Open Access
Add to BibSonomyAbstract
We report a two-colored plasmonic antenna which can control the directivity of the excitation and emission light independently and simultaneously. By carefully tuning the phase difference of the constituting elements of the antenna, unidirectional fluorescence emission and laser light scattering can be obtained. In particular, the direction of the maximum emission and minimum scattering can be tailored in the same direction resulting improvement of signal to noise ratio in single molecule experiment. A two-dipole model is applied to describe the phenomena. The radiation and scattering pattern can be further tuned by varying the antenna structure.
© 2013 Optical Society of America
1. Introduction
The control over the emission rate of a single fluorescent molecule has been a field of extensive research. By modifying the peripheral photonic environment, the emission rate of the molecule can be greatly changed [1]. In particular, engineered localized surface plasmon resonances in metal nanoparticles provide large field enhancement at nanoscale, leading to efficient radiative enhancement of single molecule fluorescence [1–4] as well as ultrahigh sensitive single molecule sensing using surface enhanced Raman scattering (SERS) [5,6]. Unlike SERS where typically one plasmonic resonance is involved, the spectral separation of the excitation and emission wavelength in fluorescent molecules generally requires careful design of plasmonic resonances covering both the absorption and the emission spectrum to maximize the fluorescence signal. Recent works try to achieve this twofold enhancement by involving multiple [7–9] or broadband [10, 11] plasmonic resonances. However, because the excitation photons are also resonantly enhanced, scattering of the incident photons brings additional unwanted noises in the experiment [12].
In this context, a plasmonic antenna with the capability of directing the scattered incident photons and emitted fluorescence into different directions can be a proper solution. The plasmonic antenna, often consists of metal nanoparticles, has the ability to enhance localized electromagnetic field at nanoscale and more importantly, to control the light propagation directions. Recently, several schemes for controlling light directions using subwavelength antenna have been proposed, including unidirectional light emission from plasmonic Yagi-Uda antenna [13,14] or compact magnetic antenna [15], as well as the use of metallic [16–18], core-shell nanoparticle arrays [19] or even single dielectric nanoparticle [20–22] for unidirectional light scattering. All of the above schemes consider only one particle-field interaction process and they focus either on the directional control over the emission or the scattering of incident photons. Rarely did any work focus on the control of excitation and emission simultaneously which is important for fluorescence experiments.
The intention of this paper is to demonstrate the complete control over the directional excitation and emission of a single molecule simultaneously and independently. More specifically, we introduce a two-colored plasmonic antenna (TCPA) to mediate the emission of the dipolar emitter as well as the incident light scattering. By tuning the phase difference between different unit nanoparticles of the antenna, the excitation and emission direction of a single photon source can be spatially separated and manipulated independently. The working principle can be well described with two-dipole theory developed in this paper, which uses two dipoles to model the resonances of the antenna respectively. In view of implementation into single molecule fluorescent experiment, we compare the peak signal to noise ratio (PSNR) of single molecule fluorescence obtained using TCPA with that obtained using single nanorod antenna (SNA). An enhancement of 35dB is achieved. At last, schemes to improve the directivity as well as the tunalibility of TCPA are discussed.
2. Excitation and emission properties of double resonant antenna
The TCPA is designed to contain two resonances corresponding respectively to the excitation and emission wavelength of the fluorescent molecule, so that both the pumping field and the emission of the single molecule can be enhanced. The molecule is placed at the spatial overlapping area of the two resonant modes to maximize the antenna effect. To meet the above requirements, the TCPA system considered here is shown in Fig. 1(a). It consists of two silver nanodimers sharing one same element in the middle. The two distinct resonances are chosen at the wavelength around 650 nm and 800 nm respectively (Fig. 1(b)) to meet the typical excitation and emission wavelength of a single photon source [13]. The background is taken to be air. However, if the TCPA is placed above a glass substrate, the radiation pattern does not change much. Because the resonance strongly depends on the distance between metallic nanodimers, the excitation and emission wavelength presented here are not specific and can be applied to other single photon sources as well. By increasing the gap between nanoparticles, the interaction between them weakens which results in blue-shifted resonance wavelength. If the aspect ratio of the nanorod increases (with increased L/a), its resonance shifts to longer wavelength. The fluorescent molecule is simulated by a z direction short current source placed 10nm above the shared element of the two dimers. The antenna can be fabricated through standard lithography process. The placement of single molecule can be achieved using chemical functionalization method [13]. The spatial overlap of the two resonances at the dipole source permits a twofold enhancement of fluorescent signal in both excitation and emission.
Fig. 1 (a) The geometry of the proposed antenna structure with g1 = 62nm g2 = 46nm a = 40nm L = 100nm d = 10nm. (b) Resonance of the antenna obtained by measuring the magnetic field Hy at point p1 and p2 in the middle of the dimer shown in Fig. 1(a). Insets show the magnetic field distribution at resonance.
The excitation and emission of the antenna are modeled with finite difference time domain (FDTD) simulation, and the permittivity of silver [23] is modeled using Drude formula with ε∞ = 4.08598, ωp = 1.3316 × 1016rad/s, γ = 1.1308 × 1014rad/s. The simulation surroundings are enclosed with perfect matched layers. A non-uniform mesh of the minimum mesh step of 1nm is used. We choose silver nanorod with square cross section as the building block of TCPA because silver nanorod exhibits higher fluorescent enhancement than gold. Besides, although nanorod with sharp top tip provides even higher charge density, thus higher field enhancement, the exact description of the structures curvature using finite mesh grid in FDTD simulation is computational excessive. As a comparison, the excitation and emission process of a SNA are also studied. The angular distribution pattern at excitation wavelength is simulated by radiating the antenna with plane wave then detecting the scattered power distribution in the far field. The emission pattern is obtained in a similar way by exciting the antenna with a dipole source near the antenna. Combining both the excitation and emission enhancement, the total enhancement factor for the fluorescent molecule is defined as [15]:
where P(θ, ϕ, λem) is the angular distribution of the emitted power with plasmonic antenna and P0(θ, ϕ, λem) is the angular distribution of the emitted power by an isolated dipole source in free space. This ratio represents the angular radiative emission enhancement of molecule in presence of the plasmonic antenna. The multiplier term |Ez,λex|2/|E0,λex|2 incorporates the enhancement at the excitation wavelength. This definition is valid as long as the excitation power does not achieve the saturation threshold of the molecule.Figure 2 presents the far-field radiation and excitation pattern for both TCPA and SNA. For both antennas, the emitted far-field power is greatly enhanced compared with the isolated dipole. The maximum enhancement factor S is about 170 for SNA (Figs. 2(g) and 2(h)) and 680 for TCPA (Figs. 2(c) and 2(d)). The superiority in maximum enhancement factor S using TCPA not only results from the excitation enhancement because of double resonance, but can also be attributed to the unidirectional emission of the TCPA. For the excitation and emission of the SNA, the far-field pattern resembles the classical doughnut pattern with evenly distributed power in the XY plane and 8 shape pattern in the XZ plane (Figs. 2(g)–2(h)). This is easy to understand since the dominant resonant mode used in this antenna is the dipole resonance. In general, both the emission and the scattered excitation power are highly symmetric, which is not good for detections since half the radiated power is lost in the opposite direction. The situation is strikingly different for TCPA. Because the dimer undergoes large phase variation due to plasmonic hybridization [15, 22, 24], the strong near field interaction of the anti-phase magnetic resonant mode provide different far-field pattern. The emitted power is reflected from +X direction resulting unidirectional emission in −X direction (Figs. 2(c) and 2(d)) whereas the direction of the scattered power mainly resides in the +X direction (Figs. 2(a) and 2(b)) resulting in a highly asymmetric pattern.
Fig. 2 Far-field pattern plot at excitation and emission wavelength of (a–d) TCPA and (e–h) a SNA with cross section axa = 40nmx40nm and length L1 = 170nm. For the excitation, the normalized electric far field pattern is plotted. For the emission, the emission enhancement factor S is plotted.
3. Two-dipole model
The observed directivity in TCPA arises from interferences between nanorods consisting TCPA. When the molecule is effectively coupled to the plasmonic antenna, its far field pattern is dominated by the far field properties of the antenna [25]. Without loss of generality, we take the TCPA system as two oscillating dipoles with phase differences. It is convenient to neglect the contribution of the third nanorod because only two of the nanorods are at resonance each time. For the excitation process, we discuss the case nanorods 1 and 2 are at resonance while for emission, nanorods 1 and 3 are at resonance as shown in Fig. 1(a). The total far field electric field is the vectorial summation of the individual nanorod with dipole moment p⃗d = |pd|eiϕd
with i = 1, j = 2 for the emission process and i = 1, j = 3 for the excitation process. Since we are only interested in the symmetry of the power pattern, we compute the sum of the Poynting vectors in +x and −x direction which yields to [16] with ϕij being the phase difference between the two nanorods. For small distance d between nanorods, the change of sin(ϕij) will induce change in the radiation symmetry. Clearly, directional scattering of the incident light occurs in a situation that the phase difference ϕ12 between the nanorods 1 and 2 is the odd multiples of π/2. Directional radiation pattern can be obtained when phase difference ϕ13 between dipoles 1 and 3 is also the odd multiples of π/2. For this reason, we plot the phase difference between dipoles extracted from FDTD simulations in Fig. 3. At the excitation wavelength of 652nm shown in Fig. 3(a), the phase difference ϕ12 = π/2 and the incident light preferentially scatters into +x direction according to Eq. (3). At the emission wavelength of 796nm shown in Fig. 3(b), the phase difference ϕ13 = −π/2 and the radiation mainly lies in −x direction. It is interesting to note at the wavelength around 620 nm, the phase difference between dipoles 1 and 2 is ϕ12 = −π/2, therefore the scattered incident light at this wavelength lies mainly in −x direction. Such a dramatic change in the scattering direction within small wavelength range is very similar to the enhanced forward or backward scattering phenomenon observed in the scattering of a single nanosphere [21] or nanopillar [20], which according to Mie theory, is due to interference between the magnetic and electric resonances [26]. Since the silver dimer presented here also possesses electric and magnetic resonances, it is expected the directional scattering has the same physical origin. A more detailed study will be carried out further to prove this point.Fig. 3 (a)Phase difference of nanorods 1 and 2 under plane wave excitation and (b) phase difference of nanorods 1 and 3 under dipole excitation.
4. Implementation in single molecule spectroscopy
To be specific, we implement the TCPA into single molecule spectroscopy (SMS) [27, 28] and evaluate it using peak signal to noise ratio (PSNR), defined as the maximum fluorescence intensity of a molecule over the intensity of scattered incident power in the same direction:
where PSMAX is the maximum power emitted by the molecule source, PN0 is the scattered incident power in this direction. For SNA, because both the excitation and emission pattern are highly symmetric, the signal and noise intensity reaches maximum in the same direction, which results in a low PSNR. But for TCPA system proposed above, the direction of maximum signal is the same as the direction of minimum noise. As a result, an improvement of 35 dB in PSNR compared with SNA is achieved. We note this increase in PSNR is different from using optical filters generally in SMS experiment. The use of optical filters weakens the fluorescent signals at the same time. As Moerner pointed out [12], the use of multiple filters in SMS loses nearly half of the fluorescent intensity. But the mechanism used in TCPA design further enhances the fluorescent signal intensity.5. Directivity enhancement using antenna array
It can be seen although the proposed TCPA has a undirectional scattering as well as a unidirectional emission property, the directivity is less than traditional Yagi-Uda antenna [13, 14]. However, the reduced size of TCPA as compared with tradiational antennas makes it promising in high integration applications. To improve the directivity of TCPA, an antenna array is used. Here, we make a 2 by 2 array shown in Fig. 4. The emission pattern of the antenna array can be simply calculated using pattern multiplication technique ignoring the mutual coupling effect. The full wave rigorous FDTD method used in the calculation has taken the coupling effect into account and the final far-field pattern is slightly different from what may be expected from pattern multiplication, but the general characteristics of narrowing down the emission pattern is clear. It is interesting to note besides the emission pattern, the scattered far-field excitation power pattern also narrows. It is because that the array pattern we chose is a half wavelength array. This array factor enhances far-field power both in the +x and −x direction. More complex array patterns can be used to further suppress the scattered far-field power.
Fig. 4 Excitation and emission of the antenna array Px = Py = 398nm. (a), (b) show the normalized farfield power pattern. (c), (d) show the field enhancement factor S.
6. Tunability of directivity
Finally, we would like to bring the attention to the tunability of the proposed plasmonic antenna. Instead of aligning the two silver dimers in a line and using backward scattering configuration, we arrange them in a perpendicular way shown in Fig. 5. The scattered far-field excitation power is directed in the +y direction while the emitted power still lies in the −x direction. By slightly tune the angle between the two metallic dimer, the far-field pattern can be tuned. This offers an extra control over the excitation and emission of the single photon source.
Fig. 5 Tunability of the excitation and emission pattern when two metallic dimers are aligned perpendicular to each other. (a), (b) are the normalized scattered far field power pattern when the plane wave is propagating in +y direction. (c), (d) are the field enhancement factor S which lies mainly in −x direction. g1 = 59nm g2 = 46nm.
7. Conclusion
In conclusion, we propose a novel plasmonic antenna structure, TCPA, which can spatially separate the excitation and emission pattern utilizing phase difference between each constituting element. The spatial isolation of the excitation and emission gives about 35dB increase in PSNR in single molecule fluorescent experiment. The directivity of TCPA can be further enhanced by antenna arrays. By slightly tune the antenna structure, the excitation and emission direction can be actively controlled, which offers good flexibility. This finding can also have potential applications in on chip photonic devices or even nonlinear process like second harmonic generation [29] where active control of antennas excitation and emission pattern is required.
Acknowledgment
This work is supported by National Key Basic Research Program of China ( 2012CB921900 and 2012CB922003), Key Program of National Natural Science Foundation of China ( 61036005) and National Natural Science Foundation of China ( 11274293, 11074240, 61177053, 61377053)
References and links
1. A. Kinkhabwala, Z. Yu, S. Fan, Y. Avlasevich, K. Müllen, and W. Moerner, “Large single-molecule fluorescence enhancements produced by a bowtie nanoantenna,” Nat. Photonics 3, 654–657 (2009). [CrossRef]
2. J. N. Farahani, D. W. Pohl, H.-J. Eisler, and B. Hecht, “Single quantum dot coupled to a scanning optical antenna: a tunable superemitter,” Phys. Rev. Lett. 95, 017402 (2005). [CrossRef] [PubMed]
3. S. Kühn, U. Håkanson, L. Rogobete, and V. Sandoghdar, “Enhancement of single-molecule fluorescence using a gold nanoparticle as an optical nanoantenna,” Phys. Rev. Lett. 97, 017402 (2006). [CrossRef] [PubMed]
4. H. Rigneault, J. Capoulade, J. Dintinger, J. Wenger, N. Bonod, E. Popov, T. W. Ebbesen, and P.-F. m. c. Lenne, “Enhancement of single-molecule fluorescence detection in subwavelength apertures,” Phys. Rev. Lett. 95, 117401 (2005). [CrossRef] [PubMed]
5. S. Nie and S. R. Emory, “Probing single molecules and single nanoparticles by surface-enhanced raman scattering,” Science 275, 1102–1106 (1997). [CrossRef] [PubMed]
6. D. Wang, W. Zhu, M. D. Best, J. P. Camden, and K. B. Crozier, “Directional raman scattering from single molecules in the feed gaps of optical antennas,” Nano Lett. 13, 2194–2198 (2013). [CrossRef] [PubMed]
7. V. Giannini and J. A. Sánchez-Gil, “Excitation and emission enhancement of single molecule fluorescence through multiple surface-plasmon resonances on metal trimer nanoantennas,” Opt. Lett. 33, 899–901 (2008). [CrossRef] [PubMed]
8. K. Thyagarajan, S. Rivier, A. Lovera, and O. J. Martin, “Enhanced second-harmonic generation from double resonant plasmonic antennae,” Opt. Express 20, 12860–12865 (2012). [CrossRef] [PubMed]
9. H. Harutyunyan, G. Volpe, R. Quidant, and L. Novotny, “Enhancing the nonlinear optical response using multi-frequency gold-nanowire antennas,” Phys. Rev. Lett. 108, 217403 (2012). [CrossRef]
10. H. Aouani, M. Navarro-Cia, M. Rahmani, T. P. Sidiropoulos, M. Hong, R. F. Oulton, and S. A. Maier, “Mul-tiresonant broadband optical antennas as efficient tunable nanosources of second harmonic light,” Nano Lett. 12, 4997–5002 (2012). [CrossRef] [PubMed]
11. H. Aouani, H. pov, M. Rahmani, M. Navarro-Cia, K. Hegnerov, J. Homola, M. Hong, and S. A. Maier, “Ultra-sensitive broadband probing of molecular vibrational modes with multifrequency optical antennas,” ACS Nano 7, 669–675 (2013). [CrossRef]
12. W. Moerner and D. P. Fromm, “Methods of single-molecule fluorescence spectroscopy and microscopy,” Rev. Sci. Instrum. 74, 3597–3619 (2003). [CrossRef]
13. A. G. Curto, G. Volpe, T. H. Taminiau, M. P. Kreuzer, R. Quidant, and N. F. van Hulst, “Unidirectional emission of a quantum dot coupled to a nanoantenna,” Science 329, 930–933 (2010). [CrossRef] [PubMed]
14. T. Kosako, Y. Kadoya, and H. F. Hofmann, “Directional control of light by a nano-optical Yagi–Uda antenna,” Nat. Photonics 4, 312–315 (2010). [CrossRef]
15. T. Pakizeh and M. Kall, “Unidirectional ultracompact optical nanoantennas,” Nano Lett. 9, 2343–2349 (2009). [CrossRef] [PubMed]
16. B. Rolly, B. Stout, S. Bidault, and N. Bonod, “Crucial role of the emitter–particle distance on the directivity of optical antennas,” Opt. Lett. 36, 3368–3370 (2011). [CrossRef] [PubMed]
17. N. Bonod, A. Devilez, B. Rolly, S. Bidault, and B. Stout, “Ultracompact and unidirectional metallic antennas,” Phys. Rev. B 82, 115429 (2010). [CrossRef]
18. D. Vercruysse, Y. Sonnefraud, N. Verellen, F. B. Fuchs, G. Di Martino, L. Lagae, V. V. Moshchalkov, S. A. Maier, and P. Van Dorpe, “Unidirectional side scattering of light by a single-element nanoantenna,” Nano Lett. 13, 3843–3849 (2013). [CrossRef] [PubMed]
19. W. Liu, A. E. Miroshnichenko, D. N. Neshev, and Y. S. Kivshar, “Broadband unidirectional scattering by magneto-electric core–shell nanoparticles,” ACS Nano 6, 5489–5497 (2012). [CrossRef] [PubMed]
20. S. Person, M. Jain, Z. Lapin, J. J. Senz, G. Wicks, and L. Novotny, “Demonstration of zero optical backscattering from single nanoparticles,” Nano Lett. 13, 1806–1809 (2013). [PubMed]
21. Y. H. Fu, A. I. Kuznetsov, A. E. Miroshnichenko, Y. F. Yu, and B. Lukyanchuk, “Directional visible light scattering by silicon nanoparticles,” Nat. Commun. 4, 1527 (2013). [CrossRef] [PubMed]
22. J. Geffrin, B. García-Camara, R. Gómez-Medina, P. Albella, L. Froufe-Pérez, C. Eyraud, A. Litman, R. Vaillon, F. González, J. S. Nieto-Vesperinas, Saenz, and F. Moreno, “Magnetic and electric coherence in forward-and back-scattered electromagnetic waves by a single dielectric subwavelength sphere,” Nat. Commun. 3, 1171 (2012). [CrossRef]
23. E. Palik, Handbook of Optical Constants of Solids (Academic press, 1998).
24. T. Shegai, S. Chen, V. D. Miljković, G. Zengin, P. Johansson, and M. Käll, “A bimetallic nanoantenna for directional colour routing,” Nat. Commun. 2, 481 (2011). [CrossRef] [PubMed]
25. T. Taminiau, F. Stefani, F. Segerink, and N. Van Hulst, “Optical antennas direct single-molecule emission,” Nat. Photonics 2, 234–237 (2008). [CrossRef]
26. M. Kerker, D.-S. Wang, and C. Giles, “Electromagnetic scattering by magnetic spheres,” J. Opt. Soc. Am. 73, 765–767 (1983). [CrossRef]
27. C. Joo, H. Balci, Y. Ishitsuka, C. Buranachai, and T. Ha, “Advances in single-molecule fluorescence methods for molecular biology,” Annu. Rev. Biochem. 77, 51–76 (2008). [CrossRef] [PubMed]
28. S. Weiss, “Fluorescence spectroscopy of single biomolecules,” Science 283, 1676–1683 (1999). [CrossRef] [PubMed]
29. S. G. Rodrigo, H. Harutyunyan, and L. Novotny, “Coherent control of light scattering from nanostructured materials by second-harmonic generation,” Phys. Rev. Lett. 110, 177405 (2013). [CrossRef] [PubMed]
