Field of Study (as a reminder )

My major interest in physics is the electromagnetic theory, especially optics, from geometry optics, wave optics to laser. I was extensively trained with electromagnetism courses. I am able to solve the Maxwell’s equations analytically in specific geometries and numerically in most geometry. My first research project on my bachelor course is about the structural color in nature, like the scales of butterflies, bees and seashells. After employed modern laboratory techniques, such as SEM (scanning electronic microscope), TEM (tunneling electronic microscope), and our group revealed that the color of butterflies is based on a very simple principle – multiple thin films reflection or interference. The color is not form pigment as most people though, it is completely form the microstructures of chintin, which is a common material in nature. By changing the thickness or geometry of the structure, we are able to change the color reflected.

The second research project is about fabrication of the structural color or the photonic crystal, by using holographic lithography. Holographic lithography records the intensity pattern from laser beams interference and forms structures on a photoresis. The size of the interference pattern is determined by the wavelength of the laser used. A UV laser is able to produce 200 to 800 nm sub-micro structures. The structures at this scale can push the optical respond range to visible range. This method is also able to produce a large sample (5 X 5 mm²) by expanded the laser beams. A large sample makes optical spectrum measurements possible.

The objective of the project is fabrication of a sub-mirco-sized gold particles array and study of the surface plasmon resonance (SPR) when EM wave incidents on it. The surface plasmon is a quantized state of the vibration of surface electrons; thus, only metallic or conductive structures have such effect. For a single gold particle, the frequency of the quantized state is given by the size and geometry. A simple imagine is the gold particle was polarized and like a giant dipole oscillating with the external field. The effect of single dipole is weak and impossible for measurement. The array of dipoles will give strong and extraordinary effect when they are in resonance. One example is the extinction, which is defined as log(1/T), there T is transmission. The high extinction means low transmission. Our group found that the extinction peak was blue shifted from elliptical spheroid to good sphere. The result is same as numerical simulation.

My master research project follows the same technique. But instead of fabricate gold particles array, I were going to fabricate spiral structures. The fabrication required interference of 1 circularly polarized beam and 6 linearly polarized beams with very fine optical alignment. Therefore, the project took a lot of time for fine-tuning and optimization of the laser beams configurations, such as incident angles, polarization, and intensity. We also used computer simulation not only in the design stage, but also in the improvement stage. We are able to find out the factors that creates defect in our sample. Our group had also designed and set-up the measurement equipment and technique for measure the full state of light polarization with limited equipments. The special effect for a polarized light pass through a chiral slab is the wavelength dependence of the change of polarization angle. The polarization angle of blue light will be rotated larger than that of red light. The result can be explained by classical EM theory with the introduction of chirality factor in the characteristic equations that connect the electric displacement field and magnetic induction with the electric field and magnetic field.

I am also interested in the atomic structure. Therefore, I took 4 related experiments on a modern laboratory course. They are Millikan oil drop, Zeeman Effect, Optical Pumping and ESR (electron spin resonance). Those experiments not only enhanced my understanding, but also polished my experimental skill. Moreover, those experiments bring me into the real world that is a lot different in the theory. There are a lot unexpected and practice issues appear when performing an experiment. That is the fun and interesting aspect of doing experiment.