Observing the individual effects of the electric and magnetic fields of light from the transverse spin of single optically trapped birefringent particles
This work discerns, for the first time, the effects of the electric and magnetic fields of light separately, by their work done on single highly birefringent particles that are optically trapped using vector beams of light.
More elaborately, the work describes an elegant scheme to discern the effects of the electric and magnetic fields of light separately, by their work done on single highly birefringent particles that are optically trapped using vector beams of light. Thus, the authors describe a scheme of generating pure transverse spin angular momentum (TSAM) in their system by tightly focusing radially and azimuthally polarized structured beams with no intrinsic angular momentum into a refractive index stratified medium. The TSAM is then transferred to single highly birefringent particles which spin in a direction parallel to the beam propagation direction. Such choice of beams ensures the breakdown of ‘electromagnetic democracy’ in the context of angular momentum in these systems, which helps demonstrate the action of the electric and magnetic fields on matter, separately.
This work is published in Lasers and Photonics Review with an Impact Factor of 11.
Posted on: December 2nd, 2023
Absence of mobility edge in short-range uncorrelated disordered model: Coexistence of localized and extended states!
In 1967, Sir Nevill F. Mott's pioneering work argued that the extended and the localized eigenstates in a 3-dimensional Anderson model are always separated by a critical energy called the mobility edge. The conventional wisdom has been that this energetic separation is true in any disordered low-dimensional system.
Our work shows that this is not always true in 1-dimensional systems. We identify the necessary criteria for realizing coexistent localized and extended states without forming any mobility edge: residual level repulsion, non-ergodicity of most bulk states, and spatial separation of localization centers. We demonstrate these ideas in a 1D disordered lattice governed by the Hamiltonians from the β-ensemble. We also explain the origin of the anomalous long-range correlations in the energy spectrum of the β-ensemble, which has been a long-standing puzzle in the random matrix theory community.
"Absence of mobility edge in short-range uncorrelated disordered model: Coexistence of localized and extended states"Adway Kumar Das, Anandamohan Ghosh, Ivan M. Khaymovich Published online 18 October 2023, appeared in 20 October 2023 issue: Physical Review Letters (Vol. 131, No. 16):
URL: https://link.aps.org/doi/10.1103/PhysRevLett.131.166401 DOI: 10.1103/PhysRevLett.131.166401
Posted on: November 21st, 2023
Spin-direction-spin coupling of quasiguided modes in plasmonic metamaterials
Simultaneous demonstration of the forward (left) and inverse (right) spin Hall effect of light in a waveguiding plasmonic crystal
Joint research of bioNap & Light Matter Lab., DPS
“Spin-Direction-Spin Coupling of Quasiguided Modes in Plasmonic Crystals” has been published in Phys. Rev. Lett. 131, 193803 (2023) and featured in the cover page. Jeeban Kumar Nayak, a PhD student of DPS and a former BS-MS student is the primary author of this paper. The other students who were involved in this research are, Shyamal Guchhait (PhD), Harley Suchiang (former BS-MS student), Subir Kumar Ray (former PhD student).
Brief description: The coupling of spin (SAM) and orbital angular momentum (OAM) degrees of freedom of light, the so-called spin orbit interactions (SOI) have led to a number of fundamental photonic effects in various light-matter interactions. These have not only provided new insights on the universal SOI phenomena in classical optical settings but also have opened up novel routes for the development of spin-orbit photonic devices. Among the various SOI effects, photonic spin-momentum locking has attracted particular attention due to its fundamental nature and potential device applications. We have observed an unusual spin-direction-spin coupling phenomena using the leaky quasiguided modes of a waveguided plasmonic crystal system. We have observed simultaneous spin controlled directional guiding of waves (spin-direction coupling) and its reciprocal effect of wavevector-dependent spin acquisition of the scattered light (direction-spin coupling) from the same nanophotonic system, which we have defined as `spin-direction-spin coupling' phenomena. While the former effect manifests as the conventional photonic spin Hall effect (SHE), the latter is known as the inverse SHE of light, which is rather new and has not been explored much. The inverse SHE of light reported in our system can be seen as an optical analogue of the spin-injection in the solid-state spintronic devices; where with unpolarized excitation (no input SAM), the scattered light acquires a particular spin polarization according to the scattering direction. These SOI effects are shown to be mediated by the evolution of space-varying polarization due to highly non-paraxial focused light and its subsequent interaction with the polarization-anisotropic quasiguided modes of the plasmonic crystal system.
The fundamental origin and the unconventional manifestation of the spin-direction-spin coupling phenomena from a relatively simple system, ability to probe and interpret the resulting phenomena in the far field through momentum-domain polarization analysis, and their regulated control in plasmonic-photonic crystals open exciting avenues in spin-orbit photonic research.
Link to the paper: https://journals.aps.org/prl/issues/131/19.
Posted on: November 13th, 2023
Trapping and exciton-exciton annihilation assisted ultrafast carrier dynamics in nanosheets of 2H-MoSe2and Cr doped 1T/2H-MoSe2
Research Highlights from Ultrafast and Terahertz spectroscopy (UFTS) group led by Dr. Kamaraju
Transition metal dichalcogenides (TMDC) in monolayer or few-layers form have attracted significant interest due to their intriguing physical properties, such as layer-dependent band structure, high exciton binding energy, valley selective optical excitation and applications like electrocatalysis, photocatalysis, optoelectronics. The preparation of TMDCs via colloidal synthesis method has been drawing considerable attention recently due to their extremely low-cost fabrication, efficient control over size and shape, and easy implementation in device fabrication via methods such as drop-casting, and spray-coating. Despite several advantages, unintentionally formed defect states in these colloidally synthesized TMDCs are crucial in device- performance and need proper characterization for efficient device formation.
In this work we have employed ultrafast time-resolved non degenerate pump–probe spectroscopy to explore the role of defects in carrier dynamics of colloidally synthesized few-layer single-phase 2H-MoSe2 and mixed-phase 1T/2H-MoSe2 nanosheets. Metastable metallic 1T-phase has been introduced via Cr doping to coexist with stable 2H-phase to form stable mixed-phase 1T/2H-MoSe2 nanosheets which has been selected for pump-probe studies to reveal the physical processes governing its enhanced photocatalytic activities compared to 2H-MoSe2. After above bandgap excitation and probing at A-exciton peak, the differential probe transmission data for both the samples 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. Pump-fluence dependent reduction of decay time constants of bound-exciton-recombination and further modelling of exciton population using higher order kinetic rate equation reveals that the two-body exciton-exciton annihilation governs the exciton recombination process initially with a decay rate of ∼10−8 cm3 s −1. Our analysis also 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, consistent with its higher photocatalytic efficiency. This work estimating crucial parameters such as trapping time, exciton-exciton annihilation rate, and shares of many-body and single-body processes in the exciton-decay should be instrumental in optimizing properties of colloidally synthesized mixed-phase MoSe2 nanosheets for applications like optoelectronics and photocatalysis.
Posted on: November 3rd, 2023
Lattice anharmonicity and incomplete soft mode
Posted on: August 21st, 2023
A Surprising Link between Cosmic Magnetism and Life
Exploration of planets in our own solar system and the discovery of more than 5000 exoplanets has the scientific community excited about the possibility of life beyond Earth. The possibility of life and the ultimate fate of planets such as the Earth are influenced by diverse factors including the nature of the stars they orbit. The strength of the invisible magnetic field - known as the magnetosphere - that surrounds a planet, as well as the strength of the magnetic field of its host star, which for us, is the Sun, have a surprisingly important role on this context, a new study from Department of Physical Sciences and the Center of Excellence in Space Sciences India finds.
In the study published in the Astrophysical Journal, PhD student Sakshi Gupta, Research Scientist Arnab Basak and Prof. Dibyendu Nandi present computer simulations exploring the interplay of cosmic magnetic fields that can result in the loss of a planet's atmospheres and their ability to host life. They go on to establish a mathematical equation which relates atmospheric mass loss rate to the relative strengths of stellar and planetary magnetic fields. Their findings are expected to aid in the identification of habitable star-exoplanet systems and catalyse observational efforts for measuring exoplanetary magnetic fields.
Reference: Gupta, S., Basak, A. and Nandy, D. 2023, “Impact of Changing Stellar and Planetary Magnetic Fields on (Exo)planetary Environments and Atmospheric Mass Loss”, Astrophysical Journal, Volume 953, Page 70
Posted on: August 5th, 2023
Radio telescopes detected the gravitational waves hum by studying pulsars
On June 29, an international group of scientists recorded the hum of the gravitational waves of the universe using six of the world’s most sensitive radio telescopes, including India’s Giant Metrewave Radio Telescope (GMRT), located near Narayangaon, Pune.
Gravitational waves were proposed by Albert Einstein in 1916 but were first detected by Laser Interferometer Gravitational-Wave Observatory (LIGO) in 2015.
Radio telescopes detected the gravitational hum by studying pulsars, a type of rapidly rotating neutron stars, which form when a massive star runs out of fuel and collapses. A pulsar acts as a cosmic lighthouse, which emits radio beams that hit Earth.
Gravitational waves are thought to change the arrival times of these radio flashes, allowing researchers to document the hum.
Fazal Kareem a Final year BS-MS Student at IISER Kolkata was involved in the discovery of the stochastic background gravitational waves, the humming of the universe, along with 40+ other researchers of the Indian Pulsar Timing Array Collaboration. The team reported significant evidence for the presence of GWB in their pulsar timing dataset. Fazal was involved with the observation of pulsars (the most important part of the experiment), data analysis, noise analysis and combination of the European Pulsar Timing Array data with InPTA data.
Figure: Artist's rendering of black hole binaries emitting gravitational waves. As the waves overlap, they produce a background of gravitational waves that creates a distinctive correlation pattern in the timing of pulses coming from pairs of pulsars.
Posted on: July 10th, 2023
Many-Body Interaction Governed Ultrafast Relaxation Dynamics of Hot Holes in CuS Nanoflakes and Photocatalytic Efficiency Enhanced CuS/Ag 2 S Nanocomposites
Research Highlights from Ultrafast and Terahertz spectroscopy (UFTS) group led by Dr. Kamaraju Natarajan
Covellite (CuS) is a direct band gap p-type semiconductor with a high intrinsic hole population causing localized surface plasmon resonance (LSPR) in NIR regime. Though these highly dense (~10 21 /cm 3 ) hole populations have immense potential to enhance photocatalytic efficiency, their ultrafast decay after photoexcitation causes a significant drawback. To enhance the photocatalytic efficiency of CuS, the relaxation of hot carriers needs to get delayed, and this can be achieved via transferring these hot carriers to another material with which CuS can stay in a composite formation. CuS/ is one such photocatalytic efficiency-enhanced nanocomposite, as observed in an earlier report. In this work, we have conducted non-degenerate ultrafast pump-probe transmission studies of CuS nanoparticles and CuS/Ag 2 S nanocomposites with varying pump fluence to explore their hot hole relaxation dynamics and determine the underlying physics of photocatalytic efficiency enhancement. The measured transient transmission data for CuS is found to originate from both photoinduced bleaching of intrinsic holes and photoinduced absorption of excited electrons. But, the localization of hot electrons in the conduction band of Ag 2 S is found to cause the disappearance of photoinduced absorption in the transient transmission of CuS/Ag 2 S at all the pump fluences except for the highest one. The first-order relaxation process based multi-exponential (ME) modeling of transient transmission data for CuS reveals three hole decay channels (∼240-410 fs, ∼24-144 ps, ≳10 µs) along with the electron decay channel of ∼1-2.6 ps. Whereas for CuS/Ag 2 S, the electron decay channel disappears, and a new hole decay channel appears making a total of four decay channels for holes (∼230-470 fs, ∼10-49 ps, ≳10 µs, ∼132-326 ps). The decay time constants are found to vary considerably with increasing pump fluence (wide range of pump fluence ranging from 0.04 mJ/cm 2 to 1.22 mJ/cm 2 ), indicating the involvement of higher-order decay processes and suggesting the need of modeling using kinetic rate equation that involves appropriate higher order terms. Extraction of transient hole densities from pump-probe measurement and modeling them using a suitable kinetic rate equation (KRE) model capable of detecting higher-order processes have been performed to reveal the exact involvement. KRE modeling extracts first, second and third order decay rate constants. The first order decay agrees with the long decay time obtained from ME fitting. And the second and third order decay rates for CuS/ get reduced by almost one order compared to CuS, indicating a reduction in overall decay rates for CuS/ consistent with its enhanced photocatalytic activity. These results are found to indicate the presence of fast trapping of hot holes and a three-hole Auger kind of process involving two free holes and one trapped hole. These observations are very much relevant for fabricating efficient light harvesting devices using CuS and its composites.
Cite This: https://doi.org/10.1021/acsaom.3c00153
Posted on: July 10th, 2023
Siddhartha Lal Reserach Group : Frustration shapes multi-channel Kondo physics: a star graph perspective
Frustration. We think we understand it, and how to deal with it. But what does frustration refer to in a quantum system? Consider classical spins (i.e., spins that have only two configurations, say, pointing up and pointing down) placed on an equilateral triangle and interacting with one another through a nearest neighbour antiferromagnetic Ising exchange interaction. It is easy to see that Neel (anti-parallel) ordering is no longer possible: for any two spins that are anti-aligned with one another, the third is left confused on which direction to choose (see Fig.1). This confusion is labelled as the frustration of the classical Neel order. Frustration can also been seen in quantum mechanical systems: while two quantum spin 1/2s with an antiferromagnetic Heisenberg interaction will form a maximally-entangled singlet state, introducing a third spin-1/2 creates a dilemma (see Fig.2) - the two-spin singlet cannot accommodate another spin (often curiously referred to as entanglement monogamy). Since the spin-flip quantum fluctuations of the system will want to lower the energy of the system by entangling all three spins, no spin can be left free and the two-spin singlet cannot be the true ground-state.
The multichannel Kondo problem involves a local antiferromagnetic Heisenberg interaction between a single spin-1/2 impurity and the electrons of several conduction bath channels (see Fig.3). Had there been only one conduction bath, the impurity moment would form a singlet together with a “cloud” of electrons from the bath. We refer to this as the screening of the impurity moment (as the singlet has no magnetisation). However, in the multichannel Kondo problem described above, the formation of a singlet between the impurity and electron from one of the conduction channels is frustrated. As a result, the so-called Kondo screening of the impurity spin’s magnetic moment is hampered. Indeed, if the total spin of the conduction bath is greater than the spin of the impurity, the multichannel Kondo problem is said to be over-screened. The screening process, as well as its breakdown, are truly many-body in nature: a macroscopic number of conduction electrons interact with a single quantum impurity, and are therefore “aware” of one another. A proper description of the physics thus requires a field-theoretic treatment of the impurity-bath interactions, and the problem has been studied using a wide variety of powerful analytic and numerical methods.
Our contribution in this work was to show that the fascinating properties of the N-channel Kondo problem could be linked to those of the associated skeletal problem: a central quantum spin-1/2 coupled to N quantum spin-1/2s (corresponding to the N conduction channels) through identical antiferromagnetic Heisenberg exchange couplings. Such a model is often referred to as a star graph, and it can be identified as a limit of the multichannel problem in which the kinetic energy of the itinerant electrons has been switched off. We show in our work that certain properties of the star graph, such as the ground-state degeneracy and the magnetisation, are linked to bulk thermodynamic properties. The star graph also sets the scattering phase shift of the conduction electrons, and the scattering phase shift then dictates how the quantum fluctuations resolve themselves in order to lead to novel features. In this way, the quantum frustration inherent in the underlying simple quantum mechanical problem is seen to offer great insights into a many-body problem which looks quite daunting otherwise. Please read our work to find out more.
Authors: Siddhartha Patra, Abhirup Mukherjee, Anirban Mukherjee, N. S. Vidhyadhiraja, A. Taraphder, and Siddhartha Lal Journal Reference: Journal of Physics: Condensed Matter 35, 315601 (2023) Link: https://iopscience.iop.org/article/10.1088/1361-648X/acd09c
Posted on: May 23rd, 2023
Ultrafast THz Spectroscopy Group: Investigation of conductivity and shielding efficiency of the free-standing PVA-GO-Ag composite thin films in terahertz regime using time domain terahertz spectroscopy
Posted on: April 23rd, 2023
Position-position classical entanglement of light from bioNap research group (Prof. Nirmalya Ghosh)
Posted on: April 22nd, 2023
Photoexcited Ultrafast Dynamics of Free Carriers and Polarons in V2O5 Microparticles through Time-Resolved Nondegenerate Pump Probe Spectroscopy (Dr. Kamaraju Natarajan )
Posted on: April 21st, 2023
Dr. H. V. Ragavendra publishes in Physical Review Letters: Observing Nulling of Primordial Correlations via the 21-cm Signal
Posted on: February 17th, 2023
EXPERIMENTAL DEMONSTRATION OF SLOWING DOWN AND ENHANCEMENT OF FLUCTUATIONS IN AN ABRUPT MOTT TRANSITION
Posted on: March 16th, 2020
DOES STRETCHING YOUR MUSCLE HELP IN HEALING WOUND? A NEW STEP TOWARDS THE PUZZLE
Posted on: June 29th, 2018
EARLY-STAGE CANCER DETECTION USING HIDDEN MARKOV MODEL BASED INTEGRATED FRAMEWORK
Posted on: December 24th, 2017
IISER KOLKATA RESEARCHERS PREDICT CORONAL STRUCTURE FOR THE GREAT AMERICAN SOLAR ECCLIPSE
Posted on: August 25th, 2017
NEW SCIENTIFIC INSIGHTS FOR TAMING BRIBERY IN PUBLIC LIFE
Posted on: March 7th, 2017