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 the only one able to capture ultrafast snapshots of single atoms. Prof. Tyler Cocker has played a central role in establishing and developing the field of THz-STM, including the recent demonstration that THz-STM can resolve femtosecond snapshots of the electron density in a single molecular orbital.
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 with the aim of recording ultrafast movies of the electron densities inside single molecules, nanostructures, and complex materials. These prospective ‘nano-movies’ will 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 in part by the Nanoscale Computing Devices and Systems Program within the US Office of Naval Research.