CAREER: Unconventional Mid-infrared and Terahertz Sources Employing Graphene Plasmonics and Intersubband Transitions in Quantum Wells

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Nontechnical description: The mid-infrared (MIR) to terahertz (THz) spectral range has its unique and crucial scientific and technological importance. Countless molecular species have strong and "fingerprint" like absorption lines in the MIR to THz spectral range, and consequently absorption spectroscopy based molecular sensing technologies with superior sensitivity and selectivity can be developed in this spectral range. Furthermore, as the demand for communication bandwidth keeps growing rapidly, wireless communication technologies are moving from the GHz frequency range towards the THz frequency range. Therefore, research advances in the MIR to THz range will continue to drive development of technologies and applications in important areas such as health care, homeland security, environmental protection, energy production, communications and internet of things. However, current device technologies for this spectral range are not as diverse and mature as those for the other spectral ranges. Aligned with the long-term career goals of the principle investigator, this CAREER proposal aims at developing new types of MIR and THz light sources based on unconventional device architectures and operating principles. Specifically, the proposed light sources will synergistically combine the advantages of graphene plasmonics and intersubband transitions in quantum wells, which are two powerful device technologies for the MIR to THz spectral range. Exploiting strong interactions between intersubband transitions in quantum wells and graphene surface plasmons (GSPs) is expected to lead to devices with improved performances, lower cost and extended functionalities. The proposed CAREER project will provide valuable opportunities and resources to help the participating students becoming the next-generation scientists and engineers who can play leading roles on the competitive global stage. The principle investigator will also actively organize and participate outreach activities which will disseminate the research outcomes and/or stimulate K-12 students' interests in the STEM disciplines. These activities will be designed to attract participation of students of diverse backgrounds, including those from the under-represented groups. Technical description: This proposed CAREER project aims at first thoroughly studying the fundamental properties of strong interactions between GSPs and intersubband transitions in quantum wells, and subsequently utilizing the obtained knowledge to develop two types of unconventional MIR and THz sources, which are (1) graphene plasmonic antenna-enhanced light emitters and (2) electrically-pumped GSP sources. Coupling between MIR or THz GSPs and resonant intersubband transitions in quantum wells establishes an intriguing physical system and a new platform for device applications. This project will focus on investigating several fundamental aspects of these interactions, including the potential modification of transition selection rules, the Purcell effect, the dependence of interaction strength on graphene carrier density and structure geometries, and the transition energy shift associated with non-vertical transitions. A comprehensive understanding of these aspects is crucial for the subsequent device development. Conventional MIR and THz light emitters suffer from the low efficiency of radiative transition process which has a lifetime by orders of magnitude longer than the non-radiative lifetime. The proposed graphene plasmonic antenna-enhanced MIR and THz light emitters aim at exploiting the strong interactions between GSPs and intersubband transitions, which are expected to lead to a drastic reduction of the radiative lifetime and hence a significant improvement of output power and power efficiency. In contrast to the conventional methods of generating GSPs via optical excitation, the proposed devices employing graphene plasmonic waveguides coupling to intersubband transitions are electrically pumped surface plasmon sources, which are much more compact and convenient to use in applications such as on-chip sensing and communications. Both types of proposed MIR and THz sources have large frequency tunability, which is another key advantage of graphene based devices.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.