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Investigation Of Graphene Based Plasmonic Filters

Posted on:2016-01-27Degree:DoctorType:Dissertation
Country:ChinaCandidate:S W ShengFull Text:PDF
GTID:1108330461484434Subject:Radio Physics
Abstract/Summary:Request the full-text of this thesis
In the past decade, we have witnessed the new technologies including silicon pho-tonics, photonic crystal and surface plasmon polartitons revolutionized many research fields. The rise of them enable us manufacturing more energy saving, ultra-small, inte-grated, very-fast and multi-functional photonic devices. The optical waveguide-based devices, such as optical splitters, couplers and filters occupy very important roles in photonic applications, so, to study miniaturizing optical waveguide and waveguide-based devices is key to promote the development of very large-scale integrated optical circuits.Silicon photonics is the study of photonic systems which use silicon as an optical medium. The refractive index of silicon is larger than silica, so the mode confinement ability a of silicon based waveguide is greater than the silica based waveguide, there-fore when operating at same wavelength, the silicon based device can be more smaller. Moreover, silicon photonic devices can be made using existing semiconductor fabri-cation techniques, and it is possible to create hybrid devices in which the optical and electronic components are integrated onto a single microchip. However, the dielectric waveguide need large radius to avoid the bend loss, so the size of dielectric optical waveguide based devices are relatively large. A promising substitute for the dielectric waveguide is the photonic crystal waveguide. A photonic crystal is an artificial optical material. Light waves in photonic band gap are forbidden to propagate in the photonic crystals. However, by introducing a line defect, it can act as a waveguide for those light, therefore photonic crystal waveguide can be constructed so that the light waves within the band gap can be stabilized long distance propagation in the photonic crystal waveguide in long distance. Also, due to the presence of the forbidden band, it can be utilized to construct a rectangular waveguide with very small bending loss, which fur-ther promoted the waveguide device itself as well as interconnecting waveguide device miniaturization. SPPs are waves that propagate between the surface of a conductor and dielectric media, it can be utilized to construct waveguide with very strong confine-ment ability, beyond the diffraction limitation, and with subwavelength size. Over the past ten years or so, researchers have focused mainly on noble metals plasmonics, with silver and gold being the predominant materials of choice. Using these material, many waveguide structures are proposed including strip shape, V shape, wedge shape, ridge shape, MIM shape etc. Today, metal plasmonics constitute the foundational pillars for applications. However, these breakthroughs were realized mainly in the visible to near-infrared frequencies. Moreover, tunning of metal SPP devices are difficult to achieve once the devices are fabricated. With those drawback, even till now, the researchers are still searching for more excellent plasmonic materials.In the ongoing search for new plasmonic materials, graphene has emerged to be a very promising candidate for terahertz to mid-infrared applications, the frequency range where its plasmonic resonance resides. Today, the terahertz to mid-infrared spectrum, is finding a wide variety of applications in information and communica-tion, medical sciences, homeland security, military, chemical and biological sensing, and spectroscopy, among many others. Unlike metals, free carriers of graphene can be induced through chemical doping or electrical gating with great ease. Researches show that solid electrolyte gating can allow for high concentration of free carriers of 0.1 per atom, which translates to a chemical potential of Ef≈1 eV. As a result, the two-dimensional material, graphene, allows for electrical tunability not possible with conventional metals.Graphene, besides being a unique material with tunable optical properties, is al-so an excellent conductor of electricity. Highest attained carrier mobility has reached 106cm2/(Vs) in suspended samples. Because of the high conductivity, stable physical and chemical property, scientists hope that the graphene nanoribbon can be used for the interconnecting material in integrated electric circuits. So the developing of elec-tric cirtuits can continue be predicted by moore’s law. In the researches of graphene nanoribbon, scientists find that the confinement ability graphene SPP is much more stronger than the graphene sheet due to the appearance of the rim. As the width of ribbon become narrower, the neff of the SPP mode supported by graphene nanoribbon can be higher than 150. If using side coupled or layered coupled graphene nanorib-bon, the neff of it can be further higher. So, graphene nanoribbon have huge potential for realzing the SPP device. Recently, many researchers devoted themselves on trans-planting the traditional dielectric and SPP waveguide based devices to the graphene nanoribbon platform, verifying the feasibility and analysing the speciality. Many kinds of two-dimensional SPP waveguide devices based on graphene nanoribbon have been proposed and studied these days, such as optical splitters, optical couplers, resonators, filters, etc. However, due to the limitation of fabrication and detecting technology, researches are solving these issues mainly depend on theory analysis and numerical method. Among many of these device, filters are very useful elements for WDM com-munication system. As the developing of optical communication, the wide application of WDM device have triggered massive need for more ultra-small and integrated op-tical filters. Filters structured as resonators coupling with waveguide are of the first choice because they can realize narrow band filter in very small size. We can get a well performance narrow band and broad FSR band stop add-drop filter by merely coupling a micro ring resonator to a waveguide. If we put the ring between two waveguide, we can get a band pass filter. If we transplanting these filters to the graphene nanoribbon platform, naturally we can realize ultra small, gate tunable and broad band applicable filters, which are very hard to achieve at same time using other materials.In this thesis, the main work is designing the plasmonic filters based on graphene nanoribbon platform. With the foundation of propagation property and guiding method of graphene SPP, we designed the filter structures, build models and analysis them by using numerical method. We explored the physics of achieving filter effect, how to tune the filter by versatile method, and the influence of many parameters on the filter transmission spectrum. The work of this thesis provide theory references for fabricating ultra small plasmonic filter based on graphene nanoribbon. According to the aforementioned research content and goal, the original work of this thesis can be listed as follows:1) In this thesis, we studied the interaction between the electromagnetic waves and the graphene, investigated the propagation property of excited SPP mode on the surface of graphene sheet. Due to different fabrication method, the graphene sheet can have different property and quality in different substrate, especially some fabrication method, the graphene sheet cannot be transferred. So, we stud-ied the dispersion property of graphene nanoribbon when it was free standing in the air, deposited on SiO2/Si layered substrate and the Si substrate. The guided SPP modes are tightly confined on the surface, lateral and the propagation direc-tion of the ribbon. The single mode operating region were identified when the ribbon is very narrower.2) By using Drude dispersion model, combined with the 3D effective dielectric con-stant, we simulated the 3D model including 2D monolayer graphene in the struc-ture using FDTD method. We designed the model of filters, calculated them with FDTD method and give a method to extract the power spectrum and the transmission spectrum of the filter. The calculated result is verified to be correct.3) Three kind of plasmonic filters based on graphene resonators coupling with graphene nanoribbon were proposed and analysed in this thesis. Firstly, a ultra-compact bandpass plasmonic filter based on two graphene ribbons coupling with a graphene nanodisk is proposed in this work. We analysed the theory and the advantage of the disk resonator. After that, the influence of depth of substrate on the transmission spectrum was analysed. We found that the central frequencies of the transmission peaks can be controlled by changing the radius of the nan-odisk and the chemical potential of the graphene. Furthermore, the bandwidths of resonance spectra can be tuned by adjusting the coupling distance between the nanodisk and the graphene ribbon.Then a plasmonic filter based on graphene nanoribbon resonator was proposed. By coupling a graphene nanoribbon resonator with a nanoribbon waveguide, we will have a simple structured bandstop plasmonic filter with -20 dB transmission dip. By using FDTD method, we analysed the theory and property of the plas-monic filter, studied the parameters having influence on the transmission spec-trum. Also the gate tuning property of the filter was investigated. As application examples, we designed and analysed a Y splitter with switch ability and a 1x3 WDM device.Lastly, a filter structured as a graphene rectangular ring resonator coupling with nanoribbon was proposed. With the strong mode confinement ability and higher neff, graphene nanoribbon can be used to build sharp bend with no bend loss. This is why the rectangular resonator can be achieved. The special double mode split was investigated. According to the coupling mode theory, we analysed ev-ery parameter having influence on the coupling and the transmission, discussed the method to achieve critical coupling.4) The filter based on resonators coupling with waveguide can achieve narrow band filtering effect, however, the bandwidth is also very important parameter of the filter. According the propagation property of graphene SPP, we proposed the tooth shaped plasmonic filter. This structure was numerically analysed by using FDTD method. The tooth shaped structure of graphene nanoribbon can induced to a sharp band-stop effect in the transmission spectrum, and the filtering charac-teristics can be analysed by the scattering matrix method. Transmission spectrum can be changed by changing the length, width of the tooth and the chemical po-tential of the graphene. In addition, transmission spectra of multi-teeth shaped plasmonic filter was also studied. This kind of structure can realize the broad band-stop filtering property. Samely, central frequency of the stop band can be influenced by the geometry parameters and the chemical potential of graphene.
Keywords/Search Tags:graphene, plasmonics, FDTD, waveguide, filter
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