|Ph.D Student||Ran Boaz|
|Subject||Phase Shift Microscopy|
|Department||Department of Physics||Supervisor||Professor Emeritus Stephen Lipson|
A Surface Plasmon Resonance (SPR) system is composed of a very thin layer of metal (about 50 nm) coated on a glass prism. If a light beam is launched within the prism toward the metal surface, at a certain angle of incidence at the metal film the electrons on its opposite surface oscillate collectively and modify the amplitude and phase of the reflected light. These oscillations are resonant, and the resonance angle is extremely sensitive to the dielectric properties of the medium in contact with the film. The SPR phenomena has been well known for about 40 years, and recently it has been widely exploited as an extremely sensitive basis for commercial analytic tools, mainly for bio-interaction measurements.
In this work, the relationship between intensity and phase modes for measuring the reflected light wave was investigated thoroughly. During the past 10 years, many authors have published theories predicting that measuring the phase of the reflected wave will improve the sensitivity by two to three orders of magnitude over that achievable by measuring its intensity. The proposed improvement in sensitivity results from a drastic phase change near the resonance, which can be optimized (theoretically) to yield infinite slope.
In this work it was shown that, contrary to the published predictions, the sensitivity of the phase is deceptive because of the assumption that the phase of the light wave is physically measurable; but unfortunately it is not. The phase can only be derived mathematically from the interferogram produced by interfering two coherent waves, where one has interacted with the resonator, and the other one is a reference wave. Since intensity and phase modes are therefore both based on intensity measurement, both are limited similarly by photon noise.
This theme was proven experimentally, theoretically and numerically. An optical bench system composed of a single beam (common path) interferometer was developed. With a simple modification, it can easily switch between intensity and interferometric modes (from which the phase can be extracted). This enables an almost direct comparison between the two measurement modes under identical conditions. Calibrated glucose solutions were measured with the system and the limiting sensitivities of the two modes were compared. The results showed that the refractive index sensitivity achieved by the interferometric measurements was about the same as that achieved using the intensity mode. The results were also shown to be in complete agreement with theoretical and numerical models.