Light transfer from quantum-dot-doped polymer nanowires to silver nanowires

Abstract

Light transfer between CdSe–ZnS core–shell quantum dot-doped polymer nanowires and silver (Ag) nanowires is achieved. Plasmons in the Ag nanowires can be effectively excited by photoluminescence emission from the quantum-dot-doped polymer nanowires under 532 nm laser excitation. Their efficient transfer makes it feasible to obtain waveguiding spectra at different propagation distances. Wavelength-converted waveguiding and wavelength-dependent dispersion effects occurred in polymer and Ag nanowires, respectively.

---

Flexible optical networks for high-speed transmission and processing of optical signals require optical interconnection with a large number of optical input and output ports on a chip to increase the transmission rates and capacity of optical signals. The large scale of optical interconnection greatly exceeds the optical diffraction limits of a traditional photonic integrated circuit (IC), and will demand the mode confinement of waveguides well below the optical diffraction limits, and also their interconnection with diffraction-limited waveguides.^1–3 Recently, considerable efforts have established a variety of plasmonic waveguides,^4–8 including microscale metal waveguides,^9,10 arrays of nanoparticles,¹¹ and plasmonic nanowires (NWs),^12–14 which demonstrates the capability of plasmonic nanophotonics. Unfortunately, the intrinsic metal loss renders it impossible to guide optical signals across the whole photonic IC chip (length of ∼1 mm) entirely with plasmonic waveguides. To realize the practical applications of photonic IC chip, plasmonic waveguides have to be interconnected with conventional photonic waveguides, which are readily connected to external light sources, guide light for long distances with low propagation loss, and distribute light to different plasmonic waveguides simultaneously.¹⁵ Silver (Ag) NWs possess unique features that enable them particularly attractive to guide and confinement light at nanometer scale.^16,17 Chemically synthesized Ag NWs have a relatively smooth surface, so the overall propagation loss of plasmons is much lower than that of Ag NWs produced via electron-beam lithography.¹⁸

Photon-to-plasmon conversion usually occurs on the cross-coupling area of NWs because of scattering, which enables plasmons to be efficiently excited at the cross-coupling area and guided with relatively low propagation loss to the tips of Ag NWs, even along sharp curves.¹⁹ Despite Ag NW plasmonic waveguides meet the significant demands of deep subwavelength mode confinement and possess low propagation loss, the important problem of how to couple light into such small Ag NWs remains. To solve this problem, a variety of methods for light coupling have been developed, including total internal reflection scheme using a prism to compensate for the mismatch between the momentums of plasmons and photons,^18,20 local excitation via directly focusing laser on the end facets of Ag NWs or on metal nanoparticles adhered to the surface of Ag NWs,^21,22 and a nano-emitter located in the near field of an Ag NW that can excite propagation plasmons because the near field of the optical dipole includes large components of momentum matching that of propagation plasmons.^23,24 Nevertheless, these coupling methods do not allow easy interconnection of plasmonic and photonic waveguides on chips to fabricate photonic ICs.

As a solution to this problem, here we develop an efficient approach to couple photons into Ag NWs by contacting a single polyvinylpyrrolidone (PVP) NW doped with cadmium selenide (CdSe)–zinc sulfide (ZnS) core–shell quantum dots (QDs). We chose these materials because PVP improves the dispersion of CdSe nanocrystals in water,²⁵ and CdSe–ZnS core–shell QDs are a hopeful nano-emitter that can serve as an active element with strong absorption/emission and optical gain.²⁶ Importantly, such QD-doped polymer NWs have already been demonstrated as feasible and active photonic waveguides in the visible region.^27,28 The large aspect ratio, excellent flexibility, and strength of QD-doped polymer NWs render their manipulation on various surfaces, so they can be assembled into flexible optical systems. The QD-doped polymer NWs also generate photoluminescence (PL) under green laser excitation and can function as both a nanoscale waveguide and light emission source, resulting in the simultaneous distribution of excited PL into multiple Ag NWs. Plasmons of Ag NW can be effectively excited by the PL from QD-doped polymer NWs generated with 532 nm laser excitation. The waveguiding spectra at different propagation distances of the Ag NW and QD-doped polymer NW light coupling structure are analyzed and would find application for sensor.^29,30 Our coupling method may be used to couple light into other metal and semiconductor NWs.

Ag NWs were chemically synthesized using an established protocol.^31,32 The diameters and lengths of the Ag NWs ranged from 300 to 850 nm and 2 to 30 μm, respectively. QDs and PVP (n = 1.48) were purchased from Zkwy Bio-Tech (Beijing, China) and Boai NKY (Henan, China), respectively. QD-doped polymer NWs were fabricated by a direct drawing method as follows: PVP (500 mg) was dissolved in anhydrous ethanol (0.6 mL) to form a homogeneous solution. QDs (250 μL, 5.2 nm in diameter) suspended in water to give a concentration of 4 μM L⁻¹ were added to the PVP solution. The mixture was stirred at room temperature for 2.5 h, and then ultrasonicated for 35 min to form a uniform solution with an appropriate viscosity for drawing. The tip of a tapered silica fiber (diameter of ∼125 μm) was immersed in the solution for 1–5 s and then pulled out at a rate of 0.1–1.5 m s⁻¹, forming a QD-doped polymer wire between the solution and fiber tip because of the very fast evaporation of ethanol. The diameter of the polymer NWs ranged from 600 to 1200 nm.

Fig. 1a shows a scanning electron microscope (SEM, HIROX, SH-5000M) image of three typical Ag NWs denoted 1, 2 and 3, which have measured diameters of 300, 800 and 500 nm, respectively, and lengths of 14, 14, and 22 μm, respectively. Meanwhile, a single straight QD-doped polymer NW had a diameter of 800 nm (Fig. 1c). To closely inspect the distribution of QDs in the polymer NW, transmission electron microscopy (TEM) at 300 kV and energy-dispersive X-ray spectroscopy (EDS) were performed. Fig. 1d depicts a TEM image of three QD-doped polymer NWs denoted 4, 5 and 6, which have diameters of 1200, 750, and 800 nm, respectively. QDs were successfully doped in the polymer NWs. The estimated maximum variations in diameter ΔD of QD-doped NWs 4–6 are 10, 20 and 15 nm over lengths L of 2.5, 4.5 and 0.65 μm, respectively. EDS analysis (Fig. 1e) confirmed the presence of S (27.01 wt%), Zn (61.44 wt%), Se (4.73 wt%), and Cd (6.82 wt%) in the QD-doped polymer NWs. The estimated concentrations of QDs in polymer NWs 4, 5 and 6 are 5, 1.8, and 1.5 × 10³ μm⁻³, respectively. By adjusting the amount of QDs added in the first step of fabrication procedure, polymer NWs with a QD concentration of 10³ to 10⁴ μm⁻³ can be obtained without obvious aggregation of doped QDs.

Fig. 1 (a) SEM image of three Ag NWs. (b) Close-up view of Ag NW 3 in (a). (c) SEM image of a single QD-doped NW. Inset is a high-resolution SEM image. (d) TEM image of three QD-doped NWs. The red arrows indicate QDs. (e) EDS spectrum of QD-doped NW 5 in (d).

To interconnect Ag NWs with QD-doped polymer NWs, the QD-doped polymer NWs were first placed on a magnesium fluoride (MgF₂, refractive index n = 1.39) substrate. Ag NWs were dispersed in deionized water (1 : 800 w/w) by ultrasonication for 3 min. The Ag NW suspension was deposited on the MgF₂ substrate with QD-doped NWs. The Ag NWs were then physically contacted with QD-doped NWs using a commercial six-axis micromanipulator (Kohzu Precision Co., Ltd, resolution of ∼50 nm) equipped with tungsten probes (300 nm tip diameter) under an up-right optical microscopy (CRAIC, 20/20 PV, U.S.A.). The orientation and position of Ag NWs with respect to QD-doped NWs was well controlled by careful micromanipulation.

A SEM image of an Ag NW with a diameter of 300 nm interconnected with a QD-doped NW with a diameter of 800 nm at a crossing angle of 60° is presented in Fig. 2a. Unfortunately, there is some pollution around the Ag NW, which comes from the procedure of cleaning substrate. The pollution attached to Ag NW would reduce the propagation length due to surface scattering. To avoid this harmful pollution, the substrate will be cleaned via ultrasonic and the NWs will be placed into clean area with the assistance of optical microscopy. To examine this cross structure in greater detail, a close-up view is provided in Fig. 2b. For practical applications, interconnection of two or more Ag NWs with a single QD-doped polymer NW is desirable. As an example of this, Fig. 2c shows an SEM image of Ag NWs 1 and 2 both with diameters of 300 nm interconnected with a QD-doped NW (800 nm diameter) at crossing angles of 60° and 65°, respectively. The distance between Ag NWs 1 and 2 is 6.4 μm. To realize high-density interconnection of Ag NWs with QD-doped NWs, more than three Ag NWs with different diameters and lengths were interconnected with a single QD-doped NW at different crossing angles and intervals, which should provide versatile functionalities in nanoscale plasmonic and photonic applications.^33–35 Fig. 2d shows a SEM image of Ag NWs 1, 2, 3 and 4 interconnected with a QD-doped NW (600 nm diameter) at crossing angles of 50°, 70°, 70° and 80°, respectively. The diameters of Ag NWs 1, 2, 3 and 4 are 850, 500, 650 and 800 nm, respectively. These Ag NWs have different lengths, orientations, and distances between them. It can be seen from Fig. 2d that Ag NWs 1 and 2 are underneath the QD-doped NW while Ag NWs 3 and 4 are above the QD-doped NW. If Ag NWs are underneath the QD-doped NW, the Ag NWs are close to substrate would resulting in more light goes into substrate side than those Ag NWs above the QD-doped NW. Thus, Ag NWs above the QD-doped NW would benefit the coupling of light into Ag NWs.³⁶ Importantly, to obtain compact structures with desirable properties, the diameter, length and performance of the NWs should be carefully selected, and the orientation and number of Ag NWs, as well as the distances between them and their relative position with respect to the QD-doped NW should be precisely controlled through micromanipulation.

Fig. 2 SEM images of (a) an Ag NW interconnected with a QD-doped NW at a crossing angle of 60°, (b) high-magnification view of the cross structure in a, (c) Ag NWs 1 and 2 interconnected with a QD-doped NW at crossing angles of 60° and 65°, respectively. (d) Ag NWs 1, 2, 3 and 4 interconnected with a QD-doped NW at crossing angles of 50°, 70°, 70° and, 80°, respectively.

Optical characterization of the light coupling structures was carried out under optical microscopy using a micro-spectrophotometer. A 532 nm green laser was used to excite the light coupling structures. PL signals were collected by a 40× objective (numerical aperture = 0.6) and directed through a dichroic mirror and 532 nm notch filter. The filtered light was split by a beam splitter and directed to a spectrometer and charge-coupled device (Sony iCY-SHOT, DXC-S500) camera for spectrum and image measurement, respectively.

Fig. 3a shows the extinction spectrum of Ag NWs while Fig. 3b shows the absorption and emission spectrum of the QD-doped polymer NWs. As PVP is optically transparent in the visible spectral range, the absorption and emission are from the QDs. Actually, the specific modes in the QD-doped PVP NW without preferred polarization direction. This is because the polarizations of those specific modes are the statistical averaging of individual QDs fluorescence polarizations and the polarization of fluorescence from individual single QD is random. Moreover, the 532 nm excitation laser used to excite the QD-doped PVP NW is an unpolarized light. It can be seen from Fig. 3 that the 580 nm emission band of the QD-doped NWs overlaps the 590 nm plasmon resonance band of Ag NWs. This is the basis for discussions of plasmon-based propagation.

Fig. 3 (a) Extinction spectrum of the Ag NWs. (b) The absorption and emission spectra of QD-doped polymer NWs.

To realize optical interconnection and analyze the resulting waveguiding spectrum, two Ag NWs were interconnected with the same QD-doped polymer NW. When plasmonic and photonic NWs are close enough contact, the plasmonic near field of a plasmonic NW (Ag NW) and optical near field of a photonic NW (QD-doped polymer NW) are strong overlap, which results in highly efficient conversion of photon to plasmon in the cross-coupling area.³⁷ In the photon-plasmon conversion area, once the plasmons of Ag NWs were excited by PL from QD-doped NW, the longitudinal mode would be dominant and the resonance of the mode will have F–P resonance peaks.^18,38 Fig. 4a shows a bright-field optical microscope image of Ag NWs 1 and 2 with diameters of 300 nm interconnected with an 800 nm diameter QD-doped NW at crossing angles of 57° and 80°, respectively. The lengths of Ag NWs 1 and 2 are 5 and 4 μm, respectively, and the distance between them is 10 μm. The sample spot was moved to desired positions to collect PL signals, thus obtaining waveguiding spectra along the Ag NWs and QD-doped NW. Fig. 4b depicts a dark-field optical microscope image of the light coupling structure in Fig. 4a excited by a 532 nm laser with an optical power of 10 mW. The excitation spot was about 15 μm in diameter. The yellow arrow indicates the propagating direction of excited 580 nm PL guiding along the QD-doped NW. The inset image was obtained by changing the focus of the microscope objective to clearly demonstrate the propagation of plasmons in two Ag NWs and measure the spectra of scattering PL signals from positions J0, J1, S1, J2, and S2. The bright yellow scattering spot J0 is caused by local defects in the QD-doped NW. The larger bright scattering spots J1 and J2 are the scattering of coupling areas caused by the metal–dielectric junctions formed between the Ag and QD-doped NWs.³⁹ It should be noted that junction scattering is very important for the efficient conversion of photons to plasmons, because it compensates for the mismatch in momentum between photons and plasmons.⁴⁰ The smaller scattering spots S1 and S2 are the tips of the Ag NWs, where the propagated plasmons converted to photons and were irradiated into free space. The Ag NWs with diameters of 300 nm strongly confine the 580 nm light into subwavelength scale, resulting in their ultra-compact interconnection with the QD-doped NW. For simplicity, the PL spectra measured at positions J0, J1, S1, J2 and S2 are plotted in a single graphic in Fig. 4c. The center wavelengths of positions J0, J1, S1, J2 and S2 are 580, 585, 595, 590, and 605 nm, respectively. The inset of Fig. 4c illustrates plasmon propagation in the Ag NWs and photon propagation in the QD-doped polymer NW. Positions J1 and J2 are photon-to-plasmon coupling areas. The QD-doped NW generated different emission colors of yellow, yellow-orange, and orange at positions J0, J1, and J2, respectively. This optical characteristic is wavelength-converted waveguiding in the QD-doped polymer NW, which we also observed and explained in our previously reported dye-doped polymer nanofiber.⁴¹ The emission color changed from yellow-orange (J1) to red-orange (S1) in Ag NW 1, which is caused by the large metal loss leading to energy dissipation, so the center wavelength red shifts from 585 to 595 nm. The emission color changed from orange (J2) to red (S2) in Ag NW 2, and the center wavelength red shifts from 590 to 605 nm. Actually, this is a wavelength-dependent dispersion effect in these two Ag NWs. The large active interface formed between thick polymer and Ag NWs also achieves the light transfer because the thick polymer NW can contain more QDs than the thin one and QDs can act as individual nano-emitter, which enables the emitted light well localized and guided along polymer NW. To interconnect multiple Ag NWs with the same QD-doped polymer NW for distributing light to numerous sites simultaneously, we carefully increased the concentration of Ag NWs in the suspension deposited on the MgF₂ substrate coated with QD-doped NWs. As an example, Fig. 2d shows an SEM image of four Ag NWs interconnected with the same QD-doped NW.

Fig. 4 (a) Bright-field optical microscope image of Ag NWs 1 and 2 interconnected with a QD-doped NW at crossing angles of 57° and 80°, respectively. Note that the black box (size: 1 × 1 μm) near Ag NW 1 is the sample spot for spectrum measurement. (b) Dark-field optical microscope image of the light coupling structure in (a) excited by a 532 nm laser with an optical power of 10 mW. The diameter of the excited spot is 15 μm. The yellow arrow indicates the propagating direction of excited 580 nm PL. The inset image was obtained by changing the focus of the microscope objective to clearly demonstrate the propagation of plasmons in the two Ag NWs and measure the spectra of scattering PL signals from positions J0, J1, S1, J2, and S2. The scale bar in (a) is applicable to (b). (c) PL spectra measured at positions J0, J1, S1, J2, and S2. Inset shows a photograph of the propagation of plasmons and photons. Positions J1 and J2 are coupling areas for photon-to-plasmon conversion.

Conclusions

We demonstrated the coupling of light from a single 800 nm diameter QD-doped polymer NW into two 300 nm diameter Ag NWs with lengths of 5 and 4 μm, respectively. In the light coupling structure, the distance between two Ag NWs is 10 μm. The plasmons of two Ag NWs were simultaneously excited by PL of the QD-doped NW under 532 nm laser excitation. Wavelength-converted waveguiding and wavelength-dependent dispersion effect occurred in polymer and Ag NWs, respectively. Simultaneous distribution of light from a single QD-doped NW to multiple Ag NWs may allow realization of ultra-compact, high-density nanophotonic circuits. This feasible coupling strategy can be extended to couple light into other metal and semiconductor NWs consisting of material such as Au, Cu, Al, Si, SiO₂, ZnO, and GaN.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (11274395) and the Program for Changjiang Scholars and Innovative Research Team in University (IRT13042).

Notes and references

1. R. F. Oulton, V. J. Sorger, D. Genov, D. Pile and X. Zhang, Nat. Photonics, 2008, 2, 496.
2. V. J. Sorger, Z. Ye, R. F. Oulton, Y. Wang, G. Bartal, X. Yin and X. Zhang, Nat. Commun., 2011, 2, 331.
3. S. Lal, J. H. Hafner, N. J. Halas, S. Link and P. Nordlander, Acc. Chem. Res., 2012, 45, 1887.
4. W. L. Barnes, A. Dereux and T. W. Ebbesen, Nature, 2003, 424, 824.
5. E. Ozbay, Science, 2006, 311, 189.
6. S. Lal, S. Link and N. J. Halas, Nat. Photonics, 2007, 1, 641.
7. H. Duan, A. I. Fernández-Domínguez, M. Bosman, S. A. Maier and J. K. Yang, Nano Lett., 2012, 12, 1683.
8. A. Kriesch, S. P. Burgos, D. Ploss, H. Pfeifer, H. A. Atwater and U. Peschel, Nano Lett., 2013, 13, 4539.
9. B. Lamprecht, J. Krenn, G. Schider, H. Ditlbacher, M. Salerno, N. Felidj, A. Leitner, F. Aussenegg and J. Weeber, Appl. Phys. Lett., 2001, 79, 51.
10. S. I. Bozhevolnyi, V. S. Volkov, E. Devaux, J.-Y. Laluet and T. W. Ebbesen, Nature, 2006, 440, 508.
11. S. A. Maier, P. G. Kik, H. A. Atwater, S. Meltzer, E. Harel, B. E. Koel and A. A. Requicha, Nat. Mater., 2003, 2, 229.
12. N.-C. Panoiu and R. Osgood, Nano Lett., 2004, 4, 2427.
13. J. Dorfmuller, R. Vogelgesang, W. Khunsin, C. Rockstuhl, C. Etrich and K. Kern, Nano Lett., 2010, 10, 3596.
14. T. P. Sidiropoulos, R. Röder, S. Geburt, O. Hess, S. A. Maier, C. Ronning and R. F. Oulton, Nat. Phys., 2014, 10, 870.
15. A. L. Pyayt, B. Wiley, Y. Xia, A. Chen and L. Dalton, Nat. Nanotechnol., 2008, 3, 660.
16. Z. Li, F. Hao, Y. Huang, Y. Fang, P. Nordlander and H. Xu, Nano Lett., 2009, 9, 4383.
17. W. Wang, Q. Yang, F. Fan, H. Xu and Z. L. Wang, Nano Lett., 2011, 11, 1603.
18. H. Ditlbacher, A. Hohenau, D. Wagner, U. Kreibig, M. Rogers, F. Hofer, F. R. Aussenegg and J. R. Krenn, Phys. Rev. Lett., 2005, 95, 257403.
19. X. Guo, Y. Ma, Y. Wang and L. Tong, Laser Photonics Rev., 2013, 7, 855.
20. R. M. Dickson and L. A. Lyon, J. Phys. Chem. B, 2000, 104, 6095.
21. A. W. Sanders, D. A. Routenberg, B. J. Wiley, Y. Xia, E. R. Dufresne and M. A. Reed, Nano Lett., 2006, 6, 1822.
22. M. W. Knight, N. K. Grady, R. Bardhan, F. Hao, P. Nordlander and N. J. Halas, Nano Lett., 2007, 7, 2346.
23. A. Akimov, A. Mukherjee, C. Yu, D. Chang, A. Zibrov, P. Hemmer, H. Park and M. Lukin, Nature, 2007, 450, 402.
24. Y. Fedutik, V. Temnov, U. Woggon, E. Ustinovich and M. Artemyev, J. Am. Chem. Soc., 2007, 129, 14939.
25. R. A. Potyrailo, A. M. Leach and C. M. Surman, ACS Comb. Sci., 2012, 14, 170.
26. C. Schieber, A. Bestetti, J. P. Lim, A. D. Ryan, T. L. Nguyen, R. Eldridge, A. R. White, P. A. Gleeson, P. S. Donnelly and S. J. Williams, Angew. Chem., Int. Ed., 2012, 51, 10523.
27. C. Meng, Y. Xiao, P. Wang, L. Zhang, Y. Liu and L. Tong, Adv. Mater., 2011, 23, 3770.
28. R. Zhang, H. Yu and B. Li, Nanoscale, 2012, 4, 5856.
29. M. Sun, Z. Zhang, P. Wang, Q. Li, F. Ma and H. Xu, Light: Sci. Appl., 2013, 2, e112.
30. Y. Huang, Y. Fang, Z. Zhang, L. Zhu and M. Sun, Light: Sci. Appl., 2014, 3, e199.
31. Y. Sun and Y. Xia, Nature, 1991, 353, 737.
32. B. Wiley, Y. Sun and Y. Xia, Langmuir, 2005, 21, 8077.
33. Y. Huang, X. Duan, Q. Wei and C. M. Lieber, Science, 2001, 291, 630.
34. Y. Xia, P. Yang, Y. Sun, Y. Wu, B. Mayers, B. Gates, Y. Yin, F. Kim and H. Yan, Adv. Mater., 2003, 15, 353.
35. R. Yan, D. Gargas and P. Yang, Nat. Photonics, 2009, 3, 569.
36. S. Zhang and H. Xu, ACS Nano, 2012, 6, 8128.
37. X. Wu, Y. Xiao, C. Meng, X. Zhang, S. Yu, Y. Wang, C. Yang, X. Guo, C.-Z. Ning and L. Tong, Nano Lett., 2013, 13, 5654.
38. T. Shegai, V. D. Miljkovic, K. Bao, H. Xu, P. Nordlander, P. Johansson and M. Kall, Nano Lett., 2011, 11, 706.
39. S.-Y. Lee, J. Park, M. Kang and B. Lee, Opt. Express, 2011, 19, 9562.
40. B. Gjonaj, J. Aulbach, P. M. Johnson, A. P. Mosk, L. Kuipers and A. Lagendijk, Nat. Photonics, 2011, 5, 360.
41. H. Yu and B. Li, Sci. Rep., 2013, 3, 1674.

---