197 Defence Science Journal, Vol. 70, No. 2, March 2020, pp. 197-200, DOI : 10.14429/dsj.70.14823  2020, DESIDOC Direct Fabrication of sub-100 nm Nanoneedles in Silver using Femtosecond Laser Direct Writing Balaji Yendeti and Venugopal Rao Soma * Advanced Centre for Research in High Energy Materials, University of Hyderabad, Hyderabad - 500 046, India * E-mail: soma_venu@uohyd.ac.in AbStrAct 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 diferent numerical aperture. Further, we have investigated the efect of confnement. Finally, the application of nanoneedles is demonstrated for trace level detection of picric acid using surface enhanced Raman spectroscopy and a feld deployable portable Raman spectrometer. Keywords: Laser processing; Raman; Femtosecond; Silver nanoneedles 1. INtroDuctIoN Metal nanostructures with needles/needle-like sharp structures are of major interest in drug delivery applications, probing biological cells, controlled delivery of nucleic acids to selected tissues and surface enhanced Raman spectroscopy (SERS) studies 1-4 . These applications are attributed to the special mechanical characteristics like high aspect ratio, electrical and optical properties of nanoneedles. There are very few methods to produce metallic and non-metallic micro/nanoneedle structures 5-8 . Optical vortex pulses were previously used to fabricate metal nanoneedles 8 . Yang 7 , et al. have used confned laser spinning (CLS) method to produce gold nanoneedles. The spinning process was described as melting and evaporation of metallic thin flm, explosion of vaporised flm and lateral propagation to form nanoneedles. In their work, laser direct writing (LDW) onto a 30 nm thin Au flm confned by ITO coated glass plate produced nanoneedles in large scale. In this letter, we followed the CLS technique with a few modifcations. Specifcally, to produce metal nanoneedles of diferent sizes in large scale, it is easier to use metal sheet rather than thin flms. There are other methods, such as metal assisted chemical etching, for producing these type of nanostructures 9 . However, these methods are not suitable for etching hard metals such as silver and gold. Whereas our proposed method is suitable for creating nanostructures in soft semiconductor materials like Si and hard materials such as silver and gold simply by controlling the laser energy. Additionally, no chemicals are involved in the present method rendering this a green technique. Herein, we present results from the LDW [using ~50 femtosecond (fs) pulses] of silver (Ag) metal sheet with diferent focusing conditions to produce and control the sizes of nanoneedles. Furthermore, we have investigated the consequences of confned and unconfned laser spinning. Additionally, the fabricated Ag nanoneedles were utilised as active SERS substrates for detecting an explosive molecule (picric acid) at low concentrations. 2. MAterIALS AND MethoDS An ultrafast laser system (~4 mJ, 1 kHz ~50 fs, 800 nm) was employed to perform the LDW experiments. The incident laser pulse energy was controlled by the combination of half wave plate and Brewster polariser. Two diferent experiments were performed as shown in Fig. 1. In the frst experiment, pulses were focused by a 20X microscope objective (MO, Newport M-20X) of numerical aperture (NA) 0.40 onto silver metal sheet (Alfa Aesar, 99% pure) of dimensions 2×1.5×0.2 cm 3 . In the second experiment, pulses were focused with a 60X MO (Newport M-60X) and 0.85 NA onto the target and was covered on top with a microscope coverslip (130-170 µm thick). In these experiments, Ag target was placed onto a 3-dimensional (XYZ) stage and interfaced to motion controller (Newport ESP 300). The stage was programmed to move in two dimensions (X and Y) with velocity of 0.2 mm/s along X-axis and 2 mm/s along Y-axis and vice-versa in cross structure fabrication (Fig. 1(a)). Third axis was used to position the focal point of the MO onto the sample. In these experiments cross structures were fabricated on the Ag target. In frst experiment, laser was focused by 20X MO directly onto the metal sheet and the Ag nanoneedles can be observed on the edges of the laser spot focused on metal sheet. In the second experiment Received : 14 August 2019, Revised : 02 January 2020 Accepted : 22 January 2020, Online published : 09 March 2020