We present an inclusive overview of the ultrafast ablation technique performed in liquids. Being a comparatively new method, we bring out the recent progress achieved, present the challenges ahead, and outline the future prospects for this technique. The review is conveniently divided into five parts: (a) a succinct preamble to the technique of ultrafast ablation in liquids (ULAL) is provided. A brief introduction to the conventional ns ablation is also presented for the sake of completeness (b) fundamental physical processes involved in this technique are elaborated (c) specific advantages of the technique compared to other physical and chemical methodologies are enumerated (d) applications of this technique in photonics; biomedical and explosives detection [using surface-enhanced Raman scattering (SERS)] is updated (e) future prospects describing the potential of this technique for creating unique nanoparticles (NPs) and nanostructures (NSs) for niche applications. We also discuss some of the recently reported significant results achieved in a variety of materials, especially metals, using this technique. Furthermore, we present some of our own experimental data obtained from ULAL of Ag, Cu, and Zn in a variety of liquids such as acetone, water, acetonitrile etc. The generated NPs (colloidal solutions) and NSs (on substrates) have been successfully utilized for nonlinear optical, SERS, and biomedical applications.

Ultrafast laser pulses induced surface nanostructures were fabricated on a copper (Cu) target through ablation in acetone, dichloromethane, acetonitrile, and chloroform. Surface morphological information accomplished from the field emission scanning electron microscopic data demonstrated the diversities of ablation mechanism in each case. Fabricated Cu substrates were utilized exultantly to investigate the surface plasmon (localized and propagating) mediated enhancements of different analytes using surface enhance Raman scattering (SERS) studies. Multiple utility of these substrates were efficiently demonstrated by collecting the SERS data of Rhodamine 6G molecule and two different secondary explosive molecules such as 5-amino-3-nitro-l,2,4-triazole and trinitrotoluene on different days which were weeks apart. We achieved significant enhancement factors of >105 through an easily adoptable cleaning procedure.

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Hybrid silver−gold targets were achieved by effortless mixing of pure silver (Ag) and gold (Au) metals at different ratios (Ag 0.65 Au 0.35 , Ag 0.5 Au 0.5 , and Ag 0.35 Au 0.65 ) and embracing a manual melting process. The obtained targets were ablated by ultrafast (∼40 fs) laser pulses in acetone ensuing the fabrication of Ag−Au bimetallic nanoparticles (NPs) and nanostructures (NSs) in a single experiment. UV−visible extinction spectra of Ag−Au colloids demonstrated the tuning of localized surface plasmon resonance (LSPR) in the spectral range of 406−524 nm. The morphologies of NSs were investigated by the field emission scanning electron microscopy (FESEM) technique. Ag−Au NPs and NSs were utilized as surface enhanced Raman scattering (SERS) platforms to detect secondary explosive molecules such as 1,1-diamino-2,2-dinitroethene (FOX-7, 5 μM concentration) and 1-nitro pyrazole (1NPZ, 20 nM concentration). Our experimental observations clearly demonstrated that the increment in gold percentage reduced the surface activity of Ag−Au NPs/NSs. The estimated enhancement factors (EFs) from the SERS data were typically >10 8 . Our detailed investigations revealed that the NPs and NSs of Ag 0.65 Au 0.35 exhibited significant EFs compared to other ratios and pure metals of Ag and Au. ■ INTRODUCTIONFabrication of pure plasmonic metal (Au, Ag, and Cu) nanoparticles (NPs) achievable through sophisticated chemical methodologies is well established and understood by the scientific community. However, many of these methods demands hours of monitoring and post production processes such as cleaning the nanomaterials to remove chemical dopants and impurities. Similarly, shape controlled production of bimetallic NPs through utilizing surfactants and reagents is also well-known in chemical methods. Many of the earlier reports revealed that bimetallic (Au−Ag, Ag−Cu, and Au−Cu) NPs were fabricated by adding individual colloidal solutions of gold, silver, and copper with different proportions to achieve hybridization of the localized surface plasmon resonances (LSPRs). Ultrafast laser ablation in liquids (ULAL) is a clean, green method which does not utilize chemicals for fabrication of NPs/NSs, and importantly, it does not necessitate extreme cleaning of NPs/NSs. Moreover, simultaneous fabrication of NPs and NSs is possible in the ULAL technique 1−15 in contrast to other chemical methods. Even though some of the solution (chemical) methods are fast (in terms of time taken) compared to various ablation techniques, the main problem with them is that the capped ligand molecules sit on surface of NPs and thus blocks the analyte molecules to achieve direct contact with NP surface. The main objective of fabricating alloy nanomaterials is to find out the exact proportion of individual metals, which exhibits superior performance (in this case, our interest is in the surface enhanced Raman signal) and, hence, are versatile and compatible in many fields such as biomedicine, spectroscopy, and photonics. For example, it has been demonstrated th...

Silicon (Si) nanoparticles (NPs) and self-organized high spatial frequency laser (HSFL) induced periodic surface structures were fabricated by means of femtosecond ablation of bulk Si target in acetone. The ablation was performed with ∼40 fs (fwhm) pulses and different input energies of ∼500, ∼200, ∼150, ∼100, ∼50, and ∼10 μJ. Fabricated NPs and nanostructures (NSs) were characterized by UV–visible absorption spectroscopy, photoluminescence (PL) spectroscopy, Raman spectroscopy, transmission electron microscopy, and field emission scanning electron microscopy. The average sizes of the NPs were estimated to be in the 4–135 nm range. From the PL studies of Si NPs of different sizes, we have observed a size-dependent shift toward blue spectral region. We could tune the observed PL peak in the spectral range of 440–515 nm. The crystalline and amorphous nature of the Si nanoparticles and nanostructures was investigated using selected area electron diffraction and Raman spectra. Complex refractive index, conduction band electron density of the Si NPs, estimated by measuring the effective spot size corresponding to each input energies, were observed to play a crucial role in determining the periodicity of HSFL induced periodic surface structures. Experimentally measured periodicity of gratings was in good agreement with the theory.