Simulation and Experimental Data

Firstly, we show the effect of SVP passing through the linear polarizer and compare the excitation of SPR between simulation and experimental data. Secondary, the effect of MIM structure will be confirm by sensing the different refractive indices.

Finally, we combined the radially polarized white light and MIM structure to detect the different concentration of salt solution.

Fig. 4-8 shows that donut-shaped intensity profile of radially polarized white light and the unique profiles which passes through different direction of transmitted axis of analyzer. The results of Fig. 4-8 demonstrate the preliminary effect of SVP, and then the effect will be further demonstrated by the excitation of SPR.

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Fig. 4-8 Radially polarized white light generated by SVP. The arrows indicate the axis direction of analyzer.

To demonstrate the excitation of chromatic SPR, we choose three specific wavelengths to compare with simulated results; the chosen wavelength is 610 nm, 530 nm, and 450 nm, respectively. For simplifying the comparison, we do not consider the effect of MIM structure; it means that the structure is only a gold monolayer.

The comparison between Fig. 4-9 (a) and Fig. 4-9 (b) shows that the experimental results strongly agree with our simulation predictions. Therefore, the effect of SVP is demonstrated. In the past, that is impossible to simply achieve. In addition, for different operation purpose, the SVP device can be extended to create arbitrary stat of polarization without loss of generality.

The effect of MIM structure which can extend the sensing range will be verified by sensing in different refractive indices. Fig. 4-10 shows the reflective disk for different sensing condition and set the radius of disk is unity. By analyzing the distance between the central to SPR dip and the difference distance between different sensing condition, we can find the MIM structure acturely supports an additional SPR

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Fig. 4-9 The (a) experimental and (b) simulated observation of reflected rainbow disk for the case of free space in contact with the gold monolayer structure, where subfigures 1 to 3 are respectively show the dark resonance ring observed at wavelength  = 610 nm, 530 nm, and 450 nm.

solution which can be excited by smaller propagating constant.

The MIM structure was proposed to overcome the barrier of detecting limitation of an objective based SPR sensor which is restricted by the NA of objective lens. The MIM structure provides provides additional 11% observation window. It

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Fig. 4-10 Comparison shows the effect of MIM structure (λ=610nm).

increases sensing range up to 1.42 and covers the general usage in bio-molecular detection

Combining with radially polarized white light and MIM structure, we are not only able to analyze the diameter change of dark ring in a larger sensing range at individual wavelength by switching color filters, but also by integrating the area of reflective disc via spectrum analyzer to monitor the refractive index change of test sample. As the results, we proposed a scenario to analyze the NaCl solution with different concentration to validate the technique and proposed model. The sample was prepared by diluting a saturated salt solution. By integrating the intensity ratio of the concentric spectral component via an additional spectrometer (Chung-Yu, USB-100), a series of differential curves shown in dash lines feature two absorption peaks, which is due to the non-uniformity of lighting source, as shown in Fig. 4-11. In order to remove the weighting bias of illumination light at different wavelength, a normalized deviation between salt solution and the condition of pure water is also demonstrated, which indicates 466 nm, 566 nm, and 650 nm are relatively sensitive observation windows in visible for monitoring the concentration change of a salt solution.

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Therefore, it provides a great feature of indicating suitable observation window for researchers to study individual bio-sample.

Fig. 4-11 (a) rainbow concentric ring in different sensing condition (b) the spectra distribution of rainbow concentric rings as the medium above the MIM structure is chosen saline with different concentration.

4.5 Summary

The advantages of PC-RPSPR sensor is not only to provide rich SPR information of a local region at one time, which reveals the resonance angle of SPR

(a)

(b)

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for different wavelength, but also to supply larger sensing range. We demonstrated a unique pattern with rainbow concentric ring which is generated by a PC-RPSPR sensor integrated with a broadband radial polarizer and the practical influence of MIM light coupler. Based on this configuration, a full color SPR wave is able to excite and use to sense test samples with refractive index up to 1.42 covering most of living cells.

The rainbow concentric ring is rich and save all of SPR information regarding to angular and wavelength spectra of a subwavelength-sized local region.

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Chapter 5

Conclusions and Future work

5.1 Conclusions

In this thesis, we demonstrate two methods to extend the DoF, one is the synthesis method of two orthogonal inhomogeneous beams, and the other is the higher-order radial polarization method. The former takes advantages of the orthogonality between two orthogonal polarizations, the polarization coded aperture is equivalent to the adding of two apertures. The latter is based on high longitudinal component to top axial distribution in focal volume. Using these concepts and based on the simulation, different combinative ratio of two inhomogeneous polarizations and different order of radial polarization all influence the effect of extended DoF.

Secondly, we demonstrate the production of chromatic radial polarization coded aperture. For sensing application, the chromatic radial polarization can excited chromatic SPR, providing the resonance angle for different wavelength of a local region at one time. Therefore, we build up a PC-RPSPR sensor; through integrating the area of reflective disc via spectrum analyzer can supply another route to monitor the characteristic properties of test sample. Furthermore, in order to optimize the sensor, we also demonstrated that the purposed MIM structure can extend the refractive index sensing range up to 1.42 in a 1.45 NA objective lens, which covers the general usage in bio-molecular detection.

Finally, we proposed a scenario to analyze the NaCl solution with different concentration to validate the capability of PC-RPSPR sensor.

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5.2 Future work

So far, we already confirm the effect of sensor system, the unique pattern with rainbow concentric ring involves the rich information in local region, which is similar to the size of a focused spot, in order to completely bring out the advantage of the PC-RPSPR sensor. Next step, the dimension of detection will be extend to two-dimensions from one-point by scanning operation, the obtained information can constitute to a data cube, the data cube can present specific images of each specific wavelength respectively. The specific images is more clearly and helpful to analyze the characteristic properties of test sample. By scanning operation, the final objective is that obtain a three-dimensional wavelength dependent refractive-index map of cell structure. As shown in Fig. 5-1.

Fig. 5-1 the productive procedure of data cube in PC-RPSPR sensor

Moreover, the defocusing error may occur in the scanning procedure, therefore, the technique of extended DoF could be employed to decrease the influence of z-axis by combining the radial polarization and azimuthal polarization. In addition to use the extended DoF technique, the sensitivity of this sensor could be further enhanced by coating gold nanoparticles on the top of metal surface. Also, it can integrate with super continuum laser source to exploit surface-enhanced Raman scattering for more applications.

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