Solitary Oxygen Dopant Emission from Carbon Nanotubes Modified by

Solitary Oxygen Dopant Emission from Carbon Nanotubes Modified by Dielectric Metasurfaces Xuedan Ma,*,†,‡,⊥ Anthony R. James,† Nicolai F. Hartmann,§ Jon K. Baldwin,§ Jason Dominguez,‡ Michael B. Sinclair,‡ Ting S. Luk,†,‡ Omri Wolf,†,‡ Sheng Liu,†,‡ Stephen K. Doorn,§ Han Htoon,§ and Igal Brener*,†,‡ †

Center for Integrated Nanotechnologies, Sandia National Laboratories, Albuquerque, New Mexico 87185, United States Sandia National Laboratories, Albuquerque, New Mexico 87185, United States § Center for Integrated Nanotechnologies, Materials Physics and Applications Division, Los Alamos National Laboratory, Los Alamos, New Mexico 87545, United States ‡

S Supporting Information *

ABSTRACT: All-dielectric metasurfaces made from arrays of high index nanoresonators supporting strong magnetic dipole modes have emerged as a low-loss alternative to plasmonic metasurfaces. Here we use oxygen-doped singlewalled carbon nanotubes (SWCNTs) as quantum emitters and couple them to silicon metasurfaces to study effects of the magnetic dipole modes of the constituent nanoresonators on the photoluminescence (PL) of individual SWCNTs. We find that when in resonance, the magnetic mode of the silicon nanoresonators can lead to a moderate average PL enhancement of 0.8−4.0 of the SWCNTs, accompanied by an average increase in the radiative decay rate by a factor of 1.5−3.0. More interestingly, single dopant polarization experiments show an anomalous photoluminescence polarization rotation by coupling individual SWCNTs to silicon nanoresonators. Numerical simulations indicate that this is caused by modification of near-field polarization distribution at certain areas in the proximity of the silicon nanoresonators at the excitation wavelength, thus presenting an approach to control emission polarization. These findings indicate silicon nanoresonators as potential building blocks of quantum photonic circuits capable of manipulating PL intensity and polarization of single photon sources. KEYWORDS: all-dielectric metasurfaces, magnetic resonance, oxygen-doped single-walled carbon nanotubes, photoluminescence, polarization

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tures where more complex geometries supporting circular current are necessary for supporting magnetic modes. Magnetic modes in dielectric metasurfaces have been used to provide control of the far-field radiation properties, such as directionality and spectral features of nearby emitters.16−18 Strong enhancement in the spontaneous emission rates of emitters close to a magnetic mirror has also been proposed theoretically.19 Therefore, magnetic modes in dielectric nanoresonators with low intrinsic losses may provide a new setting for tailoring emission from single photon emitters.18,19 However, recent theoretical20 and experimental21 works have

he potential to control and manipulate photoluminescence (PL) from single photon emitters lies at the heart of many applications, such as on-chip generation and routing of single photons,1,2 single photon transistors,3,4 and new quantum entanglement schemes for quantum cryptography.5,6 Plasmonic nanostructures can confine local electric fields to volumes far below the diffraction limit, thereby providing an excellent interface between single photons and quantum emitters.7,8 By proper shape and geometry design, they can also support magnetic modes,9−11 enabling another degree of freedom for engineering single photon emission.12 Recently, all-dielectric metasurfaces made from arrays of high index nanoresonators have attracted growing attention due to their support of strong magnetic modes whose spectral frequencies can be easily tuned by varying the nanoresonators’ size and shape,13−16 in stark contrast to plasmonic nanostruc© 2017 American Chemical Society

Received: April 28, 2017 Accepted: May 23, 2017 Published: May 23, 2017 6431

DOI: 10.1021/acsnano.7b02951 ACS Nano 2017, 11, 6431−6439

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ACS Nano demonstrated that compared to plasmonic nanostructures, dielectric nanoresonators can typically provide less local field enhancement, thus supporting much lower Purcell factors,22,23 a figure of merit widely used to characterize spontaneous emission enhancement in microcavities. The overall influence of the magnetic mode of a dielectric nanoresonator on the emission intensity of a single photon emitter relies on a complex trade-off between low intrinsic losses and small Purcell factors. While this influence is interesting and intriguing to understand for the development of all-dielectric metasurfacebased photonic devices, the modification of single photon emission by dielectric nanoresonators, especially by their magnetic dipole modes at the individual single photon emitter level, remains experimentally unexplored to date.18 Oxygen dopants in single-walled carbon nanotubes (SWCNTs) have recently been identified as a single photon source emitting at the technically important telecom-wavelength range.24,25 The deep trap states created via oxygen doping can localize the originally free-diffusing excitons, enabling single photon generation from such dopant states up to room temperature.25−29 The linearly polarized, spectraldiffusion-free emission from oxygen-doped SWCNTs establishes their potential applications as single photon sources for quantum information processing.25,27,30 Means to integrate and control single photon emission from these dopants is thus fundamentally important. In this study, we couple PL from individual oxygen-doped SWCNTs to the magnetic mode of silicon nanoresonators. Using cryogenic temperature single tube PL spectroscopy, we observe an average PL enhancement of 0.8−4.0 for the SWCNTs interacting with the dielectric metasurface, in comparison to those that are not. This PL enhancement is accompanied by an average increase in total decay rate by 1.4, from which we estimated an average Purcell factor of 1.5−3.0. More interestingly, when excited by a linearly polarized laser beam, 42.5% of the oxygen-doped SWCNTs deposited on silicon nanoresonators emit PL with linear polarization rotated more than 45° from that of the laser, in stark contrast to tubes deposited on unpatterned regions, for which the PL remained mostly polarized parallel to the laser polarization. It is known that due to their 1D structure, optical transitions in SWCNTs are strongly polarized along their tube long axis.31−33 Numerical simulations indicate that for tubes on unpatterned regions, mostly only those lying parallel to the polarization of the laser are excited, while for tubes coupled to silicon nanoresonators, modification of the near-field polarization distribution at certain areas in the vicinity of the nanoresonators leads to the excitation of tubes lying perpendicularly to the laser polarization. These findings indicate that silicon nanoresonators and their metasurfaces can act as potential building blocks of quantum photonic circuits capable of manipulating PL intensity and polarization of single photon emitters. Moreover, modification of near-field polarization distribution of nanoresonators in general can serve as an effective approach to control emission polarization.

Figure 1. (a) Sketch of the silicon nanoresonators and experimental geometry. The horizontal planes indicate simulation positions for the magnetic and electric mode profiles (middle red) and the local polarization distributions (top orange and bottom blue). (b) A scanning electron micrograph of Si640. (c) Experimentally measured (black) and numerically simulated (red) reflectance spectra of Si640 before drop-casting SWCNTs. In the simulation, the nanoresonator height is assumed to be 200 nm. Diameter of the nanoresonators is 640 nm, and center-to-center distance between neighboring nanoresonators is 840 nm. M and E indicate magnetic and electric dipole modes. (d) Numerically simulated mode profiles of Si640 at high (1300 nm, top) and low (1500 nm, bottom) resonance frequencies in a center plane horizontally cutting through silicon nanoresonators (indicated by the red plane in (a)). The white dashed lines indicate the positions of the silicon nanoresonators. The left column corresponds to electric dipole mode, and the right to magnetic dipole mode.

resonators with a height of 200 nm, diameter of 640 nm, and center-to-center distance of 840 nm between the neighboring nanoresonators (denoted as Si640). An experimentally measured reflectance spectrum of Si640 before drop-casting SWCNTs (Figure 1c, black) shows two major distinct resonance peaks at ∼1262 and 1495 nm, and is in reasonable agreement with the results from finite-difference time-domain (FDTD) simulations (Figure 1c, red), in which a plane wave is assumed to be incident perpendicular to the silicon nanoresonators (Figure 1a). The small differences of the major resonance peak positions between the experimental and simulation spectra might be due to fabrication inaccuracies.16 The additional peak at 1080 nm, only observed in the experimental spectrum, is most likely caused by Fabry−Perot resonances in the layered SOI wafers which have been observed in previous studies.16 Such layered structures were not included in our FDTD simulations due to unrealistic computation time required (based on the computational resources of our workstations). A careful inspection of the field distributions in a center plane cutting horizontally through the silicon nanoresonators (indicated by the red surface in Figure 1a) reveals that the high frequency resonance at ∼1262 nm exhibits magnetic dipole characteristics, with the magnetic field showing a dipole pattern at the center (Figure 1d, upper right) and the electric field a circulating pattern (Figure 1d, upper left). On the other hand, the low frequency resonance at ∼1495 nm exhibits electric dipole characteristics with a circulating pattern in the magnetic field (Figure 1(d), lower right) and a dipole

RESULTS AND DISCUSSION To study the influence of dielectric metasurfaces operating at the magnetic dipole mode on the PL of oxygen-doped SWCNTs, we designed silicon nanoresonator arrays with magnetic dipole modes overlapping with the emission wavelength of the dopants (Figure 1). Figure 1b shows a representative scanning electron micrograph of silicon nano6432


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ACS Nano pattern in the electric field (Figure 1(d), lower left). The large energy difference between the magnetic and electric dipoles allows for the selective study of the influence only from the magnetic dipole mode. It is worth mentioning that drop-casting SWCNTs and subsequent electron-beam deposition of 10 nm SiO2 film did not lead to any noticeable changes in the reflectance spectra of the metasurfaces. In this study, oxygen-doped SWCNTs were used as the quantum emitters. The epoxide-l functional group in oxygendoped (6,5) SWCNTs (Figure 2a) can introduce deep trap

Figure 3. (a) Representative PL spectra of individual oxygen-doped SWCNTs on Si640 (red) and in an unpatterned area (black) measured at 4 K, with the peak intensities normalized to the maximum of the PL spectra of the tube on Si640. (b) Representative normalized decay curves of individual oxygendoped SWCNTs on Si640 (red) and in an unpatterned area (black), together with the instrument response function (gray) on a semilog scale. (c) Histograms of spectrally integrated PL intensities of individual oxygen-doped SWCNTs on Si640 (upper) and in unpatterned areas (lower). The solid lines are Gaussian fittings to the histograms. (d) Histograms of average PL lifetimes of individual oxygen-doped SWCNTs on Si640 (upper) and in unpatterned areas (lower). The solid lines are Gaussian fittings to the histograms.

Figure 2. (a) Structure of an epoxide-l (l: C−O−C bond aligned parallel to the carbon nanotube axis) adduct on a segment of a (6,5) SWCNT (upper) and schematic of the potential profile of the tube with a deep trap state (red dashed line) created by the epoxide-l adduct (lower). Here E11 represents the first optical transition of SWCNTs, E*11− the transition induced by epoxide-l group, and G the ground state. (b) Spectral peak histograms of individual oxygen-doped SWCNTs. Inset: A representative PL spectrum of an individual oxygen-doped SWCNT measured at 4 K.

states capable of single photon emission in the wavelength range of 1200−1400 nm, with a peak at 1270 nm (Figure 2b).29,30 For clarity, we used a 1200 nm long-pass filter to block emission from the higher energy transitions in oxygen-doped SWCNTs along with the laser beam and background emission from the Si substrates. Therefore, only PL from the epoxide-l dopant sites of the oxygen-doped tubes was studied. To ensure that direct comparisons between oxygen-doped SWCNTs deposited on Si640 and on unpatterned regions are made at the single dopant site level, the samples were cooled to 4 K so that individual dopant sites would have distinct PL spectra due to variations in their local environments.34 Density of the SWCNTs was kept below one carbon tube/100 μm2. A representative PL image of the oxygen-doped SWCNTs is shown in Figure S1. Figure 3a shows representative PL spectra of individual oxygen-doped SWCNTs on Si640 (red) and in a blank area without silicon nanoresonators (black) normalized to the maximum of the PL spectra of the SWCNT on Si640. The much higher PL intensity of the SWCNT on Si640 might be due to coupling to the silicon nanoresonators. To describe this quantitatively, we spectrally integrated the PL spectra of 46 individual oxygen-doped SWCNTs on Si640 and 61 individual SWCNTs on unpatterned regions, respectively, and the PL intensity histograms are shown in Figure 3c. By fitting the histograms with Gaussian functions, we derived a mean PL intensity value of ∼36300 counts for the tubes on Si640, and ∼17800 counts for those in blank areas, indicating a factor of 2 increase in the mean PL intensity when the tubes are coupled to the magnetic dipole modes of Si nanoresonators. The observed PL enhancement can be caused by modification of the SWCNT excitation and/or recombination rates by nearby silicon nanoresonators. Since we excite the SWCNTs way above their emission energy with excitation

powers below their saturation level, the absorption and emission processes can be approximated as independent. We first discuss the influence of silicon nanoresonators on the excitation process of SWCNTs. The excitation rate of a SWCNT is proportional to the local electric field intensity |E|2 at its location position and the excitation wavelength.35 To estimate changes in the local electric field intensity caused by the silicon nanoresonators, we use 3D FDTD method to simulate the distributions of electric field intensity enhancement factor γ in the vicinity of silicon nanoresonators at the excitation wavelength. The enhancement factor γ is defined as the ratio of the electric field intensity |E|2 with and without the silicon nanoresonators. As shown in Figure 4a,b, the value of γ is strongly dependent on the location of the SWCNTs, and can reach up to 38. This indicates that due to coupling of SWCNTs to higher-order modes of silicon nanoresonators, modification in the excitation rate can play a significant role in the increased emission intensity. In addition to the excitation process, silicon nanoresonators can also alter the emission rates and consequently quantum yield of SWCNTs. For an oxygen-doped SWCNT not coupled to silicon nanoresonators, the ratio of its radiative decay rate kr,0 and the total decay rate, which is the sum of the radiative and nonradiative (knr,0) decay rates, determines its quantum efficiency η0 (η0 = kr,0/(kr,0 + knr,0)). When emission from a carbon nanotube is coupled to the magnetic dipole mode of Si640, energy transfer between the tube and silicon nanoresonators occurs and the silicon nanoresonators can help scatter the emission to the far-field, leading to an increase in the 6433


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quantum emitters and plasmonic nanostructures may occur resulting in reduced PL lifetime of the quantum emitters,38−40 it is quite unlikely to be the reason for the observed lifetime reduction in this study due to the application of silicon nanoresonators and the surfactant layers protecting the SWCNTs. Therefore, we attribute this decrease in lifetime to a coupling effect between the PL of SWCNTs and the magnetic dipole mode of the Si nanoresonators. From the mean lifetime of 377 ps for carbon nanotubes in unpatterned areas (i.e., kr,0 + knr,0 = (377 ps)−1) and 277 ps for tubes on Si640 (i.e., kr + knr = (277 ps)−1), we can derive that the enhancement in radiative decay rate kr/kr,0, in other words, the Purcell factor, is equal to 0.361 + 1, and the quantum efficiency of the coupled system is η

Figure 4. (a,b) Local electric field intensity enhancement factor γ at the bottom (a) and top (b) of a Si640 silicon nanoresonator at the excitation wavelength of 865 nm. Color bars are the same for both (a) and (b). (c) Purcell factor (black) and quantum efficiency (η, red) of oxygen-doped SWCNTs coupled to Si640 as a function of uncoupled SWCNT quantum efficiency (η0). The former is plotted on a semilog scale. The area marked by yellow indicates the estimated upper and lower limits of η0.

0

η = 0.267 + 0.735η0. Purcell factor and quantum efficiency of oxygen-doped SWCNTs coupled to Si640 as a function of η0 are plotted in Figure 4c. The value of the Purcell factor decreases with the increase of uncoupled SWCNT quantum efficiency η0. When η0 = 100%, i.e., the dopant is a perfect emitter without nonradiative channels, the corresponding Purcell factor reaches a minimum value of 1.4. On the other hand, quantum efficiency of carbon nanotubes coupled to Si640 (η) is always larger than 26.7%, and increases with η0. The quantum efficiency η0 of single epoxide-l dopant sites on substrates at 4 K is not known a priori. However, by adapting values from previous studies28,41 of similar systems (Supporting Information S3), we can estimate an average Purcell factor of 1.5−3.0 (Figure 4c, yellow), well in the range of the radiative decay rate enhancement factors calculated from FDTD simulations. Previous theoretical studies42,43 of electric dipoles coupled to magnetic resonance modes of silicon particles have predicted similar values of radiative decay rate enhancement factors. Aside from affecting the excitation and recombination processes of SWCNTs, silicon nanoresonators can also cause directional scattering,16,44 and potentially modify radiation patterns of the nearby quantum emitters, leading to changes in collection efficiency.45,46 Therefore, to estimate the overall PL enhancement, we should also take these two effects into consideration. We first simulate directional scattering by assuming a dopant site as an electric dipole lying at different locations with respect to silicon nanoresonators (Supporting Information S4), and found that at the wavelength of ∼1300 nm the reflectance to transmittance ratio was changed from 1.22 on SiO2 to 0.38−2.14 close to/on silicon nanoresonators, depending on the exact location of the electric dipole. This indicates that the emission could be mainly transmitted due to Si640 for certain dipole locations, consistent with previous studies,16,44 whereas for other locations, the emission is mainly reflected. Since we collected emission in a reflectance geometry, we derive an overall average PL enhancement of 0.8−4.0 by taking this directional scattering into account. We then simulated changes in radiation pattern and collection efficiency due to coupling of the SWCNTs to Si640 (Supporting Information S4). Despite the changes in the radiation patterns (Figure S4), the overall collection efficiency of the microscope objective was not significantly altered (Table S1), possibly due to the relatively large size of the silicon nanoresonators. We therefore neglect the influence of changes in radiation patterns on the average PL enhancement. Considering all these factors that may contribute to the observed PL enhancement, we conclude that (1) increase in the excitation rate of SWCNTs due to an increased electric field

radiative decay rate of the emitter. Meanwhile, due to the low intrinsic losses of silicon at near-IR wavelengths, we can neglect any dissipative losses of the silicon nanoresonators. Therefore, for an oxygen-doped SWCNT coupled to Si640, its quantum efficiency η can be defined as η = kr/(kr + knr), with kr and knr being the radiative and nonradiative decay rates of the tube coupled to Si640, respectively, and knr = knr,0. We perform 3D FDTD simulations to estimate changes in the local electric field intensity at the emission wavelength of the SWCNTs and changes in the radiative decay rates by adapting a method developed by Bharadwaj et al.36 and Chowdhury et al.37 In this method, an electric dipole is used to represent a SWCNT, and its radiative decay rate is related to the power radiated by the k P dipole through k r = P , where P and P0 are the radiated r,0

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powers of the dipole with and without nearby silicon nanoresonators. Figure S2a,b shows the simulated local electric field intensity enhancement factors at the emission wavelength of the SWCNTs. Here the electric field intensity enhancement factor is defined again as the ratio of the electric field intensity |E|2 with and without the silicon nanoresonators. Figure S2c−i presents all the simulated positions of the electric dipole, and a radiative decay rate enhancement factor of 0.5 to 15 can be observed, depending on the location and orientation of the dipole. To experimentally examine changes in the decay rates and gain an insight into the carrier dynamics, we measured PL decay curves of the carbon nanotubes using the time-correlated single photon counting method. Two representative PL decay curves of individual oxygen-doped SWCNTs on Si640 (Figure 3b, red) and on unpatterned regions (Figure 3b, black) are shown in Figure 3b. Fitting the decay curves with single exponential functions by deconvoluting the instrument response function (IRF, Figure 3b, gray) gives PL lifetimes of 252 and 425 ps for the tubes on Si640 and on unpatterned regions, respectively, indicating a reduction in lifetime and an increase in decay rate by coupling SWCNTs to Si nanoresonators. We fitted the PL decay curves of 88 individual carbon nanotubes on Si640 and 63 carbon nanotubes on unpatterned regions, and their PL lifetime histograms are plotted in Figure 3d. The mean PL lifetimes derived by fitting the histograms with Gaussian functions decreased from 377 ps for tubes on unpatterned regions to 277 ps for those on Si640. Although charge transfer between closely placed semiconductor 6434


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Figure 5. (a) Representative images of the laser beam (left) and PL spectra of oxygen-doped SWCNTs in unpatterned areas (middle) and on Si640 (right) showing both the s (top) and p (bottom) components at different emission polarization angles. The vertical direction represents different positions on the 2D InGaAs array detector. (b) Corresponding intensity of laser beam and PL spectra of SWCNTs integrated along the vertical direction of the detector. (c) Spectrally integrated s component of the laser beam (left) and PL spectra of SWCNTs (middle and right) shown in (a) as a function of emission polarization angle. The solid curves are sine-square function fits to the experimental data.

laser beam has an orientation angle of 0°, and the silicon nanoresonators cause almost no change in the laser beam polarization (Supporting Information S5). Due to their 1D structure, optical transitions in SWCNTs are strongly polarized along their tube long axis.31−33 Therefore, for SWCNTs not coupled to silicon nanoresonators, although they are randomly oriented on the substrates, mainly those oriented along the laser polarization are excited (Figure 5 middle, and Figure 6a, top), and only a few tubes with orientation angles rotated more than 45° from that of the laser were excited (Figure 6a, top). Interestingly, for SWCNTs deposited on Si640, although their PL remains linearly polarized, 37 out of 87 SWCNTs have orientation angles rotated more than 45° from that of the laser (Figure 5, right, and Figure 6a, middle). Fitting the orientation angle histogram of the SWCNTs with Gaussian functions gives a most probable orientation angle of 81° for the polarization rotated SWCNTs. This kind of anomalous emission polarization behavior from SWCNTs coupled to Si640 might be due to silicon nanoresonator induced emission modification, as was observed for quantum emitters coupled to plasmonic nanostructures whose multipolar modes alter the polarization property of a neighboring emitter.47,48 To explore this possibility, we used an electric dipole to represent individual dopant sites and FDTD to simulate field distributions of the emission projected in the far field (Supporting Information S6). For an electric dipole oriented along the x-axis lying at different positions around a silicon nanoresonator, the far-field x-component of the electromagnetic wave always dominates over the y-component, indicating that despite the interaction between the dipole and the magnetic mode of Si640, far field emission of the dipole

intensity close to the silicon nanoresonators at the excitation wavelength may contribute to the PL enhancement. This effect is strongly dependent on the relative locations of the SWCNTs. However, determining the exact location and orientation of the SWCNTs with respect to silicon nanoresonators was challenging due to the diffraction-limited optical resolution and substrate-induced electron charging in a SEM. (2) Our PL lifetime measurements show that coupling between SWCNT emission and magnetic mode of Si640 leads to an average increase in the radiative decay rate by a factor of 1.5−3.0. However, the coupling strength depends on the exact location and orientation of the SWCNTs. (3) Directional scattering of the silicon nanoresonators can modify the experimental collection efficiency of the SWCNT emission with our reflectance collection geometry. Overall, due to the uncertainty in the location and orientation of SWCNTs, it is virtually impossible to determine the exact contributions from each of these factors. Aside from PL enhancement, interestingly, we also observed very different emission polarization behavior from SWCNTs coupled to Si640 compared to those in unpatterned areas. In the experiment, a linear polarizer was placed right after the laser to form a linearly polarized excitation beam. PL from the SWCNTs was sent through a half-wave plate and then a Wollaston prism, which can spatially separate the PL into its sand p-components, and finally focused onto the spectrograph slit. Figure 5a (left) shows the laser intensity distribution between its s- and p-componentsthey are focused at different vertical locations along the spectrographas a function of the polarization angle. Plotting the laser s-component intensity as a function of polarization angle (Figure 5c, left) shows that the 6435


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To confirm our explanation that the observed emission from tubes with orientation angles rotated more than 45° from that of the laser is due to modification of local electric field polarization distribution rather than emission modification caused by the magnetic dipole mode of Si640, we designed and fabricated another type of silicon metasurface with nanoresonator diameter of 360 nm and center-to-center distance of 560 nm (Figure 6b, inset, denoted as Si360). The resonance modes in Si360 are designed to be at 996 and 1040 nm (Figure 6b), with the former exhibiting electric dipole characteristics and the latter magnetic dipole characteristics, respectively (Supporting Information S7). Due to wafer induced Fabry− Perot resonances and fabrication inaccuracies, multiple additional peaks imposed on the main resonance peaks and broader spectral width are observed in the experimentally measured reflectance spectrum (Figure 6b, black) compared to the simulated one (Figure 6b, red). However, most importantly, Si360 has no resonance peak at ∼1300 nm. Therefore, coupling effects between the silicon nanoresonator resonance modes and PL emission from SWCNTs can be excluded, and only the excitation modification effect should be considered. PL polarization measurements of 93 oxygen-doped SWCNTs on Si360 reveal that 36 tubes have emission polarization angles rotated more than 45° from that of the laser (Figure 6a, bottom). Although the percentage of tubes showing rotated PL polarization is relatively lower for tubes on Si360 than those on Si640, it is clear that Si360 can also cause PL polarization rotation. Simulations of the local electric field polarization distributions of Si360 reveal that Si360 can also cause certain polarization rotation “hot spots” that have stronger ycomponent than x-component (Figure 6c, bottom left and right), thus leading to local electric field polarization rotated from that of the incident laser beam.

Figure 6. (a) Histograms of orientation angles of oxygen-doped SWCNTs in unpatterned areas (top), on Si640 (middle) and Si360 (bottom). The orientation angle of the excitation laser beam is 0°. (b) Experimentally measured (black) and simulated (red) reflectance spectra of Si360 before depositing SWCNTs. Inset: A scanning electron micrograph of Si360. (c) Local electric field polarization distributions (|Ey|/|Ex|) at the bottom and top planes of Si640 (top) and Si360 (bottom) nanoresonators when illuminated by an x-polarized plane wave at the wavelength 865 nm. The positions of the bottom and top planes are indicated as the blue and orange planes in Figure 1a. The white dashed lines indicate the positions of the silicon nanoresonators.

remains polarized along the x-direction. Therefore, coupling to the magnetic mode of Si640 is not likely the reason for the observed emission polarization rotation. Having eliminated emission modification as a possibility, we inspect the influence of the silicon nanoresonators on the excitation of SWCNTs. For an incident x-polarized plane wave at the excitation wavelength of 865 nm, we simulate the local electric field polarization distribution by calculating the ratio between the y- and x-field components, i.e., |Ey|/|Ex|, at the bottom and top of a silicon nanoresonator (Si640) (Figure 6c, upper left and right). We find that at some areas in the vicinity of the silicon nanoresonator, |Ey|/|Ex| is much larger than unity, indicating that at these “hot spots”, the local electric field is no longer x-polarized, but instead it forms a polarization angle to the x-axis that can be as large as 88° (e.g., at the hot spots in Figure 6c upper left, |Ey|/|Ex| as large as 35 can be observed), making the field almost y-polarized. This kind of local field polarization rotation hot spot can lead to excitation of tubes that are oriented along the y-axis, causing the observed emission from tubes with polarization angle rotated more than 45° from that of the laser. This polarization rotation may also combine with excitation and radiative decay rate enhancements and contribute to the observed PL enhancement. It is worth mentioning that, in principle, it is also possible that such a large y-component may cause perpendicular excitation of tubes that are oriented along the x-axis. Recent anisotropic optical studies have revealed that optical transitions from polarization perpendicular to the tube axis can be observed although they are strongly suppressed due to the 1D geometrical anisotropy of SWCNTs.49,50 However, here, cross-polarized excitation is quite unlikely due to the large energy difference between the excitation energy used in this study (E11 phonon sideband) and the transition energy of the cross-polarized excitation (e.g., E12 or E21).

CONCLUSIONS Our single tube photoluminescence studies at cryogenic temperatures reveal that by coupling PL from oxygen-doped SWCNTs to the magnetic dipole mode of silicon nanoresonators, a PL enhancement factor of 0.8−4.0 can be obtained. From PL lifetime measurements, we deduce an increase in the radiative decay rate by a factor of 1.5−3.0. To further enhance PL intensity using dielectric metasurfaces, strong field localization in nanometer-scale gaps between dielectric dimers can be used to increase the Purcell factor significantly,51 although it imposes huge difficulties in sample fabrication. Alternatively, recently proposed metal-dielectric hybrid nanoantennas23,52 provide another approach toward high Purcell factor, low-loss systems. Another significant finding in this study is that local electric field polarization distribution can be modified by silicon nanoresonators. When excited by a linearly polarized laser beam, many tubes on Si640 showed emission polarization rotated more than 45° from that of the laser, while those in unpatterned areas had PL oriented along the laser polarization. Our numerical simulations reveal that the near-field polarization distribution at certain hot spots in the vicinity of the silicon nanoresonators at the excitation wavelength could be modified, leading to the excitation of tubes lying perpendicularly to the laser polarization. Such modification of local field polarization distribution provides a general approach toward controlling quantum emitter emission because by careful design, dielectric or plasmonic nanostructures with well-defined local field polarization distribution on the nanometer scale can be used for polarization control in 6436


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and Purcell factor, simulations of directional scattering of silicon nanoresonators and changes in the collection efficiency, polarization of laser beam on silicon metasurfaces, far field emission polarization simulations, resonance modes in Si360 (PDF)

MATERIALS AND METHODS Dielectric Metasurface Fabrication. All-dielectric metasurfaces made from silicon nanoresonators used in this study were fabricated following the method described in ref 16 by electron-beam lithography of silicon-on-insulator (SOI) wafers using a negative-tone resist NEB31A. After development in MF-321, the patterns were used as an etch mask for a reactive-ion etching process. Finally, the remaining resist was removed by oxygen plasma. Optical reflectance spectra of the silicon metasurfaces were measured with an FTIR spectrometer (Bruker) using an InGaAs detector, and further confirmed by a homebuilt reflectance spectroscopy microscope. In both methods, white light was focused onto samples either by a lens or a microscope objective. Reflectance from the samples was collected by the same lens/objective and guided to the spectrometers and InGaAs detectors. Single-Walled Carbon Nanotube Preparation. Oxygen-doped single-walled carbon nanotubes in this study were fabricated by a solidstate doping approach.25,30 Specifically, chirality enriched (6,5) SWCNTs wrapped by 1 wt% sodium dodecylbenzenesulfonate (SDBS) were prepared by the two-step aqueous two-phase extraction method,55,56 drop-cast on silicon metasurfaces, onto which was deposited a 10 nm thick layer of SiO2 at the rate of 0.2 nm/s by electron-beam deposition. Photoluminescence Imaging and Spectroscopy Measurements. For single tube optical measurements, samples were loaded into a continuous-flow, liquid-He cryostat equipped on a home-built confocal laser microscope. All the single nanotube PL measurements were performed at 4 K. The tubes were excited at their E11 phonon sideband of 865 nm either by a continuous-wave or a pulsed (150 fs, 90 MHz) tunable Ti:sapphire laser. The excitation power was kept below 15 kW/cm2 to avoid exciton−exciton annihilation that may further complicate the recombination process. A microscope objective (100×, NA = 0.65) was used to focus the laser beam and collect emission from the samples. PL images and spectra were taken with a 2D InGaAs array mounted on a 300 mm spectrograph and 1D InGaAs linear array on a 150 mm spectrograph, respectively. A superconducting nanowire single-photon detector (Single Quantum) together with HydraHarp electronics (PicoQuant) was used to perform time-resolved lifetime measurements. A 1200 nm long-pass filter was used to block the laser beam, background emission from the silicon substrates, and high energy transitions in the oxygen-doped SWCNTs. FDTD Simulations. 3D FDTD simulations were performed with the software FDTD Solutions from Lumerical Solutions, Inc. To calculate the reflectance spectra and local electric field distributions of the silicon metasurfaces, a plane wave propagating perpendicularly to the sample surface is used as the light source. The electric field enhancement factor is calculated by dividing the local electric field intensities of the silicon metasurfaces with that of blank surfaces. To calculate the radiative decay rates of the SWCNTs with and without nearby silicon nanoresonators, we adapt the method described in ref 37 and use an electric dipole to represent the SWCNTs. A set of six frequency-domain surface monitors were used to create a box around the system (including the silicon metasurfaces and electric dipole), and the total radiated power by the system is normalized to the power radiated by the electric dipole, which is calculated by another set of six frequency-domain surface monitors surrounding the dipole. This ratio gives us the radiative decay rate enhancement factor.

AUTHOR INFORMATION Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Xuedan Ma: 0000-0002-3163-1249 Nicolai F. Hartmann: 0000-0002-4174-532X Stephen K. Doorn: 0000-0002-9535-2062 Han Htoon: 0000-0003-3696-2896 Present Address ⊥

Center for Nanoscale Materials, Argonne National Laboratory, 9700 South Cass Avenue, Argonne, Illinois 60439, United States

Notes

The authors declare no competing financial interest.

ACKNOWLEDGMENTS We thank Dr. Hou-Tong Chen for use of the pulsed Ti:sapphire laser. This work was supported by the U.S. Department of Energy, Office of Basic Energy Sciences, Division of Materials Sciences and Engineering, and performed, in part, at the Center for Integrated Nanotechnologies, an Office of Science User Facility operated for the U.S. Department of Energy (DOE) by Los Alamos National Laboratory (Contract DE-AC52-06NA25396) and Sandia National Laboratories (Contract DE-NA-0003525). Sandia National Laboratories is a multimission laboratory managed and operated by National Technology and Engineering Solutions of Sandia, LLC., a wholly owned subsidiary of Honeywell International, Inc., for the U.S. Department of Energy's National Nuclear Security Administration under contract DE-NA0003525. N.F.H., S.K.D. and H.H. acknowledge partial support by Los Alamos National Laboratory, Directed Research and Development Funds. REFERENCES (1) Reithmaier, G.; Kaniber, M.; Flassig, F.; Lichtmannecker, S.; Müller, K.; Andrejew, A.; Vučković, J.; Gross, R.; Finley, J. J. On-Chip Generation, Routing, and Detection of Resonance Fluorescence. Nano Lett. 2015, 15, 5208−5213. (2) Kumar, S.; Kristiansen, N. I.; Huck, A.; Andersen, U. L. Generation and Controlled Routing of Single Plasmons on a Chip. Nano Lett. 2014, 14, 663−669. (3) Chang, D. E.; Sørensen, A. S.; Demler, E. A.; Lukin, M. D. A Single-Photon Transistor Using Nanoscale Surface Plasmons. Nat. Phys. 2007, 3, 807−812. (4) Hwang, J.; Pototschnig, M.; Lettow, R.; Zumofen, G.; Renn, A.; Götzinger, S.; Sandoghdar, V. A Single-Molecule Optical Transistor. Nature 2009, 460, 76−80. (5) Martín-Cano, D.; González-Tudela, A.; Martín-Moreno, L.; García-Vidal, F. J.; Tejedor, C.; Moreno, E. Dissipation-Driven Generation of Two-Qubit Entanglement Mediated by Plasmonic Waveguides. Phys. Rev. B: Condens. Matter Mater. Phys. 2011, 84, 235306. (6) Bernien, H.; Hensen, B.; Pfaff, W.; Koolstra, G.; Blok, M. S.; Robledo, L.; Taminiau, T. H.; Markham, M.; Twitchen, D. J.;

ASSOCIATED CONTENT S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsnano.7b02951. PL image of oxygen-doped SWCNTs, simulation of radiative decay rate enhancement factors, estimation of η 6437


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Solitary Oxygen Dopant Emission from Carbon Nanotubes Modified by

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