Graphene travelling wave amplifier for integrated millimeter-wave/terahertz systems

Abstract

Terahertz (THz) technology offers exciting possibilities for various applications, including high resolution biomedical imaging, long-wavelength spectroscopy, security monitoring, communications, quality control, and process monitoring. However, the lack of efficient high power easy-to-integrate sources and highly sensitive detectors has created a bottleneck in developing THz technology. In an attempt to address this issue, this dissertation proposes a new type of graphene-based solid state travelling wave amplifier (TWA).

Inspired by the unique properties of electrons in graphene two-dimensional (2D) fluid, the author proposes a new type of TWA in which graphene acts as the sheet electron beam. These properties include higher mobility and drift velocity at room temperature, zero effective mass, relativistic behavior, and a truly 2D configuration. Since the plasma properties of 2D electron fluid become more pronounced as the effective mass of electrons decreases and electron mobility increases, THz devices based on graphene with massless quasiparticles significantly outperform those made of relatively standard semiconductor heterostructures. Another significant advantage of graphene over semiconductors is that while the high drift velocity and electron mobility of semiconductors 2D electron gas (2DEG) are achieved only at very low temperatures, graphene has high mobility and drift velocity at room temperature.

This thesis describes the theoretical and practical methods developed for the analysis, design, and fabrication of a graphene-based THz TWA. It investigates the interaction between the electromagnetic wave and the drifting plasma wave in graphene by two methods. In the first approach, electrons in graphene are modelled as a 2D Fermi liquid, and the hydrodynamic model derived from a relativistic fluid approach is used to find the conductivity. In the second approach, the travelling wave interaction is analyzed using a quantum mechanical model. The drifting Fermi distribution function is applied to the linear conductivity response function of graphene obtained from random phase approximation. The conductivity of graphene is obtained as a function of frequency, wave number, chemical potential, and drift velocity. The result is consistent with the hydrodynamic approach. Both methods show that negative conductivity, and thus gain, is obtained when the drift velocity is slightly greater than the phase velocity. It is shown that the two methods produce comparable results.

In the next step, a slow-wave grating structure is designed and an estimate of the actual gain is obtained for the proposed graphene TWA structures. The Floquet mode analysis of top grated slab and rectangular silicon waveguides is presented. Here, a new theoretical method is developed to accurately estimate the field distribution of the first order space harmonic of a hybrid mode inside a periodic top-grated rectangular dielectric waveguide. This method gives explicit expressions for the interaction impedance of the slow wave grating structures that are then used to design the waveguide and the grating. To verify the proposed approximation method, the results obtained with this approach are compared with the simulation results.

Finally, a prototype structure is fabricated. The recipes developed for different parts of the structure are presented. These parts include: a nanometer size grating, a sub-millimeter dielectric waveguide, and biasing contacts on top of the graphene layer. The developed recipes ensure reliable fabrication processes for large-area graphene devices. In addition, two different methods used to fabricate long uniform gratings are compared. This work ends by showing the measurement results obtained for the fabricated devices.