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OPTICS LETTERS / Vol. 39, No. 8 / April 15, 2014
Amplified spontaneous emission via the coupling between Fabry–Perot cavity and surface plasmon polariton modes Jian-Juan Jiang,1,2 Yu-Bo Xie,1,2 Zheng-Yang Liu,1,2 Xia-Mei Tang,1,2 Xue-Jin Zhang,1,3,4 and Yong-Yuan Zhu1,2,5 1
Key Laboratory of Modern Acoustics and National Laboratory of Solid State Microstructures, Nanjing University, Nanjing 210093, China 2 School of Physics, Nanjing University, Nanjing 210093, China 3
College of Engineering and Applied Sciences, Nanjing University, Nanjing 210093, China 4
e-mail: [email protected] 5 e-mail: [email protected]
Received January 13, 2014; revised March 14, 2014; accepted March 15, 2014; posted March 17, 2014 (Doc. ID 204635); published April 9, 2014 We demonstrated amplified spontaneous emission by embedding dye molecules within a dielectric layer of a metal-dielectric-metal subwavelength structure. It was reinforced when a strong coupling occurred between the Fabry–Perot mode supported by the dielectric layer and the surface plasmon polariton mode supported by the adjacent metallic grating. Here, we adjust the two mode interaction via tuning the depth of the metallic grating grooves. The stronger the interaction, the smaller the full width at half-maximum of the emission spectra and the lower the threshold of the amplified spontaneous emission. © 2014 Optical Society of America OCIS codes: (050.2230) Fabry-Perot; (050.2770) Gratings; (140.4480) Optical amplifiers; (240.6680) Surface plasmons; (260.2510) Fluorescence; (050.6624) Subwavelength structures. http://dx.doi.org/10.1364/OL.39.002378
Fluorescence detection has become a dominant technology in analytical chemistry [1,2], biosciences [3], medical diagnosis [4], and sensing [5]. Recently, surface plasmon polaritons (SPPs) or localized plasmon resonance have been widely used to enhance the fluorescence of dye molecules [6–13]. Techniques that enhance fluorescence emission [14,15] can reduce measurement times, increase sensitivity, or reduce the amount of material required for accurate detection in medical diagnostics and biotechnology today. Meanwhile, much attention has been paid to the radiative decay engineering by the metal-dielectric-metal (MDM) cavity structure. The MDM structure can effectively change the fluorescence emission characters by modifying the quantum yields, anisotropy, and directionality of the emission [16–25]. Owing to the efficient concentration and storage of electromagnetic energy at certain wavelengths, a planar microcavity would produce a great field enhancement and, consequently, has been widely used for numerous light emission and detection applications [26,27]. In this Letter, we demonstrate that the emission from randomly oriented dye molecules can be precisely tuned by a subwavelength Ag-PMMA-Ag cavity structure with a 1D periodic Ag grating on the top. This structure can support both the Fabry–Perot (FP) cavity mode as well as the SPP mode that can couple together by tuning the thickness of the center PMMA film, the period, groove width, or groove depth of the 1D grating. At the coupling, the full width at half-maximum (FWHM) of the fluorescence spectra becomes the narrowest, the fluorescence intensity is the largest, and even amplified spontaneous emission can be easily realized. The results pave the way for realizing the lasing of dye molecules [28]. The fabrication process of our samples can be described as follows. First, three layers of film were successively coated on the substrate [Fig. 1(a)]. A 50 nm Ag film 0146-9592/14/082378-04$15.00/0
was coated on a well cleaned silicon substrate by e-beam evaporation, and then the C33 H33 ClN2 O9 dye-doped polymethylmethacrylate (PMMA) film with molar concentration of about 1 × 10−3 mol∕L was spin coated on it at 4000 rpm. The resultant thickness of the PMMA layer is about 185 nm. After that, about 250 nm Ag film was coated further. Second, an A-B paste was used to bind the top Ag film with another silicon substrate. After the A-B paste was solidified at room temperature for more than 24 h, the three layer film was then peeled off from the bottom [Fig. 1(b)]. The Ag surface roughness can reach several nanometers with the peeling off process, leading to less propagation loss. Third, a 1D
Fig. 1. (a)–(c) Schematic diagram of the fabricating process of the three-layer structure with a periodic grating on the top. (d) The fluorescence spectra of the dye molecule. The inset shows the molecule structure and formula. (e) The scanning electron microscope (SEM) image of the multilayer structure at 52° visual angle. (f) The schematic diagram of the enlarged cross section corresponding to the red dashed part in (e). © 2014 Optical Society of America
April 15, 2014 / Vol. 39, No. 8 / OPTICS LETTERS
periodic grating was fabricated on the Ag film by a focused ion beam system (strata FIB 201, 69 FEI Co., 30 keV Ga ions) [Fig. 1(c)]. Figure 1(d) presents a typical fluorescence spectrum of C33 H33 ClN2 O9 dye molecules. The SEM image of one sample is shown in Fig. 1(e). The structure parameters are depicted in Fig. 1(f). Although the excitation wavelength influences the fluorescence performance [29], our attention is mainly paid to the coupling effect between FP cavity mode and SPP mode within the MDM structure at the emission wavelength of the C33 H33 ClN2 O9 dye molecules. We simulated the dispersion relation of the Ag-PMMA-Ag cavity structure with a 1D periodic Ag grating on the top by using 3D finite-difference time-domain (FDTD) software (Lumerical FDTD Solutions). When the thickness of the PMMA film was set to be 185 nm, a FP cavity mode occurred in the center emission wavelength of the C33 H33 ClN2 O9 dye molecule, around 695 nm. We designed two different grating periods, P 650 nm and P 680 nm, to represent the interaction between the FP cavity mode and the SPP band edge mode. In Fig. 2, a horizontal line corresponds to a FP cavity mode, and others SPP band edge modes. It is well known that there is a quite large SPP density of states (SPP DOS) at the band edge, and the SPP DOS will increase with the band gap, which can be tuned by the groove width and depth. To obtain a large band gap (about 30 nm width) the groove width was set to 170 nm. The experimental dispersion curves were obtained by mapping the reflectivity in a plane conjugate to the Fourier (back focal) plane of the objective with an angular resolution of 1° [8]. Our experimental results of the dispersion relation [Figs. 2(c) and 2(d)] were in good agreement with the simulated ones [Figs. 2(a) and 2(b)]. In this work, the PMMA film was made a constant thickness, thus the FP cavity mode was located at a wavelength of around 695 nm, while the position of the SPP band gap modes was tuned by the grating period and the groove depth. When P 650 nm, the dispersion curve of the FP cavity mode overlaps with the upper SPP band edge at λ 695 nm. The calculated field distribution at zero angle is shown in Fig. 2(e); the field intensity is quite large due to the strong coupling between the FP cavity mode and the SPP mode. By comparison, when P 680 nm, the upper band edge of the SPPs redshifted
Fig. 2. (a), (b) Simulated and (c), (d) experimental results of dispersion relation for structures with the same groove depth, h 25 nm, but different grating periods. (a), (c) for P 650 nm, and (b), (d) for P 680 nm. (e)–(g) show the simulated field distribution corresponding to the coupling or decoupling of the FP cavity mode and the SPPs mode at normal incidence. (e) for P 650 nm, and (f), (g) for P 680 nm.
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to 725 nm, resulting in the decoupling of the dispersion curves of the FP cavity mode and SPP modes. According to the calculated field distribution of the two modes at zero angle, the field at 695 nm wavelength [Fig. 2(f)] is mainly confined within the cavity layer, while the field at 725 nm wavelength [Fig. 2(g)] on the top grating. The coupling of the two modes can not only provide a large field intensity but also enable the emission of the dye in the cavity layer transfer across the top Ag film. The emission properties of the dye molecules embedded in the Ag-PMMA-Ag cavity structure with a 1D Ag grating were investigated by microphotoluminescence (μ-PL) spectroscopy. An argon-ion laser at λ 488 nm, as an excitation source, was focused by an optical microscope objective onto the sample, and the emission was collected by the same objective. The μ-PL measurements were performed for samples with different grating depth, h, but the same period, P 650 nm, groove width, d P∕4, and top Ag thickness of around 50 nm. We chose the top Ag grating layer to be about 50 nm in the experiments, considering the excitation can be across the top Ag grating. The strongest emission takes place when the grating depth, h, is about 25 nm. This is due to the strong coupling between the FP cavity mode and the SPP mode. For the gratings with h 25 nm, the upper band edge of SPP mode lies in 695 nm, shown in Fig. 2(a). Therefore, the FP cavity mode and the SPP mode overlap at the emission wavelength of the dye molecules. As is shown in Fig. 3(a), the fluorescence spectra of h 0, h 25 nm, and on silica are all centered at 695 nm. Compared to the other h
Fig. 3. (a) Fluorescence spectra of the structures with different groove depth, h; the thickness of the center PMMA layer is fixed at 185 nm and the grating period P 650 nm. The inset shows the FWHM at various h values. (b) The experimental (dotted line) and simulated (solid line) extinction spectra of the MDM structure with h 15, 25 and 35 nm.
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values, the optimized structure of h 25 nm not only shows the largest fluorescence enhancement ratio of 32-fold, but also the narrowest FWHM of 15 nm. The band gap of a 1D periodic grating will be broadened with the groove depth, thus, the upper band edge will move toward, cross over, and move away from the cavity mode curve during the broadening process of the SPP band gap. The emission peak position then redshifts gradually with grating depth. That is, the movement of the SPP band gap curves determines the redshift result. The separation of the SPP band edge and the FP cavity mode results in the decrease of the fluorescence intensity, accompanied by redshifted or blueshifted emission spectra and increased FWHM. The extinction spectra of the MDM structure with h 15, 25, and 35 nm shown in Fig. 3(b) may help to understand the above fluorescence behavior. It can be seen that the resonance wavelength of the structure with h 25 nm is located at 695 nm while those with h 15 and 35 nm are blue/redshifted, corresponding to the tendency shown in Fig. 3(a). To confirm the above SPP-involved coupling mechanism, we studied the polarization characters of the samples. The polarization-dependent measurements were carried out by a home-built fluorescence polarization microscope. To excite the dye, an incident wavelength of around 550 nm from a mercury lamp was selected. We take the sample with grating period P 650 nm and depth h 25 nm as an example. As revealed in Figs. 2(a) and 2(c), the FP cavity mode couples with the upper SPP band edge at 695 nm. The excitation and emission polarization-related integrated fluorescence intensities are shown in Fig. 4. The fluorescence intensity changes slightly with the excitation polarization direction, but remarkably with the emission polarization direction, and the maximum fluorescence intensity exists at the polarization direction perpendicular to the grating, which is consistent with the SPP generation condition. The excitation wavelength (centered at 550 nm) is far from the SPP wavelength (centered at 695 nm) of our interest, which causes the weak excitation polarization dependence. The sample was designed for a strong coupling between the FP cavity mode and the SPP mode at 695 nm. According to Fermi’s golden rule, the fluorescence will transfer mostly into the coupled mode, which
Fig. 4. Obtained fluorescence intensity at different polarization angles by a CCD on a Zeiss microscope. The red solid circles indicate the intensity of fluorescence collected at different incident angles, while the black solid squares denote the intensity of fluorescence collected at different output angles. The four square pictures are the fluorescence images obtained at different input or output angles.
Fig. 5. Threshold of structures measured at room temperature for different groove depth, h.
possesses a large photonic DOS [Figs. 2(a) and 2(c)], and a strong field intensity [Fig. 2(e)]. In this case, the fluorescence generated at the central PMMA layer can be effectively outcoupled into far field via the top Ag grating. By calculating the ratio of the two orthogonal polarized intensities of the emission polarization case, I ⊥ − I B ∕I ∥ − I B ≈ 4.3, where I ⊥ , I ∥ , and I B denote the output intensity vertical to the grating, parallel to the grating, and the background intensity, respectively. It can be estimated that about 81% of the emitted energy of the dye molecules is transmitted into the coupled field mode (TM mode) and about 19% of the emitted energy of the dye molecules is transmitted into the TE mode. Many practical applications can be envisioned based on the coupling effect accompanying large photonic DOS and strong field intensity. Here, amplified spontaneous emission behavior of our proposed structure was surveyed, as shown in Fig. 5. The emission intensity rapidly increases with pump power when the power of incident light is above a threshold. As expected, the structure with grating depth h 25 nm shows the smallest threshold, of about 5.1 W∕cm2 . In summary, we proposed a planar MDM subwavelength cavity structure with resolved dye molecules embedded in the center and 1D periodic gratings on the top, and investigated the coupling effect in the MDM structure on the fluorescence emission of the dye. The structure parameters, such as the thickness of the dielectric layer, the grating period, and the groove width and depth can be used to adjust the emission spectrum of dye molecules. The smallest FWHM of the emission spectra and the lowest threshold of the amplified spontaneous emission were observed when the FP mode coupled with the SPP band gap edge mode. The authors would like to express thanks to Dr. Yonghong Ye (NJNU) for her support on the e-beam evaporation of the Ag film. We thank Dr. Wenchao Liu and Wei Wang for the efficient discussion of the fluorescence spectrum measurement. 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, 11174128 and 11374150) and by PAPD.
April 15, 2014 / Vol. 39, No. 8 / OPTICS LETTERS References 1. C. Zander, J. Enderlein, and R. A. Keller, Single Molecule Detection in Solution (Wiley, 2002). 2. H. Craighead, Nature 442, 387 (2006). 3. K. Bacia, S. A. Kim, and P. Schwille, Nat. Methods 3, 83 (2006). 4. M. Pitschke, R. Prior, M. Haupt, and D. Riesner, Nat. Med. 4, 832 (1998). 5. S. Wu, P. F. Guo, W. X. Huang, S. J. Xiao, and Y. Y. Zhu, J. Phys. Chem. C 115, 15205 (2011). 6. Y. H. Chen, J. F. Li, M. L. Ren, and Z. Y. Li, Small 8, 1355 (2012). 7. Y. H. Chen, J. F. Li, M. L. Ren, B. L. Wang, J. X. Fu, S. Y. Liu, and Z. Y. Li, Appl. Phys. Lett. 98, 261912 (2011). 8. J. J. Jiang, F. Xu, Y. B. Xie, X. M. Tang, Z. Y. Liu, X. J. Zhang, and Y. Y. Zhu, Opt. Lett. 38, 4570 (2013). 9. E. Fort and S. Grésillon, J. Phys. D 41, 013001 (2008). 10. Y. X. Zhang, K. Aslan, and M. J. R. Previte, Appl. Phys. Lett. 90, 053107 (2007). 11. E. Hwang, I. I. Smolyaninov, and C. C. Davis, Nano Lett. 10, 813 (2010). 12. H. Aouani, O. Mahboub, E. Devaux, H. Rigneault, T. W. Ebbesen, and J. Wenger, Nano Lett. 11, 2400 (2011). 13. J. A. Schuller, E. S. Barnard, W. Cai, Y. C. Jun, J. S. White, and M. L. Brongersma, Nat. Mater. 9, 193 (2010). 14. J. Wenger, D. Gérard, H. Aouani, and H. Rigneault, Anal. Chem. 81, 834 (2009). 15. J. Wenger, D. Gérard, J. Dintinger, O. Mahboub, N. Bonod, E. Popov, T. W. Ebbesen, and H. Rigneault, Opt. Express 16, 3008 (2008).
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16. Y. Jin, J. Feng, X. L. Zhang, Y. G. Bi, Y. Bai, L. Chen, T. Lan, Y. F. Liu, Q. D. Chen, and H. B. Sun, Adv. Mater. 24, 1187 (2012). 17. M. A. Schmidt, D. Y. Lei, L. Wondraczek, V. Nazabal, and S. A. Maier, Nat. Commun. 3, 1108 (2012). 18. S. D. Choudhury, R. Badugu, K. Nowaczyk, K. Ray, and J. R. Lakowicz, J. Phys. Chem. Lett. 4, 227 (2013). 19. A. Chizhik, F. Schleifenbaum, R. Gutbrod, A. Chizhik, D. Khoptyar, and A. J. Meixner, Phys. Rev. Lett. 102, 073002 (2009). 20. V. J. Sorger, R. F. Oulton, J. Yao, G. Bartal, and X. Zhang, Nano Lett. 9, 3489 (2009). 21. A. Muller, E. B. Flagg, P. Bianucci, X. Y. Wang, D. G. Deppe, W. Ma, J. Zhang, G. J. Salamo, M. Xiao, and C. K. Shih, Phys. Rev. Lett. 99, 187402 (2007). 22. A. Mitra, H. Harutyunyan, S. Palomba, and L. Novotny, Opt. Lett. 35, 953 (2010). 23. M. Kuttge, F. J. G. de Abajo, and A. Polman, Nano Lett. 10, 1537 (2010). 24. W. Lukosz, J. Opt. Soc. Am. 69, 1495 (1979). 25. M. Lieberherr, C. Fattinger, and W. Lukosz, Surf. Sci. 189–190, 954 (1987). 26. J. M. Gérard, B. Sermage, B. Gayral, B. Legrand, E. Costard, and V. Thierry-Mieg, Phys. Rev. Lett. 81, 1110 (1998). 27. S. Hayashi, Y. Ishigaki, and M. Fujii, Phys. Rev. B 86, 045408 (2012). 28. O. Zaitsev and L. Deych, J. Opt. 12, 024001 (2010). 29. 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).
OPTICS LETTERS / Vol. 39, No. 8 / April 15, 2014
Amplified spontaneous emission via the coupling between Fabry–Perot cavity and surface plasmon polariton modes Jian-Juan Jiang,1,2 Yu-Bo Xie,1,2 Zheng-Yang Liu,1,2 Xia-Mei Tang,1,2 Xue-Jin Zhang,1,3,4 and Yong-Yuan Zhu1,2,5 1
Key Laboratory of Modern Acoustics and National Laboratory of Solid State Microstructures, Nanjing University, Nanjing 210093, China 2 School of Physics, Nanjing University, Nanjing 210093, China 3
College of Engineering and Applied Sciences, Nanjing University, Nanjing 210093, China 4
e-mail: [email protected] 5 e-mail: [email protected]
Received January 13, 2014; revised March 14, 2014; accepted March 15, 2014; posted March 17, 2014 (Doc. ID 204635); published April 9, 2014 We demonstrated amplified spontaneous emission by embedding dye molecules within a dielectric layer of a metal-dielectric-metal subwavelength structure. It was reinforced when a strong coupling occurred between the Fabry–Perot mode supported by the dielectric layer and the surface plasmon polariton mode supported by the adjacent metallic grating. Here, we adjust the two mode interaction via tuning the depth of the metallic grating grooves. The stronger the interaction, the smaller the full width at half-maximum of the emission spectra and the lower the threshold of the amplified spontaneous emission. © 2014 Optical Society of America OCIS codes: (050.2230) Fabry-Perot; (050.2770) Gratings; (140.4480) Optical amplifiers; (240.6680) Surface plasmons; (260.2510) Fluorescence; (050.6624) Subwavelength structures. http://dx.doi.org/10.1364/OL.39.002378
Fluorescence detection has become a dominant technology in analytical chemistry [1,2], biosciences [3], medical diagnosis [4], and sensing [5]. Recently, surface plasmon polaritons (SPPs) or localized plasmon resonance have been widely used to enhance the fluorescence of dye molecules [6–13]. Techniques that enhance fluorescence emission [14,15] can reduce measurement times, increase sensitivity, or reduce the amount of material required for accurate detection in medical diagnostics and biotechnology today. Meanwhile, much attention has been paid to the radiative decay engineering by the metal-dielectric-metal (MDM) cavity structure. The MDM structure can effectively change the fluorescence emission characters by modifying the quantum yields, anisotropy, and directionality of the emission [16–25]. Owing to the efficient concentration and storage of electromagnetic energy at certain wavelengths, a planar microcavity would produce a great field enhancement and, consequently, has been widely used for numerous light emission and detection applications [26,27]. In this Letter, we demonstrate that the emission from randomly oriented dye molecules can be precisely tuned by a subwavelength Ag-PMMA-Ag cavity structure with a 1D periodic Ag grating on the top. This structure can support both the Fabry–Perot (FP) cavity mode as well as the SPP mode that can couple together by tuning the thickness of the center PMMA film, the period, groove width, or groove depth of the 1D grating. At the coupling, the full width at half-maximum (FWHM) of the fluorescence spectra becomes the narrowest, the fluorescence intensity is the largest, and even amplified spontaneous emission can be easily realized. The results pave the way for realizing the lasing of dye molecules [28]. The fabrication process of our samples can be described as follows. First, three layers of film were successively coated on the substrate [Fig. 1(a)]. A 50 nm Ag film 0146-9592/14/082378-04$15.00/0
was coated on a well cleaned silicon substrate by e-beam evaporation, and then the C33 H33 ClN2 O9 dye-doped polymethylmethacrylate (PMMA) film with molar concentration of about 1 × 10−3 mol∕L was spin coated on it at 4000 rpm. The resultant thickness of the PMMA layer is about 185 nm. After that, about 250 nm Ag film was coated further. Second, an A-B paste was used to bind the top Ag film with another silicon substrate. After the A-B paste was solidified at room temperature for more than 24 h, the three layer film was then peeled off from the bottom [Fig. 1(b)]. The Ag surface roughness can reach several nanometers with the peeling off process, leading to less propagation loss. Third, a 1D
Fig. 1. (a)–(c) Schematic diagram of the fabricating process of the three-layer structure with a periodic grating on the top. (d) The fluorescence spectra of the dye molecule. The inset shows the molecule structure and formula. (e) The scanning electron microscope (SEM) image of the multilayer structure at 52° visual angle. (f) The schematic diagram of the enlarged cross section corresponding to the red dashed part in (e). © 2014 Optical Society of America
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periodic grating was fabricated on the Ag film by a focused ion beam system (strata FIB 201, 69 FEI Co., 30 keV Ga ions) [Fig. 1(c)]. Figure 1(d) presents a typical fluorescence spectrum of C33 H33 ClN2 O9 dye molecules. The SEM image of one sample is shown in Fig. 1(e). The structure parameters are depicted in Fig. 1(f). Although the excitation wavelength influences the fluorescence performance [29], our attention is mainly paid to the coupling effect between FP cavity mode and SPP mode within the MDM structure at the emission wavelength of the C33 H33 ClN2 O9 dye molecules. We simulated the dispersion relation of the Ag-PMMA-Ag cavity structure with a 1D periodic Ag grating on the top by using 3D finite-difference time-domain (FDTD) software (Lumerical FDTD Solutions). When the thickness of the PMMA film was set to be 185 nm, a FP cavity mode occurred in the center emission wavelength of the C33 H33 ClN2 O9 dye molecule, around 695 nm. We designed two different grating periods, P 650 nm and P 680 nm, to represent the interaction between the FP cavity mode and the SPP band edge mode. In Fig. 2, a horizontal line corresponds to a FP cavity mode, and others SPP band edge modes. It is well known that there is a quite large SPP density of states (SPP DOS) at the band edge, and the SPP DOS will increase with the band gap, which can be tuned by the groove width and depth. To obtain a large band gap (about 30 nm width) the groove width was set to 170 nm. The experimental dispersion curves were obtained by mapping the reflectivity in a plane conjugate to the Fourier (back focal) plane of the objective with an angular resolution of 1° [8]. Our experimental results of the dispersion relation [Figs. 2(c) and 2(d)] were in good agreement with the simulated ones [Figs. 2(a) and 2(b)]. In this work, the PMMA film was made a constant thickness, thus the FP cavity mode was located at a wavelength of around 695 nm, while the position of the SPP band gap modes was tuned by the grating period and the groove depth. When P 650 nm, the dispersion curve of the FP cavity mode overlaps with the upper SPP band edge at λ 695 nm. The calculated field distribution at zero angle is shown in Fig. 2(e); the field intensity is quite large due to the strong coupling between the FP cavity mode and the SPP mode. By comparison, when P 680 nm, the upper band edge of the SPPs redshifted
Fig. 2. (a), (b) Simulated and (c), (d) experimental results of dispersion relation for structures with the same groove depth, h 25 nm, but different grating periods. (a), (c) for P 650 nm, and (b), (d) for P 680 nm. (e)–(g) show the simulated field distribution corresponding to the coupling or decoupling of the FP cavity mode and the SPPs mode at normal incidence. (e) for P 650 nm, and (f), (g) for P 680 nm.
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to 725 nm, resulting in the decoupling of the dispersion curves of the FP cavity mode and SPP modes. According to the calculated field distribution of the two modes at zero angle, the field at 695 nm wavelength [Fig. 2(f)] is mainly confined within the cavity layer, while the field at 725 nm wavelength [Fig. 2(g)] on the top grating. The coupling of the two modes can not only provide a large field intensity but also enable the emission of the dye in the cavity layer transfer across the top Ag film. The emission properties of the dye molecules embedded in the Ag-PMMA-Ag cavity structure with a 1D Ag grating were investigated by microphotoluminescence (μ-PL) spectroscopy. An argon-ion laser at λ 488 nm, as an excitation source, was focused by an optical microscope objective onto the sample, and the emission was collected by the same objective. The μ-PL measurements were performed for samples with different grating depth, h, but the same period, P 650 nm, groove width, d P∕4, and top Ag thickness of around 50 nm. We chose the top Ag grating layer to be about 50 nm in the experiments, considering the excitation can be across the top Ag grating. The strongest emission takes place when the grating depth, h, is about 25 nm. This is due to the strong coupling between the FP cavity mode and the SPP mode. For the gratings with h 25 nm, the upper band edge of SPP mode lies in 695 nm, shown in Fig. 2(a). Therefore, the FP cavity mode and the SPP mode overlap at the emission wavelength of the dye molecules. As is shown in Fig. 3(a), the fluorescence spectra of h 0, h 25 nm, and on silica are all centered at 695 nm. Compared to the other h
Fig. 3. (a) Fluorescence spectra of the structures with different groove depth, h; the thickness of the center PMMA layer is fixed at 185 nm and the grating period P 650 nm. The inset shows the FWHM at various h values. (b) The experimental (dotted line) and simulated (solid line) extinction spectra of the MDM structure with h 15, 25 and 35 nm.
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values, the optimized structure of h 25 nm not only shows the largest fluorescence enhancement ratio of 32-fold, but also the narrowest FWHM of 15 nm. The band gap of a 1D periodic grating will be broadened with the groove depth, thus, the upper band edge will move toward, cross over, and move away from the cavity mode curve during the broadening process of the SPP band gap. The emission peak position then redshifts gradually with grating depth. That is, the movement of the SPP band gap curves determines the redshift result. The separation of the SPP band edge and the FP cavity mode results in the decrease of the fluorescence intensity, accompanied by redshifted or blueshifted emission spectra and increased FWHM. The extinction spectra of the MDM structure with h 15, 25, and 35 nm shown in Fig. 3(b) may help to understand the above fluorescence behavior. It can be seen that the resonance wavelength of the structure with h 25 nm is located at 695 nm while those with h 15 and 35 nm are blue/redshifted, corresponding to the tendency shown in Fig. 3(a). To confirm the above SPP-involved coupling mechanism, we studied the polarization characters of the samples. The polarization-dependent measurements were carried out by a home-built fluorescence polarization microscope. To excite the dye, an incident wavelength of around 550 nm from a mercury lamp was selected. We take the sample with grating period P 650 nm and depth h 25 nm as an example. As revealed in Figs. 2(a) and 2(c), the FP cavity mode couples with the upper SPP band edge at 695 nm. The excitation and emission polarization-related integrated fluorescence intensities are shown in Fig. 4. The fluorescence intensity changes slightly with the excitation polarization direction, but remarkably with the emission polarization direction, and the maximum fluorescence intensity exists at the polarization direction perpendicular to the grating, which is consistent with the SPP generation condition. The excitation wavelength (centered at 550 nm) is far from the SPP wavelength (centered at 695 nm) of our interest, which causes the weak excitation polarization dependence. The sample was designed for a strong coupling between the FP cavity mode and the SPP mode at 695 nm. According to Fermi’s golden rule, the fluorescence will transfer mostly into the coupled mode, which
Fig. 4. Obtained fluorescence intensity at different polarization angles by a CCD on a Zeiss microscope. The red solid circles indicate the intensity of fluorescence collected at different incident angles, while the black solid squares denote the intensity of fluorescence collected at different output angles. The four square pictures are the fluorescence images obtained at different input or output angles.
Fig. 5. Threshold of structures measured at room temperature for different groove depth, h.
possesses a large photonic DOS [Figs. 2(a) and 2(c)], and a strong field intensity [Fig. 2(e)]. In this case, the fluorescence generated at the central PMMA layer can be effectively outcoupled into far field via the top Ag grating. By calculating the ratio of the two orthogonal polarized intensities of the emission polarization case, I ⊥ − I B ∕I ∥ − I B ≈ 4.3, where I ⊥ , I ∥ , and I B denote the output intensity vertical to the grating, parallel to the grating, and the background intensity, respectively. It can be estimated that about 81% of the emitted energy of the dye molecules is transmitted into the coupled field mode (TM mode) and about 19% of the emitted energy of the dye molecules is transmitted into the TE mode. Many practical applications can be envisioned based on the coupling effect accompanying large photonic DOS and strong field intensity. Here, amplified spontaneous emission behavior of our proposed structure was surveyed, as shown in Fig. 5. The emission intensity rapidly increases with pump power when the power of incident light is above a threshold. As expected, the structure with grating depth h 25 nm shows the smallest threshold, of about 5.1 W∕cm2 . In summary, we proposed a planar MDM subwavelength cavity structure with resolved dye molecules embedded in the center and 1D periodic gratings on the top, and investigated the coupling effect in the MDM structure on the fluorescence emission of the dye. The structure parameters, such as the thickness of the dielectric layer, the grating period, and the groove width and depth can be used to adjust the emission spectrum of dye molecules. The smallest FWHM of the emission spectra and the lowest threshold of the amplified spontaneous emission were observed when the FP mode coupled with the SPP band gap edge mode. The authors would like to express thanks to Dr. Yonghong Ye (NJNU) for her support on the e-beam evaporation of the Ag film. We thank Dr. Wenchao Liu and Wei Wang for the efficient discussion of the fluorescence spectrum measurement. 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, 11174128 and 11374150) and by PAPD.
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