Direct fabrication of rough gold nanoholes and investigation of their second-harmonic generation

In this study, we introduce a simple and cost-effective technique called one-photon absorption-based direct laser writing (OPA-based DLW) to directly fabricate noncentrosymmetric gold nanoholes (Au NHs). This technique relies on the optically induced local thermal effect at the focusing spot using a 532 nm excitation wavelength, which induces the evaporation of the Au thin film and thereby creating metal NHs. By controlling the exposure doses, including fabrication powers and writing velocities, we achieve Au NHs with small sizes around 300 nm and a periodicity of 500 nm. Moreover, due to the non-uniform heat transfer in an imperfectly flat Au film, the formed NHs lack perfect circularity, showing roughness and asymmetry. Thanks to this irregular shape of Au NHs, second-harmonic generation (SHG) signals are generated. This nonlinear signal can be amplified thanks to near electric field intensity enhancement at the borders of the holes. By moving the focusing spot, this DLW method allows us to fabricate any desired two-dimensional patterns, which exhibit SHG signals. This ability could be advantageous for applications in optical data storage and nonlinear imaging.