Ultrafast Scanning Probe Microscopy
Ultrafast Scanning Probe Microscopy
Research Group Melanie Müller
Research Group Melanie Müller

Research

Our Motivation

Sketch of photoexcited THz-STM. Coupling THz-pulses to the STM junction allows for control of tunneling currents on femtosecond time scales, maintaining the spatial resolution of the STM.

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. In nanostructures, molecules and spatially inhomogeneous systems, the dynamics will be dictated by the local environment, and will spatially vary on nanometer or even Angstrom 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.

With THz-gated Scanning Tunnelin Microscopy (THz-STM), we envision to study the dynamics of charge carriers, quasiparticles and local transient changes of the electronic and atomistic structure directly on atomic length scales. In particular, ultrafast photoexcitation of the THz-gated STM junction 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.

Instrumentation

Our room-temperature THz-STM was operational from February 2019 to April 2021. Since spring 2022, we are back to operation in a new individual lab, with the STM at cryogenic (LHe or LN) temperatures. We are happy to see first THz-induced signals in the STM using the spintronic THz emitter als ultrabroadband single-cycle THz source!

Impressions form our lab

After previously employing broadband VIS-NIR ultrashort laser pulses from a broadband OPCPA system for photoexcitation of the STM and spintronic THz emitter (STE), we now switched gears and installed a new versatile laser system for THz-STM operation. Gating of the tunnel junction by ultrabroadband single-cycle THz-pulses emitted from the STE 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.

Ongoing and future research projects include:

  • Spatio-temporal probing and imaging of femtosecond carrier dynamics at metal surfaces and 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

Time-domain sampling of tip-enhanced THz near-fields

THz near-field sampling in the STM and comparison of far-field and near-field waveforms and amplitude spectra, respectively.

We developed a routine to characterize the STM tip-enhanced THz field inside the STM junction directly in the time-domain with sub-cycle resolution. We therefore employ the sensitivity of photoexcited currents excited by femtosecond optical pulses on the instantaneous THz voltage applied to the STM. Taking care of the absence of non-instantaneous effects, this allows for direct sampling of the THZ near-field waveforms in the tip-sample gap, i.e., for the applied voltage transient, whose exact knowledge is essential for optimized THz-STM operation. Moreover, this sllows for the detailed characerization of the tip antenna, whose coupling efficiency depends on the THz frequency.