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Virtual Journal for Biomedical Optics

| EXPLORING THE INTERFACE OF LIGHT AND BIOMEDICINE

  • Editor: Gregory W. Faris
  • Vol. 1, Iss. 11 — Nov. 13, 2006
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Fluorescence enhancement by surface gratings

Yu-Ju Hung, Igor I. Smolyaninov, Christopher C. Davis, and Hsuan-Chen Wu  »View Author Affiliations


Optics Express, Vol. 14, Issue 22, pp. 10825-10830 (2006)
http://dx.doi.org/10.1364/OE.14.010825


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Abstract

Fluorescence from a layer of Rhodamine 6G (R6G) is observed to be enhanced strongly if a dielectric grating deposited onto a gold film is used as a substrate. The fluorescence enhancement has been studied as a function of the grating periodicity and the angle of incidence of the excitation light. The enhancement mechanism is consistent with excitation of surface-plasmon-polaritons on the metal film surface. The observed phenomenon may be promising in sensing applications.

© 2006 Optical Society of America

1. Introduction

2. Experiment

2.1 Comparison between gratings deposited onto a metal and a dielectric substrate

Fig. 1. Device structures
Fig. 2. (a) AFM image of the nano-stripe gratings; (b) Dimensions of the pattern
Fig. 3. The intensity comparison of R6G/PMMA gratings on (a) ITO/glass substrate; (b) Au/glass substrate.

2.2 Polarization and periodicity dependence

In order to understand how the fluorescence signal taken with the FOM is affected by the periodicity of the gratings, we made a sample with regions of different periodicity varying from 400nm to 1μm. The sample geometry is illustrated in Fig. 4(a), in which the grating periodicity is given in nanometers. Figures 4(b,c) indicate that the fluorescence enhancement depends strongly on the grating periodicity. We also studied the polarization dependence of the observed effect. In this experiment a mercury lamp filtered by a film polarizer was used as the excitation source at normal incidence. The sample was rotated so that the polarization direction was changed with respect to the grating trenches. The results of these experiments are shown in Figs.4 and 5. The exposure time for (b) and (c) was 250s and 700s respectively. Figure 5 shows the normalized digital value taken from the images. Every value is normalized to the background and the exposure time. The fluorescent efficiency is 10 times higher when the E field is parallel to the grating trenches.

It seems clear that some kind of surface plasmon polariton excitation is involved in the phenomena observed in our experiments. If we compute the SP dispersion and try to match the k-momentum, we find that for 640nm emission, the plasmon mode and radiation mode are strongly coupled because of k-vector momentum matching provided by the grating and the emission angle is zero degree when the grating periodicity is 736 nm. In order to study the enhancement mechanism in more detail we have studied how the excitation angle affects the fluorescence excited at various grating periodicities.

2.3 Rotation of incident angle

A one-dimensional (1D) PMMA grating on an Au film surface acts like a 1D plasmonic crystal [16

16. T. Okamoto, F. H’Dhili, and S. Kawata, “Towards plasmonic band gap laser,” Appl. Phys. Lett. 85, 3968 (2004). [CrossRef]

]. In order to relate the fluorescence enhancement with plasmonic crystal properties of our substrates we have performed more detailed measurements of fluorescence at different angles of the excitation light. In these experiments the incident laser light was tilted at an angle θ with respect to the z-axis and rotated by an angle α with respect to the y-axis in the x-y plane as shown in Fig. 6. The emission intensity of each pattern is recorded with the α rotation of every 10 degrees. Figure 7 shows two patterns illuminated at different rotation angles a. In Fig. 7(a), 411nm and 840nm patterns emit efficiently, while in Fig. 7(b), the most efficient fluorescence comes from the 693nm pattern. Fig. 7(c) indicates the position and periodicity of different gratings. The fluorescence signal measured as a function of angle is shown in Fig. 8 for different periodicities of the PMMA gratings. The background signal was subtracted from every data point and normalized to the CCD exposure time. The angle α is scanned from -10 to 90 degrees.

Fig. 4. Fluorescence under normal excitation - (a) the arrangement of grating periodicity (in nm). (b) FOM pictures taken under the polarized Hg Lamp. E field is parallel to the grating trenches (exposure time: 250s). (c) The sample was rotated 90 degrees clockwise. E field is perpendicular to the grating trenches (exposure time: 700s).
Fig. 5. Polarization effect on gratings with normal incidence to the sample surface.

3. Discussion

The angle α, which corresponds to the maximum of the fluorescence signal, can be determined from Fig. 8. The error of the measured angle is in a range of ± 2.5°. The reason for the unsymmetrical intensity at -10° and +10° is that α is not accurately tuned to a symmetrical position. To explain the angle effect, the incident wave vector k0=2π/532nm is decomposed as the projected wave vector k0sinθ in the x-y plane and k0cosθ along the z-axis. The component k0sinθ can be decomposed into x and y components as k0sinθsinα and k0sinθcosα, respectively. The grating k vector 2πn/a can provide momentum matching along the x direction as shown in Eq. (1), while the y component remains unchanged as shown in Eq. (2). If a surface plasmon is excited, the k-vector of the incoming photons mediated by the grating periodicity should match the k vector of surface plasmons as shown in Eq. (3):

kx*=kosinθsinα+(2πna)=ksp
(1)
Fig. 6. The geometry of the incident laser beam and angle definitions.
Fig. 7. (a) α=30° Patterns with 841nm and 411nm periodicity are excited most strongly. (b) α=30° The pattern with 693nm periodicity fluoresce most strongly. (c) The pattern arrangement of (a) and (b).
Fig. 8. Fluorescence emission vs. angle α for different gratings.
ky*=kosinθcosα=ksp
(2)
(k*)2=(kosinθcosα)2+(kosinθsinα+2πna)2=(ksp)2
(3)

where n is an integer, and k* is the composite k value in the x-y plane. In our experiments the incident angle θ was about 44°. The angle α is determined from the peak intensity shown in Fig. 8. The theoretical ksp for long-range SPPs on the vacuum/Au interface is 2π ∙106 2.087 m-1 at 532nm. Figure 9 shows k* for n=-2, -1, 0, and 1. At least one good integer order n can be fitted to the theoretical ksp for every periodicity. Table 1 shows the coupling order n, the maximum excitation angle α and the maximum digital value of the image intensity for each periodicity. The digital values for each period are comparable to each other because they are normalized to the exposure time. For 604nm and 693nm, the coupling order is ± 1. These gratings exhibit a higher fluorescence intensity compared to other gratings for which only one diffraction order is coupled efficiently.

Fig. 9. k* for different order n

Table 1:. The relation between the coupling order n and the digital value of the image intensity.

table-icon
View This Table

4. Conclusion

We have observed strongly enhanced fluorescence from a layer of Rhodamine 6G (R6G) on a dielectric grating deposited on top of a thin gold film. The fluorescence enhancement has been studied as a function of the grating periodicity and the angle of incidence of the excitation light. The enhancement mechanism is consistent with excitation of surface plasmon polaritons on the metal film surface. The observed phenomenon may be promising in sensing applications.

Acknowledgments

We thank Professor Robert Gammon and Dr. Vildana Hodzic for helpful discussions and Professor Michael Fuhrer for providing us with access to the electron-beam lithography system. This work has been supported by NSF.

References and Links

1.

K. Li, W. Lukosz, and R. E. Kunz, “Light emission by magnetic and electric dipoles close to a plane interface. I. Total radiated power,” J. Opt. Soc. Am. 67, 1607 (1977). [CrossRef]

2.

W. Lukosz and R. E. Kunz, “Light emission by magnetic and electric dipoles close to a plane interface. II. Radiation patterns of perpendicular oriented dipoles,” J. Opt. Soc. Am. 67, 1615 (1977). [CrossRef]

3.

W. Lukosz “Light emission by magnetic and electric dipoles close to a plane interface. III. Radiation patterns of dipoles with arbitrary orientation,” J. Opt. Soc. Am. 69, 1495 (1979). [CrossRef]

4.

W.H. Weber and C. F. Eagen, “Energy transfer from an excited dye molecule to the surface plasmons of an adjacent metal,” Opt. Lett. 4, 236 (1979). [CrossRef] [PubMed]

5.

R.R. Chance, A. Prock, and R. Silbey, “Molecular fluorescence and energy transfer near interfaces,” in Advances in Chemical Physics, I. Prigogine and S.R. Rice, eds., pp. 1–65, (Wiley, New York, 1978).

6.

J. Enderlein, T. Ruckstuhl, and S. Seeger, “Highly efficient optical detection of surface-generated fluorescence,” Appl. Opt. 38, 724 (1999). [CrossRef]

7.

J. Enderlein, “Single-molecule fluorescence near a metal layer,” Chem. Phys. 247, 1 (1999). [CrossRef]

8.

A. Minardo, R. Bernini, F. Mottola, and L. Zeni, “Optimization of metal-clad waveguides for sensitive fluorescence detection,” Opt. Express 14, 3512 (2006). [CrossRef] [PubMed]

9.

D. Neuschafer, W. Budach, C. Wanke, and S.D. Chibout,“Evanescent resonator chips: a universal platform with superior sensitivity for fluorescence-based microarrays,” Biosens. Bioelectron. 18, 489 (2003). [CrossRef] [PubMed]

10.

J.R Lakowicz, J. Malicka, I. Gryczynski, Z. Gryczynski, and C.D. Geddes, “Topical Review: Radiative decay engineering: the role of photonic mode density in biotechnology,” J. Phys. D 36, 240–249 (2003). [CrossRef]

11.

J.R. Lakowicz, “Radiative decay engineering 3: surface plasmon-coupled directional emission,” Anal. Biochem. 324, 153–169 (2004). [CrossRef]

12.

C.D. Geddes, I. Gryczynski, J. Malicka, Z. Gryczynski, and J.R. Lakowicz,“Fluorescence News- Directional Surface Plasmon Coupled Emission”, J. Fluoresc. l14, 119–123 (2004). [CrossRef]

13.

J. R. Lakowicz,“Radiative decay engineering 5: metal-enhanced fluorescence and plasmon emission,” Anal. Biochem. 337, 171–194 (2005). [CrossRef] [PubMed]

14.

J. Enderlein and T. Ruckstuhl, “The efficiency of surface-plasmon coupled emission for sensitive fluorescence detection,” Opt. Express 13, 8855 (2005). [CrossRef] [PubMed]

15.

F.D. Stefani, K. Vasiliev, N. Bocchio, N. Stoyanova, and M. Kreiter, “Surface-plasmon-mediated single-molecule fluorescence through a thin metallic film,” Phys. Rev. Lett. 94, 023005 (2005). [CrossRef] [PubMed]

16.

T. Okamoto, F. H’Dhili, and S. Kawata, “Towards plasmonic band gap laser,” Appl. Phys. Lett. 85, 3968 (2004). [CrossRef]

OCIS Codes
(240.6680) Optics at surfaces : Surface plasmons
(300.2530) Spectroscopy : Fluorescence, laser-induced

ToC Category:
Optics at Surfaces

History
Original Manuscript: August 7, 2006
Revised Manuscript: September 20, 2006
Manuscript Accepted: October 8, 2006
Published: October 30, 2006

Virtual Issues
Vol. 1, Iss. 11 Virtual Journal for Biomedical Optics

Citation
Yu-Ju Hung, Igor I. Smolyaninov, Christopher C. Davis, and Hsuan-Chen Wu, "Fluorescence enhancement by surface gratings," Opt. Express 14, 10825-10830 (2006)
http://www.opticsinfobase.org/vjbo/abstract.cfm?URI=oe-14-22-10825


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References

  1. K. Li, W. Lukosz, and R. E. Kunz, "Light emission by magnetic and electric dipoles close to a plane interface. I. Total radiated power," J. Opt. Soc. Am. 67,1607 (1977). [CrossRef]
  2. W. Lukosz and R. E. Kunz, "Light emission by magnetic and electric dipoles close to a plane interface. II. Radiation patterns of perpendicular oriented dipoles," J. Opt. Soc. Am. 67, 1615 (1977). [CrossRef]
  3. W. Lukosz "Light emission by magnetic and electric dipoles close to a plane interface. III. Radiation patterns of dipoles with arbitrary orientation," J. Opt. Soc. Am. 69,1495 (1979). [CrossRef]
  4. W.H. Weber and C. F. Eagen, "Energy transfer from an excited dye molecule to the surface plasmons of an adjacent metal," Opt. Lett. 4, 236 (1979). [CrossRef] [PubMed]
  5. R.R. Chance, A. Prock, and R. Silbey, " Molecular fluorescence and energy transfer near interfaces," in Advances in Chemical Physics, I. Prigogine and S.R. Rice, eds., pp.1-65, (Wiley, New York, 1978).
  6. J. Enderlein, T. Ruckstuhl, and S. Seeger, "Highly efficient optical detection of surface-generated fluorescence," Appl. Opt. 38, 724 (1999). [CrossRef]
  7. J. Enderlein, "Single-molecule fluorescence near a metal layer," Chem. Phys. 247, 1 (1999). [CrossRef]
  8. A. Minardo, R. Bernini, F. Mottola and L. Zeni, "Optimization of metal-clad waveguides for sensitive fluorescence detection," Opt. Express 14, 3512 (2006). [CrossRef] [PubMed]
  9. D. Neuschafer, W. Budach, C. Wanke, and S.D. Chibout," Evanescent resonator chips: a universal platform with superior sensitivity for fluorescence-based microarrays," Biosens. Bioelectron. 18,489 (2003). [CrossRef] [PubMed]
  10. J.R Lakowicz, J. Malicka, I. Gryczynski, Z. Gryczynski, and C.D. Geddes, "Topical Review: Radiative decay engineering: the role of photonic mode density in biotechnology," J. Phys. D 36, 240-249 (2003). [CrossRef]
  11. J.R. Lakowicz, "Radiative decay engineering 3: surface plasmon-coupled directional emission," Anal. Biochem. 324, 153-169 (2004). [CrossRef]
  12. C.D. Geddes, I. Gryczynski, J. Malicka, Z. Gryczynski, and J.R. Lakowicz,"Fluorescence News- Directional Surface Plasmon Coupled Emission," J. Fluoresc. l14, 119-123 (2004). [CrossRef]
  13. J. R. Lakowicz,"Radiative decay engineering 5: metal-enhanced fluorescence and plasmon emission, " Anal. Biochem. 337,171-194 (2005). [CrossRef] [PubMed]
  14. J. Enderlein and T. Ruckstuhl, "The efficiency of surface-plasmon coupled emission for sensitive fluorescence detection," Opt. Express 13, 8855 (2005). [CrossRef] [PubMed]
  15. F.D. Stefani, K. Vasiliev, N. Bocchio, N. Stoyanova, and M. Kreiter, "Surface-plasmon-mediated single-molecule fluorescence through a thin metallic film," Phys. Rev. Lett. 94, 023005 (2005). [CrossRef] [PubMed]
  16. T. Okamoto, F. H’Dhili, and S. Kawata, "Towards plasmonic band gap laser," Appl. Phys. Lett. 85,3968 (2004). [CrossRef]

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