Efficient Coupling of a Single Diamond Color Center to Propagating

Feb 15, 2013 - We report on coupling of a single nitrogen-vacancy (NV) center in a nanodiamond to the propagating gap mode of two parallel placed ...
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Efficient Coupling of a Single Diamond Color Center to Propagating Plasmonic Gap Modes Shailesh Kumar,* Alexander Huck, and Ulrik L. Andersen Department of Physics, Technical University of Denmark, Building 309, 2800 Kongens Lyngby, Denmark S Supporting Information *

ABSTRACT: We report on coupling of a single nitrogenvacancy (NV) center in a nanodiamond to the propagating gap mode of two parallel placed chemically grown silver nanowires. The coupled NV-center nanowire system is made by manipulating nanodiamonds and nanowires with the tip of an atomic force microscope cantilever. An efficient coupling of an NV-center to an easily accessible gap plasmon mode is demonstrated and we measure an enhancement of the spontaneous emission decay rate by a factor of 8.3.

KEYWORDS: NV-center, plasmon, gap mode, silver wire

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(x,y) plane for a plasmon supported by a single metallic nanowire located atop a silica substrate. We clearly see that the maximized field intensity is located at the boundary between the wire and the substrate that is difficult to access experimentally with nanocrystals, thus limiting the achievable coupling strength.13,14 Moreover, in case the maximized field on the metal surface could be accessed by the dipole it will be damped nonradiatively due to ohmic losses of the metallic wire. To circumvent these problems, in this Letter we propose a nanostructured system in which the plasmonic mode mainly propagates in free-space and is supported by a pair of nanowires, a dual nanowire system. Such a system supports two bound fundamental modes that we refer to as the symmetric (S) and the antisymmetric (AS) mode, depending on the charge distribution of induced dipoles in the transverse plane of the two nanowires.23 We demonstrate an efficient coupling of an NV-center to the easily accessible gap plasmon mode supported by two silver nanowires. A decay rate enhancement by a factor of 8.3 is observed. The highly confined and propagating AS mode, with a (+.−) charge distribution, has its maximum field intensity in the freespace intergap region between the two nanowires, which is illustrated in Figure 1b, where the Poynting vector distribution of the field is plotted. As the maximized plasmon intensity is located in free-space, it is easy to access and furthmore, the decay into ohmic loss channels of the wires will be less pronounced. For completeness, we also show the Poynting vector distribution for the S mode in Figure 1c. The field distribution of this mode is localized at the interface between

itrogen-vacancy (NV) centers in diamond are promising contenders for quantum information processing and quantum magnetometry because of their long spin coherence time,1 and the possibility of coupling their spin information to optical photons.2,3 However, to enable distributed quantum computing4,5 and high-efficiency magnetometry,6,7 the photons emitted from the NV-center must be harvested with high efficiency in a single mode. This can be done by coupling the NV-center to a cavity mode8−11 or a waveguide mode,12−14 but the performance of these approaches is either limited by diffraction or by the difficulty in placing the emitter in the field maximum. Surface plasmons supported by metallic nanostructures can be confined to subwavelength regions and thus provide a route to the miniaturization of optical circuitry and the realization of strong coherent coupling between quantum emitters and single optical modes.10 This combination of strong coupling and extreme miniaturization may enable the realization of deterministic quantum gates,15,16 single photon generators,16 qubit−qubit entanglement,17 and high-efficiency quantum sensors in nanoscale quantum devices.6,7 Because of these promising prospects, several experiments on coupling single emitters to either localized or propagating surface plasmons have been conducted.14,18−22 In particular, the coupling to a propagating single plasmonic mode on a silver nanowire has been achieved with colloidal quantum dots22 and NV-centers,13,14 resulting in an enhancement of the decay rate of the single emitters as well as the production of single plasmons in a single mode. The enhancement of the decay rates in these realizations, however, was modest as a result of the difficulty in accessing the strongly confined plasmon mode. This inaccessibility is clearly illustrated in Figure 1a where we plot the z-component of the normalized Poynting vector in the © 2013 American Chemical Society

Received: December 19, 2012 Revised: February 14, 2013 Published: February 15, 2013 1221

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Figure 1. Poynting vector and decay rate into plasmonic modes. (a−c) Normalized Poynting vector along the z-axis of a single silver wire (a), the antisymmetric (b), and the symmetric (c) mode of a dual silver nanowire. (d−f) Expected decay rate into the plasmonic mode(s) with respect to the rate in free space of a single silver nanowire (d), the antisymmetric (e), and symmetric (f) mode of a dual silver nanowire. In all plots, the nanowire diameter is 110 nm, the substrate is fused silica glass with n = 1.46 and superstrate is air with n = 1. The gap between the wires in (b), (c), (e), and (f) is 9 nm. These plots have been obtained for a vacuum wavelength of λ0 = 700 nm. The dielectric constant for silver has been used for λ0 = 700 nm.24

the wires and the substrate and thus not suitable for strong field-dipole interaction. Next, we investigate the expected enhancement of the decay rate into the plasmonic mode for the three systems in Figure 1a−c. We use a numerical model to estimate the plasmonic decay rate, Γpl, relative to the rate in vacuum, Γ0, via Γpl Γ0

=

3πε0 |E(x , y)|2 Re{k 02 ∫ (E × H*) ·z 0 dA} A∞

(1)

where ε0 is the vacuum permittivity, E and H are the electric and magnetic fields of the plasmon mode, respectively, k0 = 2π/ λ0 is the wavenumber in vacuum, z0 is a unit vector along the zdirection, and A∞ denotes integration over the transverse plane25 (see Supporting Information for more details). We compare the coupling into plasmonic modes of the dual nanowire system to a single nanowire with the same diameter as each of the wires in the dual nanowire system. In Figure 1d− f, we plot Γpl/Γ0 resolved in the transverse plane for a single wire (Figure 1d) of diameter 110 nm and the dual wire system with wires of diameter 110 nm and a gap of 9 nm for the AS (Figure 1e) and the S (Figure 1f) mode. Clearly, the coupling to the dual wire system is dominated by the AS mode in the gap region (more details in Supporting Information). In this case, coupling to the AS mode is an order of magnitude higher in comparison to a single nanowire. Therefore, in addition to a better accessibility of the free-space mode of the dual-wire system, it also possesses a significantly higher coupling strength than the single-wire system for identically sized wires. A more detailed comparison between the two configurations is presented in Figure 2, where the plasmonic decay enhancement is plotted versus the propagation losses for the single wire system (black curve) and the dual wire system. In the single wire case, the wire diameter is changed along the

Figure 2. Plasmonic decay rates as a fuction of propagation losses of the fundamental mode of single silver nanowires as the diameter is changed (black curve) and maximum plasmonic decay rates as functions of propagation losses of the antisymmetric mode of dual silver nanowire systems as the gap size is changed for different wire diameters (colored curves). The arrow indicates the direction of decreasing gap size along the colored curves for the dual nanowire systems.

curve where the leftmost point on the curve is associated with a diameter of 130 nm and the rightmost point to a diameter of 45 nm. It is clear that for acceptable propagation losses, only moderate plasmonic decay rates can be achieved in this configuration. This is very different for the dual wire system in which large decay rates at relatively low losses can be attained, as illustrated in Figure 2 by the colored curves. Here the different curves correspond to different wire diameter while the gap size is continuously changed along the individual curves. As the gap size is reduced, the decay rate increases and eventually surpasses the single wire decay rate with a gap size of around 30 nm. For example, for a gap size of around 10 nm, the plasmonic decay rate of the dual system is about an order of magnitude 1222

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larger than that of the single wire system with diameters aforementioned. To carry out the experiment, we use a standard confocal microscope in combination with an atomic force microscope (AFM, NT-MDT). In the setup, the AFM is mounted on top of the sample where the cantilever tip can be coaxially aligned to the focal spot of the confocal microscope. The AFM can be operated in tapping mode for the acquisition of sample topography images and in contact mode for cutting the nanowire into two halves as well as for the positioning of nanodiamonds and nanowires. As excitation light source in the confocal microscope, a 532 nm laser (CW or pulsed with 5.05 MHz repetition rate and a pulse-width of ∼4.6 ps) is used. Behind the objective, the fluorescence light is split by a symmetric beam splitter. One beamsplitter output port is directly imaged on a pinhole, while the beam path of the other output port could be modified by a galvanometric mirror before being imaged on another pinhole. The light passing through either pinhole was band-pass filtered with a transmission window between 647 and 785 nm to remove any residual pump light and detected with avalanche photodiodes (APDs, PerkinElmer). The two APDs together with the symmetric beamsplitter form the standard Hanbury Brown and Twiss setup for measuring the second order correlation function g(2)(τ). All time correlated data was recorded with a time to amplitude converter (PicoQuant). For a schematic of the setup we refer the reader to Huck et al.14 The samples for this experiment were made by spin coating the solutions containing nanodiamonds (Microdiamant AG, Switzerland) and monocrystalline silver nanowires27 on top of a fused silica substrate. In the experiment, we first identified a silver nanowire with a length of ∼10 μm and a diameter of 110 ± 1.5 nm. This nanowire was cut into two halves using the tip of an AFM cantilever. The parts were located next to each other with a separation of about 0.5 μm by moving one of the wires using the tip of the AFM cantilever (details on cutting and moving of silver nanowires can be found in the Supporting Information). We then located and characterized a diamond crystal with a height of 28 nm containing an NV-center in the dielectric environment of the substrate−air interface. At this position, the diamond nanocrystal was well separated from the nanowires and therefore did not feel their presence. This NV-center was measured to have a lifetime of τ1 ≈ 45.2 ns. A measurement of the second order correlation function g(2)(τ) confirmed the emission of single photons. The crystal was then brought into the near vicinity of one of the silver nanowires. An AFM topography image of the resulting system is presented in Figure 3a. The NV-center was thus coupled to the plasmonic mode propagating along the single nanowire. This is further evidenced by the observation of photon emission from the top (B) and bottom (C) wire ends shown in Figure 3b, obtained while continuously exciting the NV-center. In addition to this, the NV-center’s lifetime decreased to the value of τ2 ≈ 11.9 ns (see red trace in Figure 4a), which yields a spontaneous emission rate enhancement of 3.8 relative to the emission in the environment of the substrate−air interface. We then position the second wire (the one to the right in Figure 3a) closer to the wire−NV-center system such that the NV-center lies in the gap between the closely spaced nanowires. Figure 3c shows an AFM topography image of the final dual nanowire−NV-center system, where the location of the NVcenter is indicated by the arrow. A schematic of the crosssection containing the nanodiamond is also presented as an

Figure 3. AFM topography image (a) and photoluminescence image (b) of the NV-center in a nanodiamond of height 28 nm coupled to the single nanowire. The photoluminescence image was taken while continuously exciting the NV-center. Similarly, AFM image (c) and photoluminescence image (d) of the coupled NV-center−dual nanowire system. In both photoluminescence images, emission spot A corresponds to radiative emission from the NV-center, while the spots labeled B and C correspond to emission from the upper and lower wire ends, respectively. In AFM images, the location of the NVcenter containing diamond is indicated by the black arrow. The insets in (a) and (c), respectively, show the cross sections when a nanodiamond (blue disc) is moved close to a single nanowire with the AFM cantilever and when the two wires are placed close to each other with a “gap”.

Figure 4. (a) NV-center lifetime measured in the homogeneous environment of the substrate (black trace), after coupling to the single wire (red trace), and after coupling to the dual wire system (blue trace). (b) Second order correlation function g(2) (τ) measurement of the NV-center−dual nanowire system. The dots correspond to measurements for which detector 1 was aligned to emission spot A and detector 2 to emission spot A (black), spot B (blue), and spot C (red) of the image shown in Figure 3d. The lines of same color as the dots are double exponential fits to the data.26 1223

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quantum information processing and for studying new regimes of physics.29,30

inset in Figure 3c. At the NV-center’s position the gap between the wires was estimated to be 8.4 ± 3.4 nm by measuring the distance between the highest points on the nanowires and relating it to the nanowire diameter of 110 ± 1.5 nm (details on gap estimation can be found in the Supporting Information). In Figure 3d, we present the photoluminescence image of the coupled system taken under continuous excitation of the NVcenter with the same power as used for the single wire case. In the dual wire case, the emission from the wire ends is more than 4 times brighter than that in the single nanowire system, although the emission from the NV-center spot, (A) in Figure 3b,d, remains the same.28 We also want to emphasize in Figure 3d that spot C is brighter than the spot at the NV-center (A) by around 500 counts/s. This is observed despite the lower outcoupling efficiency from the ends of a dual wire system relative to a single wire system. This lower outcoupling efficiency is expected due to the higher confinement and higher effective index (2.1) of the AS mode when compared to a single wire mode (effective mode index 1.49). This indicates that the coupling to the dual wire system is much stronger than to the single wire system. This conclusion is further corroborated by observing a strong reduction of the NV-center’s lifetime. It is measured to be τ3 = 5.4 ns as shown by the blue trace in Figure 4a. Thus, with the realized dual nanowire system we demonstrate an increase of the spontaneous emission rate by a factor of 2.2 compared to the single nanowire system and a total factor of 8.3 compared to the emission rate when the nanodiamond was in the environment of the substrate−air interface. Measurement results of the second order correlation function g(2) i,j (τ) of the dual nanowire system are presented in Figure 4b, where the subscripts i and j describe detector alignments to the emission spots {A}, {B}, and {C} of Figure 3d. The black dots (2) correspond to g(2) A,A (τ), the red triangles to gA,B (τ), and the blue (2) triangles to g(2) (τ). A value of g (0) < 0.5 verifies that all A,C i,j three radiation spots in Figure 3d are due to the emission of single photons by the NV-center. It should be emphasized that the device efficiency can be improved by tapering the gap. The region of a small gap should then be used for efficient plasmon excitation and a subsequent increased gap can be used to reduce the propagation losses. In addition to reduced losses, the larger gap also facilitates an improved evanescent coupling to a nearby optical waveguide.16 Such a tapering is also seen in our system in Figure 3c, which improves the outcoupling from the confined AS mode. We have shown experimentally that by using a dual wire configuration as opposed to a single wire configuration much stronger coupling rates between an NV-center in diamond and a single mode of the metallic system can be attained. The experiment was carried out using chemically grown silver nanowires with a diameter of 110 ± 1.5 nm and a diamond nanoparticle with a height of 28 nm which means that the emitter was not placed exactly in the center of the gap. As a future outlook, it would be interesting to lift the NV center into the maximized field of the plasmon by employing nanosized diamond pillars.12 In such a system, a dramatic increase of the enhancement is expected. Such a strong enhancement combined with a direct accessibility of the plasmonic mode will boost the capabilities of optical control of single emitters from the generation of single photons on demand to the control of propagating plasmons where generation, absorption, and propagation of plasmons is controlled at the quantum level. This may eventually lead to a miniaturized network for

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S Supporting Information *

Additional information. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We gratefully acknowledge support from the Villum Kann Rasmussen Foundation, the Danish Agency for Science, Technology and Innovation, and the Carlsberg Foundation.

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