Tailoring the ultrafast dynamics and electromagnetic surface modes on silicon upon irradiation with mid-infrared femtosecond laser pulses using SiO2 coatings

A key issue in the use of high-power mid-infrared (mid-IR) laser sources for a plethora of applications is the investigation of the exciting laser-driven physical phenomena taking place in materials coated with dielectric films. Here, we present a theoretical investigation of the ultrafast processes and thermal response upon excitation of two-layered complexes consisting of fused silica thin films placed on silicon substrates with ultrashort pulsed lasers in the mid-IR spectral regime. Through the development of a theoretical model, we demonstrate that the control of the underlying ultrafast phenomena and the damage threshold (DT) of the substrate are achieved via an appropriate modulation of the thickness of the SiO₂ film. It is shown that a decrease of DT by up to {~}30% compared with the absence of coating is feasible, emphasizing the impact of coatings of lower refractive index than the substrate. The conditions for surface plasmon (SP) excitation on the interface between SiO₂ and Si and the influence of the film thickness on the SP features are also discussed as such electromagnetic modes can initiate structuring on the interface. Our results manifested a striking impact of the presence and the coating thickness on the SP characteristics. It is shown that the SP excitation can be tailored by changing the superstrate thickness which also determines the competition of the air and the dielectric in the SP characteristics. These remarkable predictions can be employed for the development of optical coatings and components for nonlinear optics and photonics for a large range of mid-IR laser-based applications.