Showing papers by "Somnath Ghosh published in 2020"

Exceptional Point and toward Mode-Selective Optical Isolation

Abstract: Exceptional points (EPs) in non-Hermitian systems have recently attracted considerable attention owing to unique state-flipping and peculiar phase accumulation features. The dynamical encirclement ...

Flank wear and rake wear studies for arc enhanced HiPIMS coated AlTiN tools during high speed machining of nickel-based superalloy

Abstract: Several studies are available on wear behaviour of the physical vapour deposited as well as chemical vapour deposited coated cutting tools during continuous cutting operations. However, there still exists deficiency in a comprehensive analysis of the dominating wear mechanisms. The present study investigates the wear behaviour of the AlTiN coating on the cemented carbide developed by novel arc enhanced high impulse magnetron sputtering technique (HiPIMS). In this study, micro abrasive blasting (MAB) has been adopted as a surface modification as well as the coating treatment technique. This was done primarily to enhance the performance of the cutting tool by altering the surface and sub-surface properties so that a comparison can be made with only conventionally coated and uncoated tools in terms of wear performance. In order to study the tool wear behaviour, aerospace material Nimonic C 263, possessing high strength owing to the presence of the metallic carbides and work hardening properties, was selected as workpiece material for longitudinal turning. The process parameters such as feed, depth of cut and cutting speed were kept constant to 0.12 mm/rev, 1 mm and 60 m/min respectively. Scanning electron microscopy (SEM) was carried out for detailed investigation of the worn-out regions at the rake and flank faces while energy dispersive X-ray spectroscopy (EDX) was utilized for observing the variation in elemental composition during machining. Diffusion wear studies were carried out under dynamic condition by cutting the cross-section of the worn-out inserts after the machining operation for both uncoated and coated samples, while diffusion under static condition was carried out by coupling the workpiece and tool and then heating at varying temperature. The results indicate the efficacy of the MAB as a surface treatment technique for improving the wear performance of the cutting tools during the machining of the nickel-based superalloy.

Third-order exceptional point and successive switching among three states in an optical microcavity

Abstract: One of the most intriguing topological features of open systems is that they exhibit exceptional point (EP) singularities. Apart from the widely explored second-order EPs (EP2s), the exploration of higher-order EPs in any system requires more complex topology, which is still a challenge. Here, we encounter a third-order EP (EP3) with the simultaneous presence of multiple second-order EPs in a simple fabrication feasible gain-loss assisted trilayer optical microcavity. Using the scattering-matrix formalism, we study the simultaneous interactions between three successive coupled states via avoided-resonance-crossing (ARC) phenomena, and we identify two EP2s near two ARC regimes. Such an occurrence of two EP2s inside a closed two-dimensional parametric space associated with an unbalanced gain-loss profile leads to the functionality of a cube-root branch point, i.e., an EP3. Following an adiabatic variation of two control parameters around the embedded EP3 in the presence of two identified EP2s, we present a robust successive-state-conversion mechanism among three coupled states. The proposed scheme indeed opens up a unique platform to manipulate light in integrated photonic devices.

Nonlinearity-induced anomalous mode collapse and nonchiral asymmetric mode switching around multiple exceptional points

Abstract: The dynamical encirclement around a second-order exceptional point (EP) and corresponding chirality driven nonadiabatic modal dynamics have attracted enormous attention in the topological study of various non-Hermitian systems. However, dynamical encirclement around multiple second-order EPs in a multistate system is yet to be explored. Here, exploiting an exclusive design of a planar gain-loss assisted three-mode supported optical waveguide with local Kerr nonlinearity, we encounter multiple second-order EPs. Judiciously, choosing a specific parameter space by varying the unbalanced gain-loss profile, we encircle multiple EPs simultaneously, and explore the beam dynamics toward corresponding chiral or nonchiral aspects of the device. While propagating through the designed waveguide, three coupled modes are collapsed into a specific dominating mode, owing to corresponding nonadiabatic corrections around multiple EPs. We reveal that in the absence of chirality the same amount of focusing- and defocusing-type nonlinearity gives different dominating output, irrespective of the choice of inputs, for the same topological structure of the waveguide. An analytical model to describe this anomalous mode collapsing phenomenon due to interplay between nonlinearity and the EP has been developed. Our findings should provide a promising opportunity to switch or retrieve a selective mode from a multimode signal in integrated devices.

Chirality breakdown in the presence of multiple exceptional points and specific mode excitation.

Abstract: The dynamical parametric encirclement around a second-order exceptional point (EP) enables the time-asymmetric nonadiabatic evolution of light, which follows the chirality of the underlying system. Such light dynamics in the presence of multiple EPs and the corresponding chiral aspect is yet to be explored. In this Letter, we report a gain–loss assisted four-mode-supported optical waveguide that hosts a parameter space to dynamically encircle multiple EPs. In the presence of multiple EPs, we establish a unique nonadiabatic behavior of light, where beyond the chiral aspect of the system, light is switched to a particular mode, irrespective of the choice of the input mode. Proposed scheme certainly opens a step-forward approach in light manipulation to facilitate next-generation integrated photonic systems.

Nitric oxide monooxygenation (NOM) reaction of cobalt-nitrosyl {Co(NO)}8 to CoII-nitrito {CoII(NO2-)}: base induced hydrogen gas (H2) evolution.

Abstract: Here, we report the nitric oxide monooxygenation (NOM) reactions of a CoIII-nitrosyl complex (1, {Co-NO}8) in the presence of mono-oxygen reactive species, i.e., a base (OH−, tetrabutylammonium hydroxide (TBAOH) or NaOH/15-crown-5), an oxide (O2− or Na2O/15-crown-5) and water (H2O). The reaction of 1 with OH− produces a CoII-nitrito complex {3, (CoII-NO2−)} and hydrogen gas (H2), via the formation of a putative N-bound Co-nitrous acid intermediate (2, {Co-NOOH}+). The homolytic cleavage of the O–H bond of proposed [Co-NOOH]+ releases H2via a presumed CoIII-H intermediate. In another reaction, 1 generates CoII-NO2− when reacted with O2−via an expected CoI-nitro (4) intermediate. However, complex 1 is found to be unreactive towards H2O. Mechanistic investigations using 15N-labeled-15NO and 2H-labeled-NaO2H (NaOD) evidently revealed that the N-atom in CoII-NO2− and the H-atom in H2 gas are derived from the nitrosyl ligand and OH− moiety, respectively.

Ultrasensitive light confinement: Driven by multiple bound states in the continuum

Abstract: We propose a framework to study the topological properties of an optical bound state in the continuum (BIC). Here, the formation of a BIC has been shown using the interaction between proximity resonances undergoing avoided resonance crossing in a simple fabrication, feasible gain-loss assisted microcavity. Similar to a Friedrich-Wingten (FW)-type BIC, the formation of an ultrahigh-quality ($Q$) mode due to the precise destructive interference between resonances has been reported. The enhancement in the $Q$ factor was found to be more than four orders of magnitude greater than the other participating resonances. Furthermore, multiple such FW-type high-$Q$ operating points from the same set of proximity resonances have been identified. Upon further inspection, we report the formation of a special-BIC line in the system parameter space connecting the locations of these operating points. Further, we closely investigate the light dynamics of systems near single and multiple quasi-BICs. Aiming to develop a scheme to enhance the performance of optical sensing in a microcavity, we study the sensitivity of transmission coefficients and quality factor to sense even ultrasmall perturbations in the system configuration. Our proposed scheme would open up a vast potential for an enhanced desired optical response in enhanced nonlinear applications, low-threshold nano- and microlasers, and device-level sensors.

Nitric oxide dioxygenation (NOD) reactions of CoIII-peroxo and NiIII-peroxo complexes: NOD versus NO activation

Abstract: Here, we report the nitric oxide dioxygenation (NOD) reactions of NiIII-peroxo, [(12-TMC)NiIII(O22−)]+ (NiIII–O22−, 1), and CoIII-peroxo, [(12-TMC)CoIII(O22−)]+ (CoIII–O22−, 2), complexes bearing N-tetramethylated cyclam (TMC) ligands. The reaction of complex 1 with nitric oxide (NO) generates NiII-nitrato complex [(12-TMC)NiII(NO3−)]+ (NiII-NO3−, 3). However, in contrast to the NOD reaction of 1, compound 2 produces CoII-nitrito complex, [(12-TMC)CoII(NO2−)]+ (CoII-NO2−, 4), with O2 gas formation. Results suggested the formation of a presumptive Ni-peroxynitrite ([Ni-PN]+, [Ni-ONOO–]+) intermediate, in the NOD reaction of 1, prior to the generation of NiII-NO3− (3). Although the end product of the reaction of 2 with NO is different from that of 1, the mechanistic study explored by using 2,4-di-tert-butylphenol (2,4-DTBP) suggested that the reaction of 2 with NO also occurs via a presumed Co-peroxynitrite ([Co-PN]+, [Co-ONOO−]+) intermediate. Furthermore, exploring these NOD reactions and tracking the oxygen atom using 18O-labeled hydrogen peroxide (H218O2) revealed that two oxygen atoms of 3 (NiII-18O2N16O−) and one oxygen atom of 4 (CoII-18ON16O−) are derived from the peroxo (O22−) moieties of 1 and 2, respectively. Furthermore, we have explored the M-peroxo regeneration from NOD products (3 and 4), and for the first time, we have observed the formation of MIII–O22− complexes, 1 and 2, upon treatment with H2O2 and triethylamine (TEA), respectively. This work is the first-ever report which showed the exact NO dioxygenase (NOD) enzyme catalytic cycle. Furthermore, we have also explored the NO activation of 3 and 4, and surprisingly, we observed the formation of Co-nitrosyl (5) from 4 upon reaction with NO; in contrast, 3 was unreactive towards NO. Evidence supporting the formation of [Ni-PN]+ and [Co-PN]+ intermediates is also presented.

Lévy flight of superdiffusive light in coupled waveguide lattices

Abstract: This paper reports on exotic behavior of light in one dimensional optical waveguide lattices with selective doping. The presence of gain in such lattices reduces the critical transition distance from ballistic to diffusive domain and guides the light to propagate in super-diffusive regime after certain distance. These findings may may lead to disordered lasing and also assist in image projection/transport.

Effect of a local nonlinearity on the light dynamics around an exceptional point: A quantitative analysis

Abstract: This paper investigates the effect of local nonlinearity on exceptional points-aided asymmetric modal dynamics using the framework of a dual-mode optical waveguide and reports on a non-chiral asymmetric mode conversion scheme, where depending on only the type of nonlinearities, the waveguide can deliver different dominating mode, irrespective of input and also the direction of light propagation

Modal dynamics around a fourth-order exceptional point in a planar optical waveguide

Abstract: We report a planar few-mode supported optical waveguide with a multilayer length-dependent gain-loss profile to host a dynamical encirclement scheme around a fourth-order exceptional point (EP4) and explore a chirality-driven higher-order asymmetric mode conversion scheme.

Exotic light dynamics around a fourth order exceptional point

Abstract: The physics of exceptional point (EP) singularities, has been a key to a wide range of unique physical applications in open systems. In this context, the mutual interactions among four coupled states around a fourth-order EP (EP4) in a physical system is yet to be explored. Here, we investigate the unique features of an EP4 in a fabrication feasible planar optical waveguide with a multilayer gain-loss profile based on only two tunable parameters. A unique `fourth-order $\beta$-switching' phenomenon due to quasi-static gain-loss variation around EP4 has been explored. An exclusive chiral light dynamics following the dynamical variation of the gain-loss profile has been reported for the first time, which enables a special type of asymmetric higher-order mode conversion scheme. Here, all the coupled modes associated with an EP4 are fully converted into different specific higher-order modes based on the choice of encirclement directions. The proposed scheme would present EP4 as a new light manipulation tool for integrated photonic devices.

Complementary Optical Systems to Host Conjugate Exceptional Points

Abstract: We report the hosting of two complex conjugate exceptional points (EPs) in two complementary optical waveguides and explore the nonadiabatic chiral and reverse-chiral modal dynamics in two waveguides following dynamical EP encirclement schemes.

Transverse Localization of light: Towards a paradigm shift from trivial to non-trivial lattice

Abstract: Direct transition of trivial optical lattice to its non-trivial counterpart verified by customization of bandgap is reported. Numerical simulations show that no localization transition is observed in non-trivial lattices due to deliberate refractive-index disorder. ©2019 The Author(s).

Ultra-high sensitivity near multiple bound states in the continuum in microcavity resonators

Abstract: We study the topological properties of multiple optical bound states in the continuum using continuous parameter tuning. We utilize the formation of ultra-high- Q resonances to investigate the potential of quality-sensing in a speciality optical microcavity.

Photonic Crystal Based Ultra-Sensitive Interferometric Sensor with Spatial Resolution up to 1 nm

Abstract: We report a very high precision interferometric sensor with resolution up to ~{\lambda}/1024, exploiting hollow photonic bandgap waveguide-based geometry for the first time. Here sensing has been measured by a complete switching in the direction of the outgoing beam, owing to transverse momentum oscillation phenomena. Using a 1.32 {\mu}m source and core-width of 7.25 {\mu}m, a complete switching cycle is obtained even due to a small change of ~1 nm in the core-width. Using hollow-core photonic bandgap waveguide, Talbot effect, revivals of the initial phase, oscillation in the transverse momentum along with multi-mode interference served as the backbone of the design. The ultra-sensitive multi-mode interferometric sensor based on photonic crystals will certainly open up a paradigm shift in interferometer-based sensing technologies toward device-level applications in photonic sensing/switching and related precision measurement systems.

A Specialty Endless-Core Photonic Bandgap Fiber with Ultra-wide Bandwidth for Short Pulse Propagation

Abstract: We present a novel fiber design concept based on the formation of concentric effective optical cores in a photonic bandgap geometry supporting ultra-large bandwidth for stable propagation of short optical pulses over longer distances.

Highly sensitive refractive index sensor based on degeneracy in specialty optical fibers: a new approach

Abstract: We study the phenomenon of deliberate inter-modal interactions in a specially designed index guided Microstructured Optical Fiber (MOF) by exploiting multipole expansion method (White et al. 2002). The MOF is designed in such a way that the first layer of holes is judiciously filled with a material having refractive index slightly greater than the background material or core and remaining holes are filled with air. Accordingly, we find an interesting phenomenon of mode crossing between the fundamental mode and a targeted defect mode while tuning the wavelength. Exploring this transition wavelength of the mode crossing, we propose a design of a fiber optic sensor for refractive index measurement (Silva et al. 2014) with enhanced sensitivity.

A Mini Review on the Corona Various- A Universal Long Standing Problem: T-Cell Therapy Might Show Some Light in New Vaccine Development

Abstract: Stimulation of the body’s immune system might be important to cure many diseases. The induction of cellular immune responses has thus far received little attention in respiratory virus vaccine development. CD8 T cells are critical for mediating clearance following many acute viral infections in the lung. In addition, memory CD8 T cells are also capable of providing protection against secondary infections. The present mini reviewed the present status of corona virus and was attempted to elucidate the future danger of corona virus. A suggestion for the development of T-cell based therapies has been made to fight against Corona virus. The enhancement of immunity using T-cell therapy might show a new light in this regard. Because of genetic diversity and frequent mutational change, it appears to be difficult to find a permanent solution of the corona problem in the near future. Vaccinations that solely promote the induction of neutralizing antibodies may not be optimal in providing protection against the corona virus respiratory infections, in particular. Therefore, human civilization is going to face unprecedented experiences, if not social and individual cares are always maintained and common people understand the seriousness of the virus.

Photonic Crystal Based Ultra-Sensitive Interferometric Nanometer Displacement Sensor

Abstract: A very high precision interferometric sensor with a resolution up to ~λ/1940 is reported, exploiting Talbot effect, revivals of the initial phase, oscillation in transverse momentum, multimode interference inside a hollow photonic bandgap waveguide.