Coherent phonon dynamics in topological insulator (TI): We investigated photogenerated coupled phonon-surface Dirac Plasmon in Sb₂Te₃ TI for various thicknesses using displacive excitation of coherent phonon (DECP) spectroscopy. In this project, we find that the transient coherent phonon spectra obtained at different time scale exhibits an asymmetric line shape (characterized by Fano-like line shape) in the high frequency side of the A_1g² optical phonon mode, which is attributed to quantum interference between continuum like coherent Dirac like Plasmon and phonons. The transient Fano-like resonance is persisted up to 1 ps after photo-excitation. The transient Fano resonance whose relaxation profile is well captured by the Gaussian decay <200 fs, suggesting the coherence time of the Dirac Plasmon is about to 200 fs. [Richarj Mondal et. al. Physical Review B 97 (14), 144306 (2018).]

Observation a cross-over from 3D to 2D TI by nonlinear magneto-optical effect: In this project, we proposed a novel technique, helicity dependent time-resolved Kerr measurement, to detect the Dirac surface state in topological insulators (TIs). We have investigated a critical thickness, where a crossover from a 3D-TI to 2D insulator occurs upon changing the thickness of the TI layer down to a few nanometers, by means of photon helicity dependent time-resolved Kerr measurement. The helicity dependence of the time-resolved Kerr signal exhibits a robust change in periodicity at a critical thickness 4 quintuple layer (QL) as shown in figure. We demonstrated that such observation can be interpreted based on the cascading nonlinear magneto-optical Kerr effect (OKE) induced via strong spin-orbit coupling, from which it is possible to detect the formation of a Dirac cone in TI with several thick samples. [Richarj Mondal et. al.Scientific reports 8 (1), 3908 (2018).]

Topological phase monitored in chalcogenide superlattice: Chalcogenide superlattice (CSL), GeTe/Sb₂Te₃, is a promising candidate for the next generation non-volatile electronic memory. This newly formed CSL, referred to as interfacial phase change memory (iPCM), is crystalline in both the RESET (low-conductivity) and SET (high conductivity) phases. Theoretical calculations based on ab initio, it has been predicted that GeTe/Sb₂Te₃ CSLs becomes TIs, Dirac semimetals, or NIs depending upon the thickness of the individual blocks of GeTe and Sb₂Te₃. In this project, we unraveled the topological insulating state in GeTe/Sb₂Te₃ chalcogenide superlattice by means of a helicity dependent time-resolved Kerr measurement. The helicity dependent Kerr signal exhibits a four-cycle oscillation with π/2 periodicity, which suggests the existence of a Dirac-like cone in CSLs. Furthermore, we investigated topological phase controlled by tuning the GeTe layer and we found an indication of a phase transition from a Dirac semimetal state to a normal insulating state with increasing the thickness of the GeTe layer (shown in Fig. 3b). [Richarj Mondal et. al.ACS applied materials & interfaces 10 (31), 26781-26786 (2018).]

Pauli blocking dynamics in the self-assembled InAs quantum dots: Quantum dots (QDs) have atom-like discrete energy levels owing to their three-dimensional quantum confinement. The carrier (spin) relaxation among these different quantum states in QDs is expected to be influenced by the Pauli’s exclusion principle. We investigated Pauli blocking dynamics of carriers in InAs self-assembled QDs through steady-state PL measurements by using excitation correlation (EC) spectroscopy. The time evolution of the EC signals measured at two different excitation fluences (I) for the first three QD states (denoted by G, X1, and X2, respectively) is shown in figure. It is seen that near zero delay, the signal from the state G is snubbed while that from the X1 state can either be smaller or larger than the observed signal at longer delays, depending on the excitation fluence. The results can be interpreted in terms of Pauli blocking in QDs. We proposed a minimal theoretical model, which quantitatively reproduces the observations. [Richarj Mondal et. al.Physical Review B 87 (11), 115317 (2013).]

Coherent oscillation of the heavy-hole-exciton in GaAs quantum wells: In this project, we performed the spectrally-resolved pump-probe reflectivity measurement on a high quality of GaAs/AlGaAs quantum wells (QW) sample at 4K. Spectrally-resolved signal shows a non-zero coherent oscillations signal in the negative delay between the probe and the pump beam (shown in figure). The coherent oscillations are attributed to the perturbed free-induction decay of coherent polarization at the excitonic resonance of the heavy-hole exciton (hh-x). A theoretical framework in the negative delay is constructed based on the optical Bloch equations to describe the induced polarization. The experimental observations are nicely captured by the theoretical framework as shown in figure. [Richarj Mondal et. al. J. Phys.: Condens. Matter (2018).]

Measurements of the Electric Field of Zero-Point Optical Phonons in GaAs Quantum: The band theory predicts that the density of states of insulators must have a sharp cutoff at the bandedge, while experiments have long shown a tail that extends into the theoretically forbidden gap. We have studied the temperature dependent photoluminesces (PL) from a high quality of GaAs/AlGaAs quantum wells (QWs) sample. A specific type of lifetime broadening was observed resulting in the well-known exponential “Urbach tail” density of states within the energy gap of an insulator. In this study, we proposed that there is a fundamental zero-temperature contribution to the Urbach slope resulting from the electric field of the zero-point optical phonons. The value of this electric field is estimated experimentally. [Rupak Bhattacharya et. al. Physical Review Letter 114, 047402 (2015).]