Heterogeneous integration of two-dimensional materials for on-chip optical interconnects

Abstract

Two-dimensional materials have emerged as promising candidates to augment existing optical networks for metrology, sensing, and telecommunication. Their structural nature lends themselves remarkable flexibility to be conformally transferred and "glued" strongly onto arbitrary bulk semiconductor substrates by van der Waal forces. This offers a simple approach to construct heterogeneous photonic architectures, which is currently challenging for silicon-based photonics integrated with germanium and III-V semiconductors due to mismatched lattice constants and thermal properties. In addition, the entire family of 2D materials can exhibit a rich variety of physical behaviors, ranging from that of a wide-bandgap insulator to a narrow-gap semiconductor to a semimetal or metal. Previous demonstrations suggest that generic building blocks for a photonic integrated circuit including light sources, modulators, and photodetectors can be accomplished using 2D materials. Beyond conventional components, distinct 2D materials can form a variety of 2D heterostructures with high quality, enabling potential optoelectronic devices that were not feasible using silicon and other bulk semiconductors. In this dissertation, I present several classes of active photonic components in a 2D materials-based heterogeneous architecture. First, by depositing graphene onto silicon photonic crystal nanocavities, it is possible to reach near unity absorption into graphene. The cavity-graphene system enables a high-contrast (> 10 dB) electrooptic modulation by electrically tuning the Fermi level of graphene. High-speed modulation is possible using high-speed capacitive gating, such as through a double layer graphene stack encapsulated in 2D hexagonal boron nitride layers. The demonstrated modulation speed exceeds 1.2 GHz. The cavity also enables dramatically enhanced and spectrally selective photodetection in graphene. To further enable broadband photodetection, a similar scheme that couples graphene to the evanescent field of nanophotonic waveguides extends the interaction length of graphene with light dramatically, achieving high-speed (> 40 GHz) photodetection with high responsivity. The responsivity of the detector reaches a maximum (0.36 A/W) when the doping of graphene is controlled at a level that maximizes photothermoelectric, photovoltaic and bolometric effects in graphene. Finally, light sources based on cavity-integrated 2D molybdenum disulfide show pronounced fluorescence enhancement owing to the Purcell effect in an optical cavity. An electrically-driven thermal light source of graphene in a cavity provides a new route to control the thermal emission spectrum at a temperature beyond 2000 K. These heterogeneous architectures and devices combine the advantages of 2D materials and dielectric photonic structures, promising for an ease-of-fabrication, large-scale and high-performance optical interconnect.