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Optics Express

Optics Express

  • Editor: C. Martijn de Sterke
  • Vol. 20, Iss. 16 — Jul. 30, 2012
  • pp: 18085–18090
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Saturated excitation of fluorescence to quantify excitation enhancement in aperture antennas

Heykel Aouani, Richard Hostein, Oussama Mahboub, Eloïse Devaux, Hervé Rigneault, Thomas W. Ebbesen, and Jérôme Wenger  »View Author Affiliations


Optics Express, Vol. 20, Issue 16, pp. 18085-18090 (2012)
http://dx.doi.org/10.1364/OE.20.018085


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Abstract

Fluorescence spectroscopy is widely used to probe the electromagnetic intensity amplification on optical antennas, yet measuring the excitation intensity amplification is a challenge, as the detected fluorescence signal is an intricate combination of excitation and emission. Here, we describe a novel approach to quantify the electromagnetic amplification in aperture antennas by taking advantage of the intrinsic non linear properties of the fluorescence process. Experimental measurements of the fundamental f and second harmonic 2f amplitudes of the fluorescence signal upon excitation modulation are used to quantify the electromagnetic intensity amplification with plasmonic aperture antennas.

© 2012 OSA

1. Introduction

Optical antennas are efficient devices to generate strong electromagnetic fields and control the light emission in nanoscale volumes, with major applications in molecular sensing, light-emitting devices, and photovoltaics [1

1. L. Novotny and N. van Hulst, “Antennas for light,” Nat. Photonics 5, 83–90 (2011). [CrossRef]

]. One of the most important features of an optical antenna relates to its amplification of the local excitation intensity [2

2. J. A. Schuller, E. S. Barnard, W. S. Cai, Y. C. Jun, J. S. White, and M. L. Brongersma, “Plasmonics for extreme light concentration and manipulation,” Nature Mater. 9, 193–204 (2010). [CrossRef]

]. Enhancement factors are generally estimated from fluorescence spectroscopy [3

3. A. Kinkhabwala, Z. F. Yu, S. H. Fan, Y. Avlasevich, K. Mullen, and W. E. Moerner, “Large single-molecule fluorescence enhancements produced by a bowtie nanoantenna,” Nat. Photonics 3, 654–657 (2009). [CrossRef]

], Raman scattering [4

4. H. Ko, S. Singamaneni, and V. V. Tsukruk, “Nanostructured surfaces and assemblies as SERS media,” Small 4, 1576–1599 (2008). [CrossRef] [PubMed]

], or nonlinear photoluminescence measurements [5

5. P. Mühlschlegel, H. J. Eisler, O. J. F. Martin, B. Hecht, and D. W. Pohl, “Resonant optical antennas,” Science 308, 1607–1609 (2005). [CrossRef] [PubMed]

]. However, all these techniques quantify the overall response of the coupled emitter-antenna system, merging excitation and emission processes into a single output. For the given example of fluorescence spectroscopy, the signal enhancement near the antenna is a complex combination of modifications on excitation intensity, fluorescence quantum yield, and collection efficiency [3

3. A. Kinkhabwala, Z. F. Yu, S. H. Fan, Y. Avlasevich, K. Mullen, and W. E. Moerner, “Large single-molecule fluorescence enhancements produced by a bowtie nanoantenna,” Nat. Photonics 3, 654–657 (2009). [CrossRef]

, 6

6. Y. Fu and J. R. Lakowicz, “Modification of single molecule fluorescence near metallic nanostructures,” Laser Photon. Rev. 3, 221–232 (2009). [CrossRef]

]. In some configurations, the quenching phenomenon may prevent detecting enhanced fluorescence although the excitation intensity is locally enhanced by the antenna.

Here, we present a novel approach to quantify the electromagnetic amplification on aperture antennas by taking advantage of the intrinsic non linear property of the fluorescence process. Our approach relies on the fact that the fluorescence signal is a nonlinear function of the excitation intensity, while it linearly depends on the number of emitters, their quantum yield, and the microscope collection efficiency. This technique builds on a method that we developed recently to quantify separately the local excitation intensity from the number of emitters [7

7. R. Hostein, H. Aouani, H. Rigneault, and J. Wenger, “Deciphering fluorescence signals by quantifying separately the excitation intensity from the number of emitters,” Opt. Lett. 36, 3317–3319 (2011). [CrossRef] [PubMed]

], which also relates to saturated excitation fluorescence microscopy (SAX) [8

8. K. Fujita, M. Kobayashi, S. Kawano, M. Yamanaka, and S. Kawata, “High-resolution confocal microscopy by saturated excitation of fluorescence,” Phys. Rev. Lett. 99, 228105 (2007). [CrossRef]

,9

9. M. Yamanaka, S. Kawano, K. Fujita, N. I. Smith, and S. Kawata, “Beyond the diffraction-limit biological imaging by saturated excitation microscopy,” J. Biomed. Opt. 13, 050507 (2008). [CrossRef] [PubMed]

]. For the first time, we apply this extended method to quantify the excitation intensity enhancement on plasmonic aperture antennas. The use of incoherent detection via fluorescent molecules as a probe also improves the detection accuracy by eliminating the effects from local refractive index change.

The technique can be summarized as follows: to probe the nonlinear fluorescence response induced by laser excitation, we modulate the excitation intensity at a given fundamental frequency f = 5 kHz, and demodulate the fluorescence signal at the first and second harmonic frequencies f and 2f (Fig. 1(a)). The main source for the second harmonic generation of fluorescence upon excitation modulation relates to the intrinsic non-linear dependence of the fluorescence signal on the excitation intensity, which is a natural consequence of the fluorescence saturation and finite lifetime [7

7. R. Hostein, H. Aouani, H. Rigneault, and J. Wenger, “Deciphering fluorescence signals by quantifying separately the excitation intensity from the number of emitters,” Opt. Lett. 36, 3317–3319 (2011). [CrossRef] [PubMed]

9

9. M. Yamanaka, S. Kawano, K. Fujita, N. I. Smith, and S. Kawata, “Beyond the diffraction-limit biological imaging by saturated excitation microscopy,” J. Biomed. Opt. 13, 050507 (2008). [CrossRef] [PubMed]

]. As we will demonstrate below, the ratio ρ = F2f/Ff between the fluorescence amplitudes Ff, F2f at the fundamental and second harmonic frequencies forms a metric only sensitive to the local excitation intensity. This ratio is used as a direct probe of the electromagnetic intensity enhancement brought by single and corrugated metallic nanoapertures (Fig. 1(b)). Such aperture antennas have been the topic of intensive studies for the last decade [10

10. H. J. Lezec, A. Degiron, E. Devaux, R. A. Linke, L. Martin-Moreno, F. J. Garcia-Vidal, and T. W. Ebbesen, “Beaming light from a subwavelength aperture,” Science 297, 820–822 (2002). [CrossRef] [PubMed]

, 11

11. F. J. Garcia-Vidal, L. Martin-Moreno, T. W. Ebbesen, and L. Kuipers, “Light passing through subwavelength apertures,” Rev. Mod. Phys. 82, 729–787 (2010). [CrossRef]

], and several studies have been performed to enhance and control the fluorescence emission of emitters located inside the central aperture [12

12. H. Aouani, O. Mahboub, N. Bonod, E. Devaux, E. Popov, H. Rigneault, T. W. Ebbesen, and J. Wenger, “Bright unidirectional fluorescence emission of molecules in a nanoaperture with plasmonic corrugations,” Nano Lett. 11, 637–644 (2011). [CrossRef] [PubMed]

14

14. H. Aouani, O. Mahboub, E. Devaux, H. Rigneault, T. W. Ebbesen, and J. Wenger, “Large molecular fluorescence enhancement by a nanoaperture with plasmonic corrugations,” Opt. Express 19, 13056–13062 (2011). [CrossRef] [PubMed]

]. An exhaustive exploration of the design parameters defining the corrugated aperture is now at hand for the optical transmission process [15

15. O. Mahboub, S. Carretero Palacios, C. Genet, F. J. Garcia-Vidal, S. G. Rodrigo, L. Martin-Moreno, and T. W. Ebbesen, “Optimization of bulls eye structures for transmission enhancement,” Opt. Express 18, 11292–11299 (2010). [CrossRef] [PubMed]

]. This makes aperture antennas an ideal platform to validate the saturated excitation fluorescence spectroscopy technique.

Fig. 1 (a) Principle of saturated excitation of fluorescence: upon excitation intensity modulation at a frequency f, the deformed fluorescence signal contains the fundamental frequency f and also harmonics at higher frequencies (2f, 3f ...) which are recorded by lock-in detection. (b) Scanning electron microscope images of single aperture and corrugated aperture with five grooves of 440 nm period.

2. Characterization procedure by saturated excitation of fluorescence

To detect the nonlinear components in the fluorescence emission signal of molecules, we consider an incident laser excitation temporally modulated at the fundamental frequency f with a modulation amplitude α. Under these conditions, the temporal dependence of the excitation intensity Ie(t) is given by:
Ie(t)=I¯e(1+αcos(2πft))
(1)
where Īe is the average intensity of the modulated signal Ie(t). Under steady-state conditions, the fluorescence intensity emitted by a population of N fluorophores can be expressed as [7

7. R. Hostein, H. Aouani, H. Rigneault, and J. Wenger, “Deciphering fluorescence signals by quantifying separately the excitation intensity from the number of emitters,” Opt. Lett. 36, 3317–3319 (2011). [CrossRef] [PubMed]

]:
F=NκϕσIe1+Ie/Isat=NκϕσI¯e(1+αcos(2πft))1+(I¯e/Isat)(1+αcos(2πft))
(2)
where κ denotes the collection efficiency of the experimental setup, ϕ the quantum yield of the emitter, σ the excitation cross section, and Isat the saturation intensity. The quantum yield and saturation intensity can be expressed as function of the emitter’s radiative and non-radiative rates kr and knr as ϕ = kr/(kr + knr) and Isat = (kr + knr)/σ. The modulated fluorescence intensity F(t) is therefore an even f-periodic function of time, which can be decomposed into a Fourier series:
F(t)=F0+Ffcos(2πft)+F2fcos(4πft)+F3fcos(6πft)+
(3)
Here Fif denotes the Fourier coefficient of the i-th harmonic, which can be computed as:
Fif=Nκkr2i1αi(I¯e/Isat)i(1+I¯e/Isat)i+1
(4)
Hence,
Ff=NκkrαI¯e/Isat(1+I¯e/Isat)2andF2f=Nκkr2α2(I¯e/Isat)2(1+I¯e/Isat)3
(5)

Equation (5) demonstrate that at low excitation intensities, the fundamental frequency amplitude Ff grows linearly with Īe, while the second harmonic F2f grows quadratically. Moreover, every harmonic amplitude depends linearly on the number of fluorescent emitters N and the emission rate κkr. Hence it is possible to separate the contributions of excitation and emission from the fluorescence signal. To quantify the local excitation intensity independently on the emission properties, we introduce the ratio ρ of the first and second harmonic amplitudes:
ρ=F2fFf=α2I¯eI¯e+Isat
(6)
As expected, ρ neither depends on the number of fluorescent emitter N nor on the emission properties κkr. In the excitation regime below fluorescence saturation (ĪeIsat), Eq. (6) reduces to:
ρ=α2I¯eIsat
(7)

To quantify the excitation intensity enhancement ηexc = Īe–ant/Īe–sol brought by an optical antenna, we compare the ratio ρ found in the vicinity of an antenna to a free solution case in the regime below fluorescence saturation:
ηρ=ρantρsol=I¯eantI¯esolIsatsolIsatant=ηexcηIsat
(8)
Measuring the ratios ηρ and ηIsat from Ff and F2f finally quantifies the excitation enhancement ηexc = ηρηIsat. This can be done without any knowledge of the number of emitters involved in the experiments, and without any complex fluorescence temporal dynamics analysis as used in fluorescence correlation spectroscopy and time-correlated single photon counting [12

12. H. Aouani, O. Mahboub, N. Bonod, E. Devaux, E. Popov, H. Rigneault, T. W. Ebbesen, and J. Wenger, “Bright unidirectional fluorescence emission of molecules in a nanoaperture with plasmonic corrugations,” Nano Lett. 11, 637–644 (2011). [CrossRef] [PubMed]

, 14

14. H. Aouani, O. Mahboub, E. Devaux, H. Rigneault, T. W. Ebbesen, and J. Wenger, “Large molecular fluorescence enhancement by a nanoaperture with plasmonic corrugations,” Opt. Express 19, 13056–13062 (2011). [CrossRef] [PubMed]

].

Concerning the emission properties, the ratio Ff2/F2f reduces to 2Nκkr in the excitation regime below saturation (ĪeIsat). Therefore, this ratio quantifies the product of the number of emitters times their radiative rate and collection efficiency. This is a limit of the method, which does not provide direct access to the antenna’s influence on the emission properties κkr independently on the number of emitters. To solve this issue, some assumptions have to be made to set the quantity N. For such task, fluorescence correlation spectroscopy (FCS) [12

12. H. Aouani, O. Mahboub, N. Bonod, E. Devaux, E. Popov, H. Rigneault, T. W. Ebbesen, and J. Wenger, “Bright unidirectional fluorescence emission of molecules in a nanoaperture with plasmonic corrugations,” Nano Lett. 11, 637–644 (2011). [CrossRef] [PubMed]

] appears a more relevant method, as it characterizes directly the number of emitters. Hereafter, we rather focus on the main interest of the modulation technique, which is quantify the excitation intensity enhancement brought by the antenna.

3. Results and discussion

To validate the method we investigate single apertures milled in 200 nm thick gold film with 135 nm diameter. Some apertures are decorated with a set of five shallow concentric grooves of 65 nm depth and 440 nm period, as in reference [12

12. H. Aouani, O. Mahboub, N. Bonod, E. Devaux, E. Popov, H. Rigneault, T. W. Ebbesen, and J. Wenger, “Bright unidirectional fluorescence emission of molecules in a nanoaperture with plasmonic corrugations,” Nano Lett. 11, 637–644 (2011). [CrossRef] [PubMed]

]. The experimental setup is based on a confocal microscope with a NA= 1.2 water-immersion objective. Temporal modulation of the incident excitation laser beam (CW HeNe linearly polarized operating at 633 nm) is performed with an acousto-optic modulator (AA Optoelectronic, MT200-A0.2-VIS) driven at f = 5 kHz. Alexa Fluor 647 molecules (A647, Invitrogen, Carlsbad CA, absorption / emission peaks respectively at 650 and 672 nm) diluted in a standard water-based phosphate buffered saline (PBS) solution are used as fluorescent probes. Fluorescence emission is detected within the 670±20 nm spectral range by a photomultiplier tube (Hamamatsu, H10721-20), and the fundamental and second harmonic amplitudes are extracted through a lock-in amplifier (Signal Recovery, 7280-DSP).

First, we calibrate the experiment for the open solution case (no aperture). Figure 2(a) displays the evolution of the fundamental Ff and the second harmonic F2f fluorescence amplitudes while increasing the average excitation power. As expected from Eq. (5), Ff and F2f grow respectively linearly and quadratically with average excitation power in the low excitation regime. The good agreement between our model based on fluorescence saturation and the experimental data confirms fluorescence saturation is the main source for the second harmonic of fluorescence (Fig. 2). The evolution of Ff and F2f versus the excitation power are respectively presented in Fig. 2(b) and 2(c) for the single and corrugated apertures. In both cases, the second harmonic amplitude F2f can be detected at lower excitation powers as compared to the open solution, which is a direct consequence of the excitation enhancement inside the apertures.

Fig. 2 Demodulated fluorescence amplitudes at f = 5 kHz and 2 f = 10 kHz versus average excitation power for (a) the open solution, (b) a single 135 nm aperture and (c) a single aperture with a set of five circular corrugations. Lines are numerical fits based on Eq. (5).

To quantify the excitation intensity enhancement, we compute the ratio ρ = F2f/Ff for each sample. The results versus the excitation power are presented on Fig. 3(a) along with numerical interpolation according to Eq. (6). The slopes near origin for the three cases of reference solution, single aperture and corrugated aperture amount to 0.022, 0.080, and 0.146 μW−1 respectively, while the saturation intensities equal 215, 228 and 252 μW. These figures are used to compute the ratios ηρ = ρant/ρsol and ηIsat = Isat−ant/Isat−sol, and finally to quantify the excitation intensity enhancement ηexc = ηρ ηIsat. Results for the excitation intensity enhancement are presented in Fig. 3(b) and compared to the values probed with fluorescence correlation spectroscopy found previously in reference [12

12. H. Aouani, O. Mahboub, N. Bonod, E. Devaux, E. Popov, H. Rigneault, T. W. Ebbesen, and J. Wenger, “Bright unidirectional fluorescence emission of molecules in a nanoaperture with plasmonic corrugations,” Nano Lett. 11, 637–644 (2011). [CrossRef] [PubMed]

]. Satisfactory correspondence is shown between the different methods, well within the error margins which appear of similar magnitude for both saturated excitation fluorescence and correlation spectroscopy.

Fig. 3 (a) Fluorescence amplitudes ratio ρ = F2f/Ff for the different samples. (b) Excitation intensity enhancement deduced from the data set in (a) (filled bars), and compared to the results using fluorescence correlation spectroscopy (FCS, hatched bars) [12].

4. Conclusion

Acknowledgments

The research leading to these results has received funding from the European Commissions Seventh Framework Programme (FP7-ICT-2011-7) under grant agreements 288263 NanoVista and 227577 Plasmonics. We also acknowledge funding by C’Nano PACA and the Provence-Alpes-Côte d’Azur Region.

References and links

1.

L. Novotny and N. van Hulst, “Antennas for light,” Nat. Photonics 5, 83–90 (2011). [CrossRef]

2.

J. A. Schuller, E. S. Barnard, W. S. Cai, Y. C. Jun, J. S. White, and M. L. Brongersma, “Plasmonics for extreme light concentration and manipulation,” Nature Mater. 9, 193–204 (2010). [CrossRef]

3.

A. Kinkhabwala, Z. F. Yu, S. H. Fan, Y. Avlasevich, K. Mullen, and W. E. Moerner, “Large single-molecule fluorescence enhancements produced by a bowtie nanoantenna,” Nat. Photonics 3, 654–657 (2009). [CrossRef]

4.

H. Ko, S. Singamaneni, and V. V. Tsukruk, “Nanostructured surfaces and assemblies as SERS media,” Small 4, 1576–1599 (2008). [CrossRef] [PubMed]

5.

P. Mühlschlegel, H. J. Eisler, O. J. F. Martin, B. Hecht, and D. W. Pohl, “Resonant optical antennas,” Science 308, 1607–1609 (2005). [CrossRef] [PubMed]

6.

Y. Fu and J. R. Lakowicz, “Modification of single molecule fluorescence near metallic nanostructures,” Laser Photon. Rev. 3, 221–232 (2009). [CrossRef]

7.

R. Hostein, H. Aouani, H. Rigneault, and J. Wenger, “Deciphering fluorescence signals by quantifying separately the excitation intensity from the number of emitters,” Opt. Lett. 36, 3317–3319 (2011). [CrossRef] [PubMed]

8.

K. Fujita, M. Kobayashi, S. Kawano, M. Yamanaka, and S. Kawata, “High-resolution confocal microscopy by saturated excitation of fluorescence,” Phys. Rev. Lett. 99, 228105 (2007). [CrossRef]

9.

M. Yamanaka, S. Kawano, K. Fujita, N. I. Smith, and S. Kawata, “Beyond the diffraction-limit biological imaging by saturated excitation microscopy,” J. Biomed. Opt. 13, 050507 (2008). [CrossRef] [PubMed]

10.

H. J. Lezec, A. Degiron, E. Devaux, R. A. Linke, L. Martin-Moreno, F. J. Garcia-Vidal, and T. W. Ebbesen, “Beaming light from a subwavelength aperture,” Science 297, 820–822 (2002). [CrossRef] [PubMed]

11.

F. J. Garcia-Vidal, L. Martin-Moreno, T. W. Ebbesen, and L. Kuipers, “Light passing through subwavelength apertures,” Rev. Mod. Phys. 82, 729–787 (2010). [CrossRef]

12.

H. Aouani, O. Mahboub, N. Bonod, E. Devaux, E. Popov, H. Rigneault, T. W. Ebbesen, and J. Wenger, “Bright unidirectional fluorescence emission of molecules in a nanoaperture with plasmonic corrugations,” Nano Lett. 11, 637–644 (2011). [CrossRef] [PubMed]

13.

Y. C. Jun, K. C. Y. Huang, and M. L. Brongersma, “Plasmonic beaming and active control over fluorescent emission,” Nature Commun. 2, 283 (2011). [CrossRef]

14.

H. Aouani, O. Mahboub, E. Devaux, H. Rigneault, T. W. Ebbesen, and J. Wenger, “Large molecular fluorescence enhancement by a nanoaperture with plasmonic corrugations,” Opt. Express 19, 13056–13062 (2011). [CrossRef] [PubMed]

15.

O. Mahboub, S. Carretero Palacios, C. Genet, F. J. Garcia-Vidal, S. G. Rodrigo, L. Martin-Moreno, and T. W. Ebbesen, “Optimization of bulls eye structures for transmission enhancement,” Opt. Express 18, 11292–11299 (2010). [CrossRef] [PubMed]

16.

H. Aouani, S. Itzhakov, D. Gachet, E. Devaux, T. W. Ebbesen, H. Rigneault, D. Oron, and J. Wenger, “Colloidal quantum dots as probes of excitation field enhancement in photonic antennas,” ACS Nano 8, 4571–4578 (2010). [CrossRef]

17.

N. Djaker, R. Hostein, E. Devaux, T. W. Ebbesen, H. Rigneault, and J. Wenger, “Surface enhanced Raman Scattering on a single nanometric aperture,” J. Phys. Chem. C 114, 16250–16256 (2010). [CrossRef]

OCIS Codes
(050.1220) Diffraction and gratings : Apertures
(240.0240) Optics at surfaces : Optics at surfaces
(300.6280) Spectroscopy : Spectroscopy, fluorescence and luminescence

ToC Category:
Optics at Surfaces

History
Original Manuscript: May 16, 2012
Revised Manuscript: June 27, 2012
Manuscript Accepted: June 29, 2012
Published: July 23, 2012

Citation
Heykel Aouani, Richard Hostein, Oussama Mahboub, Eloïse Devaux, Hervé Rigneault, Thomas W. Ebbesen, and Jérôme Wenger, "Saturated excitation of fluorescence to quantify excitation enhancement in aperture antennas," Opt. Express 20, 18085-18090 (2012)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-20-16-18085


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References

  1. L. Novotny and N. van Hulst, “Antennas for light,” Nat. Photonics5, 83–90 (2011). [CrossRef]
  2. J. A. Schuller, E. S. Barnard, W. S. Cai, Y. C. Jun, J. S. White, and M. L. Brongersma, “Plasmonics for extreme light concentration and manipulation,” Nature Mater.9, 193–204 (2010). [CrossRef]
  3. A. Kinkhabwala, Z. F. Yu, S. H. Fan, Y. Avlasevich, K. Mullen, and W. E. Moerner, “Large single-molecule fluorescence enhancements produced by a bowtie nanoantenna,” Nat. Photonics3, 654–657 (2009). [CrossRef]
  4. H. Ko, S. Singamaneni, and V. V. Tsukruk, “Nanostructured surfaces and assemblies as SERS media,” Small4, 1576–1599 (2008). [CrossRef] [PubMed]
  5. P. Mühlschlegel, H. J. Eisler, O. J. F. Martin, B. Hecht, and D. W. Pohl, “Resonant optical antennas,” Science308, 1607–1609 (2005). [CrossRef] [PubMed]
  6. Y. Fu and J. R. Lakowicz, “Modification of single molecule fluorescence near metallic nanostructures,” Laser Photon. Rev.3, 221–232 (2009). [CrossRef]
  7. R. Hostein, H. Aouani, H. Rigneault, and J. Wenger, “Deciphering fluorescence signals by quantifying separately the excitation intensity from the number of emitters,” Opt. Lett.36, 3317–3319 (2011). [CrossRef] [PubMed]
  8. K. Fujita, M. Kobayashi, S. Kawano, M. Yamanaka, and S. Kawata, “High-resolution confocal microscopy by saturated excitation of fluorescence,” Phys. Rev. Lett.99, 228105 (2007). [CrossRef]
  9. M. Yamanaka, S. Kawano, K. Fujita, N. I. Smith, and S. Kawata, “Beyond the diffraction-limit biological imaging by saturated excitation microscopy,” J. Biomed. Opt.13, 050507 (2008). [CrossRef] [PubMed]
  10. H. J. Lezec, A. Degiron, E. Devaux, R. A. Linke, L. Martin-Moreno, F. J. Garcia-Vidal, and T. W. Ebbesen, “Beaming light from a subwavelength aperture,” Science297, 820–822 (2002). [CrossRef] [PubMed]
  11. F. J. Garcia-Vidal, L. Martin-Moreno, T. W. Ebbesen, and L. Kuipers, “Light passing through subwavelength apertures,” Rev. Mod. Phys.82, 729–787 (2010). [CrossRef]
  12. H. Aouani, O. Mahboub, N. Bonod, E. Devaux, E. Popov, H. Rigneault, T. W. Ebbesen, and J. Wenger, “Bright unidirectional fluorescence emission of molecules in a nanoaperture with plasmonic corrugations,” Nano Lett.11, 637–644 (2011). [CrossRef] [PubMed]
  13. Y. C. Jun, K. C. Y. Huang, and M. L. Brongersma, “Plasmonic beaming and active control over fluorescent emission,” Nature Commun.2, 283 (2011). [CrossRef]
  14. H. Aouani, O. Mahboub, E. Devaux, H. Rigneault, T. W. Ebbesen, and J. Wenger, “Large molecular fluorescence enhancement by a nanoaperture with plasmonic corrugations,” Opt. Express19, 13056–13062 (2011). [CrossRef] [PubMed]
  15. O. Mahboub, S. Carretero Palacios, C. Genet, F. J. Garcia-Vidal, S. G. Rodrigo, L. Martin-Moreno, and T. W. Ebbesen, “Optimization of bulls eye structures for transmission enhancement,” Opt. Express18, 11292–11299 (2010). [CrossRef] [PubMed]
  16. H. Aouani, S. Itzhakov, D. Gachet, E. Devaux, T. W. Ebbesen, H. Rigneault, D. Oron, and J. Wenger, “Colloidal quantum dots as probes of excitation field enhancement in photonic antennas,” ACS Nano8, 4571–4578 (2010). [CrossRef]
  17. N. Djaker, R. Hostein, E. Devaux, T. W. Ebbesen, H. Rigneault, and J. Wenger, “Surface enhanced Raman Scattering on a single nanometric aperture,” J. Phys. Chem. C114, 16250–16256 (2010). [CrossRef]

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