Theoretical Research On Low-dispersion And Low-loss Plasmonic Slow-light Waveguides

This dissertation studies the devices and mechanisms which are based on metal–insulator-metal (MIM) structures. We have designed a low-dispersion slow-light waveguide and a high-Q resonant cavity, proposed a method to compensate losses and extend propagation distance, and realized large delay-bandwidth products (DBPs). These achievements could lead to a number of important fundamental and technological advances in the field of plasmonics.This dissertation is organized as follows:Chapter 1 is an introduction mainly about the theories and backgrounds our study in relation to. We first introduce the meaning and application of Plasmonics, the fundamental conceptions and properties of surface plasmon (SP), the typical waveguide structures, and the typical solutions to propagation loss. Then, we describe slow light with group velocity, dispersion, and pulse distortion. Last, we theoretically analyse the parametric gain (PG) process.In Chapter 2, we design a SP-based low-dispersion slow-light waveguide with a detailed analysis of dispersion relation, field distribution, and time flux. It is shown that the MIM structure can support slow light propagation and suppress the pulse distortion into some tolerable value simultaneously, when setting the frequency in an appropriate region in the dispersion curve. This waveguide can be utilized in the slow-light system with high bit rate capacity.In Chapter 3, we design a cavity with a high quality factor, which is based on the straight waveguide proposed in Chapter 2. The field distribution and the relationship between cavity length and resonant frequency/quality factor are analysed. It is shown that the mode with zero group velocity in nonzero wavevector can enhance the quality factor of the cavity which is a section of the waveguide. With the cavity length increasing, the resonant frequency will converge to the zero-group-velocity frequency gradually. This cavity has the importance because of its subwavelength volume.In Chapter 4, we present a parametric gain (PG) mechanism to compensate slow light propagation loss in a specially designed nanoscale MIM plasmonic waveguide. By utilizing the unique property of antisymmetric TM mode in MIM structures and introducing nonlinear material as gain medium for optical parametric amplification, a significant enhancement (100%) of the propagation length has been demonstrated. The demonstrated mechanism for compensating the absorption loss in metal can be expected as a foundation for the design of more complex nanoplasmonic devices.In Chapter 5, we improve the mechanism proposed in Chapter 4, and make a comparison between them in frequency domain. It is shown that the improved mechanism has a better amplification effect on the signal light. Through amplifying nonlinear effect by ultraslow light (0.005c), propagation length has a larger enhancement (400%).In Chapter 6, we analyse the major limiting factors in the improvement of DBP. By utilizing the flexibility of the dispersion curve in MIM waveguide, large DBPs can be achieved in several bands.In Chapter 7, we analyse the relationship between structural parameters and dispersion curves in the IMI waveguide. By setting the thickness of central metal layer and the dielectric permittivity of lateral insulator layers at appropriate values, a desirable slow light property can be achieved. The comparison with other ones shows that IMI waveguide has the longest propagation distance because of the decreased energy in metallic material.Chapter 8 summarizes the results of this work. Future research topics are proposed.