Super Resolution THz Imaging of Nanostructures

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Recently invented super-resolution microscopes are having a large impact on the fields of chemistry, biology, and materials science, due to these microscopes providing previously unheard-of images of objects with dimensions approaching that of molecules. However, important chemical and electrical features of conducting and semiconducting nanomaterials cannot be visualized with existing microscopes, because of energy limitations associated with the light used. With support from the Chemical Measurement and Imaging Program, and partial co-funding from the Macromolecular, Supramolecular, and Nanochemistry Program in the Division of Chemistry and the Electronic and Photonic Materials Program in the Division of Materials Research of the National Science Foundation, Professor Hartland and his group at the University of Notre Dame are developing a new super-resolution microscope for recording images of materials through a novel approach that takes advantage of low-energy light in the terahertz region of the electromagnetic spectrum. The smallest sized feature the terahertz microscope can resolve is a few hundred nanometers, which is 100 to 1000 times better than previously possible. The newly unleashed power of the terahertz microscope is being used by Professor Hartland and graduate and undergraduate students to study materials that hold promise for more efficient solar cells and nanomaterials that may one day be used in ultra-sensitive chemical detection systems. It is anticipated that the new terahertz microscope will be adopted by researchers who study biologicals, superconductors, and integrated circuit devices, and ways to identify trace amounts of chemicals, such as residues from explosives. Efforts go beyond training of students at the university level, with a major focus being participation of high school students and teachers recruited from the Elkhart Community Schools, a local school district with students who come from highly diverse socioeconomic backgrounds. Sustained impact of the Elkhart School collaboration is being achieved by the high school teachers receiving graduate credits for their summer research activities. This program leads to teacher participants being able to teach Indiana University dual-enrollment courses, thereby broadening the educational opportunities provided by the Elkhart Community Schools.Terahertz spectroscopy is widely used to examine semiconductors and plastics; however, currently existing terahertz microscopes have very low spatial resolution due to the diffraction limit. In a traditional terahertz microscope, the minimum feature size that can be resolved is determined by the wavelength of the impinging light divided by two, which is approximately 50 micrometers in the terahertz region. The unique approach taken by the Hartland group is based on a tightly focused visible probe beam that monitors absorption of a terahertz pump beam through the photothermal effect. The resolution in these experiments is dictated by the visible probe beam, and, as a result, is several hundred nanometers rather than tens of micrometers. The photothermal terahertz microscope is being used to examine photo-conductivity and the terahertz spectroscopy of thin films and individual nanostructures. These experiments are providing new information about the spatial distribution and motion of photo-excited charge carriers in the different structures, and the decay pathways for the charge carriers. This information is important for developing materials for photo-voltaic solar cells. Terahertz spectroscopy experiments are also being performed on the low frequency resonances of nanomaterials, which is generating new information about how these materials interact with their environment.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.