Advanced phase detection in surface plasmonic microscopy

Abstract

Surface plasmon resonance sensing is a technique widely used for real-time, high axial-sensitivity and label-free monitoring for bio-molecular interactions. By measuring the phase change around excitation angle, phase-based Surface Plasmon Resonance sensing has been found to have better sensitive than intensity-based in recent years. In addition we have developed methods based on confocal microscopy to improve the measurement localization. This thesis addresses methods to combine both microscopic and phase measurement on samples supporting surface plasmons. To measure phase two beam interferometry is most commonly used, however, the optical setup for interference measurement is more challenging as its configuration is more complicated than the conventional intensity-based measurement. Recently, our group developed an embedded confocal surface plasmon microscope to recover excitation angle by interfering the leakage radiation of SPs and the direct reflection along normal incident angle. This provides a compact and stable form of interferometry. As the system is a confocal scanning system, it requires a series of confocal measurements to perform the measurement. Our group proposed an advanced modulation strategy to improve the speed of the acquisition process. In this thesis, we apply an optical vortex approach to which can enhance the speed of data acquisition by an approximately a factor of 4 without sacrificing the signal-to-noise ratio. In addition we also address a Hilbert transform approach to reduce the data acquisition time. On the other hand, the system not only allows us to locate the excitation angle of the SP, it can also be used to determine the characteristics of the sample by measuring the phase information around SPs. In this thesis, we show that different loss mechanisms, due to reradiation and absorption, of the SP can be separated by imposing additional Goos-Hanchen phase in the surface plasmon microscope. The experimental result is validated by separating the loss mechanism on gold sample with different thickness. Thanks to the phase information of SP, more insight of the SP can be investigated. Therefore, it will be great if we can measure all the phase information on the reflection spectrum. In this thesis, ptychography algorithm is applied to recover the complex field of the BFP (the phase and the modulus of the field can be recovered). Experiment shows that the phase on BFP can be recovered successfully. With the recovered field, it allows us to perform more advanced post processing techniques to extract more information. For example, the field from the back focal plane can be projected using virtual optics to any plane in the imaging system. We demonstrate several results including the attenuation results can be obtained with this method. In other words we show that by measuring the phase we can obtain similar or indeed superior results to those we previously obtained by manipulating the phase with the spatial light modulator. In this thesis, it will be divided into two main parts, (1) advanced beam profile modulation based on SLM phase modulation and (2) Quantitative (amplitude and phase) measurement of SPR based on ptychography will be discussed.