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In this report, we present our results from various studies to qualitatively discriminate the common military explosives viz. RDX, TNT and HMX in their pure form at a distance of ~6.5 m in standoff mode using femtosecond (fs) filament induced breakdown spectroscopy technique (fs FIBS) together with principal component analysis. A ~30 cm length fs filament obtained by a two-lens configuration was used to interrogate those energetic molecules in the form of pressed pellets (150 mg each). The plasma emissions were collected by a Schmidt-Cassegrain telescope (SCT) from a distance of ~8 m away from the investigation zone. Additionally, a few significant results obtained from the LIBS-based investigations of nitroimidazoles with respect to the standoff distance (~2 m) are discussed. Furthermore, we have also summarised a few important results from our recent investigations of bulk energetic materials in various configurations (including those with fs filaments). The results obtained from various fs FIBS configurations corroborate that the filament generation and its properties, the size and f-number of collection optics influence signal strength in the FIBS technique. These results project the fs FIBS technique as a potential technique for investigations aimed at hazardous materials and harsh environments in the standoff mode. [ABSTRACT FROM AUTHOR]

Two new series of β-triphenylamine-appended porphyrins (MTPP(TPA)2X, (where M = 2H, Co(II), Ni(II), Cu(II), Zn(II) and X = NO2/CHO) have been synthesized and characterized by various spectroscopic techniques, namely, UV-vis, fluorescence, NMR spectroscopy, mass spectrometry, cyclic voltammetry, density functional theory and ultrafast nonlinear optical (NLO) studies. They exhibited 16–22 nm and 39–58 nm red-shifts in the Soret and Qx(0,0) bands, respectively, as compared to MTPPs due to the resonance and inductive effects of β-substituents on the porphyrin π-system. The first reduction potential of CuTPP(TPA)2NO2 and CuTPP(TPA)2CHO exhibited an anodic shift by 0.44 and 0.36 V, respectively, as referenced to CuTPP, due to the electronic nature of β-substituents (NO2 and CHO), which led to their easier reduction compared with CuTPP. H2TPP(TPA)2NO2 and H2TPP(TPA)2CHO exhibited the largest resultant dipole moments (7.66 D and 4.55 D, respectively) as compared to H2TPP (0.052 D) due to the cross-polarized push–pull effect of β-substituents (NO2/CHO and triphenylamino groups) and the nonplanarity of the macrocyclic core. Third-order nonlinear optical properties of MTPP(TPA)2NO2 and MTPP(TPA)2CHO (M = 2H and Zn(II)) were investigated in a broad spectral range (680–850 nm) using the Z-scan technique with femtosecond 80 MHz pulses. These materials demonstrate strong nonlinear optical coefficients, endowing them with potential for prominent photonic applications. [ABSTRACT FROM AUTHOR]

Novel methods for production of nanomaterials are urgently needed for various applications, especially in defence. In this work, we propose a direct method to produce silver nanoneedles using the femtosecond laser direct writing (LDW) technique. Femtosecond pulses were focused by a microscope objective on to a metal sheet to produce the nanoneedles. Nanoneedles of required dimensions were fabricated with a simple replacement of microscope objective of different numerical aperture. Further, we have investigated the effect of confinement. Finally, the application of nanoneedles is demonstrated for trace level detection of picric acid using surface enhanced Raman spectroscopy and a field deployable portable Raman spectrometer. [ABSTRACT FROM AUTHOR]

We have investigated femtosecond-laser-induced microstructures (on the surface and within the bulk), gratings, and craters in four different polymers: polymethyl methacrylate, polydimethylsiloxane, polystyrene, and polyvinyl alcohol. The structures were achieved using a Ti:sapphire laser delivering 100-fs pulses at 800 nm with a repetition rate of 1 kHz and a maximum pulse energy of 1 mJ. Local chemical modifications leading to the formation of optical centers and peroxide radicals were studied using ultraviolet-visible absorption and emission, confocal micro-Raman and electron spin resonance spectroscopic techniques. Potential applications of these structures in microfluidics, waveguides, and memory-based devices are demonstrated. [ABSTRACT FROM AUTHOR]

The technique of femtosecond laser-induced breakdown spectroscopy (FLIBS) was employed to investigate seven explosive molecules of nitropyrazole in three different atmospheres: ambient air, nitrogen, and argon. The FLIBS data illustrated the presence of molecular emissions of cyanide (CN) violet bands, diatomic carbon (C2) Swan bands, and atomic emission lines of C, H, O, and N. To understand the plasma dynamics, the decay times of molecular and atomic emissions were determined from time-resolved spectral data obtained in three atmospheres: air, argon, and nitrogen. The CN decay time was observed to be longest in air, compared to nitrogen and argon atmospheres, for the molecules pyrazole (PY) and 4-nitropyrazole (4-NPY). In the case of C2emission, the decay time was observed to be the longest in argon, compared to the air and nitrogen environments, for the molecules PY, 4-NPY, and 1-methyl-3,4,5-trinitropyrazole. The intensities of the CN, C2, C, H, O, and N emission lines and various molecular/atomic intensity ratios such as CN/C2, CNsum/C2sum, CN/C, CNsum/C, C2/C, C2sum/C, (C2+ C) / CN, (C2sum+ C)/CNsum, O/H, O/N, and N/H were also deduced from the LIBS spectra obtained in argon atmosphere. A correlation between the observed decay times and molecular emission intensities with respect to the number of nitro groups, the atmospheric nitrogen content, and the oxygen balance of the molecules was investigated. The relationship among the LIBS signal intensity, the molecular/atomic intensity ratios, and the oxygen balance of these organic explosives was also explored. [ABSTRACT FROM AUTHOR]

The present study investigates the effects of input wavelength (1064, 532, and 355 nm) and surrounding liquid environment (distilled water and aqueous NaCl solution) on the picosecond laser ablation on silver (Ag), gold (Au), and Ag/Au alloy targets. The efficacy of the laser ablation technique was meticulously evaluated by analyzing the ablation rates, surface plasmon resonance peak positions, and particle size distributions of the obtained colloids. The nanoparticles (NPs) were characterized using the techniques of UV-visible absorption, transmission electron microscopy, and energy-dispersive X-ray spectroscopy. Furthermore, NPs of various sizes ranging from 6 to 35 nm were loaded onto a filter paper by a simple and effective drop-casting approach to achieve flexible surface-enhanced Raman spectroscopy (SERS) substrates/sensors. These substrates were tested using a simple, portable Raman device to identify various hazardous chemicals (malachite green, methyl salicylate, and thiram). The stability of the substrates was also systematically investigated by determining the decay percentages in the SERS signals over 60 days. The optimized SERS substrate was subsequently employed to detect chemical warfare agent (CWA) simulants such as methyl salicylate (a CWA simulant for sulfur mustard) and dimethyl methyl phosphonate (has some structural similarities to the G-series nerve agents) at different laser excitations (325, 532, and 633 nm). A notably higher SERS efficiency for CWA simulants was observed at a 325 nm Raman excitation. Our findings reveal that a higher ablation yield was observed at IR irradiation than those obtained at the other wavelengths. A size decrease of the NPs was noticed by changing the liquid environment to an electrolyte. These findings have significant implications for developing more efficient and stable SERS substrates for chemical detection applications., Competing Interests: The authors declare no conflicts of interest., (Copyright © 2024, Moram et al.)

Lead-free halide double perovskite nanocrystals (DPNCs) are emerging materials, recently explored as potential candidates in light-emitting, photovoltaic, and other optoelectronic applications. This Letter reveals unusual photophysical phenomena and nonlinear optical (NLO) properties of Mn-doped Cs 2 AgInCl 6 nanocrystals (NCs) via temperature-dependent photoluminescence (PL) and femtosecond Z-scan measurements. The PL emission measurements suggest that self-trapped excitons (STEs) are present, and more than one STE state is possible for this doped double perovskite. We observed enhanced NLO coefficients, owing to the improved crystallinity arising from the Mn doping. From the closed aperture Z-scan data, we have calculated two fundamental parameters, Kane energy (29 eV) and exciton reduced mass (0.22m 0 ). We further obtained the optical limiting onset (1.84 mJ/cm 2 ) and figure of merit as a proof-of-concept application to demonstrate the potential in optical limiting and optical switching applications. Highlighting the self-trapped excitonic emission and NLO applications, the multifunctionality of this material system is demonstrated. This investigation provides an avenue to design novel photonic and nonlinear optoelectronic devices.

The technique of ultrafast laser ablation in liquids has evolved and matured over the past decade, with several impending applications in various fields such as sensing, catalysis, and medicine. The exceptional feature of this technique is the formation of nanoparticles (colloids) and nanostructures (solids) in a single experiment with ultrashort laser pulses. We have been working on this technique for the past few years, investigating its potential using the surface-enhanced Raman scattering (SERS) technique in hazardous materials sensing applications. Ultrafast laser-ablated substrates (solids and colloids) could detect several analyte molecules at the trace levels/mixture form, including dyes, explosives, pesticides, and biomolecules. Here, we present some of the results achieved using the targets of Ag, Au, Ag-Au, and Si. We have optimized the nanostructures (NSs) and nanoparticles (NPs) obtained (in liquids and air) using different pulse durations, wavelengths, energies, pulse shapes, and writing geometries. Thus, various NSs and NPs were tested for their efficiency in sensing numerous analyte molecules using a simple, portable Raman spectrometer. This methodology, once optimized, paves the way for on-field sensing applications. We discuss the protocols in (a) synthesizing the NPs/NSs via laser ablation, (b) characterization of NPs/NSs, and (c) their utilization in the SERS-based sensing studies.

Recently, filter paper (FP)-based surface-enhanced Raman scattering (SERS) substrates have stimulated significant attention owing to their promising advantages such as being low-cost, easy to handle, and practically suitable for real-field applications in comparison to the solid-based substrates. Herein, a simple and versatile approach of laser-ablation in liquid for the fabrication of silver (Ag)-gold (Au) alloy nanoparticles (NPs). Next, the optimization of flexible base substrate (sandpaper, printing paper, and FP) and the FP the soaking time (5−60 min) was studied. Further, the optimized FP with 30 min-soaked SERS sensors were exploited to detect minuscule concentrations of pesticide (thiram-50 nM), dye (Nile blue-5 nM), and an explosive (RDX-1,3,5-Trinitroperhydro-1,3,5-triazine-100 nM) molecule. Interestingly, a prominent SERS effect was observed from the Au NPs exhibiting satisfactory reproducibility in the SERS signals over ~1 cm2 area for all of the molecules inspected with enhancement factors of ~105 and relative standard deviation values of <15%. Furthermore, traces of pesticide residues on the surface of a banana and RDX on the glass slide were swabbed with the optimized FP substrate and successfully recorded the SERS spectra using a portable Raman spectrometer. This signifies the great potential application of such low-cost, flexible substrates in the future real-life fields.

A zeroth-order, non-diffracting Bessel beam, generated by picosecond laser pulses (1064 nm, 10 Hz, 30 ps) through an axicon, was utilized to perform pulse energy-dependent (12 mJ, 16 mJ, 20 mJ, 24 mJ) laser ablation of silver (Ag) substrates in air. The fabrication resulted in finger-like Ag nanostructures (NSs) in the sub-200 nm domain and obtained structures were characterized using the FESEM and AFM techniques. Subsequently, we employed those Ag NSs in surface-enhanced Raman spectroscopy (SERS) studies achieving promising sensing results towards trace-level detection of six different hazardous materials (explosive molecules of picric acid (PA) and ammonium nitrate (AN), a pesticide thiram (TH) and the dye molecules of Methylene Blue (MB), Malachite Green (MG), and Nile Blue (NB)) along with a biomolecule (hen egg white lysozyme (HEWL)). The remarkably superior plasmonic behaviour exhibited by the AgNS corresponding to 16 mJ pulse ablation energy was further explored. To accomplish a real-time application-oriented understanding, time-dependent studies were performed utilizing the AgNS prepared with 16 mJ and TH molecule by collecting the SERS data periodically for up to 120 days. The coated AgNSs were prepared with optimized gold (Au) deposition, accomplishing a much lower trace detection in the case of thiram (~50 pM compared to ~50 nM achieved prior to the coating) as well as superior EF up to ~10 8 (~10 6 before Au coating). Additionally, these substrates have demonstrated superior stability compared to those obtained before Au coating.

In this study, we report a novel, to the best of our knowledge, instrumentation procedure in the automation of laser beam steering for raster/spiral scanning of the samples used in standoff femtosecond laser-induced breakdown spectroscopy (LIBS) experiments. We have used a readily available and easy-to-handle Arduino-based computerized numerical control (CNC) shield along with the free software, universal G-code sender, for the automation. Standoff femtosecond filamentation-induced breakdown spectra (St-Fs-FIBS) of metals, three compositions of Ag-Au alloy, and polyvinyl chloride, unplasticized polyvinyl chloride, and chlorinated polyvinyl chloride plastic samples were recorded using the developed automated experimental setup. The St-Fs-FIBS spectra were recorded at a standoff distance of ∼5 m utilizing a simple hand-held spectrometer. Furthermore, principal component analysis technique was utilized for the successful classification of three compositions of Au-Ag alloy spectra using their St-Fs-FIBS spectral data.

We report the fabrication and performance evaluation of hybrid surface-enhanced Raman scattering (SERS) substrates involving laser ablation and chemical routes for the trace-level detection of various analyte molecules. Initially, picosecond laser ablation experiments under ambient conditions were performed on pure silver (Ag) and gold (Au) substrates to achieve distinct nanosized features on the surface. The properties of the generated surface features on laser-processed portions of Ag/Au targets were systematically analyzed using UV-visible reflection and field emission scanning electron microscopy studies. Later, hybrid-SERS substrates were achieved by grafting the chemically synthesized Au nanostars on the plain and laser-processed plasmonic targets. Subsequently, we employed these as SERS platforms for the detection of a pesticide (thiram), a molecule used in explosive compositions [ammonium nitrate (AN)], and a dye molecule [Nile blue (NB)]. A comparative SERS study between the Au nanostar-decorated bare glass, silicon, Ag, Au, and laser-processed Ag and Au targets has been established. Our studies and the obtained data have unambiguously determined that laser-processed Ag structures have demonstrated reasonably good enhancements in the Raman signal intensities for distinct analytes among other substrates. Importantly, the fabricated hybrid SERS substrate of "Au nanostar-decorated laser-processed Ag" exhibited up to eight times enhancement in the SERS intensity compared to laser-processed Ag (without nanostars), as well as up to three times enhancement than the Au nanostar-loaded plain Ag substrates. Additionally, the achieved detection limits from the Au nanostar-decorated laser-processed Ag SERS substrate were ∼50 pM, ∼5 nM, and ∼5 μM for NB, thiram, and AN, respectively. The estimated enhancement factors accomplished from the Au nanostar-decorated laser-processed Ag substrate were ∼10 6 , ∼10 6 , and ∼10 4 for NB, thiram, and AN, respectively., Competing Interests: The authors declare no competing financial interest., (© 2022 The Authors. Published by American Chemical Society.)

A new series of "push-pull"meso-substituted trans-A 2 BC porphyrins, where A = mesityl, B = phenothiazine (push) and C = o/p-nitrophenyl moiety (pull) and M = 2H, Ni(ii), Cu(ii), and Zn(ii), were synthesized. These trans-A 2 BC porphyrins were characterized by various techniques viz. UV-Vis, fluorescence and NMR spectroscopy, matrix assisted laser desorption ionization-time of flight (MALDI-TOF) mass spectrometry, cyclic voltammetry, single-crystal X-ray diffraction analysis and density functional theory (DFT) studies. The single crystal structure of the newly synthesized free base trans-H 2 A 2 BC porphyrin (2) revealed the orientation of meso-substituents and the planar conformation of the porphyrin core. All newly synthesized free base trans-H 2 A 2 BC porphyrins (1 or 2) and their metal complexes exhibited a sharp Soret band. The first reduction potential of all synthesized trans-MA 2 BC porphyrins showed an anodic shift as compared with that of meso-tetraphenylporphyrins (MTPPs) due to the strong electron withdrawing nature of the nitrophenyl group. In general, the first oxidation potential of trans-MA 2 BC porphyrins shows a cathodic shift due to the electron donating phenothiazine moiety as compared to that of MTPPs. p-Nitrophenyl appended trans-A 2 BC porphyrins exhibited large ground state dipole moment values (8.59-9.64 D) as compared to MTPPs (0.0013-0.052 D) owing to the polarized "push-pull" effect of meso-substituents such as phenothiazine (push) and nitrophenyl (pull) moieties. Femtosecond nonlinear optical (NLO) studies performed with kHz pulses at 800 nm revealed strong two-photon absorption coefficients (∼0.082-0.0953 cm GW -1 ) and cross-sections (∼1.71-1.95 × 10 3 GM) for these "push-pull" porphyrins. We have also attempted to understand the NLO coefficients in terms of the structural changes in these porphyrin derivatives. A comparison that has been accomplished with similar porphyrin molecules and under similar experimental conditions revealed the superior performance of the title molecules. Furthermore, femtosecond transient absorption studies demonstrated several ultrafast processes from various excited states in these porphyrins, useful for identifying the processes relevant to optical switching applications.

We demonstrate the detection of dipicolinic acid, (DPA), a biomarker of bacterial spores for Bacillus anthracis, 2,4-Dinitrotoluene (DNT) and picric acid (PA) nitroaromatic hazardous chemicals on ultra-sensitive, reusable femtosecond laser textured Au nanostructures decorated with hierarchical AuNPs as a SERS substrate. The AuNPs were achieved by ablating an Au sheet using two different laser scan speeds (1 and 0.1 mm/s) in linear and crossed patterns. The morphological studies revealed dense hierarchical nanostructures decorated with spherical AuNPs possessing 30-40 nm in size in 0.1 mm/s laser scan. The limits of detection (LOD) of the sensor were determined from the detailed SERS measurements and were estimated to be 0.83 pg/L, 3.6 pg/L and 2.3 pg/L for DPA, DNT, and PA, respectively. To the best of our knowledge, the achieved sensitivity is nearly 2 orders improved for DPA when compared with the currently reported LODs using other techniques and 1 order in the case of SERS. Moreover, for DNT and PA the LODs were found to be either superior or comparable with recent reports. We have also demonstrated the competence of our SERS substrates by testing a few real samples (water spiked with these analytes) and again obtained very good sensitivity., (Copyright © 2020 Elsevier B.V. All rights reserved.)

Two new series of β-triphenylamine-appended porphyrins (MTPP(TPA) 2 X, (where M = 2H, Co(ii), Ni(ii), Cu(ii), Zn(ii) and X = NO 2 /CHO) have been synthesized and characterized by various spectroscopic techniques, namely, UV-vis, fluorescence, NMR spectroscopy, mass spectrometry, cyclic voltammetry, density functional theory and ultrafast nonlinear optical (NLO) studies. They exhibited 16-22 nm and 39-58 nm red-shifts in the Soret and Q x (0,0) bands, respectively, as compared to MTPPs due to the resonance and inductive effects of β-substituents on the porphyrin π-system. The first reduction potential of CuTPP(TPA) 2 NO 2 and CuTPP(TPA) 2 CHO exhibited an anodic shift by 0.44 and 0.36 V, respectively, as referenced to CuTPP, due to the electronic nature of β-substituents (NO 2 and CHO), which led to their easier reduction compared with CuTPP. H 2 TPP(TPA) 2 NO 2 and H 2 TPP(TPA) 2 CHO exhibited the largest resultant dipole moments (7.66 D and 4.55 D, respectively) as compared to H 2 TPP (0.052 D) due to the cross-polarized push-pull effect of β-substituents (NO 2 /CHO and triphenylamino groups) and the nonplanarity of the macrocyclic core. Third-order nonlinear optical properties of MTPP(TPA) 2 NO 2 and MTPP(TPA) 2 CHO (M = 2H and Zn(ii)) were investigated in a broad spectral range (680-850 nm) using the Z-scan technique with femtosecond 80 MHz pulses. These materials demonstrate strong nonlinear optical coefficients, endowing them with potential for prominent photonic applications.

While the unabated race persists in achieving record efficiencies in solar cells and other photonic/optoelectronic devices using lead halide perovskite absorbers, a comprehensive picture of the correlated third-order nonlinear optical (NLO) properties is yet to be established. The present study is aimed at deciphering the role of dopants in multiphoton absorption properties of intentionally engineered CsPbBr 3 colloidal nanocrystals (NCs). The charge separation of the plasmon-semiconductor conduction band owing to the hot electron transfer at the interface was demystified using the dynamics of the bleached spectral data from femtosecond (fs) transient absorption spectroscopy with broadband capabilities. The NLO properties studied through the fs Z-scan technique revealed that Ni-doped CsPbBr 3 NCs exhibited strong third-order NLO susceptibility of ∼10 -10 esu. The exotic photophysical phenomena in these pristine and Ni-doped CsPbBr 3 colloidal two-dimensional (2D) NCs reported herein are believed to provide the avenues to address the critical variables involved in the structural differences and their correlated optoelectronic properties.

The femtosecond filament-induced breakdown spectroscopy (FIBS) technique coupled with principal component analysis (PCA) is demonstrated for standoff (ST) analysis of metals, alloys (Al, Cu, brass, stainless steel), and bimetallic strips (Ag@Cu, Ag@Au with varying weight percentages). The experiments were performed by analyzing the filament-produced plasma at ∼6.5  m from the laser. The plasma emissions were collected using a Schmidt-Cassegrain telescope (6″ f/10) at ∼8  m away. The variations in intensities of persistent atomic transitions in the FIBS spectra clearly reflected the varying weight percentage in bimetallic strips. Furthermore, PCA was successfully utilized to discriminate the metals, alloys, and bimetallic strips batch wise and altogether. Our results demonstrate the capability of femtosecond ST-FIBS for ST analytical applications.