Physikalische Chemie - Direktor: Prof. Dr. Martin Wolf
Department Online Seminar
Chair: Alexander Paarmann

Monday, December 6, 2021, 11:00 am
Dr. Markus Huber
Universität Regensburg
Nanovideography of ultrafast charge carrier dynamics in van der Waals materials
The terahertz and mid-infrared spectral domain host a multitude of interesting low-energy elementary excitations, such as phonons, plasmons and magnons. Ultrafast optical spectroscopy has provided key insights into the dynamics of these collective excitations. Unfortunately, the spatial resolution of such (far-field) studies is intrinsically limited to the scale of the probing wavelength by diffraction. Thus, the optical response cannot resolve individual nano-objects, confined polariton waves, or local surface effects.
In this talk, I will show how we use ultrafast scanning near-field optical microscopy to bypass this limitation and gain access to the transient, nanoscale dielectric functions of materials after photo-excitation by ultrashort laser pulses. We have applied this technique – simultaneously achieving a 10 nm spatial and 10 fs temporal resolution – to study carrier dynamics on their intrinsic length and time scales. I will focus on van-der-Waals bonded materials in this seminar which show great promise for future ultrathin electronics. We directly observed photo-activated surface polaritons in real space and traced their decay dynamics in black phosphorus. Using the tomographic sensitivity of the near fields, we extracted the surface dielectric function on the three-dimensional topological insulator (Bi0.5Sb0.5)2Te3. Recently, we introduced a new contact-free nanoscopy technique centered around probing the local polarizability of photogenerated electron–hole pairs with evanescent terahertz fields. This concept is capable of tracing ultrafast charge carrier dynamics even in insulating materials. Thus, we were able to observe the ultrafast tunneling of electron-hole pairs across two transition metal dichalcogenide monolayers with a temporal resolution faster than a single cycle of light.
Our results demonstrate the potential of this technique for revealing how ultrafast charge dynamics governs the properties of a large variety of solid-state systems.

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