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Research Directions of N. Kamaraju’s group, IISER Kolkata: Ultrafast and Terahertz Spectroscopy of Quantum Materials
Research Directions of N. Kamaraju’s group, IISER Kolkata: Ultrafast and Terahertz Spectroscopy of Quantum Materials
The modern era of condensed matter physics is witnessing the rise of what many call the “Quantum Age,” driven by materials whose properties emerge from the collective behavior of electrons, spins, and lattice degrees of freedom. Strong correlations, topology, and entanglement give rise to complex interactions that are challenging to probe and control with conventional approaches. Electromagnetic radiation across a broad spectral range, particularly ultrafast pulses (~10 fs–2 ps) in the UV, visible, and infrared regimes, along with terahertz (THz) pulses, provides a versatile toolkit to access electron, spin, and lattice dynamics at their intrinsic timescales. By driving systems far from equilibrium and monitoring their relaxation, these techniques allow direct investigation of quasiparticle interactions, collective phenomena, and emergent phases, bridging fundamental understanding with potential applications in spintronics, photonics, and quantum technologies.
Focus on Emergent Phenomena in Quantum Materials
Our broader research vision integrates ultrafast and THz spectroscopy to interrogate coherent quasiparticle dynamics—phonons, magnons, polarons—and their interactions with spins and carriers in quantum materials. We aim to uncover hidden phases, quantify many-body interactions, and leverage ultrafast control for quantum devices, spintronic systems, and energy-harvesting platforms. Complementary directions include THz imaging for condensed matter and biomedical applications, and advanced characterization of layered and nanoscale materials. By combining experimental precision, theoretical modeling, and materials synthesis, our group pushes the frontiers of time-resolved spectroscopy, enabling fundamental discoveries and technological innovations in quantum materials.
A particularly exciting subset of quantum materials is two-dimensional (2D) magnetic van der Waals (vdW) compounds. These atomically layered magnets provide a tunable platform to explore ultrafast spin dynamics, coherent quasiparticles, and emergent collective behavior at the nanoscale—key ingredients for next-generation quantum and spintronic technologies. They also enable studies of excitonic condensation and its interplay with other degrees of freedom. Unlike quantum Hall systems, layered vdW magnets may exhibit excitonic condensation at higher temperatures and without external fields, making them attractive for both fundamental and applied research.
We aim to probe such condensates and their interactions with carriers, magnons, phonons, polarons, polaritons, and strain. These couplings span femtosecond to nanosecond timescales and UV to THz spectral domains, necessitating an integrated ultrafast approach. Pump–probe spectroscopy, nonlinear optics, and THz time-domain techniques are employed to resolve the temporal and spectral evolution of these excitations, revealing hidden phases, collective behavior, and strain dynamics.
Research Highlights and Directions
Magnetic Dimensional Crossover (MDC) in 2D vdW Magnets: Picosecond acoustic strain pulses in CrSiTe₃ directly track the transition from short-range 2D intraplanar order to long-range 3D interplane magnetic order, revealing contributions from thermoelastic, deformation potential, and magnetic strains (Anjankumar NM. et. al., Phys. Rev. B 111, L140414, 2025). Recently in our group, ultrafast spin-orbit-entangled excitons coupled to magnetic ordering are being explored in the antiferromagnet NiPS₃ (submitted, S. Sahu et. al., arXiv:2509.04900, 2025).
Topological and Magnetic Materials via THz and ultrfast Spectroscopy: THz time-domain spectroscopy studies of MnBi₂Te₄ and Sb-doped MnBi₂Te₄ reveal Fano interference between phonons and surface electrons and temperature-dependent phonon modulation near the Néel temperature (S. Mukherjee et. al., Phys. Rev. B 110, 195401, 2024). Coherent acoustic phonons and carrier dynamics in BiSbTe₁.₅Se₁.₅ thin films using pump-probe reflection spectroscopy highlight interactions between topological surface states and lattice excitations (A. Chauhan et. al., Phys. Rev. B 112, 054311 (2025)).
Carrier and Polaron Dynamics in Oxides and Semiconductors: Ultrafast studies on V₂O₅ (Anjankumar NM. et. al., J. Phys. Chem. C 126, 20535–20541 (2022)), CoV₂O₆ (Anjankumar NM. et. al., J. Phys. Chem. C 128, 14717–14725 (2024)), and hematite nanoforms probe free carriers, polarons, self-trapped excitons, and shallow/deep traps. In V₂O₅, polaron-assisted bimolecular decay occurs within ~4.5 ps. CoV₂O₆ exhibits carrier trapping in shallow traps within ~ 2ps and deep traps within ~ 30 ps, influencing optoelectronic performance. Hematite nanoforms reveal nonlinear interactions between self-trapped excitons and free excitons, informing ultrafast optoelectronics strategies (Anjankumar NM et. al., Appl. Phys. Lett., 121, 202102, 2022).
Transition Metal Dichalcogenides (TMDCs) and Hot Carrier Dynamics: Studies of 2H-MoSe₂ and Cr-doped 1T/2H-MoSe₂ nanosheets reveal defect-mediated exciton trapping, exciton-exciton annihilation, and many-body interactions, providing insights into enhanced photocatalytic efficiency (S. Mukherjee., J. Chem. Phys. 159, 164705, 2023). CuS and CuS/Ag₂S nanocomposites exhibit multi-exponential hole relaxation and higher-order Auger processes, explaining improved photocatalytic performance (S. Mukherjee., ACS Appl. Opt. Mater., 1, 1332–1342, 2023).
Ultrafast Electron-Hole Plasma Dynamics in Oxide Nanomaterials: ZnO and α-Bi₂O₃ nanorods demonstrate multi-exponential electron-hole plasma decay and nonlinear electron-phonon interactions, guiding optoelectronic device design (J. Sarkar et. al., J. Appl. Phys. 124, 243103, 2018; J. Sarkar et. al., J. Phys. Chem. C 123, 10007, 2019).
Methodological Contributions in THz Spectroscopy: A review on numerical methods for film thickness determination in THz-TDS provides a benchmark for non-contact characterization (S. Mukherjee et. al., Eur. Phys. J. Spec. Top. 230, 4099–4111, 2021). Conductivity and shielding studies in PVA-GO-Ag composite films illustrate THz applications for materials metrology (S. Mukherjee et. al., Appl. Phys. A 129, 343, 2023).
Selected Recent Publications and Submitted Work
Ultrafast Dynamics of Spin-Orbit Entangled Excitons Coupled to Magnetic Ordering in van der Waals Antiferromagnet NiPS₃, S. Sahu et. al., arXiv:2509.04900 (2025, submitted).
Ultrafast dynamics of carriers, coherent acoustic phonons and strain pulses in BiSbTe₁.₅Se₁.₅ thin films, A. Chauhan et. al., Phys. Rev. B 112, 054311 (2025).
Probing of magnetic dimensional crossover in CrSiTe₃ through picosecond strain pulses, Anjan Kumar NM. et. al., Phys. Rev. B 111, L140414 (2025).
Investigation of magnetic order influenced phonon and electron dynamics in MnBi₂Te₄ and Sb-doped MnBi₂Te₄ via THz-TDS, S. Mukherjee et. al., Phys. Rev. B 110, 195401 (2024).
Probing shallow and deep oxygen traps in α-CoV₂O₆ using ultrafast pump-probe spectroscopy, Anjan N.M et. al., J. Phys. Chem. C 128, 14717–14725 (2024).
Trapping and exciton-exciton annihilation assisted ultrafast carrier dynamics in 2H-MoSe₂ and Cr-doped 1T/2H-MoSe₂ nanosheets, S. Mukherjee et. al., J. Chem. Phys. 159, 164705 (2023).
Many-body interaction governed ultrafast relaxation dynamics in CuS nanoflakes and CuS/Ag₂S nanocomposites, S. Mukherjee et. al., ACS Appl. Opt. Mater., 1, 1332–1342 (2023).
Photo-excited ultrafast dynamics of free carriers and polarons in V₂O₅ microparticles, Anjan Kumar NM. et. al., J. Phys. Chem. C 126, 20535–20541 (2022).
Investigation of self-trapped excitonic dynamics in hematite nanoforms, Anjan Kumar NM. et. al., Appl. Phys. Lett., 121, 202102 (2022).
Ultrafast carrier dynamics in undoped and Ho³⁺-doped α-Bi₂O₃ micro-rods, J. Sarkar et. al., J. Phys. Chem. C 123, 10007 (2019).
Ultrafast electron-hole plasma dynamics in ZnO and Ag-doped ZnO nanorods, J. Sarkar et. al., J. Appl. Phys. 124, 243103 (2018).
Review on numerical methods for thickness determination in THz-TDS, S. Mukherjee et. al., Eur. Phys. J. Spec. Top. 230, 4099–4111 (2021).
Major Experimental Tools used in our investigations:
Pump-Probe Spectroscopy:
Femtosecond pulses are divided into two part, (1) strongly (pump) and (2) weakly intense (probe). The pump excites/perturbs the condensed matter system under study, i.e creating photo excited carriers, Coherent quasi particle excitations like phonons, magnons, excitons,...etc that modulate the refractive index, dn. This modulation in refractive index is detected by the weaker pulse/probe's reflectivity/transmissivity changes in the presence of pump pulse.
Terahertz Spectroscopy:
In contrast to visible radiation, which primarily interacts with valence electrons, THz waves (photon energy of 4-20 meV @ 1-5 THz) allow direct access to numerous low energy excitations such as molecular rotations, lattice vibrations, spin waves, internal excitations of bound electron-hole pairs, and Cooper pairs, through both resonant and non-resonant excitation. With the capability to produce strong THz fields up to MV/cm, highly nonlinear collective responses in condensed matter and artificial metamaterials can be investigated.
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