Irradiation of ultrafast laser with laser fluence near the ablation threshold creates unique 1-dimensional (1D) or 2D periodic nanostructures on surfaces of various materials, including metals, semiconductors and dielectric materials. Such structures called laser-induced periodic surface structures (LIPSS) have been considered for the applications to wettability control, sensor, surface coloring, etc. [1-3]. In this presentation, we present our recent research on the fabrication of LIPSS on single crystal zinc oxide substrates by ultrafast laser. Specifically, we demonstrate the influence of laser processing parameters, including laser fluence, wavelength, scanning speed, pulse width, pulse number and polarization on morphologies of the formed LIPSS. The formation mechanism is briefly discussed based on the current theories of electromagnetic and matter reorganization models. The linearly polarized beam generates typical 1D LIPSS which is a nanostripe array, while the circularly polarized beam creates a nanodot array (2D LIPSS), as shown in Figures. By optimizing the laser processing parameters, the nanogaps in both 1D and 2D LIPSS with a width narrower than 20 nm can be fabricated. Interestingly, we find that the 2D LIPSS is oriented to rotate slightly according to the rotation direction of circularly polarized beam, forming an inwardly rotated structure. In addition, the morphology of periodic structure is highly influenced by other parameters such as the laser fluence, pulse numbers and pulse width. For example, the formation of high spatial frequency LIPSS (HSFL) or low spatial frequency LIPSS (LSFL) is governed by the laser fluence and pulse number, meaning there are critical conditions to allow the transition between HSFL and LSFL.

  The fabricated LIPSS is used for the surface-enhanced Raman scattering (SERS). SERS is an optical near-field effect, which can significantly enhance Raman scattering [4]. To create the SERS substrate, the periodic nanostructure of zinc oxide is coated with a thin noble metal layer. The influence of periodic nanostructure (such as period, gap size) and metal layer quality (such as thickness) on SERS intensity is investigated to explore the best conditions for achieving the highest performance for SERS. Owing to the narrow gaps formed on zinc oxide, a couple of applications for trace detection are illustrated to demonstrate the versatility of the SERS substrates.

References

[1] C. Florian, E. Skoulas, D. Puerto, A. Mimidis, E. Stratakis, J. Solis and Jan Siegel . Controlling the Wettability of Steel Surfaces Processed with Femtosecond Laser Pulses. ACS Applied Materials & Interfaces 10, 36564−365711 (2018).

[2] S. Bai, D. Serien, Y. Ma, K. Obata and K. Sugioka Attomolar Sensing Based on Liquid Interface-Assisted Surface-Enhanced Raman Scattering in Microfluidic Chip by Femtosecond Laser Processing. ACS Applied Materials & Interfaces 12, 42328−42338 (2020).

[3] J. Huang, L. Jiang, X. Li, A. Wang, Z. Wang, Q. Wang, J. Hu, L. Qu, T. Cui and Yongfeng Lu. Fabrication of highly homogeneous and controllable nanogratings on silicon via chemical etching-assisted femtosecond laser modification. Nanophotonics 8, 869-878 (2019).

[4] S. Bai, K. Sugioka. Recent Advances in the Fabrication of Highly Sensitive Surface-Enhanced Raman Scattering Substrates: Nanomolar to Attomolar Level Sensing. Light: Advanced Manufacturing 2, 13 (2021).