Flash Ablation of Tunable and Deep-Subwavelength Nanogap by Using a Spatially Modulated Femtosecond Laser Pulse for Plasmonic Application

The fabrication of gold nanostructures on a dielectric substrate with a nanogap between the opposite tips is suitable for the excitation of hot spots. In this study, a novel and efficient one-step method is reported for fabricating a gold bowtie nanogap by using a spatially shaped femtosecond laser pulse. The size of the nanogap could be flexibly controlled by adjusting the pulse energy. The smallest nanogap produced by the proposed method measures 30 nm (≈1/26 of the laser wavelength). The formation mechanism of the different morphologies is investigated. Combined with a phase- and amplitude-based beam-shaping technique, the bridge could be manipulated. Both experimental surface-enhanced Raman scattering spectra and simulated finite-difference time-domain results are used to characterize the plasmonic properties of the fabricated bowtie nanogap. The manufacturing scalability of the proposed method is demonstrated through the fabrication of a matrix of nanostructure, which exhibits great uniformity, achieving a density of 2.13 × 10⁶ devices cm^-2. Large-area microgaps can be patterned on a terahertz metasurface through the proposed method. The proposed technique provides a simple, flexible, and efficient alternative method for nanogap fabrication. The method can be extensively implemented in biosensing, photovoltaics, and nanophotonics.