Research

Amongst many ways to control and probe condensed matter in all its phases, electromagnetic radiation has been the most flexible impetus.  With the arrival of ultrafast (pulse width of ~10 fs - 2 ps) pulses in the ultraviolet (UV), visible (VIS) infrared (IR) electromagnetic region, there has been a rapid  development  of  high  speed  electronic  devices  along with the introduction of  the relatively  new  areas  of photonics  and  spintronics  (spin-based electronics)  as alternatives  to  traditional  electronics, crossing the boundaries of physics, materials science, and electrical engineering (Here 1 femtosecond (fs)  = 10-15 s and 1 picosecond (ps) = 10-12 s). 

Over the time, the spectroscopy using these  ultrafast  pulses (commonly called as ultrafast spectroscopy)  has  also  attained prominence  in  condensed  matter physics, chemistry and biology due to its ability to resolve dynamics at the fundamental timescales of electron and lattice motion. The most  straightforward  ultrafast spectroscopic technique  is pump-probe  spectroscopy,  which  uses  an intense ultra-short  (mostly femtosecond) laser pulse (pump) to excite a sample into a non-equilibrium state and a weaker time-delayed ultra-short pulse (probe) to measure the relaxation back to equilibrium. 

Another important technological advance in this field of femtosecond lasers is its ability to produce electromagnetic pulses in the  terahertz (THz) regime. 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.

At our group, we are interested in developing various tools using femtosecond lasers to probe condensed matter world. Right now, our interests span from strongly correlated materials, various 2D materials, and topological insulator samples and superconductors. Some of the tools that will be used in our investigations are Pump-Probe spectroscopy, THz spectroscopy, Nonlinear Spectroscopy (Second Harmonic Generation).

Research Interests & highlights:

1. Understanding ultrafast photo-excitation and the subsequent carrier dynamics in various nano and bulk insulating materials (Ultrafast multi-species kinetics models and investigations in various condensed matter systems)

• Investigation of ultrafast electron dynamics in ZnO & Bismuth Oxide nano forms: using femtosecond pump-probe spectroscopy and the time and frequency resolved terahertz (THz) spectroscopy. Nonlinear response of the  condensed matter will be investigated using femtosecond pulses in this electromagnetic region. One of the main direction in this regard is to look at the electron phonon interaction and many body interaction decay, in various condensed matter systems using ultrashort pulses. Our publications on ZnO ( Jit Sarkar e. al., Journal of Applied Physics 124, 243103; https://doi.org/10.1063/1.5058121) and Bismuth oxide (Jit Sarkar e. al., J. Phys. Chem. C, 123 (15), 10007) nano and micro systems have promising results at room temperature that has triggered us to pursue in that direction. 

• Investigation of self-trapped exciton dynamics in hematite nano-forms through non-degenerate pump–probe transmission spectroscopy

• Hematite is a potential material for a wide range of applications from optoelectronics to light harvesting. Despite its exciting features, its performance is restricted by the short minority charge diffusion distance, poor oxidizing kinetics and most importantly the short life time due to the presence of trap states. Hence, understanding and controlling the underlying recombination processes in the hematite-based systems is crucial for both device performance and fundamental physics. In the recent Extreme UV(XUV) pumped transient white light probe, and optically pumped THz time resolved experiments, the presence of self-trapped excitons (STEs) and polarons are confirmed to play a major role in the photo-excited hematite although previous transient absorption data were speculated in terms of free carriers and free excitons. Thus, there was an urgent need to re-look at the photo-excited hematite through non-degenerate pump-probe measurements as that would enable to look at exclusively the photo-excited species at the conduction band from where free excitons and self-trapped excitons can be tracked. In this work, a non-degenerate pump probe transmission spectroscopy (Pump-3.15 eV and Probe 1.57 eV) is used to explore the self-trapped exciton (STE) dynamics in hematite nanoforms at various pump fluences. The kinetics of STE formation and annihilation were studied using coupled rate equation based kinetic model for the first time. According to our study, the free excitons and STEs interact nonlinearly to annihilate one another in a way similar to the trap-assisted bi-molecular Auger recombination. Another notable observation from our studies is the strong carrier concentration dependence of the kinetics of STE formation and the exciton decay. The exact variation of these various processes with carrier density is understood by using the Coulombic screening between the carriers for carrier densities greater than ~3.3x10 17 cm -3 . This analysis yields an estimated average STE density of ~3.8x10 18 cm-3 for hematite nanoforms. We also observed that the nature of exciton dynamics is not significantly altered when hematite nanoforms are doped with K and Ni indicating a possible presence of STEs in hematite. The application of modified kinetic model of exciton to the non-degenerate pump probe data has enabled the new analysis based on STEs and free excitons and the corresponding findings in our article. Our results from this analysis indicate that the dynamics through bimolecular Auger processes involving STEs and free excitons in hematite nanoforms could be tuned with excitation density and we expect this will inspire more studies on hematite. Link to the published article: https://aip.scitation.org/doi/10.1063/5.0123246.

• Photo excited ultrafast dynamics of free carriers and polarons in irregular V2O5 microparticles through time resolved non-degenerate pump probe spectroscopy:  V2O5 is a very promising material with a wide range of applications, including light harvesting, energy storage, photocatalysis, photodetectors, solar cells, light-emitting diodes, and wave guides. However, for optimal device performance as well as from a fundamental physics point of view, a proper understanding of carrier dynamics in V2O5 based systems is essential. Despite such huge potential, not much time resolved experiments were performed on these materials, leaving the ultrafast dynamics obscure.  This article is focused to unravel the ultrafast dynamics of photo-excited carriers in V2O5 nanoform. We have used time-resolved non-degenerate pump-probe transmission spectroscopy to investigate the photo-excited carrier dynamics in V2O5 nanoforms. Tri exponential function has been used to fit the time-resolved absorption data derived from transient transmission data.  A closer look utilizing pump fluence-dependent analysis reveals that the decay is considerably impacted by the pump fluence, indicating that fitting with first order decay process is insufficient to explain the decay dynamics. To understand further, a numerically solved first order coupled differential equations based polaronic kinetic model is used on the time-resolved absorption data to study the formation of polarons and their various decay channels. Through this analysis, it is found that the free carriers are trapped in polaronic traps within an average time scale of ~4.5ps, and these polarons subsequently interact with the free carriers to undergo polaron-assisted bi-molecular decay. Using the pump fluence-dependent experiments, the variation of the various underlying processes could be explained through the carrier screening concepts. It is estimated that the average density of polarons here, is ∼2.58×1017cm−3 in our study. Link to the published article is here: https://pubs.acs.org/doi/full/10.1021/acs.jpcc.2c06894

2. Investigation of accurate thickness extraction algorithms for THz spectroscopy. Have a look at our review: "A review on numerical methods for thickness determination in terahertz time-domain spectroscopy", Soumya Mukherjee, N. M. Anjan Kumar, Prashanth C. Upadhya, and N. Kamaraju, Eur. Phys. J. Spec. Top. 230: 4099-4111 (2021). Link to our published article is here: https://link.springer.com/article/10.1140/epjs/s11734-021-00215-9. 

3. THz spectroscopy of disordered materials: We investigate various methods to model the THz conductivity of the the disordered condensed matter systems.

4. Ultrafast and THz Coherent quasi particle dynamics (Coherent acoustic and optical Phonons, magnons and other quasi particle excitations) in various condensed matter systems.

Some examples of excellent candidates to use THz and femtosecond pulses are quantum Materials like 2DEG/2DHG, 2D layered materials, topological insulator systems, and novel compounds and nano-materials, solar cells, organic semiconductors, strongly correlated systems and artificially created metamaterials apart from bulk systems. 

5. Another direction of our interest is on terahertz medical imaging and spectroscopy, and in developing methods and components for terahertz imaging and terahertz spectroscopy, also discovering new techniques to better characterize quantum materials using ultrafast spectroscopy. 

Major Experimental Tools used in our investigations: