Statement of Objectives:

Several roadblocks exist to expanding the capabilities of the optical systems currently used for long and short-distance communication. Not only is the bandwidth of components limited but also there is a high energy cost of optical communication, which is mainly a consequence of the poor energy efficiency of modulators and optical sources, and the high loss and large sizes of the critical optical components. New frontiers in scientific and engineering research of both optical materials and photonic devices need to be opened up to search for disruptive technologies that can overcome these power and bandwidth limitations for both civilian and military needs.

The objective of this proposal is to develop novel nano-optoelectronic systems based upon two- dimensional (2D) heterostructures for such new technologies. Our 2D heterostructure devices will feature monolayer transition metal dichalcogenides (TMDCs), which have recently been found to exhibit extraordinary optoelectronic properties unattainable in conventional materials1,3- 15, including direct bandgap in the visible, robust and tunable 2D excitons, strongly coupled and optically addressable spin-valley degrees of freedom, and outstanding nonlinear optical properties. In combination with other 2D crystals such as graphene and boron nitride, these heterostructures can be utilized for revolutionary integrated optoelectronics and photonics. To carry out the necessary science, innovation and development we have formed a strong and coherent team consisting of experimental and theoretical physicists (Xu, Cobden, Xiao), materials and spectroscopy scientists (Shen, Moore) and electrical engineers (Xia, Li, Koester). Our specific aims include:

1. To develop improved, scalable growth methods using physical and chemical vapor deposition, including molecular beam epitaxy, for synthesizing 2D TMDCs and creating heterostructures with precise control of purity and stoichiometry;
 to investigate doping control and contact formation, carrier mobility, fundamental effects of heterogeneous integration of various monolayer or bilayer TMDCs with BN and graphene, carrier diffusion, edge effects, interface states and band alignments;

2. To determine fundamental optical properties of 2D TMDCs and their heterostructures, including the dynamics of valley excitons, valley depolarization and decoherence rates, exciton diffusion lengths, edge effects, and second- and third-order nonlinear optical coefficients;
 to demonstrate a 2D heterostructure-based ultrafast fast photodetector, ultra-high gain phototransistor, and spin-polarized light emitting diodes;

3. To demonstrate integrated 2D heterostructure/waveguide devices for chip-scale nonlinear nanophotonics, including second-harmonic generation for telecommunication, optical parametric amplification and oscillation, and frequency comb generation with low pump power; and
 to develop integrated chip-scale nanolasers, including electrically pumped p-n junction nanolasers and double heterojunction nano lasers.

We thank AFOSR for financial supports.