Kamaraju Natarajan
@iiserkol.ac.in
Associate Professor and Department of Physical Sciences
Indian Institute of Science Education and Research Kolkata
RESEARCH, TEACHING, or OTHER INTERESTS
Condensed Matter Physics, Atomic and Molecular Physics, and Optics, Acoustics and Ultrasonics, Materials Science
Scopus Publications
Scholar Citations
Scholar h-index
Scholar i10-index
Scopus Publications
Soumya Mukherjee, Anjan Kumar NM, Ayan Mondal, Venkataramanan Mahalingam, and N. Kamaraju
AIP Publishing
Nanosheets of transition metal dichalcogenides with prospects of photocatalysis and optoelectronics applications have significant potential in device fabrication due to their low-cost production and easily controllable morphology. Here, non-degenerate pump-probe differential transmission studies with varying pump-fluence have been carried out on single-phase 2H–MoSe2 and mixed-phase 1T/2H–MoSe2 nanosheets to characterize their excited carrier dynamics. For both the samples, the differential probe transmission data show photo-induced bleaching at earlier pump-probe delay followed by photo-induced absorption unveiling signatures of exciton-state filling, exciton trapping, defect-mediated photo-induced probe absorption and recombination of defect bound excitons. The exciton trapping and photo-induced absorption by the trapped-carriers are estimated to occur with time constant of ∼430 to 500 fs based on multi-exponential modelling of the differential transmission till pump-probe delay of ∼3.5 ps. Biexponential modeling of the subsequent slow-recovery of the negative differential transmission at pump-probe delay ≳3.5 ps reveals that the exciton recombination happens via two distinct decay channels with ∼25 to 55 ps (τ1) and ≳1 ns (τ2) time constants. Pump-fluence dependent reduction in τ1 and further modelling of exciton population using higher order kinetic rate equation reveals that the two-body exciton-exciton annihilation governs the exciton recombination initially with a decay rate of ∼10−8 cm3s−1. The detailed analysis suggests that the fraction of total excitons that decay via long decay channel decreases with increasing exciton density for 2H–MoSe2, in contrast to 1T/2H–MoSe2 where the fraction of excitons decaying via long decay channel remains constant.
Soumya Mukherjee, Anjan Kumar NM, Saranya Ramesh, B. Karthikeyan, and N. Kamaraju
American Chemical Society (ACS)
Soumya Mukherjee, N. M. Anjan Kumar, B. Karthikeyan, and N. Kamaraju
Springer Science and Business Media LLC
Anjan Kumar N M, Soumya Mukherjee, Saranya Ramesh, Ayan Mondal, Venkataramanan Mahalingam, and N. Kamaraju
American Chemical Society (ACS)
Anjan Kumar N M, Soumya Mukherjee, Anoop Sunny, B. Karthikeyan, and N. Kamaraju
AIP Publishing
Non-degenerate pump–probe transmission spectroscopy is used to examine the ultrafast dynamics of photo-excited carriers in hematite nanoforms at various pump fluences. Using coupled rate equations, the kinetics of self-trapped exciton (STE) formation and its interaction with free excitons resulting in exciton annihilation were studied. It is shown from this model that the majority of the excitons were trapped by polaronic trap states to form self-trapped excitons within ∼3.5 ps. The findings indicate that free excitons and STEs interact non-linearly, similar to trap-assisted bi-molecular Auger recombination to annihilate one another. It is observed that there is substantial dependence of kinetics of STE formation and exciton decay on photo-excited exciton density, and the nature of this dependence indicates the reduced screening of electron–phonon interaction. Using the screening model applied to the rate constants of STE formation and decay, we estimate the saturation exciton density to be ∼3.3 × 1017 cm−3 and the average STE density to be ∼3.8 × 1018 cm−3 in the hematite nanoforms. We also noticed that doping K and Ni to hematite nanoforms up to 5% did not remarkably change the nature of the exciton dynamics.
Soumya Mukherjee, N. M. Anjan Kumar, Prashanth C. Upadhya, and N. Kamaraju
Springer Science and Business Media LLC
Jit Sarkar, Shankar G. Menon, N. Prasad, B. Karthikeyan, and N. Kamaraju
American Chemical Society (ACS)
We report the femtosecond two-color pump–probe studies on bismuth oxide microrods. The undoped data show three distinct decays: (1) fastest decay (τ1 ∼ 0.3–0.5 ps) attributed to intraband nonlinear effects (2) intermediate two-body decay (τ2 ∼ 3 ps) attributed to the trap-assisted Auger recombination; and (3) slow single-body decay (τ3 ∼ 36 ps) attributed to the Shockley–Read–Hall recombination. Upon doping with Ho3+, the sample shows an additional slow decay (τ4 ∼ 12 μs), attributed to the Shockley–Read–Hall recombination due to the trap states formed by the doping, along with the three decays observed in the undoped sample. The imaginary parts of third-order susceptibility values for both doped and undoped samples were estimated from the pump–probe studies, and the values are found to decrease inversely with pump fluence suggesting the plausible contribution from hot electrons.
Jit Sarkar, Shankar G. Menon, C. Madhumitha, B. Karthikeyan, and N. Kamaraju
AIP Publishing
The ultrafast dynamics of femotosecond laser excited carriers in the assembly of chemically pristine and Ag-doped ZnO nanorods is studied using the two-color pump-probe spectroscopy technique with the central photon energy of the pump pulse equal to the bandgap of the nanorods ensemble (∼3.14 eV) and the central photon energy of the probe pulse kept at 1.57 eV. On photo-excitation at 3.14 eV with an excited carrier density of ∼1020cm−3, about an order higher than the Mott density of bulk ZnO, the time delayed probe transmission displays tri-exponential decay exhibiting three decay mechanisms: (i) ultrafast electron-phonon thermalization of the order of ∼0.40 ps, (ii) trap mediated decay with a time constant of ∼9 ps, and (iii) e-h recombination with a time constant of ∼650 ps. It is observed that as the carrier density increases, the first two decay processes get longer on contrary to the recombination time which gets shorter. A detailed analysis of the faster decay process through the Two Temperature Model of ultrafast carrier relaxation supports our observation. Doping of Ag in these nanorods is found to introduce new defect states which alter the carrier dynamics significantly.
Yuanmu Yang, N. Kamaraju, Salvatore Campione, Sheng Liu, John L. Reno, Michael B. Sinclair, Rohit P. Prasankumar, and Igal Brener
American Chemical Society (ACS)
We demonstrate the ultrafast formation of terahertz (THz) metasurfaces through all-optical creation of spatially modulated carrier density profiles in a deep-subwavelength GaAs film. The switch-on of the transient plasmon mode, governed by the GaAs effective electron mass and electron–phonon interactions, is revealed by structured-optical pump THz probe spectroscopy, on a time scale of 500 fs. By modulating the carrier density using different pump fluences, we observe a wide tuning of the electric dipole resonance of the transient GaAs metasurface from 0.5 THz to 1.7 THz. Furthermore, we numerically demonstrate that the metasurface presented here can be generalized to more complex architectures for realizing functionalities such as perfect absorption, leading to a 30 dB modulation depth. The platform also provides a pathway to achieve ultrafast manipulation of infrared beams in the linear and, potentially, nonlinear regime.
N. Kamaraju, W. Pan, J. Reno, Q. Zhang, Junichiro Kono, A. J. Taylor, and R. P. Prasankumar
OSA
We used optical-pump, terahertz-probe spectroscopy to track carrier dynamics in a two-dimensional electron gas under a magnetic field, revealing photoinduced changes to the cyclotron frequency and scattering time, along with an unexpected higher frequency mode.
P. Bowlan, N. Kamaraju, D. A. Yarotski, A. J. Taylor, and R. P. Prasankumar
IEEE
We have used ultrafast THz spectroscopy to both excite and probe low energy excitations in two-dimensional electron gases, topological insulators, and multiferroics.
Kamaraju Natarajan, Qi Zhang, Michael R. Williams, Wei Pan, John L Reno, Junichiro Kono, Antionette Taylor, and Rohit Prasankumar
OSA
Kamaraju Natarajan, Wei Pan, Ulf Ekenberg, Dejan M Gvozdić, stephane Boubanga-Tombet, Prashanth C Upadhya, John Reno, Antoinette J. Taylor, and Rohit P. Prasankumar
OSA
N. Kamaraju, W. Pan, U. Ekenberg, D. M. Gvozdić, S. Boubanga-Tombet, P. C. Upadhya, J. Reno, A. J. Taylor, and R. P. Prasankumar
AIP Publishing
Two-dimensional hole gases (2DHGs) have attracted recent attention for their unique quantum physics and potential applications in areas including spintronics and quantum computing. However, their properties remain relatively unexplored, motivating the use of different techniques to study them. We used terahertz magneto-optical spectroscopy to investigate the cyclotron resonance frequency in a high mobility 2DHG, revealing a nonlinear dependence on the applied magnetic field. This is shown to be due to the complex non-parabolic valence band structure of the 2DHG, as verified by multiband Landau level calculations. We also find that impurity scattering dominates cyclotron resonance decay in the 2DHG, in contrast with the dominance of superradiant damping in two-dimensional electron gases. Our results shed light on the properties of 2DHGs, motivating further studies of these unique 2D nanosystems.
A. Gutin, A. V. Muraviev, N. Kamaraju, X. Shen, Y. Yamaguchi, M. S. Shur, D. But, N. Dyakonova, W. Knap, S. Rudin,et al.
IEEE
We demonstrate detection of individual pulses of terahertz radiation generated in femtosecond laser systems by InGaAs plasma-wave terahertz detectors. Nonlinearity of the detection mechanism is analyzed experimentally by comparison of saturation effects at femtosecond and nanosecond terahertz excitations. Good sensitivity and wide dynamic range make these detectors promising for short-pulsed terahertz applications.
Toru Shimada, N. Kamaraju, Christian Frischkorn, Martin Wolf, and Tobias Kampfrath
AIP Publishing
We irradiate a ZnTe single crystal with 10-fs laser pulses at a repetition rate of 80 MHz and investigate its resulting gradual modification by means of coherent-phonon spectroscopy. We observe the emergence of a phonon mode at about 3.6 THz whose amplitude and lifetime grow monotonously with irradiation time. The speed of this process depends sensitively on the pump-pulse duration. Our observations strongly indicate that the emerging phonon mode arises from a Te phase induced by multiphoton absorption of incident laser pulses. A potential application of our findings is laser-machining of microstructures in the bulk of a ZnTe crystal, a highly relevant electrooptic material.
Kan Wu, Javier García de Abajo, Cesare Soci, Perry Ping Shum, and Nikolay I Zheludev
Springer Science and Business Media LLC
N Kamaraju, Andrea Rubano, Linke Jian, Surajit Saha, T Venkatesan, Jan Nötzold, R Kramer Campen, Martin Wolf, and Tobias Kampfrath
Springer Science and Business Media LLC
SUNIL KUMAR, N. KAMARAJU, K. S. VASU, and A. K. SOOD
World Scientific Pub Co Pte Lt
We report ultrafast response of femtosecond photoexcited carriers in single layer reduced graphene oxide flakes suspended in water as well as few layer thick film deposited on indium tin oxide coated glass plate using pump–probe differential transmission spectroscopy at 790 nm. The carrier relaxation dynamics has three components: ~200 fs, 1 to 2 ps, and ~25 ps, all of them independent of pump fluence. It is seen that the second component (1 to 2 ps) assigned to the lifetime of hot optical phonons is larger for graphene in suspensions whereas other two time constants are the same for both the suspension and the film. The value of third order nonlinear susceptibility estimated from the pump–probe experiments is compared with that obtained from the open aperture23 Z-scan results for the suspension.
M. ANIJA, SUNIL KUMAR, N. KAMARAJU, NEHA TIWARI, S. K. KULKARNI, and A. K. SOOD
World Scientific Pub Co Pte Lt
We report ultrafast electron dynamics in gold nanorods investigated using 80 fs laser pulses centered at 1.57 eV. Five types of nanorod colloidal suspensions in water having their longitudinal surface plasmon peak (E LSP ) on either side of the laser photon energy (EL) have been studied. For E LSP > EL, photo-induced absorption with single decay time constant is observed. On the other hand, for E LSP < EL, photo-bleaching is observed having bi-exponential decay dynamics; the faster one between 1–3 ps and slower one between 7 ps to 22 ps both of them increasing almost linearly with the difference |EL – E LSP |. These time constants increase linearly with the pump intensity. Simulations have been carried out to understand the interplay between photo-bleaching and photo-induced absorption.
N. Kamaraju, Sunil Kumar, Surajit Saha, Surjeet Singh, R. Suryanarayanan, A. Revcolevschi, and A. K. Sood
American Physical Society (APS)
We study the generation of coherent optical phonons in spin-frustrated pyrochlore single crystals Dy${}_{2}$Ti${}_{2}$O${}_{7}$, Gd${}_{2}$Ti${}_{2}$O${}_{7}$, and Tb${}_{2}$Ti${}_{2}$O${}_{7}$ using femtosecond laser pulses (65 fs, 1.57 eV) in degenerate time-resolved transmission experiments as a function of temperature from 4 to 296 K. At 4 K, two coherent phonons are observed at $~$5.3 THz (5.0 THz) and $~$9.3 THz (9.4 THz) for Dy${}_{2}$Ti${}_{2}$O${}_{7}$ (Gd${}_{2}$Ti${}_{2}$O${}_{7}$), whereas three coherent phonons are generated at $~$5.0, 8.6, and 9.7 THz for Tb${}_{2}$Ti${}_{2}$O${}_{7}$. In the case of spin-ice Dy${}_{2}$Ti${}_{2}$O${}_{7}$, a clear discontinuity is observed in the linewidths of both the coherent phonons as well as in the phase of lower-energy coherent phonon mode, indicating a subtle structural change at 110 K. Another important observation is a phase difference of $\\ensuremath{\\pi}$ between the modes in all the samples, thus suggesting that the driving forces behind the generation of these modes could be different in nature, unlike a purely impulsive or displacive mechanism.
N. Kamaraju, Sunil Kumar, M. Anija, and A. K. Sood
American Physical Society (APS)
We report femtosecond time-resolved reflectivity measurements of coherent phonons in tellurium performed over a wide range of temperatures (3-296 K) and pump-laser intensities. A totally symmetric A(1) coherent phonon at 3.6 THz responsible for the oscillations in the reflectivity data is observed to be strongly positively chirped (i.e., phonon time period decreases at longer pump-probe delay times) with increasing photoexcited carrier density, more so at lower temperatures. We show that the temperature dependence of the coherent phonon frequency is anomalous (i.e, increasing with increasing temperature) at high photoexcited carrier density due to electron-phonon interaction. At the highest photoexcited carrier density of (1.4 x 10(21) cm(-3) and the sample temperature of 3 K, the lattice displacement of the coherent phonon mode is estimated to be as high as similar to 0.24 angstrom. Numerical simulations based on coupled effects of optical absorption and carrier diffusion reveal that the diffusion of carriers dominates the nonoscillatory electronic part of the time-resolved reflectivity. Finally, using the pump-probe experiments at low carrier density of 6 x 10(18) cm(-3), we separate the phonon anharmonicity to obtain the electron-phonon coupling contribution to the phonon frequency and linewidth.
N. Kamaraju, Sunil Kumar, and A. K. Sood
IOP Publishing
Degenerate pump-probe reflectivity experiments have been performed on a single crystal of bismuth telluride (Bi2Te3) as a function of sample temperature (3 K to 296 K) and pump intensity using ∼50 femtosecond laser pulses with central photon energy of 1.57 eV. The time-resolved reflectivity data show two coherently generated totally symmetric A1g modes at 1.85 THz and 3.6 THz at 296 K which blue-shift to 1.9 THz and 4.02 THz, respectively, at 3 K. At high photoexcited carrier density of ∼1.7×1021 cm− 3, the phonon mode at 4.02 THz is two orders of magnitude higher positively chirped (i.e the phonon time period decreases with increasing delay time between the pump and the probe pulses) than the lower-frequency mode at 1.9 THz. The chirp parameter, β is shown to be inversely varying with temperature. The time evolution of these modes is studied using continuous-wavelet transform of the time-resolved reflectivity data.