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Research Of The Optical Modulators Based On The ε Large Tunable Range Materials

Posted on:2017-01-08Degree:DoctorType:Dissertation
Country:ChinaCandidate:L Z YangFull Text:PDF
GTID:1108330488491028Subject:Microelectronics and Solid State Electronics
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As the information technology develops rapidly, the traditional communication methods based on the electronical interconnections have reached the bottleneck of breaking through. The speed of the information transportation, the power consumption and the feature size of the MOSFET are the main parts of the problems. Fortunately, the investigation of the optoelectronics opened another door for the communication technology. Some fundamental achievements were obtained by those basic optics devices composed by the III-V series materials, the germanium, the silicon and some other bulk materials. It is still necessary and urgent to seek for the new material because the photoelectric effects as well as the permittivity (ε) tunable range of the traditional materials are quite small. In 2004, graphene drew great attention all around the world after it was discovered. It possesses the large range tunability of the ε, the super high carriers’mobility, the free plasticity and it is able to switch between the metallic and the dielectric property. Since A. Vakil and his colleagues revealed the calculation method about the surface permittivity of the graphene in 2011, a great number of research achievements about the graphene optoelectronics were published like the mushrooms after rain. For instants, the photoelectric modulator with the 30 GHz speed, the all-optical modulator with the 200 GHz speed, the polarizer with the 27 dB extinction ratio and the photoelectric detector with a theoretical speed as 500 GHz were shown on the papers one by one, which demonstrates the graphene material will play a significant role in the future progress in the integrated optoelectronics territory.Utilizing the ε tunable material to control the optical signals is the foudamental approach to design the optical modulators. This doctoral thesis is based on the combination of the graphene, the AZO (Aluminum-Zinc-Oxide) and the silicon platform. By utilizing the large scale tunability of the permittivity of the new materials from the positive value to the negative one, some investigations about the silicon merged waveguide and the graphene surface plasmonics waveguide are conducted. We design several sorts of optical modulators and switches while the theoretical verifications are made through the simulation software. We expect the breakthrough mainly in three aspects including the extinction ratio, the feature size of the device and the modulation speed. The main contents are shown as below.(1) The graphene and some other oxide materials possess a unique property that the real part of the ε can be tuned to nearly zero. Such a characteristic is used for designing a sort of optical switch by combining the new material to the microring resonator. As the anisotropic property of graphene obstructs this usage, the AZO material takes the place here. The microring resonator is made by the AZO and the common silicon waveguide. The permittivity of the AZO is quite closed to the one of silicon while the loss is very small when the AZO is at the initial status. The microring is resonant for the specific range of wavelength so that the optical information can be output from the drop port. The permittivity of the AZO is tuned to be nearly zero if an external driving voltage dopes it and changes its carriers’density. The huge difference of the permittivity between the AZO and the silicon forces the optical energy to accumulate inside the AZO layer while the energy suffers a severe loss. It directly leads to the non-resonant status of the microring so that the optical information only outputs from the through port. As a result, the microring resonator forms the 2 × 2 optical switch. This device can be used for both the TE modes and the TM modes and the extinction ratio can be as large as about 30 dB.(2) The electromagnetic wave with a specific frequency is able to be squeezed and tunneled through a very narrow channel once if the channel is filled up with the ε-near-zero material and it is properly sealed. Such a property is utilized to design the ultra-compact optical modulator and switch. The periodical metamaterial, which composed of the graphene and the silica, connects two silicon waveguides as a channel. The effective permittivity of the metamaterial is tunable since we can change the chemical potential of the graphene so that the channel can be switched on and off. By replacing the perfect electric conductor by the common metal materials to seal the channel, the feasibility of this kind of device is also studied. Based on the analysis, this thesis demonstrates an optical modulator together with a 1 × 2 switch with the feature size of only 15 nm. Both of these devices obtain a 3 dB extinction ratio with the driving voltage less than 1 V while the highest extinction ratio is nearly 10 dB.(3) The real part of the permittivity of graphene can be tuned to a negative value under the particular doping conditions which means the graphene has the potential to be the waveguide for the surface plasmonics wave. Meanwhile, the hot electrons inside the graphene transit when the graphene is illuminated by the external pump light. For the specific condition, the transition and the cooling process only cost several hundreds of femtoseconds. These physical mechanisms lay the foundation for designing the ultrafast all-optical surface plasmonics modulator based on graphene. This thesis illustrates an all-optical modulator with the speed faster than 1 THz through the theoretical integration as well as the data analysis. This kind of graphene device shows the good performance not only on the speed but also on the modulation depth (16 dB/μm), the power consumption (90 pJ/bit) and the feature size (500 nm).
Keywords/Search Tags:permittivity, integrated optics, silicon waveguide, graphene, chemical potential, surface plasmonics wave, optical modulator, optical swtich
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