The PI has previously developed strong skills in quantum mechanical modeling of optical phenomena in carbon nanotubes (CNT), as required in the field of quantum computing with CNT. In this work, he plans to apply these skills to two modeling tasks with broader implications: (1) the extraction of energy from ambient radiation at microwave and terahertz frequencies ; (2) surface plasmon amplification by stimulated emission of radiation -- a "laser for plasmons."
Intellectual Merit:
This work will fill a very important hole in our understanding of what is possible with energy scavenging. A classic paper by Popovic's group calculated that it is possible to extract up to 20% of the energy of ambient disordered microwave radiation using spiral rectennas, but these calculations did not account for coherence issues and other quantum effects, and they did not really attempt to describe what happens at terahertz frequencies, where feature sizes are smaller. This PI has an excellent background for accounting for such effects, considering one possible way to implement spiral rectennas on smaller length scales -- CNT composite structures. New capabilities in surface plasmonics are also extremely important to the NSF priority Beyond Moore's Law.
Broad Impacts:
This project will also have a big impact on education at North Carolina Central University, the nation's first state-supported public liberal arts college for Afro-Americans. The cross-cutting and advanced nature of this work, and the PI's plan for integration of research and education, ensure large outreach benefits here. New sources of energy and faster computers may also result.
Over the past few years, research into the fundamental physical properties of quasi-one-dimensional (1D) carbon nanostructures has uncovered their intriguing optical attributes lending themselves to attractive device applications. The outcomes of this research project further advance the frontiers of fundamental knowledge on near-field ultra-fast quantum electrodynamical processes in carbon nanotubes, showing the pathways for a new carbon based nanotechnology – nanotube plasmonics with carbon nanotubes used as controlled absorbers of electromagnetic radiation, or optical switchers, or near-field optical sensors, or for materials nanoscale modification. New deeper understanding gained during this project implementation offers new nanotube-based optoelectronic device concepts both for fundamental research, such as cavity quantum electrodynamics and solid state quantum information, and for advanced optoelectronic materials engineering for use in energy related applications such as efficient energy rectification, recycling and storage. For pristine semiconducting carbon nanotubes, optically excited excitons are theoretically demonstrated to generate and amplify surface plasmons in individual nanotubes. Surface plasmons are coherent charge density waves due to the periodic opposite-phase displacements of the electron shells with respect to the ion cores. In general, plasmons cannot be excited by light in optical absorption since they are longitudinal excitations while photons are transverse. In small-diameter (one nanometer) semiconducting carbon nanotubes, light polarized along the nanotube axis excites excitons which, in turn, can couple to the nearest (same-band) interband plasmons. Both of these collective excitations originate from the same electronic transitions and, therefore, occur at the same (low) energies of the order of one electron-volt, as opposed to bulk semiconductors where they are separated in energy by tens of electron-volts. They do have different physical nature and properties. Their coexistence at the same energies in carbon nanotubes is a unique feature of the confined quasi-1D geometry where the transverse electronic motion is quantized to form 1D bands and the longitudinal motion is continuous. Charge density waves (plasmons) generated due to the nonradiative exciton-to-plasmon energy transfer, produce oscillating electric fields concentrated locally throughout the nanotube surface. The entire process can be controlled by an electrostatic field applied perpendicular to the nanotube axis. The strong local coherent fields generated in this way can be used in a variety of new tunable optoelectronic device applications, such as near-field optical probing and sensing, optical switching, enhanced electromagnetic absorption, and materials nanoscale modification. In hybrid metallic carbon nanotube systems (nanotubes containing extrinsic atomic type species such as semiconductor quantum dots, extrinsic atoms, or ions), low-energy plasmon resonances mediate near-field quantum effects such as resonance absorption, decay and bipartite entanglement. These can be controlled and monitored by using non-linear optical experimental techniques such as 2D photon-echo spectroscopy, or double quantum coherence spectroscopy. For more complex graphitic nanostructures such as double wall carbon nanotubes, strong overlapping plasmon resonances from both tubes warrant their stronger attraction. Nanotube chiralities possessing these collective excitation features are shown to result in forming the most favorable inner-outer tube combinations in double-wall carbon nanotube systems. This NSF funded research project provided a unique opportunity for underrepresented minority students at North Carolina Central University (NCCU) to gain necessary research experience to become more competitive in their future careers in science and engineering. The project favorably impacted the newly established Master program in Physics by providing research scholarships for two graduate Physics majors to work on their Master theses. It also promoted interdepartmental collaborative ties between NCCU's Department of Physics and Department of Chemistry where closely related experimental research into properties of tailored graphene-based assemblies is under way and seeks for theoretical support. The project integrated research and education in the Physics Department at NCCU through new graduate course (Advanced Solid State Physics) developed and taught by the project PI, which included the general theoretical description of collective excitations (excitons and plasmons) in quasi-1D quantum systems as a specific example of advanced theoretical methods as applied to cutting-edge contemporary research.
