Experimental Setup

The experimental setup of PC-RPSPR sensor can divide into two principal parts, first part is synthesis of radially polarized white light utilized an approach of spatially varying polarizer (SVP), as shown in Fig. 4-2 (a), and second part is the metal-insulator-metal (MIM) structure for extending sensing range on sensor system.

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We utilize an unpolarized white LED (model: Luxeon Star/O LXHL-NWE8) as a light source which has advantages of low cost and is speckle free. A collimated unpolarized light was converted to radially polarized light by the use of SVP. The radially polarized white light then relays to the entrance pupil of the commercial immersion objective lens (Olympus PlanApo-N 60x/1.45 Oil). Its corresponding half divergence angle is 75.16°, well beyond the SPR resonant angle SP ~ 45^o at wavelength 0 = 610 nm. After passing through the objective, the white light focus on the MIM structure, Non-coupled reflected light has been collected and guides backward into two different optical paths via the same objective lens. One optical path projected the reflected intensity distribution onto CCD image sensor from the back focal plane of the objective lens. The other optical path records the spectra of reflected beam via a spectrum analyzer. As shown in Fig. 4-2.

Fig. 4-2 Configuration of the PC-RPSPR sensor, where CL: collimated lens, SVP: spatial varying polarizer, RL: relay lens, BS: beam splitter, M: mirror, IL: image lens, MIM: metal-insulator-metal structure. The insets show (a) the photo of SVP, (b) the schematic diagram of MIM structure.

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4.2.1 Synthesis of polychromatic radially polarized light

The SVP consists of eight pieces of linear polarizer and the transmission axis of every sector aligned to individual principle radial direction, as shown in Fig. 4-3. The SVP is used to convert unpolarized white light into radially polarized white light. In microscopies, nano-optics, and spectroscopy, polychromatic radially polarized beams can supply other perspectives. Based on past reported studies, the common recipe to synthesis or generate radial polarization are designed for a specific working wavelength and rely on the use of phase element, liquid crystal, interference configuration [28, 29, 30, 31]. Those elements are wavelength dependent. This means that it cannot operate universally among different working wavelength in a fixed design. However, only few numbers of devices can generate those kinds of light in recently. The main systems are double conical reflector system which based on geometrical optics [32] and fiber-based system [33]. Unfortunately, the former system crate a discontinuous ring beam shape, which decreases the resolution due to the increment of side-lobe part in focal region and seems difficult to be apply for sensing;

the latter system need the procedure of precise optical alignment to couple incident light into fiber. In other words, these systems are not simply and convenient. On the contrary, proposed SVP is assembled by conventional polarizing element offering wavelength independent properties for polarization conversion and simplify the creative complexity of system. Also, it has a compact size with extremely low cost, but this device exchanges those benefits for power consumption.

Fig. 4-3 The SVP assembly, which is composed of eight sectors.

42 mechanism of SPR, the incidence angle, which provides sufficient wave-vectors to agree phase matching conditions, is greater than the critical angle. The SPR angle is wavelength dependent and is related to the dispersion relation of metal. Its resonance angle shifts up when working wavelength decreased. Furthermore, generally the refractive index of living cells are close to water (SPR, water ~ 77.4 ^o, 0 = 610 nm) which beyond the sensing limit for objective lens with NA = 1.45 (max ~ 75.16°, 0 = 610 nm). Under this circumstance, the SPR dips are outside the observation windows as the wavelength is smaller than 640 nm as shown in Fig. 4-4. Therefore, the MIM structure is purposed,

As the beam focused on the MIM structure, not only cavity resonance (CR) modes but also transformed surface plasmon resonance (T-SPR) modes which have broader sensing range are generated. CR modes are insensitive to the change of the refractive index of sample, but T-SPR mode is. The role of a MIM structure is to transform generated cavity resonance (CR) modes into transformed surface plasmon resonance (T-SPR) modes. The material and thickness of each layer determine the resonance property of both modes. In order to maximum the depth of surface plasmon resonant dips, we kept the symmetry property of MIM structure and set the overall thickness of gold thin film as 40 nm, also, in order to confirm the effect of MIM structure, we chose SiO2 as insulator which is identical to substrate. In our case, when the dip of CR modes exceeded the critical angle, the CR modes would yield abundant

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Fig. 4-4 Comparison with the different test sample. (a) and (c) are experimental data and (b) and (d) are simulation when test sample is air and water, respectively.(e) and (f) are the slice of (b) and (d), respectively.

The wavelength here is 610 nm.

evanescent waves. Then, the energy of evanescent wave would be transferred to a new SPR which can be excited by smaller propagating constant, because MIM structure [the thickness of insulator should be larger than 100 nm (150 nm) @  = 450 nm when test sample is chosen to be air (water)] supports an additional SPR solution in the dispersion relation diagram. As a result, this T-SPR mode provides the capability to detect sample with higher refractive index at the same incident angle, it also have obviously angular shift and linear dependence in sensitivity, as shown in Fig. 4-5.

Thus, this new angle-to-angle mappings function extended the sensing range of the refractive index up to n ~ 1.42 which is beyond typical range of living cells.

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Fig. 4-5 Obviously angular shift in T-SPR modes by comparing the different refractive index of test sample.