Understanding the spatio-temporal response of photoexcited nanostructures, low-dimensional materials and molecules at surfaces on their natural length and time scales is a key goal in surface dynamics. Most elementary processes in solids and molecular systems occur on ultrafast timescales in the range of femtoseconds. Photoexcitation of a sample creates a non-equilibrium distribution of electrons, which on characteristic time scales exchange energy with different microscopic subsystems, such as phonons, spins, or the electronic subsystem. Various powerful tools are nowadays available to probe the characteristic properties of a given subsystem with femtosecond time resolution. In particular, ultrafast spectroscopic tools are routinely employed to take ultrafast snapshots of transient electronic states of matter, providing fascinating insight into fundamental physical and chemical processes and their driving forces.
Most femtosecond spectroscopic techniques, however, lack high spatial resolution. But the dynamics of charge carriers will often be closely linked to their local environment, and can vary on nanometer down to atomic length scales. It is thus of key importance to understand the local response of a given subsystem, from a fundamental point of view as well as for nanodevice applications. In order to correlate the lifetime, scattering rates and microscopic coupling mechanisms of photoexcited carriers with the local sample structure, it is necessary to bring femtosecond spectroscopy down to the relevant length scales, i.e., nanometers and Angstroms.
We aim to study the local, atomically resolved dynamics of photogenerated charge carriers in solid state matter, molecular systems and at surfaces by combining femtosecond optical excitation with the Angstrom spatial and femtosecond temporal resolution of a THz-gated Scanning Tunnelling Microscope. Ultrafast photoexcitation of the THz-STM provides a broad access to a variety of spatio-temporal phenomena, whose detailed understanding is of great interest for fundamental research as well as modern device applications.
Our research currently focusses on the development of photoexcited THz-STM and its application for femtosecond spectroscopy and imaging at the atomic scale. We employ visible to infrared ultrashort laser pulses of a variable repetition-rate OPCPA system for photoexcitation of the STM. Subsequent gating of the tunnel junction by near-single-cycle THz-pulses will allow for the local probing of the photoexcited state, providing new insight into the atomic-scale variations of the dynamics of charge carriers.
Moreover, the application of a quasi-static femtosecond voltage pulse via THz-pulse excitation of the tunnel junction allows for the ultrafast injection of electrons into otherwise inaccessible electronic states. THz-controlled femtosecond electron injection in the STM thus provides a tool for probing ultrafast electronic and optoelectronic processes on the nanometer scale and below.
Our microscope is operational since February 2019 and we are happy to receive applications from motivated phd-candidates and postdocs as well as master students to join our team.
Ongoing and future research projects include:
- Spatio-temporal probing and imaging of femtosecond carrier dynamics in semiconductor materials
- Exploring the fundamental interaction of single-cycle THz-pulses and ultrashort (sub-10 fs) optical laser pulses with the STM junction and related prospects and limitations
- Investigation, modelling and control of THz-antenna properties of STM tips.
- Further technical development of the microscope and the optical and THz setup