Transient light-induced refractive index change made by laser microfabrication in nitroaniline-doped PMMA film

We report the observation of high light-induced change in refractive index (recognizable by observation in conventional microscope) in PMMA film doped with an optically non-linear dye 2-nitroaniline (NO₂(C₆H₄)NH₂ abbreviated as 2NA). The optically altered micrometer-sized regions were fabricated by single-shot irradiation of 120 fs laser pulses into doped PMMA film using high numerical aperture 1.3 and high magnification ×100 objective lens. The doping of films can be achieved in a wide range of 2NA concentrations (up to 40 wt%) without precipitation. This allows to control a storage time of an optically altered region up to one month by the adjusting the energy of the femtosecond (fs) recording pulse at 800 nm. Typical recording energy was 10-80 nj/pulse at the point of irradiation. Total recovery of transmission of the PMMA|2NA film was confirmed by optical transmission measurements in microscope. The light induced damage threshold (LIDT) (for permanent damage) was increased more than by 4 times (up to 40 nJ/pulse) when 2NA doping were ca. 13 wt%. While the LIDT for transient damage was decreased by 1.5-2 times. Total optical recovery was observed single exponential with decay time of ca. 0.5-1 min for moderate irradiation intensities (0.1×LIDT of permanent damage). The damage induced with at the higher intensities lasts up to month, but the recovery was not total (residual transmission changes were observable). The phenomenon can be applied for the optical memory, photonic crystal, and micro-mechanical applications. The underlying mechanism of the phenomenon is discussed in terms of anelastic α- and β-relaxation (polymer backbone and side chain relaxation, respectively).