Final Defense

Nima Dabidian, UT-Austin

"Active graphene metasurfaces for opto-electronic applications"

8:30am, RLM 11.204

Abstract: Plasmonic metasurfaces are optical components that enhance the light-matter interaction and can control the flow of light. The scalability and universality of the metasurface design enables their deployment across the entire electromagnetic spectrum. Especially attractive are the metasurfaces designed to operate in the mid-infrared part of the optical spectrum. That is due to two factors: the variety of technological applications and the limited choice of conventional optical components in the mid-IR spectral region.

Graphene has emerged as a promising optoelectronic material because its optical properties can be rapidly and dramatically changed using electric gating. In particular in the mid-IR regime, graphene has plasmonic properties and can be utilized as a tunable inductor for active modulation of the light with an ultra-fast rate. However graphene’s weak optical response, especially in the infrared part of the spectrum, remains the key challenge to developing practical graphene-based optical devices such as modulators, infrared detectors, and tunable reflect-arrays.

In this thesis, we take advantage of plasmonic metasurfaces to enhance the light interaction with graphene for crucial optoelectronic applications. The ability to modulate light at a high-speed is an important part of modern communication systems. We use the plasmonic properties of graphene in the mid-IR range to spectrally shift a narrow-width Fano resonance. Using this approach, we achieve strong modulation of amplitude and phase using plasmonic Fano-resonant metasurfaces integrated with graphene. We also demonstrate that the strong spectral shift of the plasmonic resonance can be used to extract one of the key optical parameters of graphene: the free carrier scattering rate.

Another interesting branch of optics is wave-front engineering. The ability to dynamically manipulate the phase, inspires interesting applications such as beam steering and holograms. We use a Michelson interferometry to measure the graphene-induced phase modulation. In particular, we show it is possible to modulate the phase of electromagnetic wave while the amplitude is constant. We demonstrate proof of concept application of active phase modulation in motion sensing and polarization conversion.

An emerging area of optoelectronic is ultra-fast photodetectors based on graphene and other 2-D materials. We employ Full-wave and electrostatic simulations to study the performance of a metasurface-based photodetector. Circuit analysis is utilized to provide a mathematical relation for the responsivity of the metasurface-based graphene photodetector. It is shown that electrically-connected metasurfaces can dramatically enhance the collection efficiency of graphene photodetectors.