Confocal surface plasmon microscopic sensing

Surface Plasmons provide a relatively high axial sensitivity and thus are generally used in a thin surface film sensing and imaging. Objective lens based surface plasmon microscopy enables measurement of local refractive index on a far finer scale than the conventional prism based systems. However, researchers find that a trade-off between the lateral resolution and the axial sensitivity exists in the conventional intensity based surface plasmon microscopy. In order to optimize the trade-off, interferometric surface plasmon microscopy was exploited. An interferometric or confocal system gives the so-called V(z) curve, the output response as a function of defocus, when the sample is scanned axially, which gives a measure of the surface plasmon propagation velocity. Considering the complexity of the two arm interferometric system, in this thesis, I show how a confocal system provides a more flexible and more stable alternative. This confocal system, however, places greater demands on the dynamic range of the system. Firstly, the sharp edge of the pupil on the back focal plane of the objective can generate similar effect with the surface plasmon (SPs) ripples; Secondly, the SPs ripples that convey much of the information are much smaller compared to the in focus response which means the confocal system suffers from low signal to noise ratio (SNR). In order to overcome the limitations, I proposed pupil function engineering which was to use a spatial light modulator to modulate the illumination beam profile by using the designed pupil functions with smooth edges. The results show that the sharp edge effect of confocal setup can be greatly reduced and the SNR is improved. Based on this system, I demonstrated that images obtained from the setup are comparable with the two arm interferometric SPR microscope and other wide-field non-SPR microscope. Secondly, I demonstrate the technique of V(α). A phase Spatial Light Modulator (SLM) was applied to replace the previous amplitude SLM. I show how a phase spatial light modulator (i) performs the necessary pupil function apodization (ii) imposes an angular varying phase shift that effectively changes sample defocus without any mechanical movement and (iii) changes the relative phase of the surface plasmons and reference beam to provide signal enhancement not possible with previous configurations using ASLM. Later, I extend the interferometer concept in the confocal system to produce an ‘embedded’ phase shifting interferometer in chapter 6, where I can control the phase between the reference and surface plasmon beams with a spatial light modulator. I demonstrate that this approach facilitates extraction of the amplitude and phase of the surface plasmons to measure of the phase velocity and the attenuation of the surface plasmons with greatly improved signal to noise compared to previous measurement approaches[1]. I also show that reliable results are obtained over smaller axial scan ranges giving potentially superior lateral resolution. In the end of the thesis, future work will be discussed. Firstly, I will propose another technique called ‘artificial’ plasmon. Secondly, I will recommend constructing another system and develop the ideas discussed so the system can work in aqueous environment.