Terahertz Quantum Electronics of Carbon Nanostructures: Population Inversion, Gain and Coherent Bandgap Engineering

Iowa State University, Ames IA

Investigators

Abstract

The challenge of pushing the switching speed-limit and integration density of today's logic and modulation devices into the terahertz (one trillion cycles per second) and sub-20 nanometer regime underlies the entire field of information processing, recording and communication. This challenge may be met by a novel paradigm of terahertz quantum nano-electronics based on ultrafast coherent laser pumping in graphene - one atom thick, the honeycomb-shaped carbon material - and single-walled carbon nanotubes the rolled-up sheets of graphene monolayers. Researchers will use short pulsed terahertz light, outside the visible spectrum, and an ultrafast camera technique to directly monitor the formation and time evolution of photo-excited states in these carbon nanomaterials. This novel method will allow them to capture and control their novel electromagnetic properties on the femtosecond scale, or to one quadrillionth of a second. The results will open fascinating opportunities to demonstrate their significant potential to advance, e.g., above-gigahertz light modulators, broadband gain mediums from the infrared to terahertz, radiation controlled hot-electron transistors, multi-functional devices responding to ultrabroadband electromagnetic radiations from the terahertz to visible frequency. Our success in this "ultrafast" and "ultrasmall" challenge will reveal as-yet-undiscovered physical processes for developing new generation optoelectronic device and offer perspectives for sustaining the information revolution and the 21st century's digital economy. Education is an integral and essential component in this proposal. It consists of interconnected, specific plans for education that span small college professors/undergraduates, "A Physics Day" program for high school teachers and their students; outreach to underrepresented minority students and provision of research/training opportunities to them. How coherent photoexcitations control excitonic bosons in single-walled carbon nanotubes and Dirac fermions in graphene monolayers is among the most fundamental, yet cross-cutting, issues in quantum and optoelectronic technologies. The proposal aims to explore some remarkable laser-driven quantum processes in these carbon nanostructures and demonstrate their significant potential for device applications. The primary goals are: to determine broadband gain spectrum and threshold in strongly photoexcited graphene monolayers; to demonstrate coherently photo-driven, bandgap opening near the Dirac cone using intense terahertz pulses; to investigate extreme mid-infrared and far-infrared nonlinear wave mixing in graphene; to achieve terahertz stimulated emission in single-walled carbon nanotubes using two-photon excited, dark exciton states. The approach for the timely advancement lies in the combination of ultrashort terahertz pulses, specially fabricated, high quality mono- and few-layer graphene and carbon nanotubes, and ultra-broadband probe capability from the terahertz to visible spectral regions. This proposal has identified compelling opportunities to advance one of the most poorly- addressed territories in some most exciting materials today dynamical, non-equilibrium, and nonlinear aspects of carbon nanostructures. The targeting problems are in the boundaries of several frontiers such as quantum optical control of matter, terahertz electrical transport, and ultrafast optoelectronic technology. Although sophisticated theoretical studies have been underway, the experimental schemes for exploring a wide range of the predicted fundamental phenomena, as proposed, have lagged behind. These original results are transformative, opening the possibility for graphene- and carbon nanotube- based above-terahertz speed modulators, saturable absorbers, ultra-broadband gain medium.