β-factor can reach a value of 0.97. Comparing our experimental results to numerical simulations, we obtain lower decay rate enhancements and β-factor than those expected in numerical simulations. This mismatch can be related to the position of NV center in the nanodiamond (See Figure 5(a)) and the low quantum efficiency of NV centers in nanodiamonds, as has been reported before. 33,41 A quantum efficiency of 0.3 closely matches the simulations with the experimentally obtained results. In conclusion, we have demonstrated efficient coupling of single NV centers to GPW modes resulting in decay rate enhancements of ~50 and sub-ns decay lifetime, which have not been observed for NV centers coupled to waveguides before. β-factor of ~0.82 and FOM of ~212 obtained for the coupled systems imply that photons emitted with high rates are channeled efficiently to the GPW mode. This is achieved by a combination of the topdown structuring of nano spacer layers of HSQ ensuring well-controlled gaps between silver surfaces, and bottom-up fabrication and assembly of monocrystalline silver structures. As an outlook, Bragg mirrors can be introduced at one end to obtain unidirectional emission

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and the GPW mode can be coupled efficiently to a dielectric waveguide, 42 thereby harnessing the best from plasmonic and photonic waveguide properties, which will then make an ideal system for wQED. Also, to enhance the emission only into the ZPL of a quantum emitter, a cavity can be made around the quantum emitter by structuring of HSQ into Bragg mirrors. 43 Alternatively, one can employ other quantum emitters (with much higher efficiency of the ZPL emission) with the same gap-plasmon based configuration. For scalable fabrication of the demonstrated system, one can envision utilizing lithographic techniques in combination with doped nanodiamonds to obtain deterministic positioning of NV centers in a desired pattern. 44,45 Subsequently, silver nanowires can be deposited at desirable positions using a dielectrophoresis-based technique. 46 Overall, the remarkable results demonstrated with the proposed configuration open new perspectives for development and practical realization of integrated solid-state quantum systems.

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

Supporting Information Available S1: Optical set-up, S2: Fabrication of silver flakes, S3: Fabrication of silver nanowires, S4: Comparison of NV center lifetimes on fused silica glass and silver flakes , S5: Coupled systems, S6: Effective indices and propagation lengths for gap modes

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ver AFM cantile

NV center

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