Investigation and analysis of leaf reflective structures in the plasmonic field for the application of passive components

Master's Thesis in the field of electrical engineering - field telecommunications

Abstract

Investigation and analysis of reflective sheet structures in the field of plasmonics for the use of passive components

In the design of optical integrated circuits, the diffraction limit limits the miniaturization of optical elements. Surface plasmon polaritons that propagate at the interface of metal and dielectric overcome this limitation. One of the subjects that is studied in this thesis is the laboratory investigation of dielectric-metal-dielectric plasmonic plate and channel waveguides with polymer-silver-prism structure. The plasmonic wave was excited by the well-known coupling method by a prism. A simple laboratory method was also introduced to measure the propagation length of the plasmonic wave. An interesting phenomenon that was observed in the experiments was the scattering effects of surface plasmonic waves at the end of the silver strip, which creates a discontinuity. This phenomenon can be used for coupling between conventional 3D waveguides and plasmonic waveguides. Currently, this topic is under investigation for the design of hybrid plasmonic couplers. In addition, in this thesis, alternating leaf structures that are used as reflectors and filters in optical circuits are studied. Plasmonic hybrid structures were used to design and simulate leaf reflectors. Plasmonic hybrid reflectors based on sinusoidal and toothed gratings were investigated and compared with the previously introduced rectangular grating. It is shown that the sawtooth grating shows better characteristics compared to the three gratings, and this grating has less passband distortion and a narrower cutoff bandwidth compared to rectangular and sinusoidal gratings. It is also shown that the sinusoidal grating, which can be easily made with holographic techniques, has less passband distortion and narrower cutoff bandwidth compared to the rectangular grating. In addition, the apodization operation of plasmonic hybrid arrays was investigated and it was found that by using this technique, the distortion in the passband and cutoff bandwidth can be further reduced.

Keywords: plasmonics, surface plasmon, waveguide, filter, leaf reflector.

1-1 Introduction and importance of the topic

Optics is one of the branches of science that before defining light as photon packets He had made a lot of progress. Defining light in the form of electromagnetic waves with specific wavelengths has greatly contributed to the expansion of this science. Photonics was another name for this science that came with the definition of light particles. In this definition, light was expressed as a package of massless particles with a specific momentum. Therefore, the wave and particle properties of light both led to the development of the science of light or optics. The advancement of technology has widened the use of light in the fields of telecommunications, material identification, biosensors, and circuits with nanometer dimensions.

Optical integrated circuits are among the discussions that received great attention from researchers with the advancement of optics. Of course, fundamental limitations were observed in the miniaturization of optical devices. The most important of these limitations stated that light cannot be replaced in space or space in dimensions smaller than wavelength. Scientists have specified a minimum limit for this substitution in their physical tests. This minimum limit was named the diffraction limit [3] to reduce the dimensions of the parts and the accuracy [2] of observing objects.

Surface plasmons (the theoretical discussions of which are expressed in the following chapters) which are defined by the full name of surface plasmon polaritons [4] (SSP), are surface electromagnetic waves that propagate parallel to the metal-dielectric interface. The full definition of these waves was first introduced in 1957 AD (years mentioned in the text are all in AD) by Mr. Ritcher [5] completely with optical application. In recent decades, these waves have become candidates for reducing the dimensions of optics to two dimensions, so that they have the ability to exceed the diffraction limit (which will be explained below) [1].

The advantages of manufacturing optical devices with micrometer and nanometer dimensions, along with the development of display technologies such as optical near field display [6] (SNOM), have caused more attention to research in this field of photonics. This field is called nanophotonic and plasmonic due to the nanometer dimensions of its elements.

Plasmonic is a field that has some electronic characteristics with its electromagnetic properties.. Information transmission in this area is done with much higher frequencies compared to electronics, and much more bandwidth is accessible. Optics requires larger waveguide lines compared to nano dimensions, which is difficult due to the diffraction limit of waveguide components. In addition, in electronic circuits, there is a delay in the exchange of data between the source and the destination, and the speed of the circuits is greatly reduced. In optical circuits, this speed increases in addition to the capacity of the transmission line.  But still the problem of diffraction limit is the main obstacle in the miniaturization of circuits.

Plasmonics introduces optical bandwidth with dimensions smaller than the diffraction limit. Therefore, it can miniaturize optics and its elements with much larger bandwidth and very small waveguides. So it combines all the advantages and properties of electronic and optical circuits. This technology needs further development because it has disadvantages such as the short propagation length of surface plasmon waves. In recent years, many researches have been started to reduce the disadvantages of these waves by using material properties to amplify waves and hybrid structures [7]. Measurements have also become very complicated in plasmonic science. For this reason, part of the research is spent on simplifying the measurements through the inherent characteristics of these waves in observing plasmons and measuring their propagation length.

The purpose of this thesis was to design and simulate leaf plasmonic filters for use in passive circuits. Therefore, in this thesis, after reviewing the history of plasmonic science and the brief definition of surface plasmon polaritons, we will introduce the diffraction limit and break it by plasmonic science. In the second chapter, the theory related to surface plasmons in different structures is expressed. Of course, according to the purpose of this thesis, which was to investigate the construction of IMI filters [8], and due to the time limit until the construction stage of the IMI waveguide for the first time in the country, and the introduction of a new method of measuring the propagation length, progress was achieved. In the third chapter, the experiments performed on the excitation of plasmons and the idea of ??a new method for measuring their propagation length are mentioned. In the fourth chapter, the simulation of hybrid plasmonic leaf filters is explained. 1-2 History overview Electromagnetic surface waves, which are a special type of this thesis, can move along the interface of two dissimilar environments. These waves have been investigated in the last century.

Zennik[9] encountered the propagation of these waves in 1909 during the discussions of radio communication around the globe when he assumed the upper half of space to be a dielectric and the earth to be a conductor[2]. Such a similar wave can be propagated at the metal-dielectric interface by coupling the electromagnetic beam to the oscillating charge density in metals. Of course, some researchers introduced this mode as a sound wave that propagates in the free electron gas model of the electron sea of ??metals [3].

Fano [10] in 1941 recognized that the surface waves that propagate at the interface between metal and dielectric are similar to an example of the waves that were introduced years ago by Zenick and later by Summerfield [11] in 1909. Fano's theory was fully proved with laboratory methods in 1967 by Richer and later by Bieglehl[12]. They separately and with coherent experiments expressed the excitation of surface plasmons by electronic and optical methods. Richer was able to obtain surface plasmons with the help of the scattering relation experiment [13]. He had done this for an incident beam with parallel polarization [14] while Bieglehl did it for both parallel and vertical polarization [15] [1].

It was after the research of the 1960s that surface plasmon waves became more attractive to researchers. They observed that the intensity of these waves, which are generated near the surface of the metal, can change due to the change of environment. Therefore, they can be used to identify small changes in the dielectric constant caused by molecular layering on the surface. These waves are also known as the main factor of improvement in Raman scattering [16], which today scientists use to find the chemical structures of substances even in single molecular dimensions [4].