Time-resolved terahertz near-field microscope

• Subject Area: Condensed Matter Physics
• Term: Funded in 2024
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 537610356

Time-resolved Terahertz (THz) spectroscopy is a powerful tool to investigate charge carrier dynamics, e.g. in semiconducting materials: a short and intense laser pulse excites the charge carriers; a short THz pulse probes the state of the excited carriers. As the wavelength of THz radiation, and thus the smallest possible focus size, is in the millimetre range, large samples are required for the investigation with conventional methods. Near-field techniques, where the THz radiation is measured in the vicinity of the sample and not collected in large distance, enable measurements of micrometer sized samples. Here we apply for a THz near-field microscope based on an atomic-force microscope (AFM). In this case, the THz radiation gets scattered at the AFM tip that interacts directly with the sample. The spatial resolution in this case is only limited by the radius of the AFM tip itself and reaches below 100 nm. A train of short THz pulses is generated with the help of a femtosecond laser and guided to the AFM tip by parabolic mirrors; a second set of parabolic mirrors is used to collect the scattered radiation and guide it to the detector. As the near-field signal that stems from the interaction of the tip and the sample scales nonlinearly with the distance, lock-in amplification can be exploited to measure the according signal. The short THz pulses cover a large spectral range that contains information on free electrons, excitonic states, phonons and/or other features of the sample. Furthermore, it allows time-resolved measurements: a tuneable femtosecond laser system is also guided to the sample and can be utilized to excite charge carriers resonantly. Thus, the tool features the investigation of non-equilibrium charge carriers on a picosecond and nanometer scale. This type of AFM-based time-resolved near-field microscope is not commercially available, but would have to be custom built. To allow measurements at low temperature, the system consists of a closed cycle optical cryostat that accommodates the AFM scanner. The tunable laser system for optical excitation of the samples in the visible to near-infrared range is not standard and thus will be a unique configuration. It has to by synchronized to the fiber laser in order to allow for time-resolved measurements, furthermore a delay stage has to be implemented and is fully included in the software for the microscope. Samples of interest are two-dimensional materials, heterostructures thereof, topological, and phase-change materials within our group. Besides the work in our group, the tool will be accessible to other groups within the University of Duisburg-Essen, which will be coordinated via the Interdisciplinary Center for Analytics on the Nanoscale (ICAN).

• DFG Programme: Major Research Instrumentation
• Major Instrumentation: Zeitaufgelöstes Terahertz Nahfeld-Mikroskop
• Instrumentation Group: 5091 Rasterkraft-Mikroskope
• Applicant Institution: Universität Duisburg-Essen
• Leader: Professor Dr. Martin Mittendorff