Toward Opto-phononics: photo-excitation and control of electrons and phonons in nanostructures

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When a beam of light strikes a bulk conductor, the generated heat spreads to a wide area. In stark contrast, the applicant's research group has recently discovered a phenomenon called "Heat Trap," consisting of strong localization of light-induced heat in nanostructured conductors known as carbon nanotube forests. The illuminated spot can act as a thermal insulator, yet remain electrically conductive. This combination of properties has been sought for decades and has far-reaching implications for many areas ranging from clean energy to information technologies. This proposal builds on this recent ground-breaking finding. We will investigate the fundamental mechanisms behind Heat Trap. In addition, we will pursue two novel devices based on it, each capitalizing on certain aspects of Heat Trap. The strong localization of heat, preventing loss of thermal energy, allows the efficient heating of the illuminated spot to extremely high temperatures with a low-power beam of light. Since the spot retains electrical conductivity, this enables thermionic emission of electrons into vacuum using readily-available light sources like the sun. Based on this, we will engineer the world's first Thermionic Solar Cell. This device could outperform existing solar cells in efficiency and, moreover, could have inherent storage capacity. It could thus have a major impact in renewable energy technologies. In Heat Trap, it appears that light can regulate the flow of phonons (particles associated with thermal vibrations). Based on this property, we will create the world's first optically-controlled thermal transistor. The electronic transistor is at the core of today's information technology revolution. Our device could play a similarly central role in future information technologies, using thermal signals in addition to electric signals. This effort will pave the way toward a new field of research: "Opto-phononics."