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OPTICS LETTERS / Vol. 38, No. 22 / November 15, 2013
Surface-enhanced fluorescence of a dye-doped polymer layer with plasmonic band edge tuning Jian-Juan Jiang,1,2 Feng Xu,1,2 Yu-Bo Xie,1,2 Xia-Mei Tang,1,2 Zheng-Yang Liu,1,2 Xue-Jin Zhang,1,3,4 and Yong-Yuan Zhu1,2,5 1
National Laboratory of Solid State Microstructures, Nanjing University, Nanjing 210093, China 2
3
School of Physics, Nanjing University, Nanjing 210093, China
School of Modern Engineering and Applied Science, Nanjing University, Nanjing 210093, China 4 e-mail: [email protected] 5
e-mail: [email protected]
Received May 3, 2013; revised August 7, 2013; accepted October 16, 2013; posted October 16, 2013 (Doc. ID 189908); published November 5, 2013 We investigated experimentally the influence of 1D rectangular Au gratings on fluorescence. The formation of a bandgap in the dispersion relation is confirmed by our experiment. At the edge of this bandgap, the fluorescence of the dye can be strongly enhanced due to the surface plasmon polaritons’ large density of states. By structural design we tuned the plasmonic band edges to the wavelength of the fluorescence of the dye molecules. An optimized Au grating structure with a duty ratio of 3∕4 is found to achieve up to 120 times stronger fluorescence than that of a planar metal surface. © 2013 Optical Society of America OCIS codes: (240.6680) Surface plasmons; (260.2510) Fluorescence; (350.2770) Gratings; (160.5293) Photonic bandgap materials. http://dx.doi.org/10.1364/OL.38.004570
Surface plasmon polaritons (SPPs) (or localized surface plasmons) are surface bound electromagnetic waves coupled to the conduction electrons at the interface between a metal and a dielectric [1,2]. When fluorescent molecules are near the metallic surface, the localized electromagnetic field of SPPs or localized surface plasmons can enhance the fluorescence [3]. Recently, surface-enhanced fluorescence has been widely studied for its broad application in nanoscale optical antennas [4–6], organic light-emitting diodes [7,8], and fluorescence spectroscopy [9–12]. Different metal structures including metal nanoparticles and structured thin-metal films have been researched in the past. Gontijo et al. [13] proved that the emissions of dye molecules are more likely to transfer to the SPPs than to free space, due to the spatial overlap between the dye molecules and the SPP electric field. Neal et al. observed 11-fold enhancement of emissions from dye-doped polymer layers on planar metal film without structure modulating [14]. Fluorescence enhancement on periodic gratings [15–19] has attracted much attention since Barnes and co-workers [20,21] investigated the origin of SPP bandgap. Harmonic gratings with one period will not open a bandgap at k 0, and the achieved maximal enhancement factor is up to 30-fold [22]. According to Fermi’s golden rule, the fluorescence enhancement factor is proportional to the field intensity at the emitter position and the SPP density of states (DOS): Γp ω
2π ⃗ ⃗ 2 hd · Ei gℏω: ℏ
(1)
Here d denotes the dipole moment of dye molecules, E is the field intensity at the emitter position, and gℏω is the SPP-DOS. A quite large SPP-DOS at the band edge can be 0146-9592/13/224570-04$15.00/0
obtained for the inverse proportion to the slope of SPP dispersion. Due to the energy redistribution [23,24], SPPs can greatly enhance the fluorescence of dye molecules [25,26]. Thus the more energy that is redistributed into the SPP mode, the larger the fluorescence enhancement factor will be. Here we have investigated the influence of 1D rectangular gratings with only one period on the fluorescence. We have designed the structure to open up a bandgap with the wavelength of the fluorescence at the band edges. 120-fold enhancement has been obtained. The sample was fabricated as follows. A 200 nm Au film was coated on the silicon substrate by e-beam evaporation. Au gratings with a period of P 600 nm and groove width d P∕4, P∕2, and 3P∕4 were fabricated
Fig. 1. (a) Schematic diagram of the three-layer structure with a constant grating period and Au film thickness (P 600 nm, H 200 nm). (b) The scanning electron microscope (SEM) image of an Au grating with a period of 600 nm and groove width of 150 nm. (c) The fluorescence spectra of the dye molecule. The inset shows the molecule’s structure and formula. © 2013 Optical Society of America
November 15, 2013 / Vol. 38, No. 22 / OPTICS LETTERS
by the focused ion beam (FIB) system (strata FIB 201, FEI Co., 30 keV Ga ions). The schematic diagram of our sample and the SEM image are shown in Fig. 1. To prevent fluorescence quenching, a pure polymethyl methacrylate (PMMA) film with a thickness of around 20 nm was spun onto the Au film. After baking the sample at 180° for 20 min, a 35 nm C33 H33 ClN2 O9 dye-doped PMMA film was spun onto it. The total PMMA thickness shrank to about 50 nm, because the pure PMMA film was partially dissolved during the above procedure. The resultant molar concentration of the dye molecules in the top PMMA layer is 5 × 10−4 mol∕L. To get the dispersion relation, the reflection spectra were first measured under a Zeiss microscope with a polarizer, a fiber spectrometer, and a halogen lamp as the light source. The dispersion curves could be obtained by mapping the reflectivity in a planar conjugate to the Fourier (back focal) planar of the objective with an angle resolution of 1°. For the fluorescence measurement, the emission spectra were detected in a home-built optical system. The fluorescence was collected by a microscope objective with an NA of 0.95 and measured by a Princeton spectrometer. The center emission wavelength of the C33 H33 ClN2 O9 dye molecule is 690 nm. In order to set the bandgap of the structure to the range of the emission wavelength, we design the period of the grating to be 600 nm. After it is coated with 50 nm PMMA, the bandgap can be redshifted to the emission wavelength. In this way, the fluorescence can be enhanced at the band edge. Since the structure is not harmonic, it has many Fourier components. It is well known that the width of a bandgap opened up by the nK (K 2π∕P, n 0, 1; 2…) component is proportional to the nK Fourier coefficient. Here, we study the SPP bandgap opened up by 2K, ignoring the other bandgaps caused by higher order Fourier coefficients. This bandgap is located at the Brillouin zone boundary outside the light cone. Photons are accessible only in the region within the cone. That is, photons cannot couple with the SPP directly. In our case, because of the existence of the K Fourier component, the gap can be folded from the Brillouin zone boundary to k 0, which can be easily excited at normal incidence.
Fig. 2. Calculated and simulated results for Au gratings (P 600 nm, h 20 nm) without PMMA film. The solid squares denote the calculated relationship between the 2K Fourier coefficients and the groove width. The solid circles represent the simulated results of bandgap width versus groove width.
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Figure 2 shows 2K Fourier coefficients of different d values calculated with the period of the grating being 600 nm. It can be seen that the whole picture is symmetric with respect to d P∕2. The largest 2K coefficients occur at d 150 nm and d 450 nm, respectively. The solid circle curve of the gap width shown in Fig. 2 is directly proportional to that of the 2K coefficients. We first performed simulations of Au gratings without PMMA on the surface. Figures 3(a) and 3(c) show the dispersion with P 600 nm and different groove widths d P∕4 and d 3P∕4 simulated by Rsoft software. Their dispersion relations are quite different, which is important for enhancing the fluorescence of dye molecules. They both opened up a bandgap of about 28 nm and present a large SPP-DOS at the band edges. In comparison, for P 600 nm and groove width d P∕2, no bandgap exists, as shown in Fig. 3(b), which is consistent with a zero 2K Fourier coefficient. The SPP-DOS at the band cross is much smaller than that of d P∕4 and 3P∕4. The location of the bandgap center (634 nm) is the same for the three structures. Also, there is an interesting phenomenon appearing in Fig. 3(a) in which the coupling strength of the upper band edge is very strong, while the lower one is reduced to zero. It is quite the opposite for Fig. 3(c). Figures 3(d)–3(h) show the field intensity distributions at both band edges corresponding to the dispersion relation in Figs. 3(a)–3(c) by using 2D finite-difference time-domain simulations (Lumerical FDTD Solutions). The right part of Figs. 3(d)–3(h) is the sketch of the surface charge distribution. The results indicate that the SPPs have already been excited, but these coupling modes are quite different. From the field distribution, we can approximately evaluate the field intensity of the band edge modes. It is not hard to see that the field intensity of d 150 nm is larger than that of d 450 nm, and the ratio can reach 1.3 for strong band edges (and weak ones). Both have the same band edges (λ 648 nm and λ− 620 nm). For d 150 nm, we
Fig. 3. (a)–(c) Dispersion curves of SPPs on gratings simulated by Rsoft software. (d)–(h) The field and corresponding charge distribution at the bandgap edges or the center of the band cross for different structures.
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find the field excited by λ 648 nm is much stronger than by λ− 620 nm. Figure 3(d) shows that the field is excited at both ends of the ridges, while Fig. 3(e) presents the field excited on the grooves and the center of the ridges, with the intensity apparently being weaker than that in Fig. 3(d). For d 450 nm we find that the field excited by λ− 620 nm is much stronger than by λ 648 nm, and the field excited for the former concentrates on both ends of the grooves, while that for the latter is on small ridges and the center of grooves. From the charge distribution we can find the reason for the missing band edge in Fig. 3. Just take Figs. 3(e) and 3(g), for example: the positive charges on the flat wide ridges or grooves are considerably hard to excite [21], while the modes in Figs. 3(d) and 3(h) are easy to excite. Next we performed simulations of Au gratings with 50 nm PMMA on the surface. The bandgap becomes broadened and redshifted with an increase in the PMMA thickness, which was confirmed by the experiment. The results of the dispersion relation are shown in Figs. 4(a)–4(c) and 4(d)–4(f). Compared with the gratings without PMMA in Fig. 3, the bandgaps for Figs. 4(a) and 4(c) are redshifted several dozens of nanometers and broadened from 28 nm to about 45 nm. For d P∕2 as the symmetry structure, only redshift occurs. The bandgap remains zero. The reason is that the effective refractive index of the SPP increases with the thickness of the PMMA layer. For the same reason, with more area filled with PMMA, the bandgap center is redshifted with an increase in the duty ratio. As shown in Figs. 4(a)–4(c), the bandgap centered at 713, 720, and 726 nm for d 150 nm, d 300 nm, and d 450 nm, respectively. For comparison, the bandgap center location is unchanged for different duty ratios of Au gratings without PMMA film, as shown in Figs. 3(a)–3(c). For the two complementary structures d P∕4 and d 3P∕4, both develop a strong band edge and a weak one. The measured gap widths and positions [Figs. 4(d)–4(f)] are in good
Fig. 4. (a)–(c) Simulated and (d)–(f) experimental results of the dispersion relation for 50 nm PMMA-coated gratings with different duty ratios. (g)–(i) The measured fluorescence enhancement ratios. The inset shows the simulated reflectivity (R).
agreement with the simulation [Figs. 4(a)–4(c)]. However, the weak band edge in the simulation shown in Figs. 3(a) and 3(c) is zero at normal incidence, while the experimental results in Figs. 4(d) and 4(f) show a weak coupling intensity. Also, the experimental dispersion relations are not as sharp as the simulation. The discrepancy might be mainly due to film surface roughness and the damping of the electron motion in metal as well as the limit of measurement accuracy. The bandgap can be also tuned by the depth of the gratings (the results are not shown here). With increasing depth, the gap becomes wider and redshifted. Our experiments confirm the simulation. We measured the fluorescence intensity of dye molecules on different structures (Au gratings, planar Au film, and silica substrate). The fluorescence intensity on planar Au film is larger than that on silica. Compared to planar film, the structured Au film is more efficient for fluorescence, due to the band edge enhancement. Figures 4(g)–4(i) show the measured fluorescence enhancement ratio, i.e., the photoluminescence intensity of the dye-doped PMMA layer on gratings divided by that on flat Au film. It can be seen that the fluorescence of all three structures (d P∕4, d P∕2, and d 3P∕4) has been enhanced. Due to the wide range of the fluorescence spectra of this molecule, the roles that both band edges play on the fluorescence can be shown. The two peaks of Figs. 4(g) and 4(i) just coincide with the location of the two band edges. The inset is the simulated reflection curve of the structure at normal incidence, with two dips corresponding to the band edge modes. For the structure with d P∕4, the two peaks are located at 690 and 730 nm, with the relevant enhanced fluorescence rates being, respectively, 70 and 120, corresponding to the weak lower band edge and strong upper band edge in the dispersion relation in Fig. 4(d). The intrinsic emission spectrum of the dye molecule indicates that emission at 734 nm is weaker than at 690 nm, but the field intensity at 734 nm (the upper band edge mode) is much stronger than at 690 nm (the lower band edge mode). That is, the strong coupling strength compensates the weak emission. For the structure with d 3P∕4, the two peaks are at 706 and 748 nm, and the enhanced fluorescence rates are 95 and 50 for the strong lower band edge and weak upper band edge in Fig. 4(f), respectively. For the grating of d P∕2, the enhanced fluorescence rate is 40 at about 718 nm [shown in Fig. 4(h)]. The large enhanced fluorescence rates are mainly caused by the large SPP-DOS and high field strength at the band edge. It is worthwhile noticing that the fluorescence enhancement ratio of the d 150 nm structure is larger than that of d 450 nm, because of its larger field intensity at both band edges. We compare the fluorescence enhancement ratio of the two structures and find the ratio being 1.26 for the two strong band edges and 1.4 for the two weak edges, which coincides with the field intensity ratio (1.3). In conclusion, this work demonstrates an efficient way to enhance the fluorescence of the dye molecules C33 H33 ClN2 O9 using a 1D rectangular Au grating. We observed a bandgap in the dispersion relation in our simulation and experiment results. Also, the width of bandgap can be tuned by the duty ratio of the grating. We investigated different excitation modes on both band
November 15, 2013 / Vol. 38, No. 22 / OPTICS LETTERS
edges to find the reason for a strong band edge and a weak one. At the band edge, the SPP-DOS is very large. Thus, fluorescence of the dye can be strongly enhanced. An optimized Au grating structure with duty ratio of 3∕4 is found to achieve up to 120 times stronger fluorescence than that of a planar metal surface. SPP-assisted dye molecule emission control could be extended to other metallic and semiconductor surfaces and promises to be an exciting research topic in the generation of high fluorescence and stimulated emission of dye molecules. The authors would like to express thanks to Ye Yonghong (NJNU) for her support in the fabrication of the Au film. This work was supported by the State Key Program for Basic Research of China (Grant Nos. 2010CB630703 and 2012CB921502), by the National Natural Science Foundation of China (Grant Nos. 11274159 and 11174128), and by PAPD. References 1. H. Raether, Surface-Plasmons on Smooth and Rough Surfaces and on Gratings, Vol. 111 of Springer Tracts in Modern Physics (Springer, 1988). 2. W. Knoll, Annu. Rev. Phys. Chem. 49, 569 (1998). 3. E. Fort and S. Grésillon, J. Phys. D 41, 013001 (2008). 4. H. Aouani, O. Mahboub, N. Bonod, E. Devaux, E. Popov, H. Rigneault, T. W. Ebbesen, and J. Wenger, Nano Lett. 11, 637 (2011). 5. H. Aouani, O. Mahboub, E. Devaux, H. Rigneault, T. W. Ebbesen, and J. Wenger, Nano Lett. 11, 2400 (2011). 6. S. V. Boriskina and L. Dal Negro, Opt. Lett. 35, 538 (2010). 7. P. A. Hobson, S. Wedge, J. A. E. Wasey, I. Sage, and W. L. Barnes, Adv. Mater. 14, 1393 (2002). 8. S. Wedge, A. Giannattasio, and W. L. Barnes, Org. Electron. 8, 136 (2007). 9. J. Dostálek and W. Knoll, Biointerphases 3, FD12 (2008).
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10. X. Q. Cui, K. Tawa, K. Kintaka, and J. Nishii, Adv. Funct. Mater. 20, 945 (2010). 11. X. Q. Cui, K. Tawa, H. Hori, and J. Nishii, Adv. Funct. Mater. 20, 546 (2010). 12. K. Kim, Y. Oh, W. Lee, and D. Kim, Opt. Lett. 35, 3501 (2010). 13. I. Gontijo, M. Boroditsky, E. Yablonovitch, S. Keller, U. K. Mishra, and S. P. DenBaars, Phys. Rev. B 60, 11564 (1999). 14. T. D. Neal, K. Okamoto, and A. Scherer, Opt. Express 13, 5522 (2005). 15. A. Kocabas, S. S. Senlik, and A. Aydinli, Phys. Rev. B 77, 195130 (2008). 16. Y. Fujiwara, K. Murata, M. Ojima, Y. Ogawa, H. Kubo, H. Yoshida, A. Fujii, and M. Ozaki, J. Nonlinear Opt. Phys. Mater. 19, 571 (2010). 17. K. Murata, M. Ojima, Y. Ogawa, Y. Fujiwara, H. Kubo, A. Fujii, and M. Ozaki, Appl. Phys. Express 3, 041601 (2010). 18. Y. F. Shen, X. H. Liu, Y. F. Tang, Y. F. Chen, and J. Zi, J. Phys. Condens. Matter 17, L287 (2005). 19. J. Gao, A. M. Sarangan, and Q. W. Zhan, Opt. Lett. 37, 2640 (2012). 20. S. C. Kitson, W. L. Barnes, and J. R. Sambles, Phys. Rev. B 52, 11441 (1995). 21. W. L. Barnes, T. W. Preist, S. C. Kitson, and J. R. Sambles, Phys. Rev. B 54, 6227 (1996). 22. Y. Jiang, H. Y. Wang, H. Wang, B. R. Gao, Y. W. Hao, Y. Jin, Q. D. Chen, and H. B. Sun, J. Phys. Chem. C 115, 12636 (2011). 23. G. P. Wiederrecht, J. E. Hall, and A. Bouhelier, Phys. Rev. Lett. 98, 083001 (2007). 24. T. Okamoto, J. Simonen, and S. Kawata, Opt. Express 17, 8294 (2009). 25. A. Salomon, R. J. Gordon, Y. Prior, T. Seideman, and M. Sukharev, Phys. Rev. Lett. 109, 073002 (2012). 26. W. Y. Wismanto, R. Hidayat, M. O. Tjia, Y. Fujiwara, K. Murata, Y. Ogawa, H. Yoshida, A. Fujii, and M. Ozaki, J. Nonlinear Opt. Phys. Mater. 21, 1250013 (2012).
OPTICS LETTERS / Vol. 38, No. 22 / November 15, 2013
Surface-enhanced fluorescence of a dye-doped polymer layer with plasmonic band edge tuning Jian-Juan Jiang,1,2 Feng Xu,1,2 Yu-Bo Xie,1,2 Xia-Mei Tang,1,2 Zheng-Yang Liu,1,2 Xue-Jin Zhang,1,3,4 and Yong-Yuan Zhu1,2,5 1
National Laboratory of Solid State Microstructures, Nanjing University, Nanjing 210093, China 2
3
School of Physics, Nanjing University, Nanjing 210093, China
School of Modern Engineering and Applied Science, Nanjing University, Nanjing 210093, China 4 e-mail: [email protected] 5
e-mail: [email protected]
Received May 3, 2013; revised August 7, 2013; accepted October 16, 2013; posted October 16, 2013 (Doc. ID 189908); published November 5, 2013 We investigated experimentally the influence of 1D rectangular Au gratings on fluorescence. The formation of a bandgap in the dispersion relation is confirmed by our experiment. At the edge of this bandgap, the fluorescence of the dye can be strongly enhanced due to the surface plasmon polaritons’ large density of states. By structural design we tuned the plasmonic band edges to the wavelength of the fluorescence of the dye molecules. An optimized Au grating structure with a duty ratio of 3∕4 is found to achieve up to 120 times stronger fluorescence than that of a planar metal surface. © 2013 Optical Society of America OCIS codes: (240.6680) Surface plasmons; (260.2510) Fluorescence; (350.2770) Gratings; (160.5293) Photonic bandgap materials. http://dx.doi.org/10.1364/OL.38.004570
Surface plasmon polaritons (SPPs) (or localized surface plasmons) are surface bound electromagnetic waves coupled to the conduction electrons at the interface between a metal and a dielectric [1,2]. When fluorescent molecules are near the metallic surface, the localized electromagnetic field of SPPs or localized surface plasmons can enhance the fluorescence [3]. Recently, surface-enhanced fluorescence has been widely studied for its broad application in nanoscale optical antennas [4–6], organic light-emitting diodes [7,8], and fluorescence spectroscopy [9–12]. Different metal structures including metal nanoparticles and structured thin-metal films have been researched in the past. Gontijo et al. [13] proved that the emissions of dye molecules are more likely to transfer to the SPPs than to free space, due to the spatial overlap between the dye molecules and the SPP electric field. Neal et al. observed 11-fold enhancement of emissions from dye-doped polymer layers on planar metal film without structure modulating [14]. Fluorescence enhancement on periodic gratings [15–19] has attracted much attention since Barnes and co-workers [20,21] investigated the origin of SPP bandgap. Harmonic gratings with one period will not open a bandgap at k 0, and the achieved maximal enhancement factor is up to 30-fold [22]. According to Fermi’s golden rule, the fluorescence enhancement factor is proportional to the field intensity at the emitter position and the SPP density of states (DOS): Γp ω
2π ⃗ ⃗ 2 hd · Ei gℏω: ℏ
(1)
Here d denotes the dipole moment of dye molecules, E is the field intensity at the emitter position, and gℏω is the SPP-DOS. A quite large SPP-DOS at the band edge can be 0146-9592/13/224570-04$15.00/0
obtained for the inverse proportion to the slope of SPP dispersion. Due to the energy redistribution [23,24], SPPs can greatly enhance the fluorescence of dye molecules [25,26]. Thus the more energy that is redistributed into the SPP mode, the larger the fluorescence enhancement factor will be. Here we have investigated the influence of 1D rectangular gratings with only one period on the fluorescence. We have designed the structure to open up a bandgap with the wavelength of the fluorescence at the band edges. 120-fold enhancement has been obtained. The sample was fabricated as follows. A 200 nm Au film was coated on the silicon substrate by e-beam evaporation. Au gratings with a period of P 600 nm and groove width d P∕4, P∕2, and 3P∕4 were fabricated
Fig. 1. (a) Schematic diagram of the three-layer structure with a constant grating period and Au film thickness (P 600 nm, H 200 nm). (b) The scanning electron microscope (SEM) image of an Au grating with a period of 600 nm and groove width of 150 nm. (c) The fluorescence spectra of the dye molecule. The inset shows the molecule’s structure and formula. © 2013 Optical Society of America
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by the focused ion beam (FIB) system (strata FIB 201, FEI Co., 30 keV Ga ions). The schematic diagram of our sample and the SEM image are shown in Fig. 1. To prevent fluorescence quenching, a pure polymethyl methacrylate (PMMA) film with a thickness of around 20 nm was spun onto the Au film. After baking the sample at 180° for 20 min, a 35 nm C33 H33 ClN2 O9 dye-doped PMMA film was spun onto it. The total PMMA thickness shrank to about 50 nm, because the pure PMMA film was partially dissolved during the above procedure. The resultant molar concentration of the dye molecules in the top PMMA layer is 5 × 10−4 mol∕L. To get the dispersion relation, the reflection spectra were first measured under a Zeiss microscope with a polarizer, a fiber spectrometer, and a halogen lamp as the light source. The dispersion curves could be obtained by mapping the reflectivity in a planar conjugate to the Fourier (back focal) planar of the objective with an angle resolution of 1°. For the fluorescence measurement, the emission spectra were detected in a home-built optical system. The fluorescence was collected by a microscope objective with an NA of 0.95 and measured by a Princeton spectrometer. The center emission wavelength of the C33 H33 ClN2 O9 dye molecule is 690 nm. In order to set the bandgap of the structure to the range of the emission wavelength, we design the period of the grating to be 600 nm. After it is coated with 50 nm PMMA, the bandgap can be redshifted to the emission wavelength. In this way, the fluorescence can be enhanced at the band edge. Since the structure is not harmonic, it has many Fourier components. It is well known that the width of a bandgap opened up by the nK (K 2π∕P, n 0, 1; 2…) component is proportional to the nK Fourier coefficient. Here, we study the SPP bandgap opened up by 2K, ignoring the other bandgaps caused by higher order Fourier coefficients. This bandgap is located at the Brillouin zone boundary outside the light cone. Photons are accessible only in the region within the cone. That is, photons cannot couple with the SPP directly. In our case, because of the existence of the K Fourier component, the gap can be folded from the Brillouin zone boundary to k 0, which can be easily excited at normal incidence.
Fig. 2. Calculated and simulated results for Au gratings (P 600 nm, h 20 nm) without PMMA film. The solid squares denote the calculated relationship between the 2K Fourier coefficients and the groove width. The solid circles represent the simulated results of bandgap width versus groove width.
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Figure 2 shows 2K Fourier coefficients of different d values calculated with the period of the grating being 600 nm. It can be seen that the whole picture is symmetric with respect to d P∕2. The largest 2K coefficients occur at d 150 nm and d 450 nm, respectively. The solid circle curve of the gap width shown in Fig. 2 is directly proportional to that of the 2K coefficients. We first performed simulations of Au gratings without PMMA on the surface. Figures 3(a) and 3(c) show the dispersion with P 600 nm and different groove widths d P∕4 and d 3P∕4 simulated by Rsoft software. Their dispersion relations are quite different, which is important for enhancing the fluorescence of dye molecules. They both opened up a bandgap of about 28 nm and present a large SPP-DOS at the band edges. In comparison, for P 600 nm and groove width d P∕2, no bandgap exists, as shown in Fig. 3(b), which is consistent with a zero 2K Fourier coefficient. The SPP-DOS at the band cross is much smaller than that of d P∕4 and 3P∕4. The location of the bandgap center (634 nm) is the same for the three structures. Also, there is an interesting phenomenon appearing in Fig. 3(a) in which the coupling strength of the upper band edge is very strong, while the lower one is reduced to zero. It is quite the opposite for Fig. 3(c). Figures 3(d)–3(h) show the field intensity distributions at both band edges corresponding to the dispersion relation in Figs. 3(a)–3(c) by using 2D finite-difference time-domain simulations (Lumerical FDTD Solutions). The right part of Figs. 3(d)–3(h) is the sketch of the surface charge distribution. The results indicate that the SPPs have already been excited, but these coupling modes are quite different. From the field distribution, we can approximately evaluate the field intensity of the band edge modes. It is not hard to see that the field intensity of d 150 nm is larger than that of d 450 nm, and the ratio can reach 1.3 for strong band edges (and weak ones). Both have the same band edges (λ 648 nm and λ− 620 nm). For d 150 nm, we
Fig. 3. (a)–(c) Dispersion curves of SPPs on gratings simulated by Rsoft software. (d)–(h) The field and corresponding charge distribution at the bandgap edges or the center of the band cross for different structures.
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find the field excited by λ 648 nm is much stronger than by λ− 620 nm. Figure 3(d) shows that the field is excited at both ends of the ridges, while Fig. 3(e) presents the field excited on the grooves and the center of the ridges, with the intensity apparently being weaker than that in Fig. 3(d). For d 450 nm we find that the field excited by λ− 620 nm is much stronger than by λ 648 nm, and the field excited for the former concentrates on both ends of the grooves, while that for the latter is on small ridges and the center of grooves. From the charge distribution we can find the reason for the missing band edge in Fig. 3. Just take Figs. 3(e) and 3(g), for example: the positive charges on the flat wide ridges or grooves are considerably hard to excite [21], while the modes in Figs. 3(d) and 3(h) are easy to excite. Next we performed simulations of Au gratings with 50 nm PMMA on the surface. The bandgap becomes broadened and redshifted with an increase in the PMMA thickness, which was confirmed by the experiment. The results of the dispersion relation are shown in Figs. 4(a)–4(c) and 4(d)–4(f). Compared with the gratings without PMMA in Fig. 3, the bandgaps for Figs. 4(a) and 4(c) are redshifted several dozens of nanometers and broadened from 28 nm to about 45 nm. For d P∕2 as the symmetry structure, only redshift occurs. The bandgap remains zero. The reason is that the effective refractive index of the SPP increases with the thickness of the PMMA layer. For the same reason, with more area filled with PMMA, the bandgap center is redshifted with an increase in the duty ratio. As shown in Figs. 4(a)–4(c), the bandgap centered at 713, 720, and 726 nm for d 150 nm, d 300 nm, and d 450 nm, respectively. For comparison, the bandgap center location is unchanged for different duty ratios of Au gratings without PMMA film, as shown in Figs. 3(a)–3(c). For the two complementary structures d P∕4 and d 3P∕4, both develop a strong band edge and a weak one. The measured gap widths and positions [Figs. 4(d)–4(f)] are in good
Fig. 4. (a)–(c) Simulated and (d)–(f) experimental results of the dispersion relation for 50 nm PMMA-coated gratings with different duty ratios. (g)–(i) The measured fluorescence enhancement ratios. The inset shows the simulated reflectivity (R).
agreement with the simulation [Figs. 4(a)–4(c)]. However, the weak band edge in the simulation shown in Figs. 3(a) and 3(c) is zero at normal incidence, while the experimental results in Figs. 4(d) and 4(f) show a weak coupling intensity. Also, the experimental dispersion relations are not as sharp as the simulation. The discrepancy might be mainly due to film surface roughness and the damping of the electron motion in metal as well as the limit of measurement accuracy. The bandgap can be also tuned by the depth of the gratings (the results are not shown here). With increasing depth, the gap becomes wider and redshifted. Our experiments confirm the simulation. We measured the fluorescence intensity of dye molecules on different structures (Au gratings, planar Au film, and silica substrate). The fluorescence intensity on planar Au film is larger than that on silica. Compared to planar film, the structured Au film is more efficient for fluorescence, due to the band edge enhancement. Figures 4(g)–4(i) show the measured fluorescence enhancement ratio, i.e., the photoluminescence intensity of the dye-doped PMMA layer on gratings divided by that on flat Au film. It can be seen that the fluorescence of all three structures (d P∕4, d P∕2, and d 3P∕4) has been enhanced. Due to the wide range of the fluorescence spectra of this molecule, the roles that both band edges play on the fluorescence can be shown. The two peaks of Figs. 4(g) and 4(i) just coincide with the location of the two band edges. The inset is the simulated reflection curve of the structure at normal incidence, with two dips corresponding to the band edge modes. For the structure with d P∕4, the two peaks are located at 690 and 730 nm, with the relevant enhanced fluorescence rates being, respectively, 70 and 120, corresponding to the weak lower band edge and strong upper band edge in the dispersion relation in Fig. 4(d). The intrinsic emission spectrum of the dye molecule indicates that emission at 734 nm is weaker than at 690 nm, but the field intensity at 734 nm (the upper band edge mode) is much stronger than at 690 nm (the lower band edge mode). That is, the strong coupling strength compensates the weak emission. For the structure with d 3P∕4, the two peaks are at 706 and 748 nm, and the enhanced fluorescence rates are 95 and 50 for the strong lower band edge and weak upper band edge in Fig. 4(f), respectively. For the grating of d P∕2, the enhanced fluorescence rate is 40 at about 718 nm [shown in Fig. 4(h)]. The large enhanced fluorescence rates are mainly caused by the large SPP-DOS and high field strength at the band edge. It is worthwhile noticing that the fluorescence enhancement ratio of the d 150 nm structure is larger than that of d 450 nm, because of its larger field intensity at both band edges. We compare the fluorescence enhancement ratio of the two structures and find the ratio being 1.26 for the two strong band edges and 1.4 for the two weak edges, which coincides with the field intensity ratio (1.3). In conclusion, this work demonstrates an efficient way to enhance the fluorescence of the dye molecules C33 H33 ClN2 O9 using a 1D rectangular Au grating. We observed a bandgap in the dispersion relation in our simulation and experiment results. Also, the width of bandgap can be tuned by the duty ratio of the grating. We investigated different excitation modes on both band
November 15, 2013 / Vol. 38, No. 22 / OPTICS LETTERS
edges to find the reason for a strong band edge and a weak one. At the band edge, the SPP-DOS is very large. Thus, fluorescence of the dye can be strongly enhanced. An optimized Au grating structure with duty ratio of 3∕4 is found to achieve up to 120 times stronger fluorescence than that of a planar metal surface. SPP-assisted dye molecule emission control could be extended to other metallic and semiconductor surfaces and promises to be an exciting research topic in the generation of high fluorescence and stimulated emission of dye molecules. The authors would like to express thanks to Ye Yonghong (NJNU) for her support in the fabrication of the Au film. This work was supported by the State Key Program for Basic Research of China (Grant Nos. 2010CB630703 and 2012CB921502), by the National Natural Science Foundation of China (Grant Nos. 11274159 and 11174128), and by PAPD. References 1. H. Raether, Surface-Plasmons on Smooth and Rough Surfaces and on Gratings, Vol. 111 of Springer Tracts in Modern Physics (Springer, 1988). 2. W. Knoll, Annu. Rev. Phys. Chem. 49, 569 (1998). 3. E. Fort and S. Grésillon, J. Phys. D 41, 013001 (2008). 4. H. Aouani, O. Mahboub, N. Bonod, E. Devaux, E. Popov, H. Rigneault, T. W. Ebbesen, and J. Wenger, Nano Lett. 11, 637 (2011). 5. H. Aouani, O. Mahboub, E. Devaux, H. Rigneault, T. W. Ebbesen, and J. Wenger, Nano Lett. 11, 2400 (2011). 6. S. V. Boriskina and L. Dal Negro, Opt. Lett. 35, 538 (2010). 7. P. A. Hobson, S. Wedge, J. A. E. Wasey, I. Sage, and W. L. Barnes, Adv. Mater. 14, 1393 (2002). 8. S. Wedge, A. Giannattasio, and W. L. Barnes, Org. Electron. 8, 136 (2007). 9. J. Dostálek and W. Knoll, Biointerphases 3, FD12 (2008).
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