Department of Physics

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We report an asymmetric behavior of optical pulses during their propagation through a time-varying linear optical medium. The refractive index of the medium is considered to be varying with time and complex, such that a sufficient amount of gain and loss is present to realize their effect on pulse propagation. We have exploited the universal formula for optical fields in time-varying media. Numerically simulated results reveal that pulses undergo opposite temporal shifts around their initial center position during their bi-directional propagation through the medium along with corresponding spectral shifts. Moreover, the peak power and accumulated chirp (time derivative of accumulated phase) of the output pulse in both propagation directions are also opposite in nature, irrespective of their initial state. Numerically simulated behavior of the pulses agrees well with the analytical solutions. © 2018 Optical Society of America.

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Applied Optics

Propagation and asymmetric behavior of optical pulses through time-dynamic loss–gain-assisted media

We report stable propagation of parabolic pulses with GHz range of repetition rate along with the characteristic linear chirp over few meters long chalcogenide glass based dispersion oscillating Bragg fiber [1] at 2.8 μm, © 2017 IEEE.

WRAP 2017 - Workshop on Recent Advances in Photonics

Quasi-CW Propagation of Similaritons Through Dispersion Oscillating Bragg Fiber at 2.8 μm

In this study, Pauli-Blocking has been observed in a 0D/2D MoS2 quantum dots/graphene (MoS2-QDs/graphene) hybrid system. For the observation of room temperature Pauli-Blocking in the 0D/2D system, a photodetector device based on n-type MoS2-QDs and CVD grown graphene has been fabricated using a facile and lithography free technique. The current-voltage characteristics of the device have been performed at room temperature. The fabricated device shows a negative response under visible light (λ ∼ 400 to 700 nm) illumination. The dark to photo current ratio of the device shows variation up to two orders of magnitude. This negative response, which results decrease in current under visible light illumination, may be attributed to the Pauli-Blocking due to high absorbance of photon energy in visible light range. Furthermore, it is believed that the present study may provide an insight into understanding the Pauli-Blocking in 0D/2D hybrid system at room temperature. © 2018 Author(s).

Journal of Applied Physics

A curious observation of Pauli-Blocking in MoS2-quantum dots/graphene hybrid system

Two unprecedented porphyrin fused nanographene molecules, 1 and 2, have been synthesized by the Scholl reaction from tailor-made precursors based on benzo[m]tetraphene-substituted porphyrins. The chemical structures were validated by a combination of high-resolution matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (HR MALDI-TOF MS), IR and Raman spectroscopy, and scanning tunnelling microscopy (STM). The UV-vis-near infrared absorption spectroscopy of 1 and 2 demonstrated broad and largely red-shifted absorption spectra extending up to 1000 and 1400 nm, respectively, marking the significant extension of the π-conjugated systems. © 2018 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Angewandte Chemie International Edition

Synthesis of Triply Fused Porphyrin‐Nanographene Conjugates

Biosensors and Bioelectronics

Fabrication of sensitive bioelectrode based on atomically thin CVD grown graphene for cancer biomarker detection

We analysed perovskite CH3NH3PbI3-xClx inverted planer structure solar cell with nickel oxide (NiO) and spiro-MeOTAD as hole conductors. This structure is free from electron transport layer. The thickness is optimized for NiO and spiro-MeOTAD hole conducting materials and the devices do not exhibit any significant variation for both hole transport materials. The back metal contact work function is varied for NiO hole conductor and observed that Ni and Co metals may be suitable back contacts for efficient carrier dynamics. The solar photovoltaic response showed a linear decrease in efficiency with increasing temperature. The electron affinity and band gap of transparent conducting oxide and NiO layers are varied to understand their impact on conduction and valence band offsets. A range of suitable band gap and electron affinity values are found essential for efficient device performance. © 2018 Korean Physical Society

Current Applied Physics

Inverted structure perovskite solar cells: A theoretical study

Monolayers of transition metal dichalcogenides (TMDs) have been proposed as the next generation electronic materials for nanoscale devices. We evaluated the thermodynamic stability of WS2, Janus WSSe and WSe2 monolayers under biaxial tensile and compressive strain using density functional approach. The phonon band structures of unstrained WS2, WSSe and WSe2 monolayers confirm their thermodynamic stability. The stability of these monolayers is investigated under strain and phonon softening is observed for acoustic and optical mode upto 8% tensile strain. The bond length reduction under compressive strain resulted in out of plane deformation, leading the instability of monolayers under compressive strain. The bond length W-S(Se) and bond angle W-S(Se)-W exhibit significant contribution in coupling strength between transition metal d-orbitals and chalcogen p-orbitals that mediated the electronic properties. The spin orbit coupling showed strong effect on splitting of the valence band for unstrained monolayers. The spin splitting in valence band increases with increasing the tensile strain and decreases with increasing the compressive strain. The effective masses and mobility values are 0.57me, 0.54me, 0.47me; and 0.059, 0.062, 0.072 m2V−1s−1 for unstrained WS2, WSSe and WSe2 monolayers, respectively. The tungsten dx2-y2 orbitals are mainly contributing to the conduction and valence band electronic states in compressive strained monolayers. In contrast under tensile strain tungsten dz2 orbitals contribute to conduction and valence band electronic states, causing the direct to indirect band gap transition in these monolayers. We observed that the impact of tensile strain is more sensitive as compared to that of compressive strain. © 2018 Elsevier Ltd

Superlattices and Microstructures

Strain-mediated stability and electronic properties of WS2, Janus WSSe and WSe2 monolayers

Molecular self-assembly of short peptide building blocks leads to the formation of various material architectures that may possess unique physical properties. Recent studies had confirmed the key role of biaromaticity in peptide self-assembly, with the diphenylalanine (FF) structural family as an archetypal model. Another significant direction in the molecular engineering of peptide building blocks is the use of fluorenylmethoxycarbonyl (Fmoc) modification, which promotes the assembly process and may result in nanostructures with distinctive features and macroscopic hydrogel with supramolecular features and nanoscale order. Here, we explored the self-assembly of the protected, noncoded fluorenylmethoxycarbonyl-β,β-diphenyl-Ala-OH (Fmoc-Dip) amino acid. This process results in the formation of elongated needle-like crystals with notable aromatic continuity. By altering the assembly conditions, arrays of spherical particles were formed that exhibit strong light scattering. These arrays display vivid coloration, strongly resembling the appearance of opal gemstones. However, unlike the Rayleigh scattering effect produced by the arrangement of opal, the described optical phenomenon is attributed to Mie scattering. Moreover, by controlling the solution evaporation rate, i.e., the assembly kinetics, we were able to manipulate the resulting coloration. This work demonstrates a bottom-up approach, utilizing self-assembly of a protected amino acid minimal building block, to create arrays of organic, light-scattering colorful surfaces. Copyright © 2018 American Chemical Society.

ACS Applied Materials and Interfaces

Opal-like Multicolor Appearance of Self-Assembled Photonic Array

We used two electrode electrodeposition techniques to optimize the black chrome solar selective coatings on copper and nickel plated copper substrates. The process was optimized and optimal absorptivity ∼ 0.92 and emissivity ∼ 0.4 was achieved for black chrome coatings on bare copper substrates. The addition of nickel metal layer prior to black chrome on copper substrate resulted into lower emissivity ∼ 0.14 and enhanced absorptivity ∼0.92 or more. The electrochemical bath and process conditions were optimized for the best solar thermal response. Also, the effect of post deposition treatment of deposited samples, such as washing and drying are investigated to understand their impact on solar thermal performance in conjunction with their structure-property relations. © 2018 Elsevier Ltd.

Materials Today: Proceedings

Optimization and structure-property correlation of black chrome solar selective coating on Copper and Nickel plated copper substrates

The weak short gamma-ray burst (GRB) 170817A was accompanied by the GW170817 gravitational-wave event and is believed to have been produced by an off-beam relativistic jet. Here, we use the Ep,i-Eiso and GEiso relations to determine its Lorentz factor γ and the viewing angle from the edge of the jet θ ′obs of GRB 170817A. Our results indicate that Γ = 13.4-5.59.8 and θ ′obs=4°3-1.51.8, corresponding to an on-axis = Ep,i= 415-167361 ke V Eiso =(2.4-1.91.6)erg. Therefore, the GRB was an intrinsically weak short GRB. We also find that the afterglow emission was in good agreement with the follow-up multiband observations and that the radio emissions at around 20 days may have come from the off-axis jet. Interestingly, the Doppler factor and luminosity follow a universal relation for GRBs and blazars, thus suggesting that they may share a similar radiation mechanism. ©2 017. The American Astronomical Society. All rights reserved.