Speaker: Dr Vibhuti N. Rai, Fachbereich Physik, Freie Universität Berlin, Berlin, Germany.
Title: "Light-Matter Interaction at Nanoscale"
Date: Friday, January 16, 2026 - Time: 4 PM
Tea & Coffee: 5 PM
Venue: CeNSE Seminar Hall

Abstract:

Controlling the optical response of individual quantum systems, such as molecules and atomic-scale 
defects, by minimising the effects of the local environment is essential for next-generation 
optoelectronic and quantum devices. In device geometries, the emission of an emitter is dictated 
not only by the intrinsic structure but is also influenced by its coupling to metallic electrodes, 
orientation, and any external perturbations. To fully access their inherent properties, we must (i) 
control the emitter’s coupling and alignment with respect to its local environment and 
(ii) understand the mechanisms of different excitations, such as excitons and vibrational 
modes, within that environment. To address these two aspects, I studied the electroluminescence 
of tailor-made single molecules and excitation dynamics at defects in (quasi-)two-dimensional 
transition-metal dichalcogenides (TMDCs) with ultrafast temporal resolution. Scanning tunnelling 
microscopy (STM)-induced luminescence is a unique tool for probing the op tical properties of 
emitters with atomic-scale spatial resolution. We achieved a molecular design by mounting a 
chromophore on a tripodal platform connected via a linker, exhibiting a high degree of 
electronic and mechanical decoupling, reflected in its high emission yield (10⁻³ ph/e⁻) and 
clear hot luminescence [1, 2]. This approach provides a route toward engineering emitter 
architectures with tailored optical properties for optoelectronic components. While such control 
is highly promising, conventional STM experiments lack the temporal resolution (ms to μs) required 
to understand fundamental excitation and relaxation processes, such as excitons or phonons, which 
occur at nanosecond to picosecond timescales and are strongly influenced by the local environment, 
ultimately limiting device performance [3]. To overcome this limitation, I efficiently coupled 
optical and THz radiation into an STM junction [4] to probe picosecond dynamics. I investigated 
(quasi-)two-dimensional materials such as TMDCs, which exhibit strong light-matter interactions and an 
interplay between electronic and structural degrees of freedom, i.e., electron-phonon coupling. These 
materials host various structural defects that significantly modify their physical properties and can 
potentially act as quantum dot-like single-photon sources. Compared to single-molecule emitters, 
these defects are more stable and provide an ideal platform for studying interactions with the environment, 
such as lattice vibrations. I probed the phonon dynamics of bulk 2H-MoTe₂ in a THz pump-THz probe scheme 
and was able to excite and detect long-range coherent phonon modes. We find that defects modulate the 
relative excitation efficiency of different modes due to local tip-induced band bending [5]. With access 
to ultrafast timescales and atomic-scale spatial resolution, we can directly unravel how a quantum system 
couples to its environment. This capability opens the door to probing charge-transfer mechanisms in hybrid 
systems, tracking the evolution of coherent states, and guiding the design of efficient optoelectronic devices. 

[1] V. N. Rai et al. Nat. Commun. 14 (1), 8253 (2023) Link 

[2] V. N. Rai et al. Phys. Rev. Lett. 130, 036201 (2023) Link 

[3] K. Kaiser et al. Nat. Commun 14, 4988 (2023) Link 

[4] T. Cocker et al. Nat. Photon. 7, 620-625 (2013) Link 

[5] V. N. Rai et al. Sci. Adv. 11, eadz6549 (2025) Link.

Biography:

I (Vibhuti Rai) completed my schooling in my hometown of Azamgarh. Thereafter, I attended Ramjas 
College, University of Delhi, where I pursued a Bachelor of Science (Honours in Physics) from 2009 to 2012. 
After that, I was admitted to the S. N. Bose National Centre for Basic Sciences, Kolkata as an integrated 
PhD student. After completing my master’s degree, I continued there as a Junior Research Fellow. In 2016, 
I was selected for two PhD scholarships: the Deutscher Akademischer Austauschdienst (DAAD, Germany) and the 
Monbukagakusho (MEXT, Japan). I pursued my PhD under the DAAD scholarship at the Karlsruhe Institute of Technology, 
Germany, from 2017 to 2021. The topic of my PhD was “Light Emission from Single Self-Decoupled Molecules in Scanning 
Tunnelling Microscopy", under the supervision of Prof. Wulf Wulfhekel. Following my PhD, I continued as a postdoctoral 
researcher in the same group for two additional years. The topic of my research was “Single Molecules as Single Photon 
Sources". Since 2023, I have been working in the group of Prof. Katharina J. Franke, where I use THz STM to probe 
ultrafast processes with atomic-scale spatial resolution.



Host Faculty:  Prof. Dhavala Suri