Exciting surface plasmons on metal-coated ... - OSA Publishing

Letter

Vol. 41, No. 22 / November 15 2016 / Optics Letters

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Exciting surface plasmons on metal-coated multimode optical waveguides using skew rays CHUNYANG HAN,1,2 JOHN CANNING,1,* KEVIN COOK,1 MD ARAFAT HOSSAIN,1

AND

HUI DING2

1

Interdisciplinary Photonics Laboratories, School of Chemistry, The University of Sydney, NSW 2006, Australia State Key Laboratory of Electrical Insulation and Power Equipment, School of Electrical Engineering, Xi’an Jiaotong University, Xi’an 710049, China *Corresponding author: [email protected]

2

Received 31 August 2016; revised 18 October 2016; accepted 25 October 2016; posted 25 October 2016 (Doc. ID 274884); published 14 November 2016

Multipoint surface plasmon resonance (SPR) excitation using a skew ray within a multimode plastic optical waveguide coated with gold, Au, is reported. The effect of skew rays on the performance of SPR has been studied both theoretically and experimentally. The approach also entails a novel method of measuring the SPR angle that is in agreement with theoretically predicted values. © 2016 Optical Society of America OCIS codes: (060.2310) Fiber optics; (120.4570) Optical design of instruments; (130.5460) Polymer waveguides; (240.6680) Surface plasmons. http://dx.doi.org/10.1364/OL.41.005353

Optically excited surface plasmon resonance (SPR) in thin metal films is used in a large variety of physical, chemical, and biological applications [1–3]. The resonance condition required to generate a surface wave depends on the angle of incidence, the wavelength of light, the dielectric functions of the metal and dielectric, and the geometry of the film. Typically, large areas of metal thin film approximating endless 2D layers create the sea of electrons in which the wave travels. If the wavelength of light is kept constant and the angle of incidence changed, the incident light will be absorbed at a particular angle. This angle is the SPR angle, θSPR . The Kretschmann configuration is perhaps the most widely used platform [4]. In this prism-based configuration, the base of a prism is coated with a metal film, typically Au. Light is incident on the prism-metal interface through one of the sides of a high-index prism. The intensity of the reflected light through the prism is measured at different angles of incidence until resonance is observed. This type of structure has practical limitations, including bulky size, internal and unwanted reflections, and the presence of various optical and mechanical parts. For this reason, many researchers prefer to work with more robust, field-deployable all-fiber-based systems [5–8]. Optical fibers have advantages, such as a small core diameter, that allow for very small sample quantities, potentially simple and rugged 0146-9592/16/225353-04 Journal © 2016 Optical Society of America

optical design, and capability for online monitoring and remote sensing. A number of theoretical studies or experimental demonstrations have been carried out on optical-fiber-based SPR devices [5–8]. To simplify the analysis, these studies focus on meridional rays or special fiber structures and only occasionally mention skew rays. However, robust skew rays can be readily excited within multimode fibers, with a continuum range of launch conditions typically off core center. They are, for example, used in astronomical applications as ideal spectral filters [9]. So long as the waveguide quality is sufficient, unlike meridional rays these rays never cross the fiber axis and instead follow helical paths inside the fiber where at each point of incidence with the fiber outer surface and ambient material, they have identical angular conditions. Here, we propose the concept of multipoint SPR excitation at a metal-coated multimode waveguide structure using skew rays. This approach benefits from multiple simultaneous sampling of a material under a test, potentially enabling much higher sensitivity using SPR. It is also amenable to generating SPR fiber probe-based sensors. Therefore, the effect of skew rays on the performance of SPR has been studied both theoretically and experimentally. Experiments are conducted using novel polymer waveguides made from a 3D printer filament. The approach also entails a novel method of measuring the SPR angle that is in agreement with theoretically predicted values. Figure 1 illustrates the geometry of the proposed structure. It consists of two functional components. The inside is a multimode circular fiber made of a high-index polymer (PETG compounded, HD glass, n 1.52), and the outside is a gold layer deposited by sputter with a thickness of t 30 nm. This polymer has been previously used to fabricate optical fibers directly from 3D printed preforms [10,11]. Given the difficulty of launching the correct condition for SPR generation at various skew angles, the input and output ends of the fiber are polished into an angle to reduce unwanted back Fresnel reflections (Fig. 1). In this case, both ends of the fiber (ϕ 2.8 mm, L 5 cm) are polished into an angle, θ 45°, to ensure the SPR angle, θSPR , can be obtained inside the fiber. The skew rays within such an optical multimode fiber have an angle of

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Letter d . Here, α depends on the angle of the incidence of the light beam launched into the fiber, the refractive index, n, and θI ; d depends on the launch position of the light beam from the fiber center and the width of the laser beam. The angle of reflection of skew rays inside the fiber, φ, can be calculated via (

sin θi φ arccos cos 45° − arcsin n Fig. 1. Schematic of the proposed structure. The diameter of the fiber is φ ∼ 2.8 mm, and the thickness of the gold layer is t 30 nm. The interrogation light is λ 632.8 nm, and the polished angle is θ 45°.

reflection that is not only dependent on the angle of incidence but also on the distance of the projection of the ray path on the cross section of the fiber (related to its off-center position) under the condition that the polished angle keeps constant, as shown in Fig. 2. Here, the fiber is fixed on a translation stage so that both the position and angle of the incidence of the light θi can be adjusted. The monochromatic light beam (He–Ne laser: λ 632.8 nm) is launched into one end of the fiber using the stage to adjust the position and angle within the fiber. Under appropriate launch conditions when a surface wave is excited, and since the beam divergence over this length is negligible, a dark line will be observed inside the fiber at each point of contact with the surface. Depending on the launching condition within an optical fiber, the light source can excite both meridional and skew rays. The angular dependence can also be fine-tuned and controlled to optimize coupling and to adjust launch conditions into SPR resonances. In Fig. 2, line P-Q-R stands for the ray path inside the fiber, and angle φ is the angle of reflection of the skew ray. The angle between the ray and the axial direction is α. Skew rays follow a helical path in the fiber, in which the projection on the cross-section is a regular polygon (not necessarily closed). The midpoints between successive reflections all touch a cylindrical surface of radius d . The angle of reflection of skew rays inside a perfectly uniform and round optical fiber in the direction of travel is the angle of incidence to excite the SPR, i.e., φ θSPR . This angle is determined by two parameters: α and

Fig. 2. Representation of skew ray inside the fiber. The dark center line corresponds with that light at the correct θSPR absorbed by the Au metal film.

sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 2ffi) 2d × 1− : ϕ (1)

Here, ϕ represents the diameter of fiber. To better understand the nature of our structure, the angle of reflection of skew rays inside the fiber as a function of angle of incidence under different incident conditions was simulated and is depicted in Fig. 3. Here, the incident angle ranges from θi −45° to 135° since the polish angle in our structure is 45°. It is evident that the angle of reflection inside the fiber depends heavily on the off-center distance, d . Additionally, the adjustable range of reflect angles also relies on d . For d 0 (a meridional ray), the range is φ 10° to 84°, while the range is φ 59° to 87° when d 1.2 mm. This implies that the measurement ranges of this type of configuration can be adjusted easily by changing the incident position. In other words, it provides two ways to obtain a satisfactory angle by adjusting either the incident angle or the incident position. Since different angles of reflection along the fiber involve a different propagation distance between reflection, the distance between reflections must be directly related to this angle. Therefore, it is possible to extract the angle of reflection, and consequently the SPR angle, by directly measuring this periodic propagation distance. In practice, however, measuring the distance between adjacent reflections is still a difficult task because dispersion arising from the beam width creates some uncertainty in identifying the start and end points of the ray paths. Here, we propose and demonstrate a method to obtain θSPR by measuring the period of the skew ray path when the dark line is present. The error can be reduced by measuring multiple periods at one time.

Fig. 3. Angle of reflection, φ, inside the fiber as a function of angle of incidence, θi , under different launch positions. The distance between the projection of skew rays on the cross-section and the center of the cross-section is d , the launch position.


Letter

Fig. 4. Length of pitch of skew rays, Λ, as a function of the angle of reflection, φ, inside the fiber under different off-center positions.

Figure 4 gives the relationship between the angle of reflection inside the fiber and the length of the pitch of skew ray path under different incident conditions. From the figure we can see there is a one-to-one relationship between the angle and the length, so it is possible to get the value of angle by measuring the length of period. For each launch condition, d , there must be a curve in Fig. 4 that describes the relationship between the angle of reflection and the length of the pitch of skew rays. Therefore, the angle inside the fiber can be obtained via measuring the periodic length, Λ. The proposed concept was also demonstrated by experiment. In our experiment, both the input and output ends of a short section of fiber has been polished into an angle θ 45° [as shown in Fig. 5(a)]. The He–Ne monochromatic light beam was launched into one end of the polished fiber with a fixed off-center position d . The incident angle θi is adjusted until a dark line appears at the specific angle of incidence [as shown in Fig. 5(b)]. At this special condition, the angle of reflection inside the fiber equals the SPR condition, so φ θSPR . The distance between the adjacent dark line (the pitch of the skew ray path) is the pitch, Λ. For a given d , the SPR angle θSPR can be obtained from Fig. 4. In our experiments, the length of the pitch of the dark line was Λ 3.6 mm and Λ 12.5 mm when the cladding material was air and water, respectively, under condition d 0.6 mm. The θSPR obtained from Fig. 4 is θSPR 50° and θSPR 68°. Theoretically, with a fiber with an index of n 1.52 coated with t 30 nm Au, the SPR angles are θSPR ∼ 47° and 67° with air and water cladding, respectively (the calculation follows that in [12]). Within experimental error, and ignoring dispersion, these figures are in remarkably good agreement, demonstrating the practical utility of this approach for sensing. We further measured the length of the pitch of the dark line when the fiber has different claddings. Experimental results were recorded in Table 1. Here, ΛAir and ΛWater represent the length of the pitch of dark lines when the fiber is surrounded by air and water, respectively. We can see from the table that in some conditions no dark line would be seen since there is no way to obtain a satisfactory SPR angle. To determine the experimental result, we also compared these data with theoretical analyses.

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Fig. 5. Several optical microscope images of the proposed structure: (a) one end of the polished fiber where the polished angle is θ ∼ 45°; (b) the skew rays inside the fiber and the dark lines caused by SPR can be seen; (c) when the cladding is air, the pitch is about Λ 3.5 mm; and (d) when one side of the fiber is covered with water and the other half with air, two different values are measured. One is caused by the air cladding and the other caused by the water cladding.

Theoretically, when the ambient material is selected the SPR angle θSPR is determined accordingly. Hence, the length of pitch, Λ, of the dark line depends only on the incident conditions (off-center position d as shown in Fig. 2), because the angle of reflection inside the fiber should be a constant value, which is always equal to θSPR . Otherwise no SPR would be excited and thus no dark line would appear. Therefore, we can get the relationship between d and Λ if the ambient material is selected. Comparing this relationship to the simulated curve, we can quantifiably assess the quality of the method we proposed for measuring θSPR . Figure 6 compares the theoretical calculations and experimental results. The solid line and dashed line show the length of the pitch of dark lines versus the off-center position of skew rays, d , when the cladding is air and water, respectively. The squares and triangles in the figure represent the experiment measurement results. From the figure we can find that the method to measure the SPR angle, θSPR , inside the fiber is reliable because the experimental results match the theoretical calculations well. We should note that when d is too large, the SPR cannot be excited since the SPR angle, θSPR , cannot be obtained inside the fiber. This is the reason why the length of the pitch of dark lines turns to zero when d is larger than a certain value as shown in Fig. 6.

Table 1. Length of the Pitch of Dark Lines (Λ) under Different Launch Position (d ) d (mm) 0.2 0.4 0.6 0.8 1.0 1.2

ΛAir (mm)

ΛWater (mm)

4.9 0.1 4.6 0.1 3.6 0.3 2.7 0.2 No Resonance No Resonance

12.3 0.1 12.7 0.1 12.5 0.2 11.7 0.1 10.1 0.2 5.2 0.2

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Letter has been studied theoretically and measured experimentally. In this configuration, a simple method for obtaining experimentally the SPR angle by measuring the pitch of the dark line is demonstrated, offering a number of advantages over alternative fiber methods. The angles measured are in accordance with the theoretically calculated SPR angle expected for this material with both air and water. In contrast to bulk prim methods and other more complicated fiber approaches, our approach is simple and low cost, and offers significantly higher sensitivity for practical field deployment of SPR. Funding. Australian Research Council (ARC) (DP140100975, LE0883038, LE100100098); National Natural Science Foundation of China (NSFC) (51377125).

Fig. 6. Length of the pitch of dark lines, Λ, as a function of incident position, d , under different ambient material. The solid line and dashed line represent ΛAir and ΛWater versus d via theoretical calculations. Squares and triangles stand for the experimental measurements.

Acknowledgment. Chunyang Han acknowledges the scholarship support from the China Scholarship Council (CSC) and the Graduate School of Xi’an Jiaotong University. This work was supported in part by the NSFC, which sponsored the visit of Chunyang Han to iPL, Sydney. REFERENCES

In summary, we have proposed using skew rays to excite identical, multiple SPR points along an optical waveguide, in this case a coreless polymer fiber. The material chosen is fabricated from 3D printer filament and rigid enough to be used as an ideal optical fiber probe. The multiple SPR format therefore enables a novel approach to making realizable probe-based SPR sensors. This offers several advantages including enhanced detection capability through multiple excitations and, in the case described here, a novel circular format of potential use in robust ultra-sensitive sensor dip probes (where a reflector on the end can be used to double the total path length and therefore sensitivity). The influence of launching conditions on the angle of the reflection and the length of the skew ray trajectory period

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