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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]

Femtosecond laser excitation (800 nm, ∼150 fs, 80 MHz) induced anisotropic nonlinear optical behaviour and optical limiting action of ethylenediamminium picrate (EDAPA) crystal along two prominent orientations (2 0 0) and (1 1 1) was studied using the Z‐scan technique. EDAPA exhibited reverse saturable type nonlinear absorption ascribed to two‐photon absorption (2PA) process and self‐defocusing based on nonlinear refraction. Intensity dependent nonlinear absorption studies portray the 2PA coefficient is independent of the peak intensity of excitation. Frontier molecular orbitals studies revealed the non‐availability of real states at laser excitation (1.55 eV) and the measured energy gap of 3.18 eV avails the energy states for simultaneous absorption of two photons involving virtual states leading to the occurrence of genuine 2PA. In addition, using molecular electrostatic potential (MEP) analyses polarizability (5.30×10−24 esu) and dipole moment (7.317 Debye) were calculated using the density functional theory. A variation in the NLO coefficients with respect to orientation of excitation arises due to the anisotropy of the crystal. EDAPA crystal along (1 1 1) orientation has high laser damage threshold (9.1 GW/cm2) and low optical limiting threshold (24.36 μJ/cm2) and thus the wide dynamic range make it a preferred candidate towards laser safety devices for high repetition rate ultrashort laser pulse excitation. [ABSTRACT FROM AUTHOR]

Nanoparticle (NP)-loaded filter paper (FP)-based surface-enhanced Raman scattering (SERS) substrates have been prepared using differently shaped gold (Au) NPs. The shape of Au NPs plays a significant role in the amplification of SERS signal. Here, two differently shaped Au NPs were synthesized using two different techniques: (a) femtosecond (fs) laser ablation in liquid and (b) chemical method. Spherical shaped Au NPs were obtained using fs ablation of a bulk Au target in distilled water and Au nanostars (NSs) were achieved through chemical process utilizing N-vinyl-2-pyrrolidone as a reducing/capping agent. The size and shapes of these synthesized NPs and NSs were investigated meticulously using different characterization techniques such as transmission electron microscopy, field emission scanning electron microscopy and X-ray diffraction. Both the NPs and NSs were subsequently loaded onto commercially available FP by simple drop casting method. To achieve higher number of hot spots, the aggregated spherical NPs were obtained by addition of NaCl. The non-aggregated spherical, aggregated spherical, and star Au NPs loaded on FP were used for the detection of a dye (Nile blue) and an explosive molecule (Picric acid). [ABSTRACT FROM AUTHOR]

During the past several years, numerous efforts have been accomplished on designing efficient surface‐enhanced Raman scattering (SERS) substrates with high sensitivity, good reproducibility, and recyclable nature. However, it still remains a significant challenge to realize all these qualities in a single substrate. We have fabricated ripple‐like nanostructures (NSs) on Iron (Fe) substrate using femtosecond (fs) laser irradiation of Fe target in distilled water. These ripple‐like structures had periodicities in the range of ~90–190 nm, which was confirmed from the analysis of field emission scanning electron microscope (FESEM) data. We subsequently deposited a thin layer of gold on these ripple‐like Fe NSs using thermal evaporation. Later, these coated NSs were effectively utilized as SERS substrates for methylene blue (MB) detection and were found to be sensitive evident from the data obtained at a very low concentration of 500pM. Furthermore, we observed that the SERS signal variations on the substrate were <11.2% indicating the reproducible nature of these substrates. Additionally, we demonstrate that these substrates can be reused (we establish this for three times consecutively) by detecting malachite green (MG) and an explosive molecule (picric acid, PA) followed by the mixture compound (Rhodamine 6G+MB) through employment of simple cleaning procedures. The detected molecular concentrations in terms of masses were found to be 3.2 pg, 36.5 pg, and 23 ng for MB (for a concentration of 500pM), MG (for a concentration of 5nM), and PA (for a concentration of 5μM), respectively. Furthermore, these Fe NSs exhibited superior batch‐to‐batch reproducibility with a relative standard deviation (RSD) value of ~l4%. The proposed method of fabricating ripple structures as SERS platforms are highly feasible, reliable, and have great potential for on‐site detection of several analyte molecules with an easily transportable Raman spectrometer. [ABSTRACT FROM AUTHOR]

Choosing a wavelength close to the electronic excitation of the analyte molecules is known to enhance the Surface Enhanced Raman Spectroscopy signal further. This technique is often referred to as Surface Enhanced Resonance Raman Spectroscopy (SERRS). Here we use SERRS for sensing different classes of molecules and demonstrate how it can be used for ultra-trace detection. Diverse SERS substrates were fabricated by femtosecond laser ablation of Ag–Au alloy (1:1) in the air by changing the angle of incidence. Further, we have examined the SERRS performance of each substrate. Often in laser ablation in air, the surface debris is considered undesirable. Here we have shown that this debris of nanoparticles can be used as an advantage for ultra-trace detection of different molecules. Further, with the advantage of having randomly stacked nanoparticles in debris and periodic substrates without debris from a single experiment, we have examined the much-debated relationship between enhancement and reproducibility in SERS and found that they are inversely correlated. The sensitivity of the substrate for rhodamine 6G, crystal violet, picric acid, and cysteine was found to be 10 fM, 100 fM, 100 nM, and 100 nM, respectively, with good reproducibility. • Femtosecond ablation of Ag–Au alloy (1:1) in air performed by changing incidence angle. • We have studied the effects of angle of incidence on the SERS performance. • Different class of molecules have been studied using resonant excitation. • We have examined the relationship between enhancement and reproducibility. • An inverse relationship has been found between enhancement and reproducibility in the SERS. • Sensitivity for Rh6G, CV, PA, Cysteine was 10 fM, 100 fM, 100 nM, 100 nM, respectively. [ABSTRACT FROM AUTHOR]

Molybdenum disulfide (MoS 2) quantum dots (QDs) of sizes 2.05–4.40 nm were synthesized using the femtosecond pulse laser ablation in liquid (PLAL) method. The optical and ultrafast nonlinear optical (NLO) properties of MoS 2 QDs were comprehensively studied using photoluminescence and femtosecond Z-scan method. We synthesized these QDs in ethanol (Et) and DI water (DW) with varying ablation times of 20 min and 10 min; the sample names are Et20 and Et10, DW20, and DW10, respectively. The nonlinear absorption (NLA) coefficients [three-photon absorption (3PA)], nonlinear refractive index, etc., of these QDs, were studied using the fs Z-scan method. Interestingly, Et20 possessed saturable absorption (SA) followed by reverse saturable absorption (RSA) due to the free carrier absorption (FCA) with FCA cross-section values of – (0.61–0.89) × 10−18 cm2. While for Et10, DW20, and DW10, we found 3PA, a rare finding for MoS 2 QDs. The estimated values of the second hyperpolarizability of these samples were found to be ∼10−30 esu. Due to the strong NLA phenomena, these samples have the optical limiting (OL) onset values in the range of ∼ (5.11–8.43) mJ/cm2, proving the optical limiting application potential of these QDs. The obtained optical, NLO parameters, and Kane energy values of laser-ablated MoS 2 QDs suggest that these can be suitable material systems for optoelectronic, NLO switching, logic gates, and other photonic applications. [Display omitted] • MoS 2 QDs synthesized using femtosecond pulsed laser ablation in ethanol, DI water. • Intensity-dependent OA Z-scan of QDs in ethanol with 20 min ablation exhibited SA, followed by RSA. • OA Z-scan measurements in other samples exhibited three-photon absorption (3PA). • From the Z-scan data, Kane energy and reduced mass of excitons were calculated. • The OL onset of these QDs was in the range of (5.11–8.43) mJ/cm2. [ABSTRACT FROM AUTHOR]

[Display omitted] • TiO 2 and Ag-TiO 2 nanoparticles (NPs) synthesized in deionized water using femtosecond laser ablation/irradiation techniques. • Broadband femtosecond Z-scan experiments performed with kHz and MHz pulses. • 2PA coefficients of ~10-8 cm/W and 3PA coefficients of ~10-15 cm3/W2 were retrieved with MHz excitation. • The presence of a prominent 3PA (~10-23 cm3/W2) using kHz excitation was established. • A correlation has been established with thermal-optic influence with MHz pulses. • Plasmon-enhanced nonlinear absorption in the case of 1 kHz pulses recognized. We report the results obtained from our extensive femtosecond (fs) nonlinear optical (NLO) studies of titanium dioxide (TiO 2) and silver (Ag) doped titanium dioxide (Ag-TiO 2) nanoparticles. TiO 2 as well as Ag-TiO 2 nanoparticles (NPs) were synthesized in deionized water using the techniques of fs laser ablation and simultaneous laser induced chemical reduction through irradiation, respectively. The broadband third-order nonlinear optical (NLO) behaviour of the synthesized NPs was investigated by employing the standard open and closed aperture Z-scan experiments with fs laser pulses delivered from an oscillator (~80 MHz repetition rate at wavelengths of 700 nm, 750 nm, 800 nm, 850 nm) and amplifier pulses (1 kHz repetition rate at wavelengths of 800 nm, 1000 nm, 1100 nm, 1200 nm). The corresponding NLO coefficients [two-photon absorption (2PA) coefficients of ~10-8 cm/W and three-photon absorption (3PA) coefficients of ~10-15 cm3/W2 with MHz excitation] were retrieved from the theoretical fits to the experimental Z-scan data. It was evident from the experimental observations that the Ag-TiO 2 NPs demonstrated a significant tendency towards 3PA and also crucially higher nonlinear susceptibility [χ(3)] in comparison with the pure TiO 2 NPs. In this extensive investigation, not only a remarkable correlation has been established regarding the incorporation of Ag into TiO 2 matrix with thermal-optic influence (MHz interaction) but also the fundamental plasmon-enhanced nonlinear absorption (NLA) in the case of 1 kHz pulse excitation has been decisively recognized. The synthesized nanocomposites were scrupulously probed in the near-IR regime, unveiling the presence of a prominent 3PA (with retrieved coefficients of ~10-23 cm3/W2 using kHz excitation), an essentially rare observation in these kinds of NPs. [ABSTRACT FROM AUTHOR]

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. ga1 • We developed an ultra-sensitive reusable SERS sensor based on laser ablated Au-sheet, decorated with hierarchical AuNPs. • The sensor demonstrates femtomolar sensitivity for the detection of DPA, DNT and PA with good reproducibility. • The achieved limit of detection (LOD) for DPA, DNT and PA were found to be better than the current best reported LODs. • The fabricated SERS sensors have potential to be used as an on-site sensing platform for the bio-chemical warfare agents. [ABSTRACT FROM AUTHOR]