Research

Research:

The Ultrafast Terahertz Spectroscopy (UFTS) group at IISER Kolkata investigates nonequilibrium phenomena in quantum and condensed matter materials using femtosecond optical and terahertz spectroscopy. Our research focuses on how charge, spin, lattice, and excitonic degrees of freedom evolve and interact on ultrafast timescales in layered magnetic materials, topological systems, correlated materials, oxides, semiconductors, and other functional materials. By combining pump–probe spectroscopy, nonlinear optical probes, terahertz time-domain spectroscopy, and picosecond strain-pulse methods, we study collective excitations, phase transitions, carrier dynamics, and emergent states of matter. 

Current Research Directions

1. Ultrafast dynamics in layered magnetic and van der Waals materials

A major direction of our work is the study of ultrafast dynamics in layered magnetic and van der Waals quantum materials. We are interested in how spin, lattice, and electronic degrees of freedom respond to photoexcitation and evolve on femtosecond to nanosecond timescales. In these systems, ultrafast optical probes can reveal nonequilibrium pathways associated with magnetic ordering, excitonic effects, coherent phonons, and strain dynamics. Recent work in this direction includes layered magnets such as CrSiTe₃ [Anjankumar NM et al., Phys. Rev. B 111, L140414 (2025)] and NiPS₃ [S. Sahu et al., accepted in Phys. Rev. B (2026)], where we have explored magnetic dimensional crossover, spin–lattice coupling, and exciton-related dynamics. 

2. THz spectroscopy of topological and magnetic quantum materials

We use terahertz time-domain spectroscopy to probe low-energy electrodynamics, phonons, and carrier dynamics in topological, magnetic, and correlated quantum materials. These measurements provide direct access to conductivity, lattice excitations, and low-energy collective responses that are often difficult to probe using conventional optical techniques. Current work in this area includes magnetic topological systems such as MnBi₂Te₄ [S. Mukherjee et al., Phys. Rev. B 110, 195401 (2024)] and related materials, as well as topological insulator thin films such as BiSbTe₁.₅Se₁.₅ [A. Chauhan et al., Phys. Rev. B 112, 054311 (2025)] and other low-dimensional quantum materials. 

3. Coherent Phonons, strain pulses, and nonequilibrium phase transitions 

A broader theme of our research is the use of ultrafast spectroscopy to investigate coherent quasiparticles and nonequilibrium phase transitions in condensed matter. In particular, we are interested in coherent phonons, picosecond acoustic strain pulses, and collective excitations that emerge following ultrafast photoexcitation. These studies provide insight into the coupling between electronic, lattice, and magnetic degrees of freedom and can reveal hidden dynamical pathways in quantum materials. This direction connects much of our work across magnetic materials, correlated systems, thin films, and semiconductors. 

4. Carrier, exciton, and polaron dynamics in oxides, semiconductors, and nanoscale functional materials 

Another important research direction in the group is the study of ultrafast carrier relaxation, trapping, exciton dynamics, and polaron formation in oxides, semiconductors, and nanoscale functional materials. We are interested in how photoexcited carriers relax, recombine, self-trap, or couple to phonons and defects on femtosecond to picosecond timescales, as these processes play a central role in charge transport, energy relaxation, photocatalysis, and optoelectronic functionality. In transition-metal oxides such as V₂O₅ and CoV₂O₆, our studies have revealed polaron-assisted relaxation and carrier trapping into shallow and deep defect states [Anjankumar NM et al., J. Phys. Chem. C 126, 20535–20541 (2022); J. Phys. Chem. C 128, 14717–14725 (2024)]. In hematite nanoforms, we have explored nonlinear interactions between self-trapped and free excitons [Anjankumar NM et al., Appl. Phys. Lett. 121, 202102 (2022)]. Related work on 2H-MoSe₂, Cr-doped 1T/2H-MoSe₂ nanosheets, CuS, and CuS/Ag₂S nanocomposites has uncovered defect-mediated exciton trapping, exciton–exciton annihilation, hot-carrier relaxation, and higher-order many-body processes relevant to photocatalytic performance [S. Mukherjee et al., J. Chem. Phys. 159, 164705 (2023); ACS Appl. Opt. Mater. 1, 1332–1342 (2023)]. Our ultrafast studies on ZnO and α-Bi₂O₃ nanorods have further examined electron–hole plasma decay and nonlinear electron–phonon interactions in oxide nanomaterials [J. Sarkar et al., J. Appl. Phys. 124, 243103 (2018); J. Phys. Chem. C 123, 10007 (2019)].

Selected Recent Highlights

• Magnetic dimensional crossover in layered magnetic materials using picosecond strain pulses: Picosecond acoustic strain-pulse spectroscopy has been used to study magnetic dimensional crossover and ultrafast spin–lattice dynamics in layered magnetic materials such as CrSiTe₃ [Anjankumar NM et al., Phys. Rev. B 111, L140414 (2025)].

• NiPS₃ and exciton–spin dynamics: Our recent work on NiPS₃ explores ultrafast exciton-related dynamics and their coupling to magnetic order in a layered antiferromagnet [S. Sahu et al., accepted in Phys. Rev. B (2026); arXiv:2509.04900)].

• THz studies of magnetic topological materials: We have investigated low-energy phonon and carrier dynamics in magnetic topological materials such as MnBi₂Te₄ and Sb-doped MnBi₂Te₄ using terahertz spectroscopy [S. Mukherjee et al., Phys. Rev. B 110, 195401 (2024)].

• Topological and thin-film systems: Ultrafast carrier relaxation and coherent acoustic phonon dynamics have been studied in topological and thin-film materials such as BiSbTe₁.₅Se₁.₅ [A. Chauhan et al., Phys. Rev. B 112, 054311 (2025)].

• Oxides and semiconductor systems: Ultrafast studies on V₂O₅, CoV₂O₆, and hematite nanoforms probe free carriers, polarons, self-trapped excitons, and shallow/deep trap states. In V₂O₅, polaron-assisted bimolecular decay occurs within ~4.5 ps; CoV₂O₆ exhibits carrier trapping into shallow traps within ~2 ps and deep traps on ~30 ps timescales; hematite nanoforms reveal nonlinear interactions between self-trapped and free excitons [Anjankumar NM et al., J. Phys. Chem. C 126, 20535–20541 (2022); J. Phys. Chem. C 128, 14717–14725 (2024); Appl. Phys. Lett. 121, 202102 (2022)].

    A full list of publications from the group is available on the Publications page.

Major Experimental Approaches

Our group develops and employs a range of ultrafast optical and terahertz tools built around femtosecond laser systems to investigate nonequilibrium dynamics in quantum and condensed matter materials.

Femtosecond pump–probe spectroscopy

Pump–probe spectroscopy (schematically illustrated in Fig. 1) is one of the primary tools used in our group to study carrier relaxation, coherent phonons, exciton dynamics, spin–lattice coupling, and phase-transition dynamics on ultrafast timescales. By tuning pump and probe wavelengths across the UV, visible, infrared, and terahertz regimes, we investigate how photoexcited states evolve in a wide variety of materials. 

Terahertz time-domain spectroscopy

erahertz time-domain spectroscopy is used in our group to probe low-energy conductivity, phonons, collective modes, and electrodynamic responses in quantum and functional materials. THz measurements are particularly useful for studying topological materials, magnetic systems, conducting composites, and thin films, where low-energy excitations often play a central role. A typical time-domain THz signal measured in our lab is shown in Fig. 2. 

Nonlinear optical and strain-pulse methods

In addition to pump–probe and THz techniques, we also use nonlinear optical probes such as second harmonic generation and picosecond acoustic strain-pulse methods to investigate structural, electronic, and magnetic dynamics in condensed matter systems.