Department of Physics

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IIT Jodhpur

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. © OSA 2020 © 2020 The Author(s)

World University root node

Optics InfoBase Conference Papers

A specialty endless-core photonic bandgap fiber with ultra-wide bandwidth for short pulse propagation

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. © OSA 2020 © 2020 The Author(s)

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

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. © 2020 American Physical Society.

Fulltext

Physical Review A

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

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. © OSA 2020 © 2020 The Author(s)

Complementary optical systems to host conjugate exceptional points

Exotic light behavior in certain disordered one-dimensional optical waveguide lattices with selectively doped gain/loss operating away from the PT symmetry regime is reported. Here, we establish that such open systems host a unique Levy flight even in the absence of scatterers following Levy distribution. Owing to the interplay between gain/loss and disorder, the critical transition distance from the ballistic to diffusive domain is reduced and also a superdiffusive propagation distance has been revealed after a certain propagation distance. The stable propagation characteristics of light have been investigated in the form of an intensity distribution for various gain/loss levels, effective beam width variation with length, the center of mass dynamics, and the probability density function. A qualitative analysis indicates that such fabrication feasible lattices may host disordered lasing in the form of a Levy-type distribution in the output intensity and may also find applications in image transport.

PHYSICAL REVIEW RESEARCH

Levy flight of superdiffusive light in coupled waveguide lattices

Time-asymmetric state evolution due to the dynamical encirclement around a second-order exceptional point has garnered enormous attention for the topological study of various photonic structures. The selective modeconversion in an optical waveguide can help in the designing of various integrated photonics devices. Here we explore a dual-mode planar optical waveguide and study the beam dynamics with the onset and offset of Kerr nonlinearity together with the encirclement of the identified exceptional point in the parameter space. The onset of the same amount of focusing and defocusing type nonlinearity in the waveguide gives two different outputs for a unidirectional propagation of light, beyond the chiral aspect of the device.

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

The large channel length graphene field-effect transistor (GFET) can outperform its competitors due to its larger active area and lower noise. Such long channel length devices have numerous applications, e.g., in photodetectors, biosensors, etc. However, long channel length graphene devices are not common due to their semi-metallic nature. Here, we fabricate large channel length (up to 5.7 mm) GFETs through a simple, cost-effective method that requires thermally evaporated source-drain electrode deposition, which is less cumbersome than the conventional wet-chemistry based photolithography. The semiconducting nature of graphene has been achieved by utilizing the Li+ ion of the Li5AlO4 gate dielectric, which shows current saturation at a low operating voltage (∼2 V). The length scaling of these GFETs has been studied with respect to channel length variation within a range from 0.2 mm to 5.7 mm. It is observed that a GFET of 1.65 mm channel length shows optimum device performance with good current saturation. This particular GFET shows a "hole"mobility of 312 cm2 V-1 s-1 with an on/off ratio of 3. For comparison, another GFET has been fabricated in the same geometry by using a conventional SiO2 dielectric that does not show any gate-dependent transport property, which indicates the superior effect of Li+ of the ionic gate dielectric on current saturation. © 2020 Author(s).

AIP Advances

{Lithography-free fabrication of low operating voltage and large channel length graphene transistor with current saturation by utilizing Li+ of ion-conducting-oxide {\ldots}}

Transition-metal dichalcogenides (TMDCs) with atomic thickness are promising materials for next-generation electronic and optoelectronic devices. Herein, we report uniform growth of triangular-shaped (∼40 μm) monolayer WS2 using the atmospheric-pressure chemical vapor deposition (APCVD) technique in a hydrogen-free environment. We have studied the optical and electrical behaviors of as-grown WS2 samples. The absorption spectrum of monolayer WS2 shows two intense excitonic absorption peaks, namely, A (∼630 nm) and B (∼530 nm), due to the direct gap transitions at the K point. Photoluminescence (PL) and fluorescence studies reveal that under the exposure of green light, monolayer WS2 gives very strong red emission at ∼663 nm. This corresponds to the direct band gap and strong excitonic effect in monolayer WS2. Furthermore, the efficacy of the synthesized WS2 crystals for electronic devices is also checked by fabricating field-effect transistors (FETs). FET devices exhibit an electron mobility of μ ∼6 cm2 V-1 s-1, current ON/OFF ratio of ∼106, and subthreshold swing (SS) of ∼641 mV decade-1, which are comparable to those of the exfoliated monolayer WS2 FETs. These findings suggest that our APCVD-grown WS2 has the potential to be used for next-generation nanoelectronic and optoelectronic applications. Copyright © 2020 American Chemical Society.

ACS applied materials \& interfaces

Study on the Growth Parameters and the Electrical and Optical Behaviors of 2D Tungsten Disulfide

Despite significant interest toward solid-state electrolytes owing to their superior safety in comparison to liquid-based electrolytes, sluggish ion diffusion and high interfacial resistance limit their application in durable and high-power density batteries. Here, a novel quasi-solid Li+ ion conductive nanocomposite polymer electrolyte containing black phosphorous (BP) nanosheets is reported. The developed electrolyte is successfully cycled against Li metal (over 550 h cycling) at 1 mA cm−2 at room temperature. The cycling overpotential is dropped by 75% in comparison to BP-free polymer composite electrolyte indicating lower interfacial resistance at the electrode/electrolyte interfaces. Molecular dynamics simulations reveal that the coordination number of Li+ ions around (trifluoromethanesulfonyl)imide (TFSI−) pairs and ethylene-oxide chains decreases at the Li metal/electrolyte interface, which facilitates the Li+ transport through the polymer host. Density functional theory calculations confirm that the adsorption of the LiTFSI molecules at the BP surface leads to the weakening of N and Li atomic bonding and enhances the dissociation of Li+ ions. This work offers a new potential mechanism to tune the bulk and interfacial ionic conductivity of solid-state electrolytes that may lead to a new generation of lithium polymer batteries with high ionic conduction kinetics and stable long-life cycling. © 2020 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

Advanced Functional Materials

Highly-Cyclable Room-Temperature Phosphorene Polymer Electrolyte Composites for Li Metal Batteries

Recent measurements of neutron star mass from several candidates (PSR J1614-2230, PSR J0348+0432, MSP J0740+6620) set the lower bound on the maximum possible mass for this class of compact objects ∼2 M⊙. Existence of stars with high mass brings the possibility of existence of exotic matter (hyperons, meson condensates) at the core region of the objects. In this work, we investigate the (anti)kaon (K-, K̄0) condensation in β-equilibrated nuclear matter within the framework of covariant density functional theory. The functionals in the kaonic sector are constrained by the experimental studies on K- atomic, kaon-nucleon scattering data fits. We find that the equation of state softens with the inclusion of (anti)kaon condensates, which lowers the maximum mass of neutron star. In one of the density-independent coupling cases, the K- condensation is through a first order phase transition type, which produces a 2 M⊙ neutron star. The first order phase transition results in mixed phase region in the inner core of the stars. While K̄0 condensation appears via second-order phase transition for all the models we consider here. © 2020 American Physical Society.