Laser-induced thermo-lens in ion-implanted optically-transparent polymer

A strong laser-induced thermo-lens (LITL) effect is found in optically-transparent ion-implanted polymer upon irradiation by a cw laser with a power up to 100 mW (λ = 532 nm). The effect is observed in bulk polymethylmethacrylate (PMMA) implanted with silicon ions (Si⁺). A series of PMMA specimens is examined, subjected to low-energy (50 keV) Si⁺ implantation at various dosages in the range from 10¹⁴ to 10¹⁷ ions/cm². The thermo-lensing is unambiguously attributed to the modification of the subsurface region of the polymer upon the ion implantation. Having a gradient refractive-index in-depth profile, the subsurface organic-carbonaceous layer produced in the polymer by ion implantation, is responsible for the LITL effect observed in reflection geometry. The LITL occurs due to optical absorption of the ion-implanted layer of a thickness of about 100 nm buried in a depth ~ 100 nm, and subsequent laser-induced change in the refractive index of the Si⁺-implanted PMMA. Being of importance as considering photonic applications of ion-implanted optically-transparent polymers, the LITL effect in Si⁺-implanted PMMA is studied as a function of the implant dose, the incident laser power and incidence angle, and is linked to the structure formed in this ion-implanted plastic.