Paper NS-ThM5
Femtosecond Time-Resolved Scanning Tunneling Microscopy on Nanostructures

Thursday, November 1, 2012, 9:20 am, Room 12

With the size reduction of structures in current electronic devices, differences in the electronic properties caused, for example, by the structural nonuniformity of each element have an ever-increasing effect on macroscopic functions. The study of nonequilibrium quantum dynamics in materials with small structures is of great importance not only from the fundamental viewpoint but also as a basis for the further development of functional devices. Real-space imaging of the transient carrier transport and transitions in nanostructures is desired to obtaine a deeper understanding of current semiconductor physics. Probing the effect of local electronic structures of nano-clusters on energy transfer is important for the analysis of chemical reactions in catalytic activities and also for the development of organic solar devices. To advance such studies, a method that enables the probing of local carrier dynamics with high spatial and temporal resolution is necessary.

Recently, femtosecond time-resolved scanning tunneling microscopy (STM), which enables ultrafast phenomena on a target material to be probed with the spatial resolution of STM, has been realized by the combination of STM with ultrashort-pulse laser technologies [1-4]. In time-resolved STM, the tunnel gap of STM is illuminated by a sequence of paired laser pulses, and the change in tunneling current ΔI is measured as a function of the delay time between the paired pulses (t_d). A high temporal resolution in the femtosecond range, which is limited only by the optical pulse width, is obtained simultaneously with the atomic spatial resolution of STM. This microscopy technique is applicable to systems in which the response of the tunneling current has a nonlinear dependence on the optical excitation intensity. I n the case of semiconductors, for example, the magnitude of ΔI obtained for a certain delay time t_d reflects the density of photogenerated minority carriers at t_d after the first pulse excitation, and the carrier decay processes can be observed by analyzing the delay-time dependence of ΔI. Using polarized light, spin dynamics can be probed, and the detection of signals such as phonons is also possible.

In this talk, I would like to introduce this new microscopy technique with some new results.

References

[1] Y. Terada, S. Yoshida, O. Takeuchi and H. Shigekawa, J. of Physics: Condensed Matter 22, 264008 (2010). [2] Y. Terada, S. Yoshida, O. Takeuchi and H. Shigekawa, Nature Photonics, 4, 12, 869 (2010). [3] Y. Terada, S. Yoshida, O. Takeuchi and H. Shigekawa, Advances in Optic.Tech., 2011, 510186 (2011). [4] S. Yoshida, Y. Terada, R. Oshima, O. Takeuchi and H. Shigekawa, Nanoscale, 2012, 4, 757 (2012).