Research Areas

We gratefully acknowledge the funding agencies who have generously been supporting our scientific research endevours through various research projects. We continue to work on certain frontier topics within the following areas of physics:

Physics of novel phases of condensed matter systems: Our group has strong affinity for novel materials and their probing through optical, transport and spectroscopic means.

Ultrafast, nonlinear and THz optics: We are equally interested in ultrafast optics, nonlinear optics, nonlinear interferrometry, THz optics and related processes. Ultrashort laser pulses are used for exploring the nature of light-matter interaction at ultrafast time scales and/or under extreme conditions of light field. By using them we study nonequilibrium physics of novel phases of condensed matter, light control in photonic structures, coherent THz radiation generation and detection, coherence and polarization, pulse shaping, etc.

Research Highlights: The following are few representative examples from our current research activities/interests.

1. Ultrafast THz Spintronics and Orbitronics.

Some Examples

• Ultrafast THz probing of nonlocal orbital current: THz generation from femtosecond photoexcited spintronic heterostructures has become a versatile tool for investigating ultrafast spin-transport and transient charge-current in a non-contact and non-invasive manner. The equivalent effect from the orbital degree of freedom is still in the primitive stage. Here, we experimentally demonstrate orbital-to-charge current conversion in metallic heterostructures, consisting of a ferromagnetic layer adjacent to either a light or a heavy metal layer, through detection of the emitted THz pulses. Our temperature-dependent experiments help to disentangle the orbital and spin components that are manifested in the respective Hall-conductivities, contributing to THz emission. NiFe/Nb shows the strongest inverse orbital Hall effect with an experimentally extracted value of effective intrinsic Hall-conductivity of ~195 Ohm^-1cm^-1 , while CoFeB/Pt shows maximum contribution from the inverse spin Hall effect. In addition, we observe a nearly ten-fold enhancement in the THz emission due to pronounced orbital-transport in W-insertion heavy metal layer in CoFeB/W/Ta heterostructure as compared to CoFeB/Ta bilayer counterpart.

Ref.: Nature Communications 14, 8185 (2023).

• THz Magnetometry - Ultrafast Magnetization Switching: The ultrafast optical control of magnetization in spintronic structures enables one to access to the high-speed information processing, approaching the realm of terahertz (THz). Femtosecond visible/near-infrared laser-driven ferromagnetic/nonmagnetic metallic spintronic heterostructures-based THz emitters combine the aspects from the ultrafast photo-induced dynamics and spin-charge inter-conversion mechanisms through the generation of THz electromagnetic pulses. In this Letter, we demonstrate photoexcitation density-dependent induced exchange-bias tunability and THz switching in an annealed Fe/Pt thin-film heterostructure, which otherwise is a widely used conventional spintronic THz emitter. By combining the exchange-bias effect along with THz emission, the photo-induced THz switching is observed without any applied magnetic field. These results pave the way for an all-optical ultrafast mechanism to exchange-bias tuning in spintronic devices for high-density storage, read/write magnetic memory applications.

Ref.: Applied Physics Letters 120, 202403 (2022).

2. Quantum Materials - Understanding from Photophysical Processes.

Some Examples

• Sub-bandgap activated charges transfer in a graphene-MoS2-graphene heterostructure: Monolayers of transition metal dichalcogenides are semiconducting materials which offer many prospects in optoelectronics. A monolayer of molybdenum disulfide (MoS2) has a direct bandgap of 1.88 eV. Hence, when excited with optical photon energies below its bandgap, no photocarriers are generated and a monolayer of MoS2 is not of much use in either photovoltaics or photodetection. Here, we demonstrate that large size MoS2 monolayer sandwiched between two graphene layers makes this heterostructure optically active well below the band gap of MoS2. An ultrafast optical pump-THz probe experiment reveals in real-time, transfer of carriers between graphene and MoS2 monolayer upon photoexcitation with photon energies down to 0.5 eV. It also helps to unravel an unprecedented enhancement in the broadband transient THz response of this tri-layer material system. We propose possible mechanism which can account for this phenomenon. Such specially designed heterostructures, which can be easily built around different transition metal dichalcogenide monolayers, will considerably broaden the scope for modern optoelectronic applications at THz bandwidth.

Ref.: Nano Select 2(10), 2019 (2021).

• Enhancement in optically induced ultrafast THz conductivity in MoSe2MoS2 heterobilayer: THz conductivity of large area MoS2 and MoSe2 monolayers as well as their vertical heterostructure, MoSe2MoS2 is measured in the 0.3–5 THz frequency range. Compared to the monolayers, the ultrafast THz reflectivity of the MoSe2MoS2 heterobilayer is enhanced many folds when optically excited above the direct band gap energies of the constituting monolayers. The free carriers generated in the heterobilayer evolve with the characteristic times found in each of the two monolayers. Surprisingly, the same enhancement is recorded in the ultrafst THz reflectivity of the heterobilayer when excited below the MoS2 bandgap energy. A mechanism accounting for these observations is proposed.

Ref.: Optics Express 29(3), 4181-4190 (2021).

• Phonon Bottleneck in Hot Carrier Relaxation in Graphene Oxide flakes: Temperature-dependent ultrafast hot carrier relaxation in graphene oxide (GO) flakes is measured. At all temperatures in the range from ~6 to 400 K, the time evolution of the photoexcited relaxation reveals a three-component recovery, among which the initial two components are signature time constants of graphene. We experimentally confirm that the slowest third relaxation component (t3) is not related to the radiative recombination of the carriers, rather due to the carrier scattering from defects present in GO. We observe that t1 and t3 are nearly independent of the sample temperature, while t2 linearly increases with the sample temperature providing the main bottleneck in carrier relaxation. The latter is due to the accumulation of excess number of hot phonons in the decay cascade with increasing lattice temperature. From this behavior, we have estimated the electron–phonon coupling strength in GO.

Ref.: Journal of Physical Chemistry C 125, 26583 (2021).

3. Photophysics of Topological Insulators.

Some Examples

• Unveiling surface and bulk contributions in temperature dependent THz emission from Bi2Te3: We report evolution of the pulsed terahertz (THz) emission from Bi2Te3 topological insulator in a wide temperature range, where an interplay between the topological surface and bulk contributions can be addressed in a distinguishable manner. A circular photogalvanic effect-induced topological surface current contribution to THz generation can be clearly identified in the signal, otherwise, overwhelmed by the hot carrier decoherence in the bulk states. With the decreasing temperature, an initial sharp increase in the topological surface THz signal is observed before it attains a constant value below ~200 K. The scattering channels between topological surface and bulk regions via carrier-phonon scattering are dominantly active only above the bulk-Debye temperature of ~180 K, and the temperature-independent behavior of it at lower temperatures is indicative of robust nature of topological surface states. THz emission due to ultrafast photon-drag current in the bulk states is almost independent of temperature in the entire range, while the combined photo-Dember and band-bending effects induced photocurrent is doubled at 10 K.

Ref.: New Journal of Physics 26, 033044 (2024).

• Photo-Seebeck effect in a three-dimensional topological insulator: Bismuth telluride is a low energy bulk bandgap topological system with conducting surface states. Besides its very good thermoelectric properties, it also makes a very good candidate for broadband photodetectors. Here, we report the temperature-dependent photo-Seebeck effect in a bulk single crystalline bismuth telluride. Upon light illumination, an electrically biased sample shows distinguishable contributions in the measured current due to both the Seebeck effect and the normal photo-generated carriers within a narrow layer of the sample. The detailed experiments are performed to elucidate the distinction between the Seebeck contribution and the photogenerated current. The temperature dependence of the photocurrent without Seebeck contribution shows a sign reversal from negative to positive at a specific temperature depending on the wavelength of photoexcitation light.

Ref.: Applied Physics Letters 116, 141902 (2020).

4. Light Control in Photonic Structures.

Some Examples

• Nonlinear wave-mixing in photonic crystal fibers: Terahertz radiation generation by four-wave mixing and low-loss propagation in a well optimized photonic crystal fiber has been presented here. Teflon with low-loss in the THz range has been considered where diatomic hexagonal crystalline symmetry in the core region is important for large modal overlap to efficiently generate THz radiation. The core and cladding of the fiber are made of Teflon with slightly different indices both at the pump and idler as well as THz frequencies. The core of the fiber consists of modified Teflon and Polyethylene inclusions of same radial dimension arranged in diatomic lattice with hexagonal symmetry. Input pump and idler fields are chosen to be at the frequencies of standard carbon dioxide and carbon monoxide lasers, respectively. The underlying photonic structure produces high efficiency of 15% with respect to pump power and moderately tunable THz radiation at about 9.2 THz within a very short 6.5 m propagation length. The pump power can be varied to tune the THz signal and the idler frequency in certain range. The optimized design also provides a low-loss guidance with propagation and bending losses of less than 1dB/km.

Ref.: Optics Communications 454, 124460 (2020).

• Ultrafast response of plasmonic nanostructures: Ultrafast photoresponse of plasmonic nanostrucutres, specifically, nanoparticles has been discussed here. Femtosecond laser pulses are useful not only for the time-resolved investigations but also to look at the optical nonlinearities in materials which are primarily electronic at such time-scales. Like the linear photoresponse such as absorption and scattering cross-sections, ultrafast nonlinear optical response also gets emensely enhanced at wavelenths near the surface plasmon resonace of the nanosystem under study. In a time-resolved measurement, typically the electronic scattering processes are studied, however, the confined accoustic phonons due to the finite size effects of the nanostrucutres can also modify the ultrafast time-resolved response. Although, Raman spectroscopy and infrared absorption spectroscopy are the popular techniques for studying phononic properties of the nanostructures, terahertz time-domain spectroscopy using ultrashort terahertz pulses also has shown potential to become another important characterization technique for nanoparticles specially at very low frequencies where other techniques are difficult to reach. We have attempted to present a comprehensive account of the above topics by including essential background given in the beginning of the chapter and subsequently discussing some of the important experimental results from the recent literature along with our results on metal nanoparticles.

Ref.: Reviews in Plasmonics 2015, 131-167 (2016).