1. Introduction

Surface plasmon polaritons (SPPs) are light waves trapped at the metal/dielectric interface owing to its resonant interaction with conduction electrons at the metal surface [1]. There is a growing interest in the interactions between fluorophores and SPPs [2,3]. A particular area of investigation is on the phenomenon of surface plasmon-coupled emission (SPCE) [4], which occurs due to the localization of fluorophores near a thin silver film on a transparent substrate. Radiation from fluorophores transfers to SPPs modes supported by the silver film which then enters into the substrate at the surface plasmon resonance (SPR) angle (radial angle). As a result, the fluorescence emission will form a circular ring (SPCE ring). The SPCE displays not only strong directional emission but also unique polarization property (p-polarized for all the points on the SPCE ring), which is potentially useful as a sensing mechanism in some applications [5]. In previous studies, the SPCE was observed as circular ring of uniform intensity; where fluorophores emit at the SPR angle (radial angle) with equal intensity distribution in all azimuthal directions. In these experiments, the fluorophores were dispersed in a planar thin film with random orientations [4–7]. In contrast, for directionally oriented fluorescence molecules, the SPCE pattern exhibited an anisotropic intensity distribution [8–10]. The SPCE ring pattern represents the emitting directions of the fluorescence. In this paper, we propose an interesting method to get different intensity distribution on the SPCE ring with the shape of PMMA film although the doped fluorescence molecules are of random orientation, which are different from the complete films or solution used in all the reported SPCE experiments [4–10]. By using the shaped PMMA film, the fluorescence will emit stronger at certain azimuthal angles with the fixed radial direction determined by the SPR effect, which will enhance the fluorescence collection efficiency and has applications in fluorescence related sensing or imaging.

2. Sample preparation and experiment

The samples were prepared in the following manner. A sample of Rhodamine B (RhB) powder (0.1mg/ml) was dissolved in a PMMA solution (950K PMMA, 2% in Anisole) and the solution was agitated by ultrasonic disrupter for 30 minutes. Subsequently it was allowed to stand for 48 hours, to ensure that the RhB molecules dissolved totally in solution. The RhB doped complete PMMA film was obtained by spin coating the solution onto a 45 nm-thick silver substrate and the film baked for 10min at 105°C to remove the solvent. The thickness of the PMMA films was about 80nm. An Electron Beam Lithography method (Raith GmbH, e_LiNe) was subsequently used to write different PMMA structured films on the silver substrate.

Leakage radiation microscopy (LRM) [11–14] was used to characterize the SPCE by observing fluorescence in real and reciprocal spaces respectively. The schematic of the experimental set-up was shown in Fig. 1 . A laser beam with 532 nm wavelength was expanded by a lens array to fully fill the rear aperture of an oil-immersion objective (60X, numerical aperture (N.A.), 1.42). The laser beam was tightly focused onto the shaped PMMA films to excite the doped RhB molecules. A pellicle beam-splitter (from Thorlab Corp, working wavelength 400-700nm) and a long pass filter (from SEM ROCK) were used to observe the fluorescence on the CCD. By changing the position of the CCD and the focal length of the imaging lens, we can get both the back focal plane (BFP, also named Fourier plane image) and the image plane images (also named direct-space image) of the objective. The image plane directly gives out the image of the samples and also the fluorescence, whereas, the back focal plane gives out the wave-vector information. The diameter of SPCE ring (BFP image) represents the radial angle of fluorescence emission. The intensity distributions on the ring represent the fluorescence distribution at the azimuthal angles. A fiber illuminator was used to illuminate the samples, so that bright field transmittance images of the samples can be obtained. More details on the characterization of SPCE or SPPs with LRM can be found in References [15–17].

 

Fig. 1 Schematic of the experimental set-up, leaky radiation microscopy.

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3. Result and discussion

3.1. Effect of shape

A circularly polarized laser beam was initially used as the excitation source due to its circular symmetry in exciting SPPs on the metallic film. A triangularly shape PMMA film was selected to investigate the shape effect on the SPCE pattern. The sides of the equilateral triangle are 10 µm. Figure 2(a) gives the bright field transmission image acquired directly by a CCD camera without a long pass edge filter to reject the excitation beam. The laser beam was attenuated so that the focal point of the excitation beam can be imaged without saturating the CCD. Figure 2(b) shows the direct space fluorescence image using a laser power of 0.2 mW with the long pass filter placed before the CCD to reject the excitation beam. The figure shows that there are attenuated waves propagating on the bare Ag film near the three sides of the triangle. Since the fluorescence molecules were only doped in the PMMA film, the attenuated waves can only be attributed to the SPPs-1 wave (SPPs on the interface of Air/Ag/Glass) generated by the excited fluorescence molecules [15,17]. The SPPs-1 waves marked with 1, 2, and 3 propagate mainly along the direction perpendicular to triangle side and attenuate during propagation. The SPPs-2 waves (SPPs on the Air/PMMA/Ag /Glass interface) cannot be clearly imaged because the excitation area is also the propagating area, and it is difficult to distinguish the excitation source and the propagating waves [11]. In this Letter, we mainly investigate the behavior of SPCE related to the SPPs-1 waves.

 

Fig. 2 (a): Bright-field transmission image of triangular PMMA film acquired by the CCD camera without filter. The central bright spot is the focused laser spot. (b) direct-space fluorescence image; (c) Fourier plane fluorescence image. White boxes marked with 1, 2, 3 represent the stronger area on the SPCE ring.

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Figure 2(c) demonstrates the corresponding Fourier plane fluorescence image. It shows the commonly known SPCE ring: the inner ring is caused by the SPPs-1 waves and the outer by the SPPs-2 waves. The two rings indicate that the fluorescence is mainly emitted at two angles (radial angle). The emitting angles can be estimated as about 43° and 69°, this corresponds to the calculated SPR angles (43.6°, 69.4°, n = 0.12 + 3.547i for Ag) at the peak wavelength of RhB fluorescence (576 nm). The white boxes labeled with 1, 2, 3 are three brighter arcs on the inner ring, indicating that the SPCE intensity is anisotropic in the azimuthal direction. The ratio of maximum intensity to the minimum intensity on the SPCE ring shown in Fig. 2(c) is about 2.3. This is consistent with the findings in Fig. 2(b), which show that the propagation and intensity distribution of the SPPs-1 imaged in the real space are correspondingly reflected in the reciprocal space, such as the 1, 2, and 3 areas marked in Figs. 2(b) and 2(c).

It should be noted that the fluorescence intensity in the area marked with box-1 is stronger than that marked with box-2 and 3. The reason is related with the unique properties of the SPCE fluorescence. Because the SPCE is the inverse process of SPPs excitation, and since only p-polarized beam are able to excite the SPPs on a thin metallic film, the polarization of the fluorescence on the SPCE ring is along the radial direction of the ring as illustrated in Fig. 1. This has been verified by previous experiments [5,16]. Furthermore as the reflection efficiency of the beam-splitter for s-polarized beam is larger than that of p-polarized, the fluorescence on the upper and bottom portions of the ring have both p-polarized and s-polarized components relative to the plane of beam splitter. However as the fluorescence in the middle of the ring is entirely s-polarized, therefore the fluorescence collection efficiency by the CCD on the upper and lower sides are weaker than that in the middle. As a result, the intensity of area 1 is stronger than that of area 2 and 3.

As a comparison, a circularly shape PMMA film doped with RhB molecules was investigated and shown in Fig. 3 . Figure 3(a) is the bright field transmission image observed without the filter and the center bright spot is the location of the focused laser spot on the PMMA film. Figure 3(b) is the direct space fluorescence image, which shows that the excited SPPs-1 wave propagating along the radial directions of the circular. Figure 3(c) is the corresponding Fourier plane (back focal plane shown in Fig. 1 fluorescence image. The white boxes labeled with 1, 2, 3 are at the sample positions as that of Fig. 2(c). When comparing Fig. 2(c) with Fig. 3(c) in the regions marked with the white boxes, we find that the fluorescence intensity distributions on the SPCE ring are different. The ratio of maximum intensity to the minimum intensity on the SPCE ring shown in Fig. 3(c) is about 1.2, which is mainly due to the different reflectivity of different polarizations relative to the plane of the beam-splitter. So experimental results verify that different SPCE intensity pattern can be obtained with different shape of the PMMA films on silver films.

 

Fig. 3 (a): Bright-field transmission image of circular PMMA film acquired by the CCD camera without long pass filter. The central bright spot is the focused laser spot. (b) direct-space fluorescence image; (c) Fourier plane fluorescence image.

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A point to note in the case of circular shape is that when the PMMA films are resolved around the beam axis of the objective and the irradiated position was at the center of the circular PMMA film, the intensity variations of SPCE ring was too small to be distinguished by the CCD camera. This is a verification that the RhB molecules doped in the PMMA film are of random orientations.

3.2. Effect of irradiation location

The intensity distribution of the SPCE ring can be modified by focusing the laser beam at different locations on the shaped PMMA film. Figure 4(a) shows the bright field transmission images focused at one corner of a square PMMA film. Figures 4(b) and 4(c) are the corresponding direct space and Fourier plane fluorescence images respectively. We see that the excited SPPs-1 waves propagate onto the air side and diverge away from the corner. The areas marked with 1 and 2 are the two stronger SPPs-1 waves with different propagating directions, corresponding to the two brighter arcs marked with white boxes (1 and 2) in Fig. 4(c) respectively. Figures 4(d), 4(e), and 4(f) indicate another case where the laser focus is located midway along the side of the structure. We observe that the SPPs-1 waves are strongest in the mark 3 of Fig. 4(e) and propagate mainly along the direction perpendicular to the side of the square as shown in Fig. 4(e). In Fig. 4(f), the strongest arc is marked with 3, corresponding to the strongest area as marked 3 in Fig. 4(e). Figures 4(g), 4(h), and 4(i) are the case with focal point on the up-side of the square PMMA film. Figure 4 (h) show that SPPs-1 wave are stronger in the up of the PMMA film marked with 4. In the corresponding Fourier plane image shown in Fig. 4(i), the fluorescence intensity on the top part is stronger than that in the middle part (white boxes named 4 and 5) and others parts on the ring, which is consistent with Fig. 4(h). The ratios of maximum intensity to the minimum intensities on the SPCE ring shown in Figs. 4(c), 4(f), and 4(i) are about 6.3, 3.5 and 6.5 respectively. Figure 4 demonstrates that for given shape, different irradiated position related to the shaped PMMA film give out different fluorescence intensity distributions on the SPCE ring as shown in Figs. 4(c), 4(f), and 4(i).

 

Fig. 4 Different irradiated position related to the square PMMA film. (a), (d) and (g): Bright-field transmission image of square shape PMMA film acquired by the CCD camera without long pass edge filter. The central bright point is the focused laser spot. (b), (e) and (h) are direct-space fluorescence images; (c), (f) and (i) are the Fourier plane fluorescence images.

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3.3. Polarization effect

In this section we investigate the influence of polarization on the emission profile of the SPCE. In the previous experiments, only circular polarized excitation beam was used. As a comparison, linear polarized light with different polarized directions were used to excite the RhB molecules doped in the square PMMA films. In this section, we only observe the direct space image, which is consistent with the Fourier plane image. The excitation beam is focused onto the centre of the square PMMA film. The excitation laser beam of Figs. 5(a) , 5(b), and 5(c) are circularly polarized, vertically and horizontally linearly-polarized respectively. Figure 5(a) shows that the intensity distribution of the SPPs-1 waves at the four sides of the square are the same. The propagating directions of the SPPs-1 waves are mainly perpendicular to the four square side. This is due to the shape effect of the same mechanism as shown in Fig. 2. Figures 5(b) and 5(c) display that the SPPs-1 waves are stronger along the direction of polarization. The intensity distribution and propagating patterns of the SPPs-1 waves along the polarized direction are different from the perpendicular direction, which can be attributed to the different intensity distributions of the focused linearly and circularly polarized beams [12,18]. Figure 5 displays that for a given shape, different polarization of the incident laser beam also can give out different SPPs-1 wave patterns, which means different intensity distribution of SPCE rings.

 

Fig. 5 Different polarization of the excitation beam. Direct space fluorescence image with circularly polarized (a), vertically linearly polarized (b) and horizontally linear polarized excitation beam focused onto the center of the square PMMA film.

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4. Conclusion

In summary, different fluorescence intensity distribution from the SPCE ring and propagation of SPPs on silver film were realized by using shaped PMMA films doped with RhB molecules and coated on the silver films. Various SPCE intensity ring patterns can be obtained by different PMMA shape fabricated by lithography techniques, furthermore the polarization of the incident light and location of irradiation can be used to modulate the intensity patterns. This phenomenon can attributed to the different modes excited in the two kind layers, one is the Air/Ag/glass, another is Air/PMMA/Ag/glass, which have different SPR angles [16,17]. Our experiments give out a new method to engineer the fluorescence emitting directions, which can enhance the fluorescence collection efficiency for sensing or imaging and has applications in radiative decay engineering [5,6]. With proper designed structure of the PMMA film doped with fluorescence, the fluorescence could be controlled to emit at a particular direction, just like a collimated laser beam, which can be used in fluorescence related imaging or sensing.