Optical properties of 2D materials: ultrafast dynamics of electrons, holes, excitons, phonons, plasmons, and defects (Conference Presentation)

Abstract

Two-dimensional atomically thin materials, most notably graphene and transition metal dichalcogenides (TMDs), have generated tremendous interest among researchers. The high electron mobility and strong light absorption exhibited by these materials make them attractive for opto-electronic applications. We will present our recent experimental and theoretical work on the ultrafast dynamics of collective excitations, such as excitons, phonons, and plasmons, in these materials for electronic and photonic device applications. We study the dynamics of excitons in 2D materials and optoelectronic devices using ultrafast optical/terahertz pump-probe and correlation spectroscopy. Our experimental work on metal dichalcogenide materials and devices (such as photodetectors) as well as our theoretical results show that defect assisted recombination involving capture of excitons and carriers by Auger scattering is the fastest mechanism for the non-radiative recombination of photoexcited electrons and holes. In particular, the very Coulomb interaction that resulted in the strongly bound excitons in these materials, causes extremely fast capture of the excitons by defects resulting in extremely poor quantum efficiencies in optoelectronic devices. The large sensitivity of device performance to defects is thus fundamental to 2D TMD materials. Defect-passivated 2D materials have demonstrated quantum efficiencies approaching ten percent. Our ultrafast two-pulse photovoltage correlation experiments show that the photoresponse of TMD photodetectors can be very fast making them useful for operation at frequencies in the hundreds of gigahertz range. Our recent experimental work has shown that 2D materials could be very promising for high frequency phononic devices. Our work has shown that mechanical oscillations in these atomically thin membranes can reach terahertz frequencies and are tunable from few tens of gigahertz to almost one terahertz. 2D material membranes can therefore enable MEMs resonator structures with record frequency-quality factor products at these high frequencies. Our ultrafast work in graphene plasmonic structures has revealed enormous potential for graphene based VLSI interconnects in which electrical signals are carried by plasmonic waves with much reduced propagation delays, losses, signal distortions, and cross-talk compared to conventional metal interconnects like copper.