Characteristics of elevated aerosol layer over the Indian east coast, Kattankulathur (12.82oN, 80.04°E): A northeast monsoon region.

The elevated aerosol layer (EAL) plays a vital role in weather and climate by modifying the Earth's radiation budget. In the present study, the EAL occurrence and its characteristics in the pre-monsoon season using micropulse lidar (MPL) observations during 2016-2018 and Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observation (CALIPSO) during 2007-2018 over Kattankulathur is being reported. We have collected 147 days (101 cases) of MPL (CALIPSO) observations during clear sky conditions in the pre-monsoon 2016-2018 (2007-2018), out of which EAL is observed for 56 days (61 cases). The EAL width is generally found to be ~2.0 km and occurs between ~1.0 km and 5.0 km. Three different types of EALs are categorized based on their altitudinal occurrence using the zero-crossing method. The EALs with their base at ~1.0-1.5 km, ~1.5-2.0 km, and ~ 2.0-3.0 km are taken as types I, II, and III, which occur for 9, 20, and 27 days, respectively. The EAL significantly modifies the total columnar aerosol optical depth (AOD). It is found that AOD, in total, within ABL and EAL, are ~0.72 (0.61), 0.28 (0.25), and 0.45 (0.36) using MPL (CALIPSO), respectively. The aerosols within ABL contribute ~38 % (41 %) while EAL ~ 62 % (59 %) to the total AOD obtained using MPL (CALIPSO). We observed that the ABL and EAL are characterized by different aerosol subtypes, such as dust marine (31 %) and smoke (~ 27 %) aerosols. Other aerosol subtypes, such as dust and polluted dust, commonly occur within the ABL (54 %) and EAL (52 %).

Syntheses, Crystal Structure, and Second Harmonic Generation Response of Noncentrosymmetric Layered Selenites and Tellurites of Antimony(V), ASb3X2O12 (A = K, Rb, Cs, Tl; X = Se, Te).

Eight quaternary selenites and tellurites of antimony(V), ASb3X2O12 (A = K, Rb, Cs, Tl; X = Se, Te), have been synthesized by solid state reactions and structurally characterized by single crystal X-ray diffraction. They are isostructural compounds, which crystallize in the polar noncentrosymmetric P63mc space group with hexagonal unit cell parameter values in the ranges of a = 7.1538(1)-7.2429(1) Å and c = 11.8978(4)-12.0478(3) Å. Their noncentrosymmetric layered structure consists of hexagonal-tungsten-oxide-related (Sb3X2O12)- layers interleaved with A+ ions. They have optical band gap values of 2.90-4.46 eV and showed powder second harmonic generation response equivalent to 1.0-37.2 × KDP.

Gold-Silver Bimetallic Nanoparticles Reduced with Herbal Leaf Extracts Induce ROS-Mediated Death in Both Promastigote and Amastigote Stages of Leishmania donovani.

Resistance to antileishmanial drugs such as sodium stibogluconate (SSG), amphotericin B (Amp-B), and miltefosine is on the rise, and alternate strategies for effective treatment have gained importance in recent years. Although nanoparticle (NP)-based composite drugs that have emerged recently have been found to be effective, the associated toxicity limits their usage. Bimetallic NPs produced through reduction with medicinal plant extracts are proposed to overcome the toxicity of the NPs. In the present study, three types of gold-silver bimetallic nanoparticles (Au-Ag BNPs) were synthesized through a single-step reduction process using fenugreek, coriander, and soybean leaf extracts. All of the three types of BNPs exhibited high antileishmanial effects against promastigotes with half-inhibitory concentration (IC50) values in the range of 0.03-0.035 µg/mL. The IC50 values of the BNPs are much lower compared to those of miltefosine (IC50 = 10 µg/mL). The synthesized BNPs induced the reactive oxygen species (ROS)-mediated apoptosis-like death in the promastigotes and could potentiate the antileishmanial activity of macrophages. The intracellular amastigotes were reduced by 31-46% in macrophages. The biogenic BNPs synthesized in this study and their potent antileishmanial activity provide further impetus to the ongoing quest for novel drugs to effectively manage leishmaniasis.

A thiazolo[5,4-d]thiazole-bridged porphyrin organic framework as a promising nonlinear optical material.

Porphyrin-based porous organic frameworks are an important group of materials gaining interest due to their structural diversity and distinct opto-electronic properties. However, these materials are seldom explored for nonlinear optical (NLO) applications. In this work, we investigate a thiazolo[5,4-d]thiazole-bridged porous, porphyrin framework (Por-TzTz-POF) with promising NLO properties. The planar TzTz moiety coupled with integrated porphyrin units enables efficient π-conjugation and charge distribution in the Por-TzTz-POF resulting in a high nonlinear absorption coefficient (ß = 1100 cm GW-1) with figure of merit (FoM) σ1/σ0 = 5571, in contrast to analogous molecules and material counterparts e.g. metal-organic frameworks (MOFs; ß = ∼0.3-0.5 cm GW-1), molecular porphyrins (ß = ∼100-400 cm GW-1), graphene (ß = 900 cm GW-1), and covalent organic frameworks (Por-COF-HH; ß = 1040 cm GW-1 and FoM = 3534).

Nonlinear Optical Switching in Regioregular Porphyrin Covalent Organic Frameworks.

Covalent organic frameworks (COFs) have garnered immense scientific interest among porous materials because of their structural tunability and diverse properties. However, the response of such materials toward laser-induced nonlinear optical (NLO) applications is hardly understood and demands prompt attention. Three novel regioregular porphyrin (Por)-based porous COFs-Por-COF-HH and its dual metalated congeners Por-COF-ZnCu and Por-COF-ZnNi-have been prepared and present excellent NLO properties. Notably, intensity-dependent NLO switching behavior was observed for these Por-COFs, which is highly desirable for optical switching and optical limiting devices. Moreover, the efficient π-conjugation and charge-transfer transition in ZnCu-Por-COF enabled a high nonlinear absorption coefficient (ß=4470 cm/GW) and figure of merit (FOM=σ1 /σo , 3565) value compared to other state-of-the-art materials, including molecular porphyrins (ß≈100-400 cm/GW), metal-organic frameworks (MOFs; ß≈0.3-0.5 cm/GW), and graphene (ß=900 cm/GW).

Photoluminescence and Nonlinear Optical Properties of Transition Metal (Ag, Ni, Mn) Doped ZnO Nanoparticles.

ZnO nanoparticles doped with transition metal (Ag, Ni, Mn) were prepared by using co-precipitation method. Absorption spectra showed exciton peak for ZnO nanoparticles at 367 nm and for Ag, Ni, Mn doped ZnO nanoparticles at 328, 354 nm and 342 nm respectively. Photoluminescence dynamics showed shallow level-trap and deep-level emission where the intensity of deep-level trap increases with transition metal doping due to the oxygen deficiency. Nonlinear absorption measurements showed the two-photon absorption increasing with transition metal following the trend Ag > Ni > Mn when excited with 532 nm, 30 ps laser pulses. The transition metal doped and un-doped ZnO nanoparticles can be used as optical limiters.

Preparation of Multiwall Carbon Nanotubes with Zinc Phthalocyanine Hybrid Materials and Their Nonlinear Optical (NLO) Properties.

Hybrid material of multi walled carbon nanotubes (MWCNT)-zinc phthalocyanine (ZnPcs) (MWCNT-ZnPc) composites were synthesized and compared with ZnPc. Nonlinear optical properties of MWCNT-ZnPc and ZnPc were studied. The synthesized MWCNT-ZnPc characterized by UV-visible absorption spectroscopy, photoluminescence and TEM. Their nonlinear optical properties were measured through Z-scan technique with 800 nm, 110 fs, 1 kHz Ti:Sapphire laser and 532 nm, 30 ps and 10 Hz Nd:YAG laser. The results showed that phthalocyanines were anchored onto the surface of the multi walled carbon nanotubes by using TEM. Photoluminescence spectra had shown quenching in emission of MWCNT-ZnPcs hybrid material due to the energy transfer. Reverse saturable absorption in MWCNT-ZnPcs hybrid material was compared with pure phthalocyanine, which had shown an increase in nonlinear absorption coefficient with 532 nm and reduction with 800 nm. The results concluded that MWCNT and MWCNT-ZnPc can be used as effective optical limiters.

Ultrashort vortex from a Gaussian pulse - An achromatic-interferometric approach.

The more than a century old Sagnac interferometer is put to first of its kind use to generate an achromatic single-charge vortex equivalent to a Laguerre-Gaussian beam possessing orbital angular momentum (OAM). The interference of counter-propagating polychromatic Gaussian beams of beam waist ωλ with correlated linear phase (ϕ 0 ≥ 0.025 λ) and lateral shear (y 0 ≥ 0.05 ωλ) in orthogonal directions is shown to create a vortex phase distribution around the null interference. Using a wavelength-tunable continuous-wave laser the entire range of visible wavelengths is shown to satisfy the condition for vortex generation to achieve a highly stable white-light vortex with excellent propagation integrity. The application capablitiy of the proposed scheme is demonstrated by generating ultrashort optical vortex pulses, its nonlinear frequency conversion and transforming them to vector pulses. We believe that our scheme for generating robust achromatic vortex (implemented with only mirrors and a beam-splitter) pulses in the femtosecond regime, with no conceivable spectral-temporal range and peak-power limitations, can have significant advantages for a variety of applications.

A novel blue luminescent material Na2[Co(C2O4)2(H2O)2]·6H2O: synthesis, structure, luminescence and magnetic properties.

The synthesis, crystal structure and physical properties of new disodium trans-diaquabis(oxalato)cobaltate(ii)hexahydrate {Na2[Co(C2O4)2(H2O)2]·6H2O} crystals have been investigated. Single crystal X-ray analysis reveals that this compound crystallizes in the triclinic system with the space group P1[combining macron]. The structure of this complex consists of [Co(C2O4)2(H2O)2](2-) anionic units with a slightly distorted octahedral geometry of cobalt surrounded by four oxygen atoms of two oxalate groups. The anionic units are interlinked by two Na(+) ions with different octahedral and distorted octahedral environments. The electronic absorption spectra of the compound exhibit bands at 208, 246 and 526 nm in the UV and visible regions. A strong blue luminescence was observed at room temperature when excited at 355 nm. The M(H) curve at 2 K shows a significant nonlinear behaviour with almost zero coercivity which clearly indicates an extremely weak antiferromagnetic/ferromagnetic state of the complex.

Coherently embedded Ag nanostructures in Si: 3D imaging and their application to SERS.

Surface enhanced Raman spectroscopy (SERS) has been established as a powerful tool to detect very low-concentration bio-molecules. One of the challenging problems is to have reliable and robust SERS substrate. Here, we report on a simple method to grow coherently embedded (endotaxial) silver nanostructures in silicon substrates, analyze their three-dimensional shape by scanning transmission electron microscopy tomography and demonstrate their use as a highly reproducible and stable substrate for SERS measurements. Bi-layers consisting of Ag and GeOx thin films were grown on native oxide covered silicon substrate using a physical vapor deposition method. Followed by annealing at 800°C under ambient conditions, this resulted in the formation of endotaxial Ag nanostructures of specific shape depending upon the substrate orientation. These structures are utilized for detection of Crystal Violet molecules of 5 × 10(-10) M concentrations. These are expected to be one of the highly robust, reusable and novel substrates for single molecule detection.

Design and synthesis of donor-acceptor pyrazabole derivatives for multiphoton absorption.

A series of ferrocenyl substituted pyrazaboles were prepared, which have an electron-withdrawing pyrazabole moiety as the central core, with a D-π-A-π-D structural motif (D = donor, A = acceptor) and containing a variety of π bridges. A crystallographic study reveals that the pyrazabole central core was observed in planar geometry. Strong two-photon absorption in the visible region and three-photon absorption in the near IR region were observed in the ferrocenyl substituted pyrazaboles. It was observed that the 2PA absorption cross-section increases with the conjugation length.

High quality factor 1-D Er³âº-activated dielectric microcavity fabricated by RF-sputtering.

Rare earth-activated 1-D photonic crystals were fabricated by RF-sputtering technique. The cavity is constituted by an Er3+-doped SiO2 active layer inserted between two Bragg reflectors consisting of ten pairs of SiO2/TiO2 layers. Scanning electron microscopy is employed to put in evidence the quality of the sample, the homogeneities of the layers thickness and the good adhesion among them. Near infrared transmittance and variable angle reflectance spectra confirm the presence of a stop band from 1500 nm to 2000 nm with a cavity resonance centered at 1749 nm at 0° and a quality factor of 890. The influence of the cavity on the 4I13/2 -->4I15/2 emission band of Er3+ ion is also demonstrated.

Synthesis of silver nanoparticles in poly(vinyl alcohol) matrix in solution and thin films through laser irradiation.

Here we communicate our experimental results on the synthesis of silver nanoparticles in solution and thin films using silver nitrate and poly vinyl alcohol (PVA) mixture at different concentrations and different laser irradiations. Detailed studies were carried out by varying pulse width, wavelength, exposure time, and energy of the laser. Formation of nanoparticles was confirmed through color change from transparent to yellow. Irradiated solutions and thin films at different concentrations showed plasmon peak in the absorption spectra. Formation of different sized nanoparticles at different energies with peak shift is observed. Transmission electron microscope (TEM) results confirmed the formation of nanoparticles with size of the particles varying from 2 to 200 nm. Formation of silver nanoparticles with hexagonal and different shapes were observed in particular with 355 nm laser irradiation. Influence of wavelength, pulse width, exposure time, and energy in the synthesis of silver nanoparticles is highlighted. Electron diffraction patten of a single nanoparticle in TEM showed polycrystallinity with cubic nature for the silver nanoparticles prepared. We also compared the linear and nonlinear absorption properties of the freshly prepared nanoparticles with nanoparticles solution left in a shelf for a long period of time.

Biosynthesis of silver nanoparticles using Moringa oleifera leaf extract and its application to optical limiting.

The Development of biologically inspired experimental processes for the synthesis of nanoparticles is evolving into an important branch of nanotechnology. The work presented here with the biosynthesis of silver nanoparticles using Moringa oleifera leaf extract as reducing and stabilizing agent and its application in nonlinear optics. The aqueous silver ions when exposed to Moringa oleifera leaf extract are reduced resulting in silver nanoparticles demonstrating the biosynthesis. The silver nanoparticles were characterized by UV-Visible, X-ray diffraction (XRD), Fourier transform infra-red spectroscopy (FT-IR) and transmission electron microscopy (TEM) techniques. TEM analysis shows a dispersion of the nanoparticles in a range of 5-80 nm with the average around 46 nm and are crystallized in face centred cubic symmetry. To show that these biosynthesized silver nanoparticles possess very good nonlinear properties similar to those nanoparticles synthesized by chemical route, we carried out the Z-scan studies with a 6 ns, 532 nm pulsed laser. We estimated the nonlinear absorption coefficient and compare it with the literature values of the nanoparticles synthesized through chemical route. The silver nanoparticles suspended in solution exhibited reverse saturable absorption with optical limiting threshold of 100 mJ/cm2.

Effect of input spectrum on the spectral switch characteristics in a white-light Michelson interferometer.

We report here a detailed experimental study to demonstrate the effect of source spectral characteristics such as spectral bandwidth (Deltalambda), peak wavelength (lambda(0)), and shape of the spectrum on the spectral shifts and spectral switches measured due to temporal correlation in a white-light Michelson interferometer operated in the spectral domain. Behavior of the spectral switch characteristics such as the switch position, switch amplitude, and switch symmetry are discussed in detail as a function of optical path difference between the interfering beams. The experimental results are compared with numerical calculations carried out using interference law in the spectral domain with modified source spectral characteristics. On the basis of our results we feel that our study is of critical importance in the selection of source spectral characteristics to further improve the longitudinal resolution or the measurement sensitivity in spectral-domain optical coherence tomography and microscopy.

Nanodisplacement measurement using spectral shifts in a white-light interferometer.

We report a novel experimental method to measure nanometer displacements using wavelength shifts of spectral peaks around spectral switch or singular phase points in the interference spectra due to temporal correlation in a Michelson interferometer illuminated by a broadband white-light source. Dramatic changes in the spectral characteristics are recorded as a function of path difference between the interfering beams around the spectral switch position. These are then compared with measurements far from it in order to demonstrate the higher sensitivities involved in the proposed method.

Four wave mixing at air-dielectric interfaces with a femtosecond laser excitation.

In this article we report our experimental results on non collinear four-wave mixing (FWM) at air-dielectric interfaces. We generated FWM signal from the two interfaces of a 1mm thick fused silica slide with air using two 800 nm, 100 fs pump pulses in non-collinear pump-probe type geometry. We observe that there is a maximum peak shift of approximately 1.5 nm in FWM signals corresponding to the two interfaces of the same silica slide. Further, we find that the intensity of FWM signal observed at air to silica interface is less than that from silica to air interface. We briefly elaborate on this observation with systematic study of FWM in fused silica.

In situ synthesis of metal nanoparticles in polymer matrix and their optical limiting applications.

We present an overview of the simple and environmentally benign protocol we have developed recently, for the in situ generation of metal nanoparticles inside polymer films by mild thermal annealing, leading to free-standing as well as supported thin films of nanoparticle-embedded polymer. The fabrication chemistry is discussed and spectroscopic/microscopic characterizations of silver and gold nanoparticles in poly(vinyl alcohol) film are presented. Optical limiting characteristics of the silver-polymer system are investigated in detail and preliminary results for the gold-polymer system are reported.

Nonlinear optical absorption and switching properties of gold nanoparticle doped SiO2-TiO2 sol-gel films.

The nonlinear optical absorption and switching properties of sol-gel derived of Au nanoparticle doped SiO2-TiO2 sol-gel films having different Au-surface plasmon absorption positions are reported in this paper. The Au nanoparticles are embedded in SiO2 and SiO2-TiO2 mixed glassy film matrices with different refractive index values. To study the nonlinear absorption properties, lasers with three different wavelengths are used. The optical switching behavior is studied by using the pump-probe technique with 532 nm as the excitation wavelength. Ground state conduction band, surface plasmon band, and the free carrier band are taken as three level model to explain theoretically the obtained RSA and SA behaviors.

Experimental studies of spontaneous emission from dopants in an absorbing dielectric.

We report the first measurements, to our knowledge, of the modification of spontaneous emission rates of Eu3+ ions in the visible region owing to an absorbing medium. Precise levels of the absorption coefficient are introduced by codoping with different amounts of Nd3+. We use a binary glass system PbO-B2O3 as the host, the compositional variation of which leads to a change in the real part of the refractive index. Measured lifetimes are found to follow the real cavity model, and the data are analyzed by the model proposed by Scheel et al. [Phys. Rev. A 60, 4094 (1999)]. We give estimates of the parameter that is related to the radius of the cavity around Eu3+.