Graphene Plasmonic Nanostructures for Terahertz Light Emission

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Title: Terahertz Light Sources Based on Graphene Plasmonic NanostructuresNontechnical Abstract:Electromagnetic radiation with frequency in the 1-10 THz range is ideally suited for many demanding imaging and sensing applications. Unlike visible or near-infrared light, THz radiation can propagate through many common packaging materials and therefore provides visual access to concealed objects or defects. Furthermore, many chemicals of potential interest for sensing applications, including explosives and illicit drugs, feature distinctive absorption resonances at THz frequencies and therefore can be accurately detected by THz spectroscopy. Specific areas where these capabilities can play an enabling role include security screening, medical diagnostics, manufacturing quality control, and artwork conservation. However, the widespread adoption of these technologies has so far been hindered by the lack of suitable devices for the generation of THz light. Existing sources tend to be bulky and expensive, often requiring cryogenic cooling, have limited frequency tunability, and otherwise cannot provide sufficient THz output power for most applications.This project will develop a new device technology for THz light emission that can overcome these critical limitations, by leveraging recent advances in materials science and fundamental nanophotonics. The proposed devices can operate at room temperature with the required output power levels (several milliwatt) for typical THz applications, can be manufactured at low cost with highly miniaturized form factors, and are broadly tunable across the THz spectrum. As a result, these devices are promising for a transformative impact on a diverse set of technology sectors that can benefit from the unique capabilities of THz imaging and sensing. The proposed activities will also promote education through the training of graduate and undergraduate students in relevant areas of optoelectronics, nanophotonics, and materials science, and through related curriculum development efforts. Technical Abstract:The proposed devices are based on the recently developed family of two-dimensional materials and heterostructures – specifically single-layer graphene combined with the two-dimensional semiconductor molybdenum disulfide (MoS2). The underlying radiation mechanism involves the excitation of THz plasmon polaritons (collective oscillations of the electron gas) by current injection, and their outcoupling to free-space radiation in specially designed graphene nanostructures. The proposed work focuses on maximizing the efficiency of this radiation process, through a combination of electromagnetic design simulations, materials development efforts, and device fabrication and characterization activities. The plasmonic extraction efficiency will be optimized by combining the graphene nanostructures with additional optical elements (metallic THz antennas in an open-cavity configuration) designed to promote critical coupling to free-space radiation. At the same time, the plasmonic internal emission efficiency will be enhanced through the controlled injection of non-equilibrium carrier distributions in the plasmonic nanostructures using graphene/MoS2 Schottky junctions. The latter idea is analogous to the principle of operation of light emitting diodes (LEDs) used in solid-state lighting, but applied to an entirely new material system and spectral region. In addition to its potential technological impact on THz imaging and sensing, this project will also create new knowledge and new research opportunities in the area of plasmonics and light-matter interactions in two-dimensional materials.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.