Enhanced Nonlinear Optical Responses of Nanoscale-Roughened Ag Wires via Spatially Extended Plasmonic Hotspots: Relevance to Optical Limiting Applications

Utilizing the inherent plasmonic effects of metal nanostructures and thereby attaining superior nonlinear optical responses are strongly dependent on the spatial distribution of plasmonic hotspots. Herein, we developed Ag wires (AgWs) with roughened nanostructures on the surface with a size less than 100 nm to maximize the plasmonic effect formation toward enhancement in nonlinear optical responses. The plasmon resonance enabled near-field effects induced an effective nonlinear absorption coefficient, β_eff of 48.00 cm/GW at an input pulse energy of 30 μJ, when these surface-roughened AgWs (SR-AgWs) were evaluated using an open aperture Z-scan technique with Nd:YAG nanosecond laser pulses of wavelength of 532 nm, pulse width of 7 ns, and repetition rate of 10 Hz. This β_eff value was found to be 92% higher than that of the pristine AgWs at the same input pulse energy, indicating the importance of plasmonic hotspots in nonlinear optical activity. The finite-difference time-domain (FDTD) simulations confirmed the presence of plasmonic hotspots distributed throughout the surface of SR-AgWs with an E-field intensity enhancement factor (|E|²/|E₀|²) of more than 1000. The occurrence of these hotspots was further confirmed via obtaining surface-enhanced Raman scattering signals from a 0.1 μM concentration of rhodamine 6G molecules decorated over the surface of SR-AgWs. Moreover, the optical limiting plots indicated the low limiting threshold of 3.65 J/cm², which is found to be noteworthy compared with the previously reported benchmark values. Effective nonlinear absorption and nonlinear scattering were attributed to the observed optical limiting response. These findings highlight the importance of introducing plasmonic hotspots to achieve enhanced nonlinear optical responses with potential implications for optical limiting applications.