Loading...
| Friend's email | |
| Your name | |
| Your email | |
| enter code | |
This page was sent successfuly
- Type of Document: Ph.D. Dissertation
- Language: Farsi
- Document No: 51181 (05)
- University: Sharif University of Technology
- Department: Electrical Engineering
- Advisor(s): Faez, Rahim
- Abstract:
- In this study, we simulated and analyzed a plasmonic waveguide modulator based on single layer graphene. It includes a graphene sheet, which sandwiches between two layers of silicon dioxide. Then, some gates are arranged on either side of the waveguide on a periodic structure. When an electric field is applied perpendicular to the waveguide plate, the Fermi level of graphene under the gates, changes. Detailed analysis is performed by the method of lines based on Maxwell's equations along the propagation direction of the waveguide. Computation of the multi-gate device starts by examining the effect of Fermi level. Transmission coefficient of the magnetic-field norms of the modulator is calculated by varying the parameters such as Fermi level, length, gates number and distance between the gates to achieve optimized design of the modulator device with very small dimension. The results show that at higher Fermi levels, where the imaginary part of the effective index of the waveguide is close to zero, the reflection is dominant and absorption is low. Therefore, the modulator length becomes so long that is more than one hundred nanometers. At lower Fermi level, where the amount of imaginary part of the effective index is significant, the absorption is dominant. At this range, one-gate device is sufficient for modulation. Consequently, the designed minimum device length becomes equal to six nanometers for ten-micron wavelength. Furthermore, the design is carried out in other wavelengths.Consequently, an electro-plasmonic modulator is modeled based on the plasmonic waveguide. We calculate the fundamental mode of the waveguide, the characteristic impedance, and circuit elements by considering losses when varying the Fermi level of graphene sheet. Especially, we consider the loss of a single waveguide and the loss of graphene conductance discontinuity in our research. The transmission line elements for the plasmonic waveguide including a series inductor, a series resistor, a parallel capacitor, and a parallel conductor are calculated for different Fermi levels and frequencies. Then, the discontinuity effect is investigated in the graphene conductivity, and its scattering matrix is extracted by considering losses and using the method of lines to solve Maxwell's equations. Using this circuit model, the transmission coefficient of the modulating device is monitored at several near-infrared wavelengths and at a few gate lengths and is compared to the Comsol simulation results.Afterwards, the modal analysis of the lossy coupled plasmonic waveguide is carried out where two graphene sheets are sandwiched between three layers of SiO2. Even and odd complex propagation constants are extracted for the coupled transmission lines along the direction of propagation. Besides, even and odd characteristic impedances at various Fermi levels are calculated. The first even mode has no loss, but the other even and odd modes are very lossy. Taking into account losses, coupled transmission line RLCG model including self and mutual elements, are calculated at various Fermi levels and specific frequency. Then, a lossy plasmonic directional coupler analyzed with a length of a non-quarter of plasmon wavelength based on the coupled plasmonic waveguide for a variety of different cases. Furthermore, the device is implemented by ADS. The transmission, reflection, coupling and isolation coefficients of the device are obtained for several Fermi levels and coupler length in order to maximize power on coupled terminal and minimize it on reflected and isolated terminal. The results show that the minimum length of the plasmonic coupler is 39 nm at 40 THz. Also, by increasing the frequency, the optimum device length is decreasing and the sheets Fermi level increasing. Subsequently, the coupled plasmonic waveguide is simulated by Comsol MultiPhysics using finite element method to verify even and odd modes
- Keywords:
- Modulators ; Plasmons ; Waveguides ; Couplers ; Nanomaterials ; Graphene ; Circuit Model ; Lossy Transmission Line
Digital Object List-
محتواي کتاب - view
Bookmark