Modern ultrafast science can famously trace its roots back to high-speed photography and snapshots of a racehorse in full stride. The concept of recording an image that corresponds to a particular time window endures, even though studying many of the most interesting condensed matter phenomena of today demands a high-speed ‘movie-making’ technique with not only far better time resolution, but also nanoscale spatial resolution.
The ultimate dream of watching single atoms, molecules, and electronic orbitals move on their intrinsic length and time scales has driven the development of new ultrafast microscopy techniques with spectacular capabilities. Among these, lightwave-driven terahertz scanning tunneling microscopy (THz-STM) is able to capture ultrafast snapshots of single atoms. Prof. Tyler Cocker and his group have played a central role in establishing and developing the field of THz-STM, including the recent demonstrations of lightwave-driven scanning tunneling spectroscopy and terahertz time-domain spectroscopy on the atomic scale.
In the Cocker Group at MSU, we are using the new possibilities enabled by THz-STM to explore ultrafast dynamics at the ultimate time and length scales of condensed matter physics. We have constructed a third-generation THz-STM and are now recording ultrafast movies of the electron densities inside nanostructures and complex materials. These ‘nano-movies’ reveal how new materials respond to light on the smallest length scales, and inform the design of future nanotechnology and molecular electronics.
We are also interested in other ultrafast laser and terahertz techniques, especially pump-probe spectroscopy and scattering-type scanning near-field optical microscopy (s-SNOM). The latter is a complementary approach to THz-STM that accesses the local dielectric function beneath a sharp metal tip with sub-wavelength spatial resolution. It is especially useful for studying low-energy elementary excitations in materials such as plasmons, phonons, and interlevel transitions in excitons. These processes are of particular importance for nanomaterial functionality and typically survive for only femtoseconds to picoseconds after photoexcitation.
Our research is funded by the Nanoscale Computing Devices and Systems Program within the US Office of Naval Research, including through the MURI and DURIP programs. It is also funded by the Army Research Office through the Young Investigator Program, by the Air Force Office of Scientific Research, and by the Jerry Cowen Endowment.
Disclaimer: Any opinions, findings, and conclusions or recommendations expressed in this material are those of the author(s) and do not necessarily reflect the views of the Office of Naval Research, the Army Research Office, the Air Force Office of Scientific Research, or Michigan State University.