Doubly Resonant Nanoantennas on Diamond for Spatial Addressing

Letter pubs.acs.org/NanoLett

Doubly Resonant Nanoantennas on Diamond for Spatial Addressing of Spin States Tzach Jaffe,*,† Ofir Sorias,† Lior Gal,† Rafi Kalish,‡ and Meir Orenstein† †

Department of Electrical Engineering and ‡Solid State Institute and Physics Department, Technion - Israel Institute of Technology, 32000 Haifa, Israel S Supporting Information *

ABSTRACT: The negatively charged nitrogen-vacancy (NV) color center in diamond is an important atom-like system for emergent quantum technologies and sensing at room temperature. The light emission rates and collection efficiency are key issues toward realizing NV-based quantum devices. In that aspect, we propose and experimentally demonstrate a selective and spatially localized method for enhancing the light-matter interaction of shallow NV centers in bulk diamonds. This was achieved by polarized doubly resonant plasmonic antennas, tuned to the NV phonon sideband transition peak in the red and the narrowband near infrared (NIR) singlet transition. We obtained a photoluminescence (PL) enhancement factor of about 10 from NV centers within the hot spot of the antenna area (excluding the extraction efficiency enhancement) and similar emission lifetime reduction. The functionality of the double resonance antenna is controlled by the impinging light polarization. KEYWORDS: NV centers, plasmonics, nanoantennas, field enhancement, spatial addressing, diamonds

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diameter) by using plasmonic nanoantennas. These enhanced light-matter interactions at a specific site can pave the way toward efficient highly localized access to the NVC spin state. The use of plasmonic nanoantennas, which provide efficient coupling between far-field and near-field, can serve as intermediary between the NVC and the incident light as well as the collected emitted light and thus mitigate the mismatch issue discussed above.10 Higher emission rate and collection efficiency are key-issues toward realizing NVC-based quantum devices, and without such antennas high sensitivity and resolution of NVC-based magnetometry are both compromised due to large photon shot noise. The structure we designed is facilitating spatially controlled electromagnetic field enhancement at a specific nanosized spot which enhances the photonic density of states in the NVC vicinity. The enhanced interaction is governed by the higher density of states as formulated by the Q Purcell factor,11 Fp ∝ V (λ/n)3, where Q is the quality factor, V is the modal volume, λ is the vacuum wavelength, and n is the index of refraction. One can use photonic cavities for achieving high Q-factor but usually with insufficient spatial localization due to large mode volume (V > (λ/n)3).12−14 Although the plasmonic nanoantenna has relatively low Q of 10−100, its mode volume, which is a small fraction of λ3, leads to enhanced light-matter interaction. Previous related experiments reported

n recent years, atom-like systems have been the subject of intensive research, mainly in the context of quantum information technology and sensing. Although there is a growing interest in such systems, they usually suffer from inherent problems. One of the main problems is the large spatial mismatch between the emitter dimensions (order of 1 nm in size) and the effective length scale of light (the wavelength), reducing the efficiency of light-matter interactions. The second problem is the large decoherence and dephasing while operating with solid-state based quantum systems at room temperature. This is the reason why a significant number of coherent control experiments are performed at cryogenic temperatures. By using diamond as a solid state substrate, and specifically the negatively charged nitrogen-vacancy center (NVC) in diamond, one can reach relatively long coherence times of the electronic spins with T2 ∼ 2 ms for single NVC in isotopically pure diamond at room temperature1−3 or even longer by dynamic decoupling techniques.4,5 The NVC, a localized multielectron system inside the diamond host, has a unique property: the NVC spin state can be addressed optically, providing control over initialization and readout of the spin state. By virtue of those characteristics, NVC field has seen much progress with demonstrations as a building block in quantum information processing (QIP), quantum circuitry, and nanoscale sensing.6−9 We propose and experimentally demonstrate a selective and spatially localized method for enhancing the light-matter interaction of shallow NVCs in bulk diamonds with polarization and wavelength selectivity at a highly localized spot (30−40 nm © XXXX American Chemical Society

Received: March 14, 2017 Revised: June 13, 2017

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DOI: 10.1021/acs.nanolett.7b01088 Nano Lett. XXXX, XXX, XXX−XXX

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Figure 1. (a) SEM image of an equal-dimer TCA array tuned to the NVCs visible and NIR transitions. (b) Reflection-enhancement measurement of the TCA array (a) demonstrating double resonance at 670 and 1100 nm. (c) Near-field simulation results of the intensity enhancement due to the antenna resonance at 670 nm. (d) Near-field simulation results of the intensity enhancement inside the antenna’s gap at the main resonance wavelength around 1100 nm.

combinations of nanodiamonds and plasmonic particles15 or plasmonic cavities on bulk diamond.16,17 On the contrary, our scheme is engaging bulk diamond with its long coherence time with plasmonic antenna for enhanced light-matter interactions and high radiation (extraction) efficiencies. We first introduce the specific plasmonic nanoantennas and analyze their resonances and near-field distributions. Then, we experimentally demonstrate its polarized plasmonic doubly resonant characteristics by reflection and near-field measurements using antennas tuned to the peaks of the NVCs optical emission transitions, one in the visible and the second in the NIR. Subsequently, we describe a procedure for generating the shallow NVCs in bulk diamond and finally, detail the measurement results of the interaction between the NVCs and the plasmonic nanoantennas at the main optical transition at ∼700 nm, by examining in detail the emission lifetime and photoluminescence (PL) efficiency. The localized plasmon resonance depends on the material properties and the geometry of the structure. Our antenna design, denoted here as the “templar cross” antenna (TCA), comprised of two perpendicular dimers, is made of a combination of square and triangle gold patches (Figure 1b). Plasmonic cross antennas were investigated in the past18−21 and recently were suggested for angular momentum confinement,22 but never before as an intermediary for enhanced light-matter interaction with NVCs at a highly confined spot. We demonstrate the potential of this structure to enhance lightmatter interaction inside the feed gap of the antenna and its polarization and wavelength selectivity. In the first set of experiments, we measured antenna-assisted reflection enhancement compared to the bare diamond surface (Figure 1b). The experiment was carried out by using standard reflection setup (see Supporting Information), and for a symmetric TCA structure we measured two main resonances with substantial

unpolarized characteristics, as expected for the almost identical two perpendicular dimers. The main resonance is attributed to dipole-like charge oscillations with high electric field enhancement by up to 3 orders of magnitude for the electric field intensity both at the antenna gap and at the edges of the parallel dimer as confirmed by FDTD near-field simulation at the main resonance wavelength (Figure 1d). The second resonance is contributed by the external edges of the dimer perpendicular to the incident light polarization (Figure 1c) and lacking field enhancement in the antenna gap. Although we can tune the symmetric antenna resonances to meet the two optical NVC transitions, as can be seen by the simulation results, the spatial location of the field enhancement cannot be used for interaction with the same NVCs. In order to mitigate the above spatial inconsistency, we designed and measured polarized doubly resonant TCA structure, which consists of two different orthogonal dimers. Spectral-polarization reflection measurements of this asymmetric TCA are clearly showing the polarized doubly resonant characteristics with two different resonant wavelengths at the two perpendicular polarizations (Figure 2b). One resonance is tuned to the visible transition of the NVC and the other to the narrowband transition of the NIR spectrum around 1042 nm. We measured a 30% reflection enhancement and by using the fill factor within the unit cell of the antenna array we evaluated a local enhancement factor close to 20. The results are compared with a symmetric TCA structure that exhibits nonpolarized resonance behavior (Figure 2b). We carried out near-field measurements of an asymmetric TCA on top of glass substrate, using a transmission-mode aperture-less near-field scanning optical microscope (NSOM) with spatial resolution of 10−20 nm. The measurements were performed by illuminating the sample in a transmission mode with linearly polarized beam at 900 nm using two perpendicular polarizations (aligned with B

DOI: 10.1021/acs.nanolett.7b01088 Nano Lett. XXXX, XXX, XXX−XXX

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electric-field enhancement at the antenna gap is highly dependent on the incident light polarization and it is maximal when the light polarization is parallel to the resonant dimer (Figure 2c,d). We obtained field ratio of ∼10 between the field at the antenna gap and its outer surroundings. The plasmonic field enhancement at the main resonance is highly localized and penetrates to a depth of ∼40 nm from the diamond surface as deduced from FDTD near field simulation. For external electric field excitation, polarized in the x-direction, the XZ cross-section plane shows an enhancement of almost 2 orders of magnitude within 30 nm from the diamond surface (Figure 3a). From the YZ cross section plane, it can be seen that the enhanced field distribution is about two times more confined in the Y-direction compared to the X-direction. This improved confinement is assisted by the boundaries of the perpendicular dimer. We conducted the following experiments for measuring PL enhancement and lifetime of the “red” transition at room temperature on a 1b diamond with high natural concentration of nitrogen atoms (∼1018 cm−3). In order to achieve strong light-matter interaction a significant overlap between the plasmonic field distribution and the NVC is required. To promote shallow NVC formation within 20−30 nm from the diamond surface and within the enhanced plasmonic field region, the diamond was implanted with low energy argon atom beam (40 keV). The argon atom implantation induced high vacancy concentration in the nitrogen vicinity and at the desired depth as confirmed by the vacancy depth distribution analysis using SRIM23 simulation tool (Figure 3b). We implanted half of the diamond area for comparison and performed rapid thermal annealing at 1000οC for 2 min. PL and lifetime measurements of the NVC ensemble were performed in order to evaluate the formation of shallow NVCs. An emission enhancement factor of 2.5 of the implanted area

Figure 2. (a) Reflection enhancement measurements of the symmetric TCA as a function of wavelength and polarization, demonstrating the nonpolarized enhancement around the peak of the NVC visible transition (marked with white dashed line). (b) Reflection enhancement measurements of an asymmetric TCA. The polarized doubly resonant property of the asymmetric TCA is demonstrated containing the spectral region of both, the NVC NIR transition at 1042 nm (marked with yellow dashed line) and the peak of the NVC visible transition around 700 nm. (c,d) NSOM transmission measurements of an asymmetric TCA on a glass substrate for two perpendicular polarizations of the incident light aligned to the antenna dimers (inset: SEM image of an asymmetric TCA on top of glass substrate).

the antenna dimers) and probing the near-field scattered by the metallic tip. The near-field measurements further support our analysis and coincide with the far-field measurements. The

Figure 3. (a) Calculated plasmonic field penetration depth at two cross sections using FDTD simulations with schematics emphasizing the preferred interaction with shallow NVC for enhanced “plasmon-matter” interaction. (b) SRIM simulation results of the depth distribution of vacancies generated by the damage of argon atoms. The concentration peak is positioned around 20 nm below the diamond surface. (c) PL comparison between the pristine diamond and the shallow implanted area. The extracted NVC density enhancement is about 80. (d) Lifetime measurements of the implanted and nonimplanted 1b diamond. C

DOI: 10.1021/acs.nanolett.7b01088 Nano Lett. XXXX, XXX, XXX−XXX

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Figure 4. (a) Simulation results of scattering cross-section normalized to the antenna area for variation of the length of the antenna in parallel to the impinging light polarization. (b) Schematics of the relevant NVC electronic transitions. (c) Comparison of PL measurements from area activated by shallow argon implantation with and without the antenna arrays (d) Lifetime measurements of the excited state of the NVC with and without the TCA array (inset).

We measured 100% enhancement factor (Figure 4c) and by calculating conservatively the fill factor inside a unit cell of the TCA (see Supporting Information), we got an intensity enhancement factor of ∼28. In order to rule out potential contributions of nonresonant metallic structure in the NVCs vicinity, we compared the results with PL of a very similar but gapless-antenna array. The latter did not show an enhancement nor reduction of the emission compared to the bare implanted diamond. By measuring the lifetime of the orbital excited state, we observed directly a coarse 20% reduction in the averaged lifetime due to interaction with the antennas array (Figure 4d). We also measured the emission lifetime of the NVCs in the gapless antenna area that showed no change compared to that of the bare shallow implanted area with no antennas. This result indicates that the lifetime reduction is not due to loss in the metallic structure. The temporal signal at the TCAs area has complex contributions from the lifetime of three NVCs populations, one at the antenna hot spot, the noninteracting shallow NVCs in the unit cell, and the bulk NV population (see Supporting Information). The intrinsic lifetime shortening was evaluated to be a factor of ∼10 and was extracted using a multiexponential fit and based on the lifetime measurements from the pristine diamond and the shallow implanted areas. The lifetime reduction together with the PL enhancement verify that the TCA enhances the overall interaction due to higher photonic density of states in the NVC vicinity. Two additional potential effects of the antenna: improvement of the extraction out of the high index material (diamond) was estimated by FDTD simulation to be about a factor of 2 and the beaming−improving the collection efficiency was insignificant due to high NA objective we employed. Excluding the extraction efficiency enhancement, both, PL enhancement and lifetime reduction measurements are highly correlated representing a factor of ∼10.

compared to the pristine 1b diamond area was measured (Figure 3c). We calculated the enhancement of the NVC volumetric concentration to be a factor of 80 at the implanted area (see Supporting Information). We then conducted timeresolved PL measurement excited by picosecond light pulses at 560 nm filtered from a supercontinum laser to tirgger an exponentially decaying fluoresence signal from which the lifetime information was extracted. The lifetime measurements also confirmed that indeed we are measuring a shallow population of NVCs. The measured lifetime (Figure 3d) is longer for the shallow implanted area compared to the nonimplanted diamond area. It is attributed to the decrease of the photonic density of states for the shallow NVCs, due to the reduced effective index of refraction resulting from their vicinity to air. In order to evaluate the shallow implanted diamond area lifetime, we used a biexponential slowly decaying function to fit the two NVC populations. We extracted a lifetime constant of τ ≈ 10.3 ± 0.3 ns for the shallow NVCs and τ ≈ 7.3 ± 0.2 ns for the bulk NVCs. The result is consistent with our simulation of the NVCs depth as well as a highresolution confocal scans that we conducted (see Supporting Information). In order to match the plasmonic structure resonances with the NVC transitions we conducted FDTD simulative analysis of the parameters of the antenna. By changing the length of the structure along the electric field polarization direction, we tuned the structure main resonance (Figure 4a and Supporting Information). Different-sized TCAs were patterned by electron beam lithography on the shallow implanted diamond followed by the deposition of 60 nm gold with subsequent liftoff procedure. We investigated the interaction of the NVC with the impinging light as mediated by the plasmonic antenna array, by PL and lifetime measurements of TCA array tuned to the peak of the NVC visible transition (phonon sideband) at 700 nm. D

DOI: 10.1021/acs.nanolett.7b01088 Nano Lett. XXXX, XXX, XXX−XXX

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In summary, we have demonstrated the potential of carefully designed TCA arrays to spatially address a small ensemble of NV centers at the hot spot of the antenna and enhance the efficiency of the readout signal of the NVC spin state. The doubly resonant feature of the antenna may be used further to control several optical transitions in single NVC schemes for more elaborate optical control of the spin states. The polarization control can also provide a suitable method for decoupling the optical signals from the bulk and the extremely shallow NVC at the antennas sites. Methods. Sample Preparation. We used [1 0 0] 1b diamond with high natural concentration of nitrogen atoms, (more than 1018 cm−3). We then implanted the diamond with low energy argon atom beam (40 keV) in order to promote shallow NVC formation within 20 nm from the diamond surface. We used a 1014 cm−2 ion fluence and then performed annealing in nitrogen environment for 2 min at 1000 °C inside rapid thermal annealing (RTA) chamber. We used RTA to make sure that we just activate a bonding between the shallow vacancies and adjacent nitrogen atoms while making sure that the shallow vacancies are not diffusing deeper inside the diamond substrate and outside of the desirable position of 20− 30 nm from the diamond surface. By employing high-resolution confocal scan with 1.3 NA oil immersion objective and calibrating to single NV emission, we evaluated that the shallow NV concentration is about 1016−1017cm−3. Before depositing the gold antennas, we measured the bare sample to confirm that we have promoted shallow NVC formation. The sample was spin-coated by a positive PMMA resist and then patterned by electron beam lithography for gold deposition later on. The gold antennas with different dimensions were deposited on top of the shallow implanted area in the diamond and a standard lift-off process was performed. Experimental Setup. The optical setup for reflection measurements is shown in Supporting Information, Figure S1. All the reflection measurments were conducted using a low NA objective (0.4). PL and lifetime measurements at room temperature were performed using Fianium supercontinum pulsed laser at 560 nm, with an acusto-optic filter. In the PL and lifetime measurments the laser was focused to ∼1 μm spot using 100× objective with NA of 0.95. The PL from the sample was collected by the same objective used for focusing the laser light.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Tzach Jaffe: 0000-0003-2235-841X Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research was supported by Russell Berrie Nanotechnology Institute (RBNI). We acknowledge the partial support by the Israeli ICORE program: “Circles of light”. The fabrication was performed at the Micro-Nano Fabrication Unit (MNFU), Technion. The NSOM measurements were carried out at Professor Guy Bartal’s lab at the Technion with the assistance of Kobi Cohen and Evgeny Ostrovsky.

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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.7b01088. Reflection measurement setup (S1). Near-field and scattering enhancement simulations of the TCA tuned to the NV main optical transition (S2). Volumetric concentration ratio calculation and high resolution confocal scans (S3, S4). Lifetime measurements and fitting method using multiexponential fit (S5). FDTD simulation of the intrinsic radiative enhancement factor of a dipole source (F6). Additional reflection enhancement measurements of different TCA arrays demonstrating the doubly resonant functionality of the array (S7). The fill factor correction evaluation (S8) (PDF) E

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Nano Letters (20) Brown, L. V.; Zhao, K.; King, N.; Sobhani, H.; Nordlander, P.; Halas, N. J. J. Am. Chem. Soc. 2013, 135, 3688−3695. (21) McLeod, A.; Weber-Bargioni, A.; Zhang, Z.; Dhuey, S.; Harteneck, B.; Neaton, J. B.; Cabrini, S.; Schuck, P. J. Phys. Rev. Lett. 2011, 106, 37402. (22) Klaer, P.; Razinskas, G.; Lehr, M.; Krewer, K.; Schertz, F.; Wu, X.-F.; Hecht, B.; Schönhense, G.; Elmers, H. J. Appl. Phys. Lett. 2015, 106, 261101. (23) Ziegler, J. F.; Ziegler, M. D.; Biersack, J. P. Nucl. Instrum. Methods Phys. Res., Sect. B 2010, 268, 1818−1823.

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DOI: 10.1021/acs.nanolett.7b01088 Nano Lett. XXXX, XXX, XXX−XXX