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In this study, we fabricated laser-induced periodic surface structures (LIPSS) on a germanium surface through laser ablation in air using axicon and femtosecond (fs) pulses. This novel approach permitted the nanoscale material processing outcome refinement via an fs Bessel beam. Our investigations aimed at systematically understanding the formation of periodic structures under various experimental conditions, such as (i) different pulse energies ranging from 50 µ J to 1000 µ J at a constant scan speed and (ii) constant energy with different scan speeds (0.1–3 mm s−1). By adjusting the fluences and scan speeds, we were able to identify the parametric space and alter the periodicity of the low-spatial frequency LIPSS and high-spatial frequency LIPSS on germanium, which were analyzed using field emission scanning electron microscopy. An optimal LIPSS formation over a large area of germanium was achieved at an input energy of 250 µ J and a scan speed of 0.75 mm s−1. Additionally, we measured the contact angles of the Ge nanostructures (GeNSs) to demonstrate their hydrophobic nature and non-wetting properties, providing insights into the behavior of LIPSS. Subsequently, the GeNSs were coated with a ∼15 nm thick gold (Au) film using a thermal deposition method. Utilizing these, the surface-enhanced Raman spectroscopy (SERS) technique detected diverse analytes, such as tetryl (an explosive) at a concentration of 50 µ M and thiram (a pesticide) at 500 nM. The SERS enhancement factors for tetryl and thiram molecules on GeNSs coated with a 15 nm-thick Au layer were determined to be 2.5 × 104 and 4.2 × 105, respectively. [ABSTRACT FROM AUTHOR]

Here, we report a single-step, high-yield technique for producing planar sheets of wrinkle-free multi-layer graphene (MLG) under ambient conditions by ablation of graphite powder dispersed in ethanol using a femtosecond laser. The parameters for the synthesis of MLG have been optimized and established. The number of layers and quality of MLG produced at several laser fluences (39.4 J/cm2, 98.72 J/cm2, 197.45 J/cm2, 296.16 J/cm2) was established based on detailed characterization using field emission scanning electron microscopy (FESEM), high-resolution transmission electron microscopy (HRTEM) with selected area electron diffraction (SAED), atomic force microscopy (AFM), Raman spectroscopy, X-ray photoelectron spectroscopy (XPS), and Fourier transform infrared spectroscopy (FTIR). Wrinkle-free, ultra-flat crystalline MLG sheets with 2-12 layers were produced in each case with an average lateral dimension of ∼1--3 μm. The associated oxy- functional groups were characterized and quantified using FTIR and XPS. The highest percentage of sp2 C atoms and C/O ratio for MLG was achieved at the lowest laser fluence. The higher the laser fluence higher the defects produced in the MLG sheets. The technique produces zero chemical waste, has a very high yield of ∼ 95%, and is promising for bulk-scale production of MLG. [ABSTRACT FROM AUTHOR]

We report on the influence of resistivity in picosecond (ps) laser ablation of silicon (Si) leading to the formation of diverse surface micro- and nanostructures. Subsequently, we investigated their potential in sensing applications based on the surface enhanced Raman scattering (SERS) technique. The varying resistivity (ρ 1: 1â€"10 Ω cm, ρ 2: 0.01â€"0.02 Ω cm, ρ 3: 0.001â€"0.005 Ω cm) Si wafers were subjected to cross patterned ps laser ablation in ambient air. Ladder-like microstructures embedded with numerous nano growths were formed on low resistivity Si (ρ 3) while similar micro- and nanostructures were observed on higher resistivity Si (ρ 2 < ρ 1). The structures were non-plasmonic and anti-reflecting in nature with an optical reflectance of <6% over a broad range of wavelengths (350â€"1200 nm). Non-plasmonic Si microstructures were subsequently transformed to plasmonic by means of deposition of a thin layer of gold (Au). Additionally, the effect of annealing on the evolution of nanostructures was also investigated. We employed these hybrid substrates for the trace detection of an explosive molecule, ammonium nitrate, and dye, malachite green. Our detailed SERS studies have demonstrated a superior enhancement in the trace detection of analytes for low resistivity Si substrate. However, the annealed hybrid substrates have demonstrated further improvement in the SERS signal (by at least one order of magnitude). These detailed SERS investigations provide us a proof of the sensitivity of different resistivity Si nano/microstructures. [ABSTRACT FROM AUTHOR]

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. [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]

The development of recyclable surface enhanced Raman scattering (SERS) based sensors has been in huge demand for trace level explosives detection. A simple, hybrid Silicon (Si) nanotextured target-based SERS platform is fabricated through patterning micro square arrays (MSA) on Si using femtosecond (fs) laser ablation technique at different fluences. Using the hybrid target Si MSA substrate loaded/decorated with Ag–Au alloy NPs (obtained using femtosecond ablation in liquids) we demonstrate the trace level detection of organic nitro-explosives [picric acid (PA), 2,4-dinitrotoluene (DNT), and 1, 3, 5-trinitroperhydro-1, 3, 5-triazine (RDX)] and their mixtures. The microstructures/nanostructures of MSA fabricated at an input fluence of 9.55 J/cm2, and decorated with Ag–Au alloy NPs, exhibited exceptional SERS enhancement factors (EFs) up to ∼1010 for MB, ∼106 for PA, and ∼104 for RDX with the detection limits obtained being ∼5 pM, ∼36 nM, and ∼400 nM for MB, PA and RDX respectively. Furthermore, we demonstrate these SERS substrates possess good reproducibility (RSD values < 15%) and a superior performance compared to a commercial Ag substrate (SERSitive, Poland). Three binary mixtures, i.e. MB-PA, MB-DNT, PA-DNT at different concentrations, were also investigated using the same SERS substrate to test the efficacy. Further, the SERS spectra of dyes, explosives, and complex mixtures were utilized for discrimination/classification using principal component analysis. Image 1 • SERS substrates developed by combining nanotextured Si and Ag–Au alloy NPs. • Trace level detection of explosives achieved using portable Raman spectrometer. • LOD of ∼5 pM, ∼36 nM, and ∼400 nM achieved for MB, PA, and RDX, respectively. • Reproducible SERS measurements of binary, tertiary explosive mixtures. [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]

We demonstrate the detection of picric acid (PA) and ammonium nitrate (AN) at µM concentrations by utilizing gold (Au) nanostructures (NSs) as surface enhanced Raman scattering (SERS) substrates fabricated using the technique of ultrafast laser ablation in liquids. Au NPs and NSs were also utilized for detecting Rhodamine 6G (Rh6G) and methylene blue (MB). Detection of all the molecules using the same substrates (NPs and NSs) demonstrated their versatility. Detection limits of 10 −5 , 10 −6 , 10 −7 , 10 −8 M were achieved for AN, PA, Rh6G and MB, respectively. Reproducibility of the SERS intensity using NSs and NPs as substrates demonstrate their efficacy. [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]