In order to verify our far-field scheme, we conducted scanning near-field optical microscopy (SNOM) to image the field distribution of excited SPPs. Figure 4.7 shows the field
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distributions of excited SPPs imaged by SNOM, where (a): single excitation, = 45°, on focus, (b) single excitation,= 15°, on focus, (c) double excitation, = 45°,= 45°, on focus, and (d) double excitation, = 45°, = 45°, defocus Z > 0, respectively. The insets depict the calculated SPP field distributions via FDTD calculation. In the case of a single excitation on the focus [Fig. 4.7(a) and (b)], the arc center of TM-polarized sector, 0, dominates the propagating direction of excited SPPs. The spread area of excited SPPs is directly governed by the occupied ratio of TM-polarized sector which exhibits a reciprocal dependence between each otherFigure 4.7(c) and (d) show the function of defocus subject to a double excitation scheme. As two TM portions were kept in phase and focused, two groups of plasmonic
Fig. 4. 7 SPP field distributions imaged by SNOM under different excitation scheme: (a) single excitation, = 45°, on focus, (b) single excitation, = 15°, on focus, (c) double excitation,= 45°,= 45°, on focus, and (d) double excitation,
= 45°,= 45°, defocus Z > 0, where insets represent FDTD maps.
waves not only lead to a constructive interference at the center, but also form an additional
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pair of outer edges. The angular interval of those outer edges is controlled by the size of the sandwiched TE-polarized sector. When the observation plane moves above the focus (Z > 0), the field distribution of double-excited SPPs goes back to propagating mode. Entire excited SPPs would propagate away from the center and no interference pattern would be observed.
Furthermore, the results of double excitation is similar to the work done by L. Novotny, where he used a tightly focused beam to depolarize a linear polarization and thus form a two-lobe pattern [81].
Besides the manipulation of SPP patterns, the proposed scheme enables scientists to create interference patterns with a wide range of line width and period. Figure 4.8 shows the field distribution of excited SPPs created by a double excitation scheme with different ratios of the opposite TM sector along the x-axis. The measured period (r-SPP) of SPP interfering fringes along the x-axis increased fromr-SPP275 nm to 316 nm as we increased the TM ratio from= 15° to 75°. The experimental results closely agree with FDTD results,
r-SPP273 nm at= 15° and 312 nm at = 75°. It is noted that the period of the interference pattern extends horizontally. This is because the spatial period of interference is given by r-SPP2 = x-SPP2 + y-SPP2, where obliquely propagating SPPs provide an additional ky wavevector in horizontal direction. Once a TM-polarized sector occupies the entire entrance pupil, the profile of excited SPPs will identical to a radial-polarization-generated Airy disk [23, 73, 74]. These properties ensure the modulation of the TM/TE ratio not only control the envelope of the SPP localization but also the fringe pattern.
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Fig. 4. 8 Cross-sectional cut of interference in the case of double excitation scheme with different ratios of the opposite TM sector along the x-axis (a) = 15°
and (b) = 75°, where the period of fringe pattern is increased due to more oblique component of ky wavevector in the case of = 75°.
Based on the arrangement of the split polarization and the focus position, we demonstrated a number of unique patterns by using the far-field approach. Finally, we design the polarization distribution of entrance pupil into three-fold TM-polarized sector with equal arc distance = 45°. In addition, we record the images of excited SPPs while the observation plane scans through the focus. The excited SPPs were formed either by interference between counter-propagating plasmons, or three independent propagating SPPs, as shown in Fig. 4.9. At Z < 0, three bands of in-phase plasmon waves propagated
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toward the center and yielded a 150-nm-radius dots array profiled by a 300-nm-period hexagonal shape. The dot sizes and separation distances were governed by the standing plasmonic wave, the results were close to the half SPP effective wavelength (SPP= 2Re[k0()1/2] = 598 nm). The separation distance between subwavelength holes can be manipulated by tuning the ratio of TM/TE polarized sectors. Compared with the strong steering by a converging beam, at Z > 0, three bands of unmodulated plasmonic waves would lead to three plasmonic fans propagating outward. In combination with the split engineering, other asymmetric SPP patterns can be achieved by introducing designated phase modulation at the entrance pupil.
Fig. 4. 9 Intensity distribution of SPPs under triple excitation scheme when the observation plane scanned through the focus with (a) defocus Z = -1 um, (b) on focus and (c) defocus Z = 1 um. (d), (e) and (f) represent the corresponding FDTD results.
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4.5 Short Conclusion
In conclusion, we proposed a far-field scheme for the generation of asymmetric SPPs. The excitation mechanism is based on the spatial arrangement of split polarization at entrance pupil in conjunction with a defocus technique. It provides a similar function to the near-field approach, which utilizes the subwavelength features in order to generate SPP dipoles. Also, it provides an irreplaceable flexibility to real-time manipulate SPPs into desired distribution by integrating a spatial light modulator into this scheme. Proposed method will certainly has a promising impact on carrying out various SPP excitations for lithographic applications, plasmonic waveguides, and biophotonic devices due to its simplicity and versatility.
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Chapter V
Application on Objective-based SPR sensors
In the previous chapter, we discuss the application on applying SIP beam to excite surface plasmon polaritons (SPPs) which occurs at the metal/dielectric interface based on the energy coupling of the transmitted evanescent wave. In the meantime, rest of the uncoupled light will reflect to the back focal plane of the objective lens. By collecting all of the reflected light, a reflected disk having a dark resonance ring at particular angular position will be shown which is refer to resonance of surface plasmon. By observing the radius change or deformation of the dark resonance ring, we can built a sensor with high sensitivity which is used to detect tiny deviation came from a sample in terms of the change of effective refractive index and its shape. In Chapter 5.1, a brief introduction of surface plasmon (SPR) sensor and its corresponding literatures review will be given. Following current tendency of development on SPR sensor, we proposed two radial polarization enabling SPR sensors to meliorate the capability of current SPR sensor. One is interfering SPR sensor based on coherent radially polarized light, and the other is polychromatic SPR sensor based on incoherent light. The overall view of those two radial polarization enabling SPR sensors including concept, simulation results, and experimental results will be delivered in the chapter 5.2 and Chapter 5.3, respectively.
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5.1 Introduction to SPR sensor
Surface plasmon resonance (SPR) sensors have been widely used to analyze the optical characteristics of material due to its highly axial resolution and angular/spectral sensitivity via strong confinement of SPPs at metal-dielectric interface. As the in-plane wavevector of an incident wave matches the resonant condition at the metal-dielectric interface, the surface plasmon would be excited accordingly [67, 87, 88]. SPs are generally excited by the attenuated total internal reflection (ATIR) configuration, which was firstly proposed by Otto and Kretschmann in 1960s by means of a prism coupler [89, 90]. Figure 5.1 shows the schematic diagram of a Kretchmann configuration. The interface of bottom golden monolayer and dielectric media support the existence of surface plasmon polaritons mode which can be excited by coupling a light came from a high refraction index media with high incidence angle. One can observe the evidence of energy coupling through the intensity change on reflected light. Normally, the sudden dip would be observed at the incident angle just slightly larger than total internal reflection (TIR) angle due to the phase matching condition.
Fig. 5. 1 the schematic diagram of a Kretschmann configuration for the generation
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of SPP and its corresponding reflectance versus incident angle.
Recently, H. Kano et al replaced a prism coupler with a collinear objective lens to create a universal angular wave vectors without the need for angular scanning [82, 91]. As a result, we are able to determine the resonance condition in a scan-free operation by observing the diameter change of resonance ring at the exit pupil. Based on the collinear setup, Zhan took advantages of radially polarized (RP) illumination to improve the conversion efficiency and spatial resolution [73, 74]. Impressive two-dimensional refractive index images of a bio-sample with fine details and high contrast have been obtained, but mostly by a monochromatic source [50, 51, 76, 92, 93].