Optical Energy Transfer from Photonic Nanowire to Plasmonic Nanowire

which is one of the most promising candidates to meet the contemporary information technology requirements of big data processing at high-speed. 2. Su...
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Optical Energy Transfer from Photonic Nanowire to Plasmonic Nanowire Xianguang Yang, Yuchao Li, Zaizhu Lou, Qin Chen, and Baojun Li ACS Appl. Energy Mater., Just Accepted Manuscript • DOI: 10.1021/acsaem.7b00098 • Publication Date (Web): 12 Jan 2018 Downloaded from http://pubs.acs.org on January 13, 2018

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Optical Energy Transfer from Photonic Nanowire to Plasmonic Nanowire

Xianguang Yang*, Yuchao Li, Zaizhu Lou, Qin Chen, and Baojun Li* Guangdong Provincial Key Laboratory of Optical Fiber Sensing and Communications, Institute of Nanophotonics, Jinan University, Guangzhou 511443, China *E-mail: [email protected] *E-mail: [email protected]

ABSTRACT: Rational and versatile transfer of optical energy from photonic nanowire to plasmonic nanowire is highly desirable and extremely challengeable. Herein, we demonstrate that optical energy transfer from CdSe-ZnS core-shell quantum dots doped polymer photonic nanowire to silver plasmonic nanowire with an efficiency of 32% ± 1.5% via the Förster resonance energy transfer process. The quantum dots with an emission at 650-nm wavelength were embedded in polymer nanowire and were excited by a 532-nm green laser. In the crossing region of the photonic nanowire and plasmonic nanowire, propagated surface plasmons of silver nanowire can be effectively excited by the emitted light from quantum dots embedded in polymer nanowire. This energy transfer between photonic and plasmonic nanowires would provide potential applications in optical interconnection.

KEYWORDS: Energy transfer, nanowire waveguide, photonics, plasmonics, quantum dots

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Photonics merging with electronics at nanoscale dimensions generates nano-plasmonics,1 which is one of the most promising candidates to meet the contemporary information technology requirements of big data processing at high-speed.2 Surface plasmons include localized and propagated modes in metal nano-particles and nano-waveguides, respectively. These two modes can both be effectively excited by the incident electromagnetic radiations coherently coupled with the electron plasmas oscillation in metals, possess the ability to beat diffraction limit since they achieve optical energy localization at a sub-wavelength interface between metals and dielectrics.3 Hence, the metal nanostructures used for plasmonic nano-waveguides significantly allow to the manipulation of light signals at a nanoscale dimension.1-3 High-speed transmission, processing and storage of optical signals require the optical energy transfer between photonic and plasmonic ports on a diffraction limited scale.4-11 For high density photonic and plasmonic interconnection, those ports are applicable with photonic and plasmonic nanowire (NW) waveguides.5-7,9-11 However, passive port with photonic nanowire just act as a traditional waveguide for light energy delivery, in which photon-to-plasmon conversion (donor-to-acceptor energy transfer) only occurred at distal tips of plasmonic NW via scattering compensated momentum mismatch.5,12 Rational and versatile transfer of light energy from photonic to plasmonic NW in midsection is highly desirable and extremely challengeable. Previously, a primary work on the excitation of metallic NW surface plasmon by quantum dots (QDs) has been reported.13 Herein, QDs doped polyvinylpyrrolidone NW is an active photonic waveguide, which can act as both a traditional waveguide and light emitting source (energy donor), leading to the simultaneous distribution of excited emission into two distal ends of plasmonic silver (Ag) NW (energy acceptor). The QDs with 650-nm emission were embedded in polyvinylpyrrolidone NW and were excited by 532-nm green laser excitation. In the crossing region of the photonic

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NW and plasmonic NW, propagated surface plasmons of Ag NW in midsection can be effectively excited by photonic emission via Förster resonance energy transfer process. The transfer efficiency of 32% ± 1.5% is larger than the optimal efficiency of recently published work (about 25%).4 Notably, the optimal efficiency requires experimental identification of nanostructure eigenchannels and wavefront shaping of incident beam to a specific eigenchannel.4 In contrast, our energy transfer between photonic and plasmonic NWs are less complex and more facility to provide potential applications in optical interconnection. Polymer photonic NWs are fabricated by direct drawing the mixture solution of polyvinylpyrrolidone and CdSe-ZnS QDs. The diameter of fabricated photonic NWs is ranging from 300 to 500 nm. The fabrication details can be found in our previous work.14 Figure 1a shows a scanning electron microscope (SEM) image of a single QD-doped photonic NW with 300-nm diameter. The diameter is uniform and the surface is smooth, which is benefited to the delivery of light energy. To clearly observe the distribution of QDs in the polyvinylpyrrolidone NW, transmission electron microscopy (TEM) at 300 kV operating voltage and energydispersive X-ray spectroscopy (EDS) were conducted. Figure 1b shows a TEM image of the 300nm diameter QD-doped photonic NW. The CdSe-ZnS core-shell QDs were successfully embedded in NW and their distribution is relatively homogeneous without visible aggregation. The estimated maximum variation in diameter (marked as ∆D) of QD-doped photonic NW is ∆D of 5 nm over length L of 2 µm. For element contents, EDS analysis shown in Figure 1c confirming the presence of S (4.06 wt %), Zn (11.03 wt %), Se (1.32 wt %), and Cd (1.88 wt %) in the photonic NW. The above four elements came from the doped CdSe-ZnS QDs. The other elements of C, O, and Cu came from the carbon membrane supported copper micro-grid. The estimated concentration for doped QDs in the 300 nm-diameter photonic NW is approximately

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3.5 × 103 µm−3. It is noted that the QD concentration can be controlled ranging from 3 to 4 × 103 µm−3 without obvious aggregation by adjusting the amount of added QDs in the fabrication procedure (see Methods). To ensure the doped QDs are optically active, fluorescence microscopic image of a single QD-doped photonic NW with red emission was observed and shown in Figure 1d. The doped QDs were efficiently excited by incident 532-nm laser and then emitted 650-nm wavelength fluorescence.

Figure 1. (a) SEM and (b) TEM images of a representative QD-doped photonic NW with diameter of 300 nm. (c) EDS analysis of the 300-nm diameter QD-doped photonic NW. The inset shows the percentages of individual element contents. (d) Fluorescence microscopic image of a QD-doped photonic NW with 650-nm wavelength emission.

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Plasmonic Ag NWs were chemically synthesized by an established method.15 The diameter of fabricated Ag NWs is ranging from 200 to 400 nm. Figure 2a shows the SEM image of fabricated Ag NWs, the length can up to 60 µm, which provided an opportunity to investigate the long distance delivery of plasmonic energy. Different from SEM image showing the surface information, TEM image gives internal structure information to Ag NWs, which is provided in Figure 2b. The Ag NWs lie on the micro-grid supported with carbon film, the image contrast between Ag NWs and carbon film is obvious. The high-contrast line shape is Ag NW and the low-contrast film shape is carbon film supported micro-grid. To clearly know the structure of distal end, the inset shows a close-up view of a representative distal end in 200-nm diameter Ag NW. The photonic and plasmonic NWs crossing structure was readily assembled by micromanipulating a QD-doped photonic NW to an Ag plasmonic NW using tungsten probes under an optical microscopy. The main diameter of the tungsten probe is 5 µm and the taper area is 8.8 µm long with tip diameter of about 200 nm. Figure 2c shows the optical microscopic image of an experimental used tungsten probe. Figure 2d shows a SEM image of a QD-doped photonic NW just contact with an Ag NW. The resulted crossing structure is shown in Figure 2e, and the inset gives a TEM image. The slightly different crossing angle is caused by the transfer process from SEM to TEM measurements. For the SEM measurement, the crossing structure was adhered on conducting resin. To perform the TEM measurement, the crossing structure will be peeled off the resin and placed onto the copper micro-grid (as mentioned above: the copper micro-grid is supported with carbon membrane). At the same time, the QD-doped photonic NW also crossed with another bend Ag NW 2. For optical interconnection in practical device applications, this type of one photonic NW simultaneously crossed with both a straight and a bend Ag NWs could provide multi-functionalities for photonics and plasmonics.16-18 In addition, the orientation and

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number of crossing Ag NWs, even the distance between each other can be ready controlled through micro-manipulation.

Figure 2. (a) SEM image of as synthesized Ag NWs with diameter ranging from 200 nm to 400 nm and length up to ~60 µm. (b) TEM image of six Ag NWs and the inset shows a high resolution TEM image of one distal end in 200-nm diameter Ag NW. (c) Bright field optical microscopic image of as used tungsten taper tip, the taper area is 8.8 µm long and the tip diameter is about 200 nm. (d) SEM image of a QD-doped photonic NW just contact with an Ag NW. (e) SEM image of two Ag NWs crossed with a QD-doped photonic NW, one Ag NW is straight and the other one is slightly bend. The inset shows the TEM image of the left crossing.

The optical energy transfer from a QD-doped photonic NW (400-nm in diameter) to an Ag plasmonic NW (300-nm in diameter), the photonic and plasmonic NWs are interconnected with a crossing angle of 45° (Figure 3a). When plasmonic and photonic NWs are close enough contact, the plasmonic near field of a Ag plasmonic NW and optical near field of a QD-doped photonic

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NW are strong overlap, which results in highly efficient conversion of photon to plasmon in the cross-coupling area.19 Figure 3b shows a dark-field optical microscope image of the optical energy transfer crossing structure excited by a 532-nm laser with an optical power of 5 mW. Note that the excitation spot is at the left side of the coupling area and not presented in Figure 3b. The distance between excitation spot and coupling area is about 30 µm. The size of excitation spot is about 1 µm, which is good agreement with the calculated result of 998.5 nm from the equation 1.22 × λ/NA = 1.22 × 532/0.65 nm = 998.5 nm, where NA is the numerical aperture of experimentally used 60× objective. The Ag NW is 55 µm long, and the lengths of the long (white double-headed arrow) and short (yellow double-headed arrow) regions of the Ag NW on either side of the crossing structure (hereafter referred to as 'limbs') are 40 and 10 µm, respectively. The red arrows show the propagation direction of excited 650-nm photoluminescence (PL) from the doped QDs. Generally, the momentum mismatch between plasmon and photon can usually be compensated with additional wave vectors provided by light scattering.20 However, for the junction scattering in the cross-coupling area, the light is scattered at the middle section of the Ag plasmonic NW, a cylindrical symmetry cannot supply the axial scattering along the Ag NW. Thus, the scattering assisted excitation of the plasmon modes along the Ag NW is weak. Herein, the photons guided in the QD-doped photonic NW efficiently converted to plasmons were achieved without momentum matching via the process of Förster resonance energy transfer.21 In the crossing structure, photon-to-plasmon coupling could be specifically understood by the energy transition: photons-to-excitons-to-plasmons, and without the need for momentum matching, because the near-field of QD optical dipolar contains momentum components matching those of Ag NW plasmons. Therefore, the energy transfer is not dependent on the

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crossing angle.21 It is very useful for optical interconnection to distribute light simultaneously into different directions with the same efficiency at a relatively high level (≥ 30%). In addition, the reference experiments on the coupling between Ag NW and bare polymer NW (bare means that the polymer NW is without QDs doped) indicates that the guided light in polymer NW cannot couple to a crossed Ag NW. The converted plasmons guided along the surface of Ag plasmonic NW, finally scattering from the two tips of the Ag plasmonic NW into free space. Since the cylindrical symmetry is broken at the distal tips of the Ag NW, the light scattering supplies additional wave vectors in all directions, then the momentum mismatch is compensated. Therefore, when the guided plasmons face the distal tips of the Ag NW, the guided plasmons are decoupled into photonic emissions but not at the middle section of the Ag plasmonic NW.22 To further demonstrate plasmon propagation in a single Ag plasmonic NW, the intensity profiles of excited red PL along an Ag NW were measured by using the histogram tool of Adobe Photoshop.5 Figures 3c and 3d show red PL intensity profiles along the long and short limbs of an Ag NW, respectively, plotted with an exponential decay of I(x) = I0e−x/L,5 where I(x) is the measured red PL intensity at distance x, I0 is initial intensity, x is propagation distance and L is propagation length, indicating the wave-guiding property of propagated surface plasmons. It should be pointed out that L is defined as the characteristic length at which the measured intensity I(x = L) decreases to I0/e. In addition, L is decided by the imaginary part of the complex plasmon wave vector k, which is provided by the equation L = 1/2Im{k}.8 The propagation loss of plasmons (marked as α) can be expressed as α = −10lg(1/e)/L ≈ 4.343/L.23 Specifically, L = 10 ± 0.5 µm and α = 0.43 ± 0.02 dB/µm were calculated for long limb of the Ag plasmonic NW, while L = 5 ± 0.3 µm and α = 0.87 ± 0.04 dB/µm were obtained for the short one. In addition, propagation length of L = 120 ± 6 µm and propagation loss of β = 0.036 ± 0.002 dB/µm were

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also obtained for QD-doped photonic NW. Experimentally, the propagation loss is reversible between long and short limbs when shining the excitation laser at the right side of the coupling area. Therefore, the difference of α between the long and short limbs is due to orientation angle dependent plasmon modes.24 The crossing angle between the PL propagation direction in the QD-doped photonic NW and plasmon propagation direction in the long limb is 45°, while that between the PL propagation direction and plasmon propagation direction in the short limb is 135°. This experimental analysis clearly demonstrates plasmon propagation in a single Ag plasmonic NW.

Figure 3. (a) SEM image of an optical energy transfer crossing structure assembled from an Ag NW and QD-doped NW with a crossing angle of 45°. (b) Dark-field optical microscope image of the optical energy transfer crossing structure excited by a 532-nm laser with an optical power of 5 mW. Intensity profiles of red PL along the long (c) and short (d) limbs of the Ag NW, respectively, plotted with an exponential decay. The insets and green dashed double-headed

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arrows are a visual guide. (e) PL spectra measured at coupling area, distal ends of long and short limbs.

Actually, the thin Ag NW with diameter less than 100 nm would result in larger L because they are leakage radiativeless and indeed support long propagation.25,26 However, the manipulation of thin Ag NW under optical micromanipulation is not easy due to the optical diffraction limit (size of 200 nm). At same time, the thin Ag NW is sensitive to physical vibrations, resulting in the instability of manipulation for photonic structures fabrication. Thus, we chose the thick Ag NW (200-400 nm in diameter) with relatively long propagation for practical applications to ensure the stability of photonic structures. The large-area active interface formed between thick photonic and plasmonic NWs also benefits the effective energy transfer since the thick photonic NW can contain more QDs than that embedded in the thin one. Furthermore, the embedded single QD acts individually as a nano-emitter, the NW confinement can enable the QDs emitted light energy well localized and guided along photonic NW. In addition, Figure 3e shows the PL spectra measured at coupling area, distal ends of long and short limbs with the center wavelength at 650, 660, and 654 nm, respectively. The center wavelength red shifts from 650 to 654 and 660 nm are caused by energy dissipation due to the large metal loss in Ag NW, which is a wavelength-dependent dispersion effect.27 The precise transfer efficiency of light energy from a QD-doped photonic NW to an Ag plasmonic NW is difficult to measure. For the approximated transfer efficiency (η), which is defined as the ratio between the total optical energy input into the Ag NW (IAg) and the total optical energy output from the QD-doped photonic NW (IQD) at the coupling area. It can be estimated from the intensity of emission from the end of the Ag NW (Iend) and the radiative

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coupling loss because of scattering (Iscatter) at the coupling area. That is, η = IAg/IQD = IAg/(IAg + Iscatter). Iscatter can be measured simply by integrating the light spot of scattering at the coupling area. Specifically, the coupling area is considered as a square region with side length of 1 µm (area: 1 × 1 µm2), which can totally confine the largest scattering spot with position X = 0 shown in Figures 3c and 3d. For comparison, the calculated area of all the other scattering spots is set to be of 1 × 1 µm2. Iend is related to IAg via Iend = IAge−x/L, where x is the distance from the coupling area to the distal end of the Ag NW, which was measured from the dark-field optical microscope image. Inserting Iend = IAge−x/L into η = IAg/(IAg + Iscatter), the approximated transfer efficiency can be expressed as η = Iendex/L/(Iendex/L + Iscatter), where Iend and Iscatter can be measured simply by using the histogram tool of Adobe Photoshop. Therefore, the calculated η of excited 650-nm light transfer from the QD-doped photonic NW to the Ag plasmonic NW is 32% ± 1.5%. Interestingly, by properly design the structures formed with photonic and plasmonic NWs, the hybrid photonic-plasmonic mode supported in designed structure would be investigated for tailoring enhanced light-matter interactions and/or transferring broadband light between different modes in the near future.28-30

Conclusions In summary, we demonstrated the optical energy transfer from a QD-doped photonic NW into an Ag plasmonic NW with a coupling efficiency of 32% ± 1.5% at the single NW level. The propagated surface plasmons of the Ag NW were excited in midsection by PL from the QDdoped photonic NW under 532-nm laser excitation. For a single 300-nm diameter Ag plasmonic NW, the propagation losses of plasmons were 0.43 ± 0.02 and 0.87 ± 0.04 dB/µm for its long

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and short limbs, respectively. The optical energy transfer between photonic and plasmonic NWs might be better in reality than plasmonic nano-laser coupling in an integrated circuits chip.

Materials and methods CdSe-ZnS core-shell QDs at 650-nm emission wavelength and polyvinylpyrrolidone (refractive index n = 1.47) were commercially available from Zkwy Bio-Tech (Beijing) and Boai NKY (Henan) companies, respectively. QD-doped photonic NWs were fabricated as follows: Polyvinylpyrrolidone (600 mg) was dissolved in anhydrous ethanol (0.8 mL) to form a homogeneous solution. QDs (300 µL, 5.8-nm in diameter) suspended in water to give a concentration of 8 µM/L were added to the polyvinylpyrrolidone solution. The mixture solution was stirred at room temperature for 3 h, and then ultra-sonicated for 25 min to form a uniform solution with an appropriate viscosity for drawing. The tip of a tapered silica fiber tip (diameter of ∼25 µm) was immersed in the solution for 3-8 s and then pulled out at a rate of 0.5-2.5 m/s, forming a QD-doped polyvinylpyrrolidone wire between the solution and fiber tip because of the very fast evaporation of ethanol. The diameter of the polyvinylpyrrolidone NWs ranged from 300 to 500 nm. To assemble Ag NWs with QD-doped photonic NWs, the Ag NWs were dispersed in deionized water (1:1000 w/w) by ultra-sonication for 5 min and then deposited on a magnesium fluoride (MgF2, n = 1.38) substrate. The QD-doped photonic NW was placed on the MgF2 substrate with Ag NWs. The QD-doped NW were then physically contacted with Ag NWs using a commercial six-axis micro-manipulator (Kohzu Precision, 50-nm resolution) equipped with tungsten probes (200-nm tip diameter) under an optical microscopy (CRAIC, 20/20 PV). The orientation and position of Ag NWs with respect to QD-doped photonic NW were well controlled by careful micro-manipulation.

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To characterize the microscopic structures of photonic and plasmonic NWs, SEM (HIROX, SH-5000M) and TEM (Tecnai G2 F30) working at 300 kV were used, and the corresponding EDS were conducted. For optical characterization of the crossing structure, a 532-nm green laser was used to excitation source. PL signals were collected by a 60× objective with numerical aperture of ∼0.65 and directed through a dichroic mirror and notch filter. The filtered light was split by a beam splitter and directed to a spectrometer and charge-coupled device (CCD) camera (Sony iCY-SHOT) for spectrum and imaging, respectively.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] Author ORCIDs Xianguang Yang: 0000-0002-6787-9924 Notes The authors declare no competing financial interests. ACKNOWLEDGMENT This work was supported by the National Natural Science Foundation of China (No. 21703083), the Natural Science Foundation of Guangdong Province (Nos. 2017A030313026 and 2017A030310463), and the Fundamental Research Funds for the Central Universities (No. 21617334). REFERENCES

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(27) Yang, X.; Bao, D.; Li, B. Light Transfer from Quantum-Dot-Doped Polymer Nanowires to Silver Nanowires. RSC Adv. 2015, 5, 60770-60774. (28) Peng, P.; Liu, Y.-C.; Xu, D.; Cao, Q.-T.; Lu, G.; Gong, Q.; Xiao, Y.-F. Enhancing Coherent

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Resonances. Phys. Rev. Lett. 2017, 119, 233901. (29) Xiao, Y.-F.; Liu, Y.-C.; Li, B.-B.; Chen, Y.-L.; Li, Y.; Gong, Q. Strongly Enhanced LightMatter Interaction in a Hybrid Photonic-Plasmonic Resonator. Phys. Rev. A 2012, 85, 031805 (R). (30) Jiang, X.; Shao, L.; Zhang, S.-X.; Yi, X.; Wiersig, J.; Wang, L.; Gong, Q.; Lončar, M.; Yang, L.; Xiao, Y.-F. Chaos-Assisted Broadband Momentum Transformation in Optical Microresonators. Science 2017, 358, 344-347.

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The following graphic will be used for the TOC:

TOC Graphics Manuscript title: Optical Energy Transfer from Photonic Nanowire to Plasmonic Nanowire Names of authors: Xianguang Yang, Yuchao Li, Zaizhu Lou, Qin Chen, and Baojun Li* Brief synopsis: Optical energy transfer from CdSe-ZnS core-shell quantum dots doped polymer photonic nanowire to silver plasmonic nanowire with an efficiency of 32% ± 1.5% via the Förster resonance energy transfer process.

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