Showing papers by "Somnath Ghosh published in 2019"

Exceptional Point and Toward Mode Selective Optical Isolation

Abstract: Dynamical encirclement of an Exceptional Point (EP) and corresponding time-asymmetric mode evolution properties due to breakdown in adiabatic theorem have been a key to range of exotic physical effects in various open atomic, molecular and optical systems. Here, exploiting a gain-loss assisted dual-mode optical waveguide that hosts a dynamical EP-encirclement scheme, we have explored enhanced nonreciprocal effect in the dynamics of light with onset of saturable nonlinearity in the optical medium. We propose a prototype waveguide-based isolation scheme with judicious tuning of nonlinearity level where one can pass only a chosen mode in any of the desired directions as per device requirement. The deliberate presence of EP enormously enhances the nonreciprocal transmission contrast even up to 40 dB over the proposed device length with a scope of further scalability. This exclusive topologically robust mode selective all-optical isolation scheme will certainly offer opportunities in integrated photonic circuits for efficient coupling operation from external sources and improve device performances.

The role of delayed water curing in improving the mechanical and microstructural properties of alkali activated fly ash based geopolymer paste blended with slag

Abstract: This paper aims to investigate the effect of delayed water curing on the mechanical and microstructural properties of fly ash-based geopolymer paste-blended with Ground Granulated Blast Furnace Slag (GGBS) with different rest periods.,The blended geopolymer paste was composed of GGBS (15 per cent of the total weight) and the base material, Fly Ash (FA). The blended mix was activated by activator solution (Sodium hydroxide and Sodium silicate) containing 6 per cent Na2O of total base material. The effect of delayed water curing has been studied by gradually increasing the aging period (Rest Period) from 2 hours to 24 hours in the formation of activated outcome along with Calcium Silicate Hydrate (CSH). To analyze the mechanical and microstructural properties of the resultant blended geopolymer paste, compressive strength test, FESEM and XRD have been carried out. Moreover, a long-term durability test subjected to sulphate exposure has been performed to evaluate the durability of the designed sustainable geopolymer paste.,The present paper shows that the delayed water curing incorporates secondary heat input enhancing the partial polymer formation along with CSH. Slag-blended AAFA-based geopolymer paste is seen to exhibit quick setting property. Also, AAFA-based geopolymer paste samples subjected to longer rest period show early strength gain at a high rate under water curing as compared to those subjected to the shorter rest period.,To the best of authors’ knowledge, the effect of delayed water curing on the mechanical and microstructural properties of slag-blended AAFA-based geopolymer paste has not been studied before.

Higher-order topological degeneracies and progress towards unique successive state switching in a four-level open system

Abstract: The physics of topological singularities, namely, exceptional points (EPs), has been a key to a wide range of intriguing and unique physical effects in non-Hermitian systems. In this context, exploration of the mutual interactions among the states in four-level systems around fourth-order EPs (EP4s) is lacking. Here we report a four-level parameter-dependent perturbed non-Hermitian Hamiltonian, mimicking quantum or wave-based systems, to explore the physical aspects of an EP4 analytically as well as numerically. The proposed Hamiltonian exhibits different orders of interaction schemes with the simultaneous presence of different higher-order EPs. Here an EP4 has been realized by mutual interaction between four states with proper parameter manipulation. We comprehensively investigate the dynamics of corresponding coupled eigenvalues with stroboscopic parametric variation in the vicinity of the embedded EP4 to establish a successive state-switching phenomenon among them, which proves to be robust even in the presence of different orders of EPs. Implementing the relation of the perturbation parameters with the coupling control parameters, we report a region to host multiple EP4s in a specific system. The chiral behavior of successive state exchange has also been established near the EP4. The proposed scheme, which is enriched with physical aspects of EP4s, should provide a unique light manipulation tool in any anisotropic multistate integrated system.

Effect of functionalization and concentration of carbon nanotubes on mechanical, wear and fatigue behaviours of polyoxymethylene/carbon nanotube nanocomposites

Abstract: The main focus of this work is to improve the mechanical, wear and fatigue behaviours of polyoxymethylene (POM) by reinforcing with carbon nanotubes (CNTs). To improve compatibility between CNTs and POM, the surface of the CNTs was modified by various methods of functionalization like carboxylation, silanation, carbonylation and amination. The functionalized CNTs were characterized by Fourier transform infrared spectroscopy to confirm the different functional groups attached to the surface. POM/CNT nanocomposites were developed with functionalized CNTs in different concentrations varying from 0.25 to 2 wt%. Nanocomposites with 1 wt% of silanated CNTs resulted in maximum improvement of tensile, flexural and impact properties. Furthermore, experimental results on fatigue and dry sliding wear tests revealed that the fatigue strength, specific wear rate and friction coefficient are sensitive to functionalization and concentration of CNTs.

A droplet-based microfluidics route to temperature-responsive colloidal molecules

Abstract: Small clusters of spherical colloids that mimic real molecules, so-called colloidal molecules, hold great promise as building blocks in bottom-up routes to new materials. However, their typical hard sphere nature has hampered their assembly into ordered structures, largely due to a lack of control in the interparticle interactions. To provide easy external control of the interactions, the present work focuses on the preparation of colloidal molecules from temperature-responsive microgel particles that undergo a transition from a soft repulsive to a short-range attractive state as their characteristic volume phase transition temperature (VPTT) is crossed. Preparation of the colloidal molecules starts with the use of a droplet-based microfluidics device to form highly uniform water-in-oil (W/O) emulsion droplets containing, on average and with a narrow distribution, four microgels per droplet. Evaporation of the water then leads to the formation of colloidal molecule-like clusters, which can be harvested following cross-linking and phase transfer. We use a mixture of two types of microgels, one based on poly(N-isopropylacrylamide) (PNIPAM) and the other on poly(N-isopropylmethacrylamide) (PNIPMAM), to prepare bicomponent colloidal molecules, and show that the difference in VPTT between the two allows for induction of attractive interparticle interactions between the PNIPAM interaction sites at temperatures in between the two VPTTs, analogous to the interactions among patchy biomacromolecules such as many proteins.

Mechanical, wear and fatigue behavior of functionalized CNTs reinforced POM/PTFE composites

Abstract: The main focus of this work is to improve the strength, stiffness, wear resistance and fatigue strength of polyoxymethylene (POM). For this purpose, POM is incorporated with a lubricating additive polytetrafluoroethylene (PTFE) and reinforced with silanized multiwalled carbon nanotubes (CNTs). PTFE filled POM composites with varying weight fraction of PTFE and hybrid composites with 1 wt% silanized CNTs and varying weight fractions of PTFE were fabricated by melt compounding using a twin screw extruder followed by injection molding. Tensile, flexural, wear and fatigue behavior of the developed composites were evaluated. Incorporation of PTFE resulted in a significant decline of tensile properties, flexural properties and fatigue strength, whereas specific wear rate and coefficient of friction increased significantly. Reinforcing PTFE filled POM composites with 1 wt% functionalized CNTs has led to improvement of strength, stiffness, wear resistance and fatigue strength. Hybrid composites with 10 wt% PTFE and 1 wt% functionalized CNTs resulted in improvement of strength and stiffness by 31% and 50% respectively and decrease in wear rate and friction coefficient by 67% and 44% respectively as compared to pristine POM.

Finding a new pathway for acid-induced nitrite reduction reaction: formation of nitric oxide with hydrogen peroxide.

Abstract: Here, we report a new pathway for nitrite reduction chemistry, formation of cobalt-nitrosyl ({CoII-NO}8) with H2O2 in the reaction of a CoII-nitrito complex with a one-fold acid (H+) via the formation of a CoII-nitrous acid intermediate ({CoII-ONOH}). Mechanistic investigations using 15N-labeled-15NO2− revealed that the N-atom in the {CoII-NO}8 complex is derived from the nitrito ligand, and H2O2 came from the homolysis of the ON–OH moiety. Spectral evidence supporting the formation of the CoII-ONOH intermediate and the generation of H2O2 is also presented.

Zein film functionalized with gold nanoparticles and the factors affecting its mechanical properties

Abstract: In this article, we report a simple method to synthesize biodegradable zein films functionalized with gold nanoparticles (AuNPs) with significantly improved mechanical properties, as an environmentally benign substitute to biologically hazardous polymers. Zein-coated AuNPs were synthesized using the zein protein as a reducing agent and characterized with IR, UV, CD, ζ-potential, and TEM measurements. The zein protein interaction with the negatively charged surface of AuNPs provides excellent strength to the zein thin film. For the first time, FT-IR spectral studies suggested the strong interaction between AuNPs and zein protein, which was further supported by the higher binding constant (Kb) value. The films were characterized for mechanical properties with spectroscopic and physical experimental investigations. The surface morphology of AuNP-doped zein film was explored by AFM and SEM, which suggested that the AuNPs prevent the buckling of zein film and increase the strength as well as flexibility of the film.

Asymmetric propagation and limited wavelength translation of optical pulses through a linear dispersive time-dynamic system.

Abstract: We study optical pulse propagation through a linear, dispersive, gain-loss-assisted bulk medium whose refractive index is time-varying. To analyze the dynamics, we have used a novel technique of time transformation that provides universal formulas of pulse propagation. Our analytical and numerical investigations reveal that optical pulses show asymmetric behavior while propagating in opposite direction through such a medium, in both the temporal and spectral domains. Moreover, the wavelength shift during this process is the most interesting outcome which is limited in range, but could be tuned by varying the refractive index with time. Phenomena that are observed in this Letter are novel and realizable in practical devices such as coupled waveguides where the refractive index is a function of time.

Application-specific Specialty Optical Fibers: A new Paradigm in Fiber Designs

Abstract: Specialty optical fiber for specific applications have become a new paradigm in guided wave photonics. In this invited presentation, we would review some of our application-specific specialty fiber designs for parabolic pulse generation to overcome nonlinear detriments, and all-fiber light sources for mid-IR and THz photonics.

Optical waveguide hosting multiple exceptional points: Toward selective mode conversion

Abstract: 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 ({\beta}) of the corresponding pairs of couple modes in complex {\beta}-plane. Choosing two different patterns of the parametric loop, we establish the immutable topology in {\beta}- switching phenomena. This robust mode conversion scheme shall provide a platform to realize selective mode switching devices or optical-mode converters.