Directional launching methods of surface plasmon in nanoslits and applications

Abstract

Surface plasmon polaritons (SPPs), known as quasi-particles generated by a collective oscillation of photons and electrons, have been studied by numerous researchers due to their various fascinating properties such as sub-wavelength confinement characteristics and strong electromagnetic field enhancement near metal surface. Indeed, such characteristics are quite demanded to design a novel photonic devices for future technologies such as highly integrated photonic circuit and optical storage system with ultra-high density. Hence, numerous studies have been reported about optical devices based on the physical characteristics of SPPs, so-called plasmonic devices. Although they are not yet combined to integrated system which can overcome the conventional computing system operated by electrons, optical or quantum computing via SPPs is still an impressive issue in nanophotonics. One of the necessary parts of optical computing is a switching of SPP source. The works done in this dissertation are focused on the physical mechanism and structural design of nanostructures such as nanoslit and nanoaperture which can be used to switch SPP source. After brief introduction, the first part of this dissertation discusses on the various methods for directional switching of SPP excitation from nanoslit geometry. In broad separation, two types of directional switching mechanisms are demonstrated: the first one is a plasmonic directional switching which can be modulated by polarization modulation. It is shown that the excited SPPs from nanoslit can be unidirectionally launched without any asymmetry of neither geometry nor incident momentum at certain polarization state when the light illuminates obliquely along the parallel-to-slit direction. To explain such a novel type of SPP excitation phenomenon, an induced current model that can explain an aspect of SPP excitation from nanoslit for TE and TM polarization illumination is proposed. Then, appropriate experimental results are followed to verify the possibility and performance of unidirectional SPP excitation. Then, the second type of plasmonic directional switching method, which can be modulated by phase modulation, is discussed. When two beams illuminate obliquely to nanoslit, it is found that excited SPPs from nanoslit have either symmetric or anti-symmetric profile according to the relative location between node of interference pattern and center of slit. After finding the optimized condition of nanoslit geometry which can efficiently excite both symmetric and anti-symmetric SPPs in nearly same amplitude scale, experimental demonstration of SPP switching is achieved with electromotive phase modulation done by piezo-stage. The origins of both types of switching methods can be explained by the induced charge distribution near the nanoslit. In the second part of this dissertation, various plasmonic devices that use the switching mechanism introduced in the first part of this dissertation are demonstrated. For the realization of optical signal multiplexing and spectrum analyzing in integrated chip scale, it is necessary to develop a plasmonic color splitter which can clearly split the designed region of frequencies. By applying the physical analogy discussed in the first part of this dissertation, theoretical analysis on the plasmonic dichroic splitter that can launch two different frequencies of visible light into an opposite direction is shown for the first application device. In this case, metal-insulator-metal (MIM) waveguide structure is used instead of single interface geometry to form a Fabry-Perot resonator structure at the entrance of SPP coupler. Similar to the work done in the first part, this plasmonic dichoric splitter can also switch their launching directions of SPPs when incident polarization state is changed. Moreover, beaming from surface plasmon source has great potential to generate a subwavelength scale of optical beam. For the second application device, analysis on the plasmonic beaming from plasmonic double spiral bulls eye geometry is discussed. Chiral geometry pattern of spiral bulls eye geometry can lead to the polarization dependent characteristics for beam generation. It is shown that the generated beam from spiral bulls eye can be switched on and off according to the optical polarization of incident light. Such beam switching phenomenon can be explained by the angular momentum change, which is caused by spin-orbital interaction at the nanohole and spiral gratings.