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Two-dimensional transition metal dichalcogenides (TMDCs) are promising materials for next
generation optoelectronic and photonic devices owing to their strong light- matter interaction and
unique excitonic properties. However, defects and ultrafast non-radiative relaxation processes
critically influence their performance and are difficult to investigate using conventional optical
techniques. In this talk, I will present our recent work on ultrafast visible-pump/mid-infrared-
probe spectroscopy for directly probing hidden carrier relaxation pathways in two-dimensional
quantum materials. Using a home-built ultrafast mid-IR transient absorption spectroscopy
system, we investigate intraband and intra-excitonic transitions in monolayer and few-layer
MoS₂. By resonantly exciting the A and B excitons and probing below and above the exciton
binding energy, we reveal defect-assisted non-radiative recombination, polarization-dependent
carrier relaxation, and the effects of sulfur vacancies and vanadium doping on ultrafast carrier
dynamics. Mid-IR probing further reveals how n-type & p-type doping defects alter excitonic
interactions and many-body phenomena across the exciton Mott transition, leading to significant
changes in carrier mobility and recombination dynamics. Our recent investigations on
MoS₂–MoSe₂ heterostructures demonstrate the formation and defect-mediated relaxation of
interlayer excitons, providing insights into charge-transfer processes relevant for energy
conversion and valleytronic applications.
The talk will further highlight our development of compact ultrafast mid-IR pulse-generation
techniques and microscope-integrated transient absorption spectroscopy for micron-scale
materials, establishing mid-IR spectroscopy as a powerful tool for investigating hidden electronic
states in quantum materials. Also, I will briefly introduce our complementary research on
femtosecond laser direct writing, including plasmonic nanostructures for ultrasensitive single
molecule SERS sensing, ultra-black metal surfaces for stray-light suppression, and high-
emissivity laser-textured surfaces developed for thermal management for ISRO and DRDO
applications.
Keywords: Ultrafast spectroscopy; Mid-IR transient absorption; TMDCs; Exciton dynamics;
Defect engineering, laser direct applications, Super Black Surfaces, single molecule sensing.
References:
(1) Mid-IR Probing Unveils Vanadium Doping-Induced Unsaturation of
Defect States in Monolayer MoS₂, npj 2D Materials and Applications,10,14
(2026).
(2) Anisotropic Nonradiative Recombination of Carriers in Few-Layer MoS₂
Probed by Ultrafast Mid-IR Spectroscopy, The Journal of Physical Chemistry
C,127,27,13120, (2023).
(3) Femtosecond Laser-Induced Nanoscale Texturing for Super-Black
Stainless-Steel Surfaces, Journal of Physics: Photonics,8,025030, (2026).
(4) Highly Sensitive and Stable Surface-Enhanced Raman Scattering on
Ladder-Like Nanostructures, Surface and Interfaces 28,101622, (2023). |