|M.Sc Student||Shoham Amir|
|Subject||New Method for Accurate Refractive Index Measurement with|
High Spatial Resolution
|Department||Department of Physics||Supervisor||Professor Emeritus Stephen Lipson|
Measuring the refractive index field with high lateral and refractive index resolution is of great interest in imaging low contrast samples. The existing measuring methods with high refractive index resolution rely on optical phenomena, in which the phase, amplitude or polarization states of the refracted or reflected waves are sensitive to the refractive index. These phenomena include total internal reflection, reflection from layered structures or surface plasmon resonance. In order to measure the refractive index with these phenomena, the polarization direction and the angle of the incident wave have to be fixed, because the phenomena are sensitive to these parameters as well. Moreover, in order to maximize the refractive index resolution the incident angle and the polarization have to be fixed at the values where the phenomena have the maximal sensitivity to the refractive index. The lateral resolution of optical systems depends on the numerical aperture of the illumination and the imaging lenses which is equivalent to the conical angle of the incident and reflected rays. Thus there is a trade off, in a method of refractive index measurement using standard illumination, between the lateral resolution which requires using rays from all directions and from a large conical angle and high refractive index resolution which requires collecting a narrow beam around a given value of reflection angle.
In our research, we overcome this trade off by illuminating every point on the sample with the same angle and polarization mode but in varying direction, i.e. a conical beam with axial symmetry. The use of a conical beam is a well known way to get high lateral resolution, and the use of axial polarization enables getting high sensitivity to refractive index because the incident angle and the polarization direction are kept constant.
In order to realize this required illumination we developed a new technique to polarize a visible light beam with axial symmetry, by using conical mirrors and a linear polarizer sheet. We describe several optional methods for refractive index field measurement which may be used in combination with our polarizer device and objective lenses in order to achieve the required resolutions. We choose to implement one refractive index measuring method which measured the phase shift resulting from the total internal reflection phenomenon. The method was proved to work but has not yet achieved the expected refractive index resolution. We suggest improving the optical design, especially the quality of the polarizing device.