Tailored Aggregate-Free Au Nanoparticle Decorations with Sharp Plasmonic Peaks on a U-Type Optical Fiber Sensor by Nanosecond Laser Irradiation

A novel experimental methodology is presented for fabricating U-shaped optical fiber probes decorated with aggregate-free Au nanoparticles exhibiting sharp localized surface plasmon resonance (LSPR) spectra. The U-type tip is coated with gold nanoparticles (AuNPs) using a simple and time-efficient dip-coating procedure, without initially taking any care to prevent the formation of nanoparticle aggregates in the coated area. In a second step, the coating was irradiated with a few tens of laser pulses of 5-ns duration at 532 nm with intensities in the range of 2–14 MW/cm², leading to the formation of aggregate-free LSPR optical fiber probes. The process was monitored and controlled in real time through the changes induced into the fiber’s extinction spectra by the laser irradiation, and the coated fibers were characterized by electron microscopy. The proposed methodology resulted into the fabrication of U-type optical fiber probes coated with AuNPs exhibiting a sharp plasmon peak, which is a perquisite for their application as sensing devices.