Graphene-based Nanophotonic Modulators for Near-Infrared and Mid-Infrared Applications

Abstract

Department of Electrical EngineeringGraphene, one of carbon allotropes, which has a two-dimensional hexagonal crystalline structure, has been spotlighted in optoelectronics and photonics since it has extraordinary properties such as zero bandgap, high electron mobility, and electrically-tunable chemical potential. Diverse graphene-based optoelectronic and photonic devices, including optical modulators, optical sensors, and photodetectors, have been investigated up to now. This thesis focuses on graphene-based optical modulators working in the near-infrared (near-IR) and mid-infrared (mid-IR) ranges. In the near-IR range where wavelengths are between 800 nm and 2 ??m, graphene is usually utilized as an active material for optical modulators. Optical signals can be tuned by controlling the chemical potential of graphene because the interband transitions of electrons are allowed or prohibited depending on the chemical potential and photon energy. Due to this tunable absorption by graphene, a variety of graphene-based silicon photonic electroabsorption modulators (EAMs), which can be merged into silicon (Si) photonic integrated circuits, have been proposed and demonstrated. Basically, those modulators are based on the structure in which graphene is placed on the surface of a Si waveguide. Since graphene interacts with the weak evanescent field of a Si waveguide mode, the modulation depths of the previous graphene-based Si waveguide modulators are smaller than 0.16 dB/??m. In order to reduce a device footprint, different methods of integrating graphene with a Si waveguide are essentially required. In this respect, two waveguides are investigated. One is an inverted-rib-type (IRT) Si waveguide, and the other is a metal-insulator-silicon-insulator-metal (MISIM) waveguide. In the case of the former, graphene is placed within the region where the electric field of the IRT Si waveguide mode is mainly confined. In the case of the latter, graphene is placed on the narrow insulator where the electric field of the MISIM waveguide mode is strongly confined. Consequently, both the waveguides can have enhanced light-graphene interaction compared to previous graphene-based waveguides. The EAM based on the IRT Si waveguide is theoretically investigated, and its calculated modulation depth is 0.41 dB/??m. The MISIM waveguide covered with solid-electrolyte-gated graphene is experimentally demonstrated. The measured modulation depth of the MISM waveguide is 0.276 dB/??m. In addition, for a larger electrical bandwidth, the MISIM waveguide covered with double graphene layers is theoretically investigated: its calculated modulation depth and electrical bandwidth are 0.412 dB/??m and 185 GHz, respectively. In the mid-IR range where wavelengths are between 2 ??m and 20 ??m, graphene with an appropriate chemical potential has metallic characteristics such that a graphene plasmon (GP), which is a collective electron oscillation in a graphene layer, exists in the mid-IR range. The electromagnetic wave associated with a graphene plasmon is called a graphene plasmon polariton. Graphene plasmon polaritons have unique properties such as strong field confinement around an ultrathin graphene layer and tunability based on electric gating. Various mid-IR modulators using a GP have been designed and demonstrated. However, those mid-IR GP devices have a difficulty in efficiently exciting a GP and practically using it. To solve this problem, a mid-IR modulator based on grating-assisted coupling between a hybrid plasmonic waveguide mode and a GP is proposed and theoretically investigated. The operation principle of the modulator is that the grating-assisted coupling generates a rejection band in the transmission spectrum of the hybrid plasmonic waveguide. The modulation efficiency of the modulator is estimated to be 25.25 dB. To realize this modulator, it is essential to fabricate a grating with a period of less than 200 nm on a zinc sulfide (ZnS) film used for the modulator and to check if a GP can be really excited by the grating. For the purposes, a dry etching process necessary for making ZnS nanostructures is established and it is used to make various graphene-covered ZnS gratings. It is clearly observed that GPs are effectively excited by the gratings. Finally, a mid-IR perfect absorber based on a metal-insulator-metal (MIM) structure is experimentally investigated to demonstrate the feasibility of the grating-assisted coupling between a GP and an MIM waveguide mode. The absorber has the perfect absorption caused by the magnetic dipole resonance existing in the MIM structure. When the MIM waveguide mode is coupled to the GP around the graphene embedded in the insulator, the magnetic dipole resonance is suppressed, and the perfect absorption disappears. It is observed that the realized absorber has an intensity modulation of 65 %. This work not only indicates the possibility of realizing the theoretically investigated waveguide modulator but also shows the efficient free-space mid-IR modulator.clos