Searcharxiv⌕ Search

arXiv subjects

Abhijit Biswas

Publications and source records attributed to Abhijit Biswas.

At least 55 records · Page 3Linked to original sources
arXiv2019

Optical waveguide hosting multiple exceptional points: Toward selective mode conversion

We investigate the astonishing physical aspects of Exceptional Points (EPs) in a 1D planar few-mode optical waveguide. The waveguide hosts four quasi-guided modes. Here interactions between the selected pair of modes are modulated by a spatial distribution of inhomogeneous gain-loss profile. Both the coupled pairs approach two different second-order EPs in parameter plane. Considering a proper parametric loop to encircle the identified EPs simultaneously, we establish a specific topological feature where one round encirclement in parameter space yields the switching between the propagation constants (β) of the corresponding pairs of couple modes in complex β-plane. Choosing two different patterns of the parametric loop, we establish the immutable topology in β- switching phenomena. This robust mode conversion scheme shall provide a platform to realize selective mode switching devices or optical-mode converters.

physics.optics↗
arXiv2018

Optical communication on CubeSats - Enabling the next era in space science

CubeSats are excellent platforms to rapidly perform simple space experiments. Several hundreds of CubeSats have already been successfully launched in the past few years and the number of announced launches grows every year. These platforms provide an easy access to space for universities and organizations which otherwise could not afford it. However, these spacecraft still rely on RF communications, where the spectrum is already crowded and cannot support the growing demand for data transmission to the ground. Lasercom holds the promise to be the solution to this problem, with a potential improvement of several orders of magnitude in the transmission capacity, while keeping a low size, weight and power. Between 2016 and 2017, The Keck Institute for Space Studies (KISS), a joint institute of the California Institute of Technology and the Jet Propulsion Laboratory, brought together a group of space scientists and lasercom engineers to address the current challenges that this technology faces, in order to enable it to compete with RF and eventually replace it when high-data rate is needed. After two one-week workshops, the working group started developing a report addressing three study cases: low Earth orbit, crosslinks and deep space. This paper presents the main points and conclusions of these KISS workshops.

astro-ph.IM↗
arXiv2018

Non-adiabatic Modal Dynamics around Exceptional Points in an All-Lossy Dual-Mode Optical Waveguide: Towards Chirality Driven Asymmetric Mode-Conversion

We report a 1D planar optical waveguide with transverse distribution of inhomogeneous loss profile, which exhibits an exceptional point (EP). The waveguide hosts two leaky resonant modes; where the interaction between them in the vicinity of the EP is controlled by proper adjustment of the inhomogeneity in attenuation profile only. We study the adiabatic dynamics of propagation constants of the coupled modes by quasi-static encirclement of control parameters around the EP. Realizing such an encirclement with the inhomogeneous loss distribution along the direction of light propagation, we report the breakdown of adiabatic evolution of two coupled modes through the waveguide in presence of an EP. Here, during conversion the output mode is irrespective of the choice of input excited mode but depends on the direction of light transportation. This topologically controlled, robust scheme of asymmetric mode conversion in the platform of the proposed all-lossy waveguide structure may open up an extensive way-out for implementation of state-transfer applications in chirality driven waveguide-based devices.

physics.optics↗
arXiv2018

Absence of superconductivity in pulsed laser deposited Au/Ag modulated nanostructured thin films

We have grown Au/Ag modulated nanostructured thin films by pulsed laser deposition using metallic targets. The growths performed at room temperature involved deposition of alternate layers of the two metals with larger thickness of Au and much lower thickness of Ag (two cases) on crystalline silicon (100) and quartz substrates. However, the characterization revealed that nanostructured configuration is realized in the film. We characterized the films by x-ray diffraction (XRD), Field emission scanning electron microscopy (FESEM) with energy dispersive analysis of x-rays (EDAX), magnetization (M-T) and resistivity (Rho-T) measurements. The magnetization and resistivity measurements confirm that there is no signature of superconducting transition within the temperature range of 5K to 300K studied in these films.

cond-mat.mtrl-sci↗
arXiv2017

Atomically flat single terminated oxide substrate surfaces

To achieve high quality epitaxial thin films and heterostructures of transition metal oxides with atomically controlled interfaces, one critical requirement is the use of atomically flat single terminated oxide substrates since the atomic arrangements and the reaction chemistry of the topmost surface layer of substrates determine the growth and consequent properties of the overlying films. Achieving the atomically flat and chemically single terminated surface state of commercially available substrates, however, requires judicious efforts because the surface of as-received substrates is of chemically mixed nature and also often polar. In this review, we summarize the surface treatment procedures to accomplish atomically flat surfaces with single terminating layer for various metal oxide substrates. We particularly focus on the substrates with lattice constant ranging from 4.00 to 3.70 angstrom, as the lattice constant of most perovskite materials falls into this range. For materials outside the range, one can utilize the substrates to induce compressive or tensile strain on the films and explore new states not available in bulk. The substrates covered in this review, which have been chosen with commercial availability and, most importantly, experimental practicality as a criterion, are KTaO3, REScO3 (RE = Rare-earth elements), SrTiO3, La0.18Sr0.82Al0.59Ta0.41O3 (LSAT), NdGaO3, LaAlO3, SrLaAlO4, and YAlO3. Analyzing all the established procedures, we conclude that atomically flat surfaces with selective A- or B-site single termination would be obtained for most commercially available oxide substrates. We further note that this topmost surface layer selectivity would provide an additional degree of freedom in searching for unforeseen emergent phenomena and functional applications in epitaxial oxide thin films and heterostructures with atomically controlled interfaces.

cond-mat.mtrl-sci↗
arXiv2017

Towards Ultra-Large Bandwidth and a New Class of Specialty Optical Fibers

We propose a scheme to enhance the effective photonic bandwidth exploiting bandgap overlapping of same order or different orders through judiciously chosen aperiodic geometries is spatial dimension. To implement the scheme, we design a specialty optical fiber with hybrid chirped-cladding. Our designed fiber provides an ultra-wide photonic bandwidth of ~ 3 micron. The less dispersive behavior of the structure carries the signature of zero dispersion at ~2.7micron. The proposed two material all-solid fiber geometry is based on thermally compatible chalcogenide glasses, GeAsSe and AsSe as low and high index respectively. The efficient delivery of short pulses with less distortion, and enhanced frequency broadening of specific pulses at certain conditions are demonstrated through these specialty fibers.

physics.optics↗
arXiv2017

Unconventional next nearest neighbor resonance coupling and states flipping mechanism in degenerate optical microcavities

We report a specially configured non-Hermitian optical microcavity, imposing spatially imbalanced gain-loss profile, to host an exclusively proposed next nearest neighbor resonances coupling scheme. Adopting scattering matrix (S-matrix) formalism, the effect of interplay between such proposed resonance interactions and the incorporated non-Hermiticity in the microcavity is analyzed drawing a special attention to the existence of hidden singularities, namely exceptional points (EPs); where at least two coupled resonances coalesce. We establish adiabatic flip-of-states phenomena of the coupled resonances in the complex frequency plane (k-plane) which is essentially an outcome of the fact that the respective EP is being encircled in system parameter plane. Encountering such multiple EPs, the robustness of flip-of-states phenomena have been analyzed via continuous tuning of coupling parameters along a special hidden singular line which connects all the EPs in the cavity. Such a numerically devised cavity, incorporating the exclusive next neighbor coupling scheme, have been designed for the first time to study the unconventional optical phenomena in the vicinity of EPs.

physics.optics↗
arXiv2017

Growth and engineering of perovskite SrIrO3 thin films

5d transition-metal-based oxides display emergent phenomena due to the competition between the relevant energy scales of the correlation, bandwidth, and most importantly, the strong spin-orbit coupling (SOC). Starting from the prediction of novel oxide topological insulators in bilayer ABO3 (B = 5d elements) thin-film grown along the (111) direction, 5d-based perovskites (Pv) form a new paradigm in the thin-film community. Here, we reviewed the scientific accomplishments in Pv-SrIrO3 thin films, a popular candidate for observing non-trivial topological phenomena. Although the predicted topological phenomena are unknown, the Pv-SrIrO3 thin film shows many emergent properties due to the delicate interplay between its various degrees of freedom. These observations provide new physical insight and encourage further research on the design of new 5d-based heterostructures or superlattices for the observation of the hidden topological quantum phenomena in strong spin-orbit coupled oxides.

cond-mat.str-el↗
arXiv2016

Towards self-similar propagation in a dispersion tailored and highly nonlinear segmented bandgap fiber at 2.8 micron

We numerically demonstrate self-similar propagation of parabolic optical pulses through a highly nonlinear and passive specialty photonic bandgap fiber at 2.8 micron. In this context, we have proposed a scheme endowed with a rapidly varying, but of nearly-mean-zero longitudinal dispersion and modulated nonlinear profile in order to achieve self-similarity of the formed parabolic pulse propagating over longer distances. To implement the proposed scheme, we have designed a segmented bandgap fiber with suitably tapered counterparts to realize such customized dispersion with chalchogenide glass materials. A self-similar parabolic pulse with full-width-at-half-maxima of 4.12 ps and energy of ~ 39 pJ as been achieved at the output. Along with a linear chirp spanning over the entire pulse duration, 3dB spectral broadening of about 38 nm at the output has been reported.

physics.optics↗
arXiv2016

Execution and Other Details for New Space Experiment Proposed for General Relativity

Following two dictums roadmap of Einstein (one, for incorporation of maximum of empirical facts, and the other, on long continued experimental verifications for perfecting a scientific theory) for future generations to make his General Relativity Theory (GRT) evolve to perfection, it is necessary to experimentally confirm his concept on General relativistic nature of speed of light, c, which relates to the variation of c under the influence of gravitational field, and whose experimental confirmation is long overdue. This paper gives the Execution and some Additional Details for the Space Experiment Proposed for GRT, so that any of the Space research organizations (viz., NASA, ESA, ASI, ISRO, etc.) would conduct this low cost experiment, during Space missions to Mercury, Venus, or the sun, or may choose to conduct it as an independent space mission as the cost may be brought down by sending micro satellites using the launch sharing facility of any other lending country. Even an advanced country may conduct this experiment at low cost by sending a micro satellite (called a co-passenger satellite) by arranging with the Space research organization of any lending country, which is sending a primary satellite for a space mission to any of the desired locations. At present, even a small country interested in making a historic contribution for making GRT of Einstein evolve, but not having a Space research organization, may contract with an experienced organization, or hire the services of an experienced project director who * can organize the planning, procurement, and launching of a micro satellite carrying the laser device, from launch pad of another country (adopting the piggy back launching method), and * can guide (else, can hire the services of a Principal Investigator, from some renowned institute) remote operation of the laser device for a few days.

physics.gen-ph↗
arXiv2016

Metal-Insulator Transitions and non-Fermi Liquid Behaviors in 5d Perovskite Iridates

Transition metal oxides, in particular, 3d or 4d perovskites have provided diverse emergent physics that originates from the coupling of various degrees of freedom such as spin, lattice, charge, orbital, and also disorder. 5d perovskites form a distinct class because they have strong spin-orbit coupling that introduces to the system an additional energy scale that is comparable to bandwidth and Coulomb correlation. Consequent new physics includes novel Jeff = 1/2 Mott insulators, metal-insulator transitions, spin liquids, and topological insulators. After highlighting some of the phenomena appearing in Ruddlesden-Popper iridate series Srn+1IrnO3n+1, we focus on the transport properties of perovskite SrIrO3. Using epitaxial thin films on various substrates, we demonstrate that metal-insulator transitions can be induced in perovskite SrIrO3 by reducing its thickness or by imposing compressive strain. The metal-insulator transition driven by thickness reduction is due to disorder, but the metal-insulator transition driven by compressive strain is accompanied by peculiar non-Fermi liquid behaviors, possibly due to the delicate interplay between correlation, disorder, and spin-orbit coupling. We examine various theoretical frameworks to understand the non-Fermi liquid physics and metal-insulator transition that occurs in SrIrO3 and offer the Mott-Anderson-Griffiths scenario as a possible solution.

cond-mat.str-el↗
arXiv2016

Generation and Stability Analysis of Self Similar Pulses Through Specialty Microstructured Optical Fibers in Mid Infrared Regime

We report a numerical study on generation and stability of parabolic pulses during their propagation through highly nonlinear specialty optical fibers. Here, we have generated a parabolic pulse at 2.1 $μ$m wavelength from a Gaussian input pulse with 1.9 ps FWHM and 75 W peak power after travelling through only 20 cm length of a chalcogenide glass based microstructured optical fiber (MOF). Dependence on input pulse shapes towards most efficient conversion into self similar states is reported. The stability in terms of any deviation from dissipative self-similar nature of such pulses has been analyzed by introducing a variable longitudinal loss profile within the spectral loss window of the MOF, and detailed pulse shapes are captured. Moreover, three different dispersion regimes of propagation have been considered to study the suitability to support most stable propagation of the pulse.

physics.optics↗
arXiv2015

Metal insulator transition and magnetotransport anomalies in perovskite SrIr0.5Ru0.5O3 thin films

We investigated the nature of transport and magnetic properties in SrIr0.5Ru0.5O3, (SIRO) which has characteristics intermediate between a correlated non-Fermi liquid state and an itinerant Fermi liquid state, by growing perovskite thin films on various substrates (SrTiO3 (001), (LaAlO3)0.3(Sr2TaAlO6)0.7 (001) and LaAlO3 (001)). We observed systematic variation of underlying substrate dependent metal-to-insulator transition temperatures at 80 K on SrTiO3, 90 K on (LaAlO3)0.3(Sr2TaAlO6)0.7 and 100 K on LaAlO3) in resistivity. Resistivity in the metallic region follows a T3/2 power law; whereas insulating nature at low T is due to the localization effect. Magnetoresistance (MR) measurement of SIRO on SrTiO3 (001) shows negative MR upto 25 K and positive MR above 25 K, with negative MR proportional to B1/2 and positive MR proportional to B2; consistent with the localized-to-normal transport crossover dynamics. Furthermore, observed spin glass like behavior of SIRO on SrTiO3 (001) in the localized regime, validates the hypothesis that (Anderson) localization favors glassy ordering. These remarkable features provide a promising approach for future applications and of fundamental interest in oxide thin films.

cond-mat.str-el↗
arXiv2015

Electronic and magnetic transitions in perovskite SrRu1-xIrxO3 thin films

We have investigated the electronic and magnetic properties of perovskite SrRu1-xIrxO3 thin films grown by pulsed laser deposition on atomically-flat (001) SrTiO3 substrates. SrRuO3 has the properties of a ferromagnetic metal with Curie temperature 150 K. Substituting Ir for Ru in SrRuO3, films showed fully-metallic behavior and ferromagnetic ordering, although resistivity increased and the ferromagnetic TC decreased. Films with x = 0.25 underwent the metal-to-insulator transition at 75 K, and spin-glass-like ordering at 45 K with the elimination of ferromagnetic long-range ordering caused by the electron localization at the substitution sites. In ferromagnetic films, resistivity increased near-linearly with T, but in paramagnetic film (x = 0.25) resistivity increased as T3/2. Moreover, observed spin-glass-like (TSG) ordering with the negative magnetoresistance in film with x = 0.25; validates the hypothesis that (Anderson) localization favors glassy ordering at amply disorder limit. These observations provide a promising approach for future applications and of fundamental interest in 4d and 5d mixed perovskites.

cond-mat.str-el↗
arXiv2015

Emergence of non-Fermi liquid behaviors in 5d perovskite SrIrO3 thin films: interplay between correlation, disorder, and spin-orbit coupling

We investigate the effects of compressive strain on the electrical resistivity of 5d iridium based perovskite SrIrO3 by depositing epitaxial films of thickness 35 nm on various substrates such as GdScO3 (110), DyScO3 (110), and SrTiO3 (001). Surprisingly, we find anomalous transport behaviors in the tempeature dependent resistivity, where the temperature exponent evolves continuously from 4/5 to 1 and to 3/2 with an increase of compressive strain. Furthermore, magnetoresistance always remains positive irrespective of resistivity upturns at low temperatures. These observations imply that the delicate interplay between correlation and disorder in the presence of strong spin-orbit coupling is responsible for the emergence of the non-Fermi liquid behaviors in 5d perovskite SrIrO3 thin films. We offer a theoretical framework for the interpretation of the experimental results.

cond-mat.str-el↗
arXiv2015

Persistent semi-metal-like nature of epitaxial perovskite CaIrO3 thin films

Strong spin-orbit coupled 5d transition metal based ABO3 oxides, especially iridates, allow tuning parameters in the phase diagram and may demonstrate important functionalities, for example, by means of strain effects and symmetry-breaking, because of the interplay between the Coulomb interactions and strong spin-orbit coupling. Here, we have epitaxially stabilized high quality thin films of perovskite (Pv) CaIrO3. Film on the best lattice-matched substrate shows semi-metal-like characteristics. Intriguingly, imposing tensile or compressive strain on the film by altering the underlying lattice-mismatched substrates still maintains semi-metallicity with minute modification of the effective correlation as tensile (compressive) strain results in tiny increases (decreases) of the electronic bandwidth. In addition, magnetoresistance remains positive with a quadratic field dependence. This persistent semi-metal-like nature of Pv-CaIrO3 thin films with minute changes in the effective correlation by strain may provide new wisdom into strong spin-orbit coupled 5d based oxide physics.

cond-mat.str-el↗